i

LAPORAN AKHIR

PENELITIAN PASCA SARJANA

DANA ITS 2020

PENGEMBANGAN KAPAL TRIMARAN GREEN ENERGI UNTUK

OPTIMALISASI KEBUTUHAN POWER

Tim Peneliti :

Prof. Ir. I Ketut Aria Pria Utama, M.Sc., Ph.D. (Teknik Perkapalan/FTK)

Dr. Ir. I Ketut Suastika, M.Sc. (Teknik Perkapalan/FTK)

Egi Yuliora, S.T., M.T. (Teknik Perkapalan/FTK/ITS)

Sutiyo, ST. (Teknik Perkapalan/FTK)

DIREKTORAT RISET DAN PENGABDIAN KEPADA MASYARAKAT

INSTITUT TEKNOLOGI SEPULUH NOPEMBER

SURABAYA

2020

Sesuai Surat Perjanjian Pelaksanaan Penelitian No: 920/PKS/ITS/2020

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Daftar Isi

Daftar Isi ............................................................................................................................................ i

Daftar Tabel ...................................................................................................................................... ii

Daftar Gambar ................................................................................................................................. iii

Daftar Lampiran ............................................................................................................................... iv

BAB I RINGKASAN ....................................................................................................................... 1

1.1 Latar Belakang ........................................................................................................................ 1

1.2 Tujuan, Manfaat dan Dampak Kegiatan yang Diharapkan ..................................................... 4

1.3 Tahapan dan Metode Penelitian .............................................................................................. 4

1.3.1 Metode Numerik (CFD) ................................................................................................... 5

1.3.2 Metode Eksperimen Model Fisik di Towing Tank ...................................................... 7

1.4 Target Luaran .......................................................................................................................... 8

BAB II HASIL PENELITIAN.......................................................................................................... 9

2.1 Pembuatan Model Kapal ......................................................................................................... 9

2.2 Analisis Hambatan dan Daya Mesin Kapal ........................................................................... 11

2.3 Analisis Numerik CFD ..................................................................................................... 12

2.4 Analisis Numerik CFD ..................................................................................................... 13

2.5 Meshing dan Grid Independence Study ................................................................................ 14

BAB III STATUS LUARAN.......................................................................................................... 15

BAB IV KENDALA PELAKSANAAN PENELITIAN ................................................................ 16

BAB V DAFTAR PUSTAKA ........................................................................................................ 17

BAB VI LAMPIRAN ..................................................................................................................... 19

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Daftar Tabel

Tabel 2. 1 Ukuran Utama Kapal Trimaran........................................................................................ 9

Tabel 2. 2 Nilai Hambatan Kapal (N) ............................................................................................. 11

Tabel 2. 3 Grid Independence Study ............................................................................................... 14

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Daftar Gambar

Gambar 2. 1 Model 1: Kombinasi main hull NPL dan side hull NPL .............................................. 9

Gambar 2. 2 Model 2: Kombinasi main hull NPL (+ axe-bow) dan side hull NPL ....................... 10

Gambar 2. 3 Dimensi Kapal ............................................................................................................ 10

Gambar 2. 4 Hambatan Kapal ......................................................................................................... 12

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Daftar Lampiran

LAMPIRAN 1 Tabel Daftar Luaran

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BAB I RINGKASAN

1.1 Latar Belakang

IMO menetapkan batas atas kandungan sulfur dalam bahan bakar sebesar 0.5% yang

diberlakukan mulai 1 Januari 2020 (IMO 2018), melalui pertemuan ke-72 Komite Perlindungan

Lingkungan Laut (MEPC) yang digelar di London pada 9-13 April 2018. Kebijakan ini secara

signifikan akan mengurangi jumlah sulfur oksida yang berasal dari emisi kapal. Regulasi ini akan

memberikan dampak positif yang besar bagi kesehatan dan lingkungan, khususnya bagi penduduk

yang tinggal di dekat pelabuhan dan pantai. Pemerintah Indonesia selaku anggota Dewan

International Maritime Organization (IMO) harus melaksanakan setiap ketentuan yang telah

dikeluarkan IMO termasuk penerapan aturan Marine Pollution (Marpol).

Sulfur Oksida memiliki 16,4% bagian dari komposisi polutan yang mencemari udara.

Sedangkan, komposisi terbesarnya adalah karbon monoksida, yaitu sebesar 49,1%, lihat Gambar 1.

1. Selanjutnya penyebab utama polusi udara adalah dari kendaraan bermotor yaitu sebesar 46,2%.

ICCT (2017) melaporkan bahwa kegiatan di sektor pelayaran seluruh dunia berkonstribusi

menyumbang Gas Rumah Kaca (GRK) sebesar 11%, seperti yang ditunjukkan pada Gambar 1. 2

(Olmer, et al. 2017). Organisasi Maritim Internasional (IMO) melalui PBB mengkoordinasikan

keselamatan maritim internasional dan pelaksanaannya dengan melakukan kerja-sama antar-

pemerintah dan antar-industri pelayaran untuk meningkatkan keselamatan maritim dan untuk

mencegah polusi udara dan air laut.

Gambar 1. 1 Komposisi pencemaran udara dan faktor penyebabnya

Terdapat tiga cara bagi kapal untuk memenuhi IMO GSC 0,5% ini. Pertama adalah

menggunakan bahan bakar LNG atau bahan bakar cair bertitik nyala rendah (Low Flashpoint Fuel)

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seperti methanol. Cara kedua adalah dengan menggunakan scrubber (sistem pembersih gas buang).

Pemasangan sistem Scrubber harus memenuhi ketentuan dan persetujuan otoritas pelayaran negara

bendera. Tidak banyak galangan kapal yang memiliki kualifikasi instalasi scrubber yang diakui.

Cara ketiga dalah beralih ke BBM yang berkadar sulfur rendah (low sulphur fuel) di bawah 0.5

persen (Jurnal Maritim 2018).

Gambar 1. 2 Konstribusi pelayaran pada emisi GRK dan konsumsi BBM global (Kodjak 2015)

(a) (b)

Gambar 1. 3 Jenis Kapal (a) kapal Monohul dengan axe-bow; (b) Kapal Cepat Trimaran

Peraturan IMO secara khusus dibelakukan untuk melindungi lingkungan dari polusi gas

buang yang disebabkan oleh kapal dari pemakain bahan bakar. Selain ketiga cara tersebut (Gelling

2006) dan (Romadhoni and Utama 2015) telah melakukan penelitian yang menunjukkan bahwa

kapal monohull menggunakan axe-bow mampu mereduksi hambatan antara 8-20%. Selanjutnya,

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perkembangan teknologi rancang bangun kapal yang berkembang pesat dengan ditunjukkan dengan

pengembangan berlambung banyak termasuk kapal trimaran. Hasil peneletian (Murdijanto, Utama

and Jamaluddin 2011) menemukan bahwa kapal dengan katamaran dan trimaran dapat memiliki

hambatan yang lebih kecil dibandingkan dengan monohull dengan displasmen yang sama.

Karakteristik seakeeping kapal multihull memilki olah gerak yang sebanding dengan monohull. Ini

mengindikasikan hal yang baik yaitu bahwa katamaran/trimaran untuk sungai adalah kapal yang

efisien dan nyaman. Jika prototipe atau kapal nyata dikembangkan bisa menjadi kapal yang sangat

efisien serta kapal dengan standar keamanan tinggi. Penelitian yang dilakukan oleh (Luhulima,

Sutiyo and Utama 2017) menunjukkan bahwa kapal trimaran mampu mereduksi bahan bakar

sampai 11,7%, bahkan (Zong, et al. 2015) melakukan optimasi lambung kapal trimaran yang

mampu mengurangi hambatan sampai 21,34%. Kelebihan kapal trimaran dapat menjadi

pertimbangan untuk pembangunan kapal baru. Penggabungan dari penggunaan axe-bow dan

lambung trimaran memiliki implikasi penggunaan bahan bakar menjadi sangat efisien, lihat Gambar

1. 3.

Tantangan terhadap tekanan ekonomi dan aturan lingkungan yang cukup ketat menciptakan

kebutuhan akan inovasi baru untuk estimasi daya dorong kapal dan pilihan mesin yang sesuai, serta

desain lambung kapal yang optimal untuk bangunan baru. Untuk meminimalkan daya pendorong

dapat diatasi pada tahap desain yaitu desain bentuk lambung dan desain propulsor. Metodologi dan

prosedur desain kapal yang sesuai, dengan mempertimbangkan faktor ekonomi dan lingkungan

untuk kapal masa depan dibahas secara komprehensif oleh (Molland, et al. 2014).

Salah satu tantangan yang dihadapi oleh naval architects adalah keakurasian dalam

memprediksi karakteristik hidrodinamikanya, khususnya pada aspek hambatan, propulsi serta

penentuan mesin utama penggerak kapal yang dapat mengurangi pencemaran udara dan lingkungan

laut. Penelitian ini difokuskan pada optimalisasi kapal trimaran dengan menggunakan axe-bow yang

sesuai dengan perairan Indonesia yang difokuskan pada kajian hambatan, konsumsi bahan bakar

dan pengaruhnya terhadap lingkungan.

Perhitungan hambatan kapal dilakukan dengan 2 cara: kajian numerik dan eksperimental.

Kajian numerik dengan simulasi CFD (Computational Fluid Dynamics) menggunakan teknologi

komputer berkecepatan tinggi dan menghasilkan penyelidikan model sangat teliti. Kajian

eksperimen merupakan pengujian geometri 3-dimensi ukuran badan kapal dalam skala kecil pada

kolam uji (towing tank) yang memenuhi standard ITTC. Hasil yang diperoleh diharapkan dapat

memberikan kontribusi dalam memprediksi secara akurat terkait hambatan trimaran yang

selanjutnya dapat diaplikasikan untuk menghitung kebutuhan bahan bakar yang diperlukan. Hasil

evaluasi penelitian ini berupa kajian hambatan kapal trimaran yang efisien dalam kaitan dengan

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penentuan kapasitas mesin penggerak kapal yang dapat mengurangi pemakaian BBM serta mampu

memenuhi persyaratan IMO dan EEDI.

1.2 Tujuan, Manfaat dan Dampak Kegiatan yang Diharapkan

Tujuan penelitian adalah melakukan analisa dan evaluasi parameter hidrodinamika dan

korelasinya dan pengaruh interferensi komponen-komponen hambatan antara satu dengan lainnya

secara sistematis dan rinci dalam kajian interferensi komponen hambatan kapal trimaran. Adapun

tujuan lengkap dari penelitian ini adalah:

1. Melakukan review dari state of the art pada komponen hambatan lambung trimaran yang simetris

dengan konfigurasi bentuk main hull NPL dan axe-bow.

2. Mengetahui pengaruh jarak S/L secara melintang terhadap hambatan kapal trimaran dengan

menggunakan axe-bow.

3. Mengembangkan pengetahuan perihal interaksi dan interferensi komponen hambatan viskos dan

gelombang melalui eksperimen model fisik dan simulasi CFD.

4. Mengidentifikasi pengaruh interaksi dan interferensi pada lambung trimaran dan menjelaskan

pengaruh tersebut pada komponen-komponen hambatan serta melakukan observasi terhadap

efek hidrodinamika yang terjadi.

5. Menganalisa, mengevaluasi serta mengidentifikasi komponen interferensi hambatan bentuk

(form drag), interferensi hambatan viskos dan interferensi hambatan gelombang melalui kajian

yang sistematis dan akurat.

Hasil penelitian ini diharapkan dapat memperkaya dan memperkuat database dalam

mempresentasikan pengaruh interferensi komponen hambatan pada lambung kapal katamaran dan

selanjutnya dapat diaplikasikan secara lansung dalam perhitungan hambatan yang digunakan untuk

penentuan tenaga mesin kapal katamaran pada tahapan desain (preliminary design).

1.3 Tahapan dan Metode Penelitian

Metodologi penelitian yang digunakan untuk memecahkan masalah kapal trimaran green

energy dan pengaruhnya terhadap lingkungan tersebut dibagi dalam 2 tahap yaitu perhitungan/

simulasi numerik dan pengujian model skala fisik, seperti ditunjukkan pada Gambar 1. 4.

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Gambar 1. 4 Diagram Alir Penelitian

1.3.1 Metode Numerik (CFD)

Computational Fluid Dynamics merupakan penyelesaian numerik dari dinamika fluida

(Bertram 2011). Pada kasus kapal, CFD sangat membantu dalam mengekspresikan fenomena aliran

fluida di sekitar lambung kapal, termasuk masalah interferensi dan interaksi komponen hambatan

pada lambung katamaran dan multihull (Luhulima, Sutiyo and Utama 2017) dan (Murdijanto,

Utama and Jamaluddin 2011)

Dalam desain kerjanya, problem perlu dideskripsikan dengan menggambarkan model yang

akan dianalisa, sifat-sifat fluida di sekitar model dan penentuan kondisi batasnya. Selanjutnya dalam

solver problem akan dihitung dengan persamaan Navier-Stokes yaitu persamaan kekekalan massa,

momentum, dan energi pada setiap titik pada grid 2D atau 3D. Dari hasil perhitungan tersebut akan

diperoleh hasil output dari simulasi program CFD.

Pada proses pemodelan kapal katamaran, analisa CFD dilakukan dengan bantuan software

ICEM CFD dan CFX yang merupakan produk dari ANSYS. ICEM CFD digunakan pada tahap

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pembuatan geometri lambung tahap meshing baik pada model maupun pada fluida. Sedangkan

untuk pengerjaan tahap selanjutnya digunakan CFX. Analisa CFD yang akan dilakukan pada

pemodelan lambung katamaran ini adalah pemodelan aliran dan perhitungan besarnya hambatan

pada lambung tersebut dan visualisasi aliran fluida. Program CFD terdiri dari tiga tahap yaitu: Pre-

processor, Flow Solver (Solution), dan Post-processor.

Keakurasian hasil analisis CFD ditentukan oleh 3 (tiga) factor (ANSYS 2013) yaitu:

a. Konvergensi, yaitu analisis kebenaran internal dimana tingkat kesalahan yang dirancang

dipenuhi oleh model yang dikembangkan. Jika nilai konvergensi / variable value di bawah 10-4

untuk model benam dan 10-5 untuk model free-surface.

b. Studi grid independence, yaitu pengetahuan tentang efisiensi pemakaian grid.

c. Verifikasi, yaitu membandingkan hasil CFD dengan data lain yang ada sehingga secara realistis

kebenaran dapat diterima.

Gambar 1. 5 Diagram komputasi pada program ANSYS CFX

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Gambar 1. 5 memperlihatkan skema perhitungan dengan menggunakan program Ansys

CFX. Struktur ANSYS CFX terdiri dari 4 modul software yang memerlukan geometri dan mesh

untuk memberikan informasi yang dibutuhkan dalam menampilkan analisa CFD. Komponen

ANSYS CFX antara lain ANSYS CFX-Pre sebagai bagian dari Physics Pre-Processor, dilanjutkan

dengan ANSYS CFX-Solver yang bertautan dengan ANSYS CFX-Solver Manager sebagai bagian

untuk memecahkan atau menjalankan simulasi dan ANSYS CFD-Post yang merupakan modul

untuk menampilkan hasil simulasi yang dirangkai dengan berbagai visualisasi aliran.

1.3.2 Metode Eksperimen Model Fisik di Towing Tank

Pengujian model fisik di towing tank dilakukan berdasarkan rekomendasi ITTC

(International Towing Tank Conference), baik prosedur pengujian maupun analisa pengukuran.

Metode pengukuran hambatan pada model kapal melalui eksperimen di towing tank, umumnya,

terdiri atas dua metode (ITTC 2011) yang biasa digunakan:

a. Mengukur total hambatan dan mengaplikasikan formulasi empiris untuk hambatan gesek

(friction).

b. Mengukur lansung komponen-komponen hambatan dengan menggunakan teknik eksperimen

yang kompleks dan pengujian model yang cukup banyak.

Pada penelitian ini, metode pertama yang akan digunakan dengan melakukan teknik dan prosedur

pengukuran sebagai berikut:

• Mengukur besarnya total komponen hambatan (RT) berdasarkan variasi kecepatan dan

konfigurasi jarak antara lambung kapal (secara melintang dan membujur), termasuk:

• mengamati refleksi dan interaksi gelombang pada lambung kapal

• mengamati aliran dan gelombang disekitar lambung kapal.

• mengamati gelombang depan (bow) dan belakang (stern) yang ditimbulkan oleh mainhull dan

sidehull

Gambar 1. 6 Alat ukur stain gauge satu sumbu

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Total hambatan lambung kapal diukur dengan load cell transducer. Load cell adalah suatu

transducer gaya yang bekerja berdasarkan prinsip deformasi suatu material akibat adanya tegangan

mekanis yang bekerja. Besar tegangan mekanis berdasarkan pada deformasi yang diakibatkan oleh

regangan. Regangan tersebut terjadi pada lapisan permukaan dari material sehingga dapat terukur pada

alat sensor regangan atau strain gage yang dapat di lihat pada Gambar 1. 6. Strain gauge ini merupakan

transducer pasif yang merubah suatu pergeseran mekanis menjadi perubahan tahanan/hambatan.

Gambar 1. 7 Kolam uji model (towing tank) ITS

Dimensi partikular kolam uji (towing tank) berukuran 50 m panjang, 3 m lebar dan 2 m

kedalam air, sebagaimana yang diperlihatkan pada Gambar 1. 7. Kecepatan kereta tarik (towing

carriage) maksimum 4 m/detik.

1.4 Target Luaran

Target luaran yang diharapkan dari penelitian ini adalah:

1. Publikasi pada jurnal international terindeks Scopus Q1 (International Journal of Engineering,

IJTech) dengan judul “Experimental and Numerical Investigation into the Effect of Axe-Bow

on Drag Reduction of Trimaran Configuration.”

2. Publikasi pada seminar internasional SRCM 2020 yang diselenggarakan pada 30 November

2020 di Surabaya. Judul paper “CFD Analysis into the Drag Characteristics of Trimaran Vessel:

Comparative Study between Standard NPL 4a and the Use of Axe-Bow.”

3. Thesis (S2) atas nama Sutiyo, NRP.04111950030005 dengan Judul: “Pengembangan Kapal

Trimaran Green Energi untuk Optimalisasi Kebutuhan Power”

Kata-kata kunci: trimaran, axe-bow; IMO, hambatan, interferensi

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BAB II HASIL PENELITIAN

2.1 Pembuatan Model Kapal

Pembuatan geometri kapal trimaran dilakukan dengan menggunakan bantuan perangkat

lunak design modeler sesuai dengan ukuran utama kapal yang telah didesain. Ukuran utama dari

kapal trimaran yang digunakan dalam pengujian dapat dilihat pada Tabel 2. 1 Ukuran Utama Kapal

Trimarandan secara geometri diperlihatkan pada Gambar 2. 1 dan Gambar 2. 2.

Tabel 2. 1 Ukuran Utama Kapal Trimaran

Parameter Unit Model 1 Model 2

LOA m 1,252 1,252

LWL m 1,218 1,252

B m 0,848 0,848

T m 0,067 0,096

H m 0.75 0.75

Displacement m3 6,238 6,238

S(S/L=0.3) kg 0,376 0,376

S(S/L=0.4) m 0,501 0,501

Gambar 2. 1 Model 1: Kombinasi main hull NPL dan side hull NPL

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Gambar 2. 2 Model 2: Kombinasi main hull NPL (+ axe-bow) dan side hull NPL

Gambar 2. 3 Dimensi Kapal

Keterangan :

L adalah Panjang kapal

B adalah Lebar Kapal

S adalah Jarak Garis tengah Antara Mainhull dan Sidehull

LM adalah Panjang Maibhull

BM adalah Lebar Mainhull

LS adalah Panjang Sidehul

BS adalah Lebar Sidehull

D adalah Selisih Antara LM dan LS

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Untuk melakukan pengujian hidrodinamika ketiga lambung kapal ini dimodelkan dengan

bantuan perangkat lunak permodelan tiga dimensi untuk mendapatkan bentuk lambung (hull form)

sehingga nantinya model tersebut akan diuji performa hidrodinamikanya dengan menggunakan

perangkat lunak Computational Fluid Dynamic (CFD). Untuk memberikan gambaran terkait bentuk

model lambung kapal dapat dilihat pada Gambar 2. 3 Dimensi Kapal yang merupakan ilutrasi tiga

dimensi dari model lambung kapal.

2.2 Analisis Hambatan dan Daya Mesin Kapal

Analisis terkait nilai hambatan dari model lambung kapal dilakukan pengujian di Towing

Tank, FTK-ITS. Perhitungan hambatan kapal dilakukan pada kecepatan tertentu, pada penelitian

ini rentang kecepatan dalam pengujian nilai hambatan kapal yaitu pada Fr = 0.15 - 0.5. Hasil

pengujian hambatan dapat dilihat pada Tabel 2. 2 dan Gambar 2. 4.

Tabel 2. 2 Nilai Hambatan Kapal (N)

Trimaran Trimaran

Trimaran Trimaran

Selisih (%)

Fr NPL Axe-Bow

NPL Axe-Bow

0,3 0,4 0,3 0,4 S/L=0,3 S/L=0,4

0,15 0,6 0,6 0,8 0,8 0,7 0,7 -12,5 -15,8

0,20 1,4 1,5 1,6 1,5 1,4 1,3 -10,5 -11,1

0,25 1,6 1,8 2,3 2,1 2,1 2,0 -5,6 -2,0

0,30 2,5 2,5 3,5 3,4 3,1 2,9 -12,9 -13,6

0,4 4,2 4,0 6,3 6,1 6,0 5,6 -4,6 -7,6

0,5 7,8 7,6 9,3 9,6 10,8 9,8 15,7 1,7

Selisih rata-rata -5,1 -8,1

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Gambar 2. 4 Hambatan Kapal

2.3 Analisis Numerik CFD

Analisis numerik (CFD) didasarkan pada penggunaan Persamaan Navier-Stokes, dimana

persamaan kendali (governing equations) untuk aliran fluida dengan kekentalan yang tidak dapat

dimampatkan (incompressible viscous flow) menggunakan persamaan RANSE (Reynold–Averaged

Navier–Stokes Equations) yang paling banyak digunakan dalam analisis numerik CFD karena lebih

efisien dan murah, tetapi memberikan hasil akhir yang cukup akurat. Detail persamaan tersebut

dijelaskan sebagai berikut:

(2.1)

Dimana 𝑢𝑖, 𝑢𝑗 menandakan rata-rata waktu dari komponen kecepatan, u′i, u′j adalah fluktuasi

dari komponen kecepatan, 𝑝 adalah rata-rata waktu dari tekanan, ρ adalah koefisien viskositas

dinamis, t adalah waktu dan xi, xj adalah unit vektor pada arah i dan j. Selanjunya, untuk melengkapi

persamaan tersebut, digunakan persamaan pemodelan aliran turbulen (Shear Stress Transport, SST).

Variabel peubah dari besaran energi kinetik turbulen, k, dijelaskan sebagai berikut:

(2.2)

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Variabel transportasi kedua (ω) dinyatakan sebagai:

(2.3)

Gk, Gω menggambarkan energi kinetik turbulen akibat gradien kecepatan rata-rata. Yk, Yω adalah

komponen disipasi turbulen; Γk, Γω menyatakan komponen difusi efektif; Dω divergensi

orthogonal.

Volume aliran model (VOF) digunakan untuk melacak lokasi dan perkembangan dari permukaan

bebas [12]. Ide dasar dari VOF adalah mendefinisikan fungsi penanda (α) dalam domain diskrit dan

untuk menentukan nilai dari fungsi volume di dalam grid dalam kaitannya dengan volume fluida di

dalamnya, ketika nilai α adalah 1 atau 0, maka hanya ada 1 fluida di dalam grid. Kerika nilai α

antara 0 dan 1, maka di dalamnya terdapat 2 macam fluida, berarti bahwa di sana terdapat

permukaan bebas di dalam grid. Nilai α memenuhi persamaan transport berikut:

(2.4)

2.4 Analisis Numerik CFD

Dimensi dari domain dan kondisi batas ditunjukkan pada Gambar 2. 5. Mengingat prinsip

karakteristik simetri dari medan aliran fluida, maka hanya separuh dari benda uji yang dimodelkan.

Kondisi batas didetailkan sebagai berikut: benda uji dinyatakan sebagai batas bergerak dimana tidak

ada slip pada permukaan bebas benda; kondisi bebas slip dipasang di atas, sisi dan dasar dinding;

kondisi simetri digunakan pada bidang pusat benda; kecepatan aliran pada inlet dinyatakan sebagai

kecepatan uji; pada outlet, tekanan hidrostatik dinyatakan sebagai fungsi ketinggian air/fluida;

selanjutnya, lokasi awal dari permukaan bebas ditentukan dengan mendefinisikan fungsi fraksi

volume dari air dan udara di daerah inlet dan outlet.

Gambar 2. 5 Kondisi Batas.

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2.5 Meshing dan Grid Independence Study

Pembuatan mesh generation dalam penelitian ini menggunakan Design Modeler.

Perhitungan domain dibatasi menggunakan structured dan unstructured mesh. Berkenaan dengan

geomteri yang kompleks dari karakteristik lambung kapal, maka mesh dengan elemen segitiga

dibuat pada permukaan lambung dan lapisan batas disemournakan dengan elemen prisma yang

dibuat dengan memperpanjang node pada permukaan mesh. Daerah di sekeliling benda/kapal diisi

dengan elemen tetrahedral dengan inflasi, sementara di daerah yang jauh dari benda, unstructured

mesh dengan grid yang terdistribusi dibuat untuk mengurangi jumlah elemen.

Ukuran mesh memainkan peranan penting di dalam prosedur perhitungan, dimana mesh

yang rapat (fine mesh) akan selalu memberikan hasil yang akurat dan benar di dalam ANSYS CFX,

tetapi pada waktu yang sama akan menambah waktu perhitungan/komputasi sehingga menambah

biaya komputasi dan waktu yang dibutuhkan untuk mengerjakan model yang besar dan kompleks.

Karena itu, untuk menentukan ukuran mesh dengan akurasi numerik dan jumlah elemen yang dapat

diterima, maka dilakukan studi konvergensi pada model kapal trimaran dengan S/L=0.3 dan angka

Froude 0.4. Selanjutnya hasil analisis grid independence diperlihatkan pada Tabel 2.3 dan Gambar

2.6.

Tabel 2. 3 Grid Independence Study

Jumlah elemen (x1000) 192 398 823 1,583 2,876

Koefisien hambatan total (CT)

(x10-3) 13.365 9.6136 8.1033 7.2356 7.1153

Selisih (%) 28.07 15.71 10.71 1.66

Gambar 2. 6 Grid Independence Study

15

BAB III STATUS LUARAN

Dalam penelitian ini dihasilkan luaran berupa model kapal trimaran yang terdiri dari

lambung utama (main hull) yang tidak menggunakan dan menggunakan axe-bow dan 2 lambung

samping (side hulls). Luaran paper berupa tulisan pada seminar internasional terindeks Scopus dan

jurnal ilmiah internasional terindeks Scopus.

1. Artikel seminar ilmiah internasional dengan judul CFD Analysis into the Drag Characteristics

of Trimaran Vessel: Comparative Study between Standard NPL 4a and the Use of Axe-Bow yang

dipresentasikan pada seminar internasional SRCM 2020 yang diselenggarkan oleh DRPM ITS

pada tanggal 30 November 2020. Status paper sudah dibacakan dan akan dipublikasikan pada

IOP Conference Proceedings yang terindeks Scopus.

2. Artikel ilmiah jurnal internasional dengan judul Experimental and Numerical Investigation into

the Effect of Axe-Bow on Drag Reduction of Trimaran Configuration. Status paper berupa

draft/persiapan dan akan disubmit ke jurnal internasional (International Journal of Technology),

terindeks Scopus Q1.

16

BAB IV KENDALA PELAKSANAAN PENELITIAN

Kendala Pelaksanaan Penelitian berisi kesulitan atau hambatan yang dihadapi selama

melakukan penelitian dan mencapai luaran yang dijanjikan:

1. Kesulitan validasi terkait hasil analisis menggunakan software CFD dan penentuan jumlah grid

yang optimum dalam melakukan simulasi serta proses komputasi yang tidak stabil menyebabkan

proses komputasi menjadi lama.

2. Proses pembuatan model kapal memakan waktu cukup lama, terutama terbatasnya akses pada

saat pandemic covid-19.

3. Proses uji model di towing tank membutuhkan waktu yang cukup lama berkenaa dengan proses

kalibrasi dan akurasi hasil akhirnya.

4. Proses analisis data membutuhkan waktu yang cukup lama.

17

BAB V DAFTAR PUSTAKA

ANSYS. 2013. ANSYS CFX-SOlver Model Guide. ANSYS Academic Research .

BENTLEY. 2018. "User Manual MAXSURF RESISTANCE."

Bertram, Volker. 2011. Practical Ship Hydrodynamics, 2nd Edition. Butterworth-Heinemann.

Deng, R, D B Huang, L Yu, X K Cheng, and H G Liang. 2011. "Research on factors of a flow field

affecting catamaran resistance calculation." Harbin Gongcheng Daxue Xuebao/Journal

Harbin Eng. Univ.

Dinham-Peren, T, C Craddock, A Lebas, and A Ganguly. 2008. Use Of Cfd For Hull Form And

Appendage Design Assessment On An Offshore Patrol Vessel And The Identification Of A

Wake Focussing Effect. RINA Marine CFD.

Faltinsen, Odd M. 2005. Hydrodynamics of High-Speed Marine Vehicles. Cambridge: Cambridge

University Press.

Gelling, J L. 2006. "the Axe Bow: the Shape of Ships to Come." Internasioanl HISWA Symposium

on Yacht Design and Yacht Construction, 19th. Amsterdam.

IMO. 2018. Guidance on The Development of a Ship Implementation Plan for The Consistent

Implementation of the 0.50% Sulphur Limit under MARPOL ANNEX VI. International

Maritime Organization.

ITTC. 2011. ITTC - Recommended Procedures and Guidelines - Resistance Test (7.5-02-0.2-0.1).

ITTC.

Jurnal Maritim. 2018. Mulai 2020, IMO Tetapkan Global Sulphur Cap 0.5 Persen. Milestone

Evolusi Pelayaran Dunia? April 20. https://jurnalmaritim.com/mulai-2020-imo-tetapkan-

global-sulphur-cap-0-5-persen-milestone-evolusi-pelayaran-dunia/.

Kodjak, Drew. 2015. POLICIES TO REDUCE FUEL CONSUMPTION, AIR POLLUTION, AND

CARBON EMISSIONS FROM VEHICLES IN G20 NATIONS. Washington DC: The

International Council on Clean Transportation (ICCT).

Luhulima, R B, Sutiyo, and I Ketut Aria Pria Utama. 2017. "An Investigation into The Correlation

Between Resistance and Seakeeping Characteristics of Trimaran at Various Configuration

and with Particular Case in Connection with Energy Efficiency." International Symposium

Maritime Engineering. Tokyo.

Menter, F R. 1993. "Zonal two equation κ-ω turbulence models for aerodynamic flows." AIAA 23rd

Fluid Dyanmics, Plasmadynamics, and Lasers Conference.

18

Molland, A, S Turnock, D Hudson, and I Keteut Aria Pria Utama. 2014. "Reducing Ship Emissions:

A Review of Potential Practical Improvements in The Propulsive Efficiency of Future. ."

RINA International Journal of Marine Engineering.

Murdijanto, I Ketut Aria Pria Utama, and A Jamaluddin. 2011. "An Investigation into The

Resistance/Powering and Seakeeping Characteristics of River Catamaran and Trimaran."

MAKARA Journal of Technology.

Olmer, Naya, Bryan Comer, Biswajoy Roy, Xiaoli Mao, and Dan Rutherford. 2017.

GREENHOUSE GAS EMISSIONS FROM GLOBAL SHIPPING, 2013-2015. Washington

DC: The INternational Council on Clean Transportation (ICCT).

Romadhoni, and I Ketut Aria Pria Utama. 2015. "Analisa Pengaruh Bentuk Lambung Axe Bow

Pada Kapal High SPeed Craft Terhadap Hambatan Total." KAPAL Jurnal Ilmu Pengetahuan

dan Teknologi Kelautan 12.

Savitsky, Daniel. 1964. "Hydrodynamics Design of Planing Hulls." Marine Technolgy Vol 1 71-95.

Zong, Z, Z Hong, Y Wang, and H Hefazi. 2015. "Hull form omptimization of trimaran using self-

blending method." Applied Ocean Research.

19

BAB VI LAMPIRAN

Trimaran NPL S/L = 0.3, Fr = 0.15 – 0.5

Fr = 0.15 – 0.25 Fr = 0.3 – 0.5

Trimaran NPL S/L = 0.4, Fr = 0.15 – 0.5

Fr = 0.15 – 0.25 Fr = 0.3 – 0.5

20

Trimaran Axe-Bow S/L = 0.3, Fr = 0.15 – 0.5

Fr = 0.15 – 0.25 Fr = 0.3 – 0.5

Trimaran Axe-Bow S/L = 0.4, Fr = 0.15 – 0.5

Fr = 0.15 – 0.25 Fr = 0.3 – 0.5

21

LAMPIRAN 1 Tabel Daftar Luaran

Program : Penelitian Paska Sarjana, Dana ITS 2020

Nama Ketua Tim : Prof. Ir. I Ketut Aria Pria Utama, M.Sc., Ph.D.

Judul : Pengembangan Kapal Trimaran Green Energy untuk

Optimalisasi Kebutuhan Power

1.Artikel Jurnal

No Judul Artikel Nama Jurnal Status Kemajuan*)

1.

Experimental and Numerical

Investigation into the Effect of Axe-

Bow on Drag Reduction of

Trimaran Configuration

International Journal of

Technology (IJTech) Draft / Persiapan

*) Status kemajuan: Persiapan, submitted, under review, accepted, published

2. Artikel Konferensi

No Judul Artikel

Nama Konferensi (Nama

Penyelenggara, Tempat,

Tanggal)

Status Kemajuan*)

1.

CFD Analysis into the Drag

Characteristics of Trimaran Vessel:

Comparative Study between

Standard NPL 4a and the Use of

Axe-Bow

SRCM (DRPM ITS,

Surabaya, 30 November

2020

Presented

*) Status kemajuan: Persiapan, submitted, under review, accepted, presented

3. Paten

No Judul Usulan Paten Status Kemajuan

*) Status kemajuan: Persiapan, submitted, under review

4. Buku

No Judul Buku (Rencana) Penerbit Status Kemajuan*)

*) Status kemajuan: Persiapan, under review, published

5. Hasil Lain

No Nama Output Detail Output Status Kemajuan*)

*) Status kemajuan: cantumkan status kemajuan sesuai kondisi saat ini

6. Disertasi/Tesis/Tugas Akhir/PKM yang dihasilkan

22

No Nama Mahasiswa NRP Judul Status*)

1. Sutiyo 04111950030005 Pengembangan Kapal Katamaran

Green Energi untuk Optimalisasi

Kebutuhan Power

Sedang

dikerjakan.

*) Status kemajuan: cantumkan lulus dan tahun kelulusan atau in progress

CFD Analysis into the Drag Characteristics of Trimaran

Vessel: Comparative Study between Standard NPL 4a and the

use of Axe-Bow

Sutiyo, I Ketut Aria Pria Utama

Institut Teknologi Sepuluh Nopember, Surabaya, 60111

sutiyo.19041@mhs.its.ac.id

Abstract. Recently, there has been an increased demand for multihull vessels for

military and commercial applications. This demand for multihull vessel in order to

balance speed with payload requirements. One such hull form is the trimaran. This

research conducted a CFD analysis of resistance for trimaran hull forms as a

parameter range of practical hull forms is established round bilge trimaran hull forms

based on the NPL systematic series and Axe-Bow Modification at main hull. The

resistances of trimaran hull forms are therefore, estimated by using ANSYS CFX, a

commercial CFD software package. Trimaran model was analysed on variation of

mainhull and side independently, S/L = 0.3 and S/L=0.4 at Fr = 0:15, 0.2, 0:25, 0.3,

0.4 and 0.5. The results showed that the trimaran ship with the Axe-Bow modification

showed better results when compared to the NPL Hull, with an average value of

26.6%. This shows a positive influence with the use of Axe-Bow on the modification

of the NPL Hull in the trimaran ship configuration.

1. Introduction

There is an increase interest in trimaran vessels due to its advantages and applications [1]. Because of

the stability gained from the side hulls, the trimaran can use slender hulls that reduce residuary

resistance. Therefore, the trimaran reduces fuel consumption compared to an equivalent monohull.

The trimaran’s three hulls have the flexibility to accommodate many propulsion plant arrangements.

An important feature of the trimaran form is the additional upper deck and upper ship space that is

created. For the same displacement or volume as a monohull, the trimaran form will generate a ship

with a greater length and, in the useful central region, greater upper deck beam that is extreme breadth

between the two side hulls, giving the possibility for many potential uses.

Considering the advantages of the trimaran concept, a lot of research has been done during the last

decades. When the literature on trimarans is examined in general, it can be clearly seen that the most

important parameter in resistance optimization is the configuration of the out-riggers because of the

flow interference effect between center-hull and outriggers [2]. Optimum placement of them will

result in an interaction between the wave train produced by the center-hull and the wave trains

produced by the outriggers that ideally counteract each other at the primary speed(s) of interest [3].

Preliminary researches of trimaran was carried out [4]. In this study, resistance characteristics of a

trimaran hull form with different arrangements were investigated to verify the theoretical prediction

with comparing towing test. CFD method was utilized by [5] to analyze the hydrodynamic

performance of a trimaran hull form with small-sized outriggers to determine optimum outrigger

positions for minimum wave resistance performance. They also considered the wave interactions

between the center-hull and outriggers to predict the total wave-making resistance.

Mahmood and De-bo [6] investigated the prediction of wave resistance on the trimaran hull forms

using a CFD software. Three different mesh sizes and two different turbulence models were used to

investigate the effect of mesh structure and turbulence models on the prediction of the resistance. CFD

analyses were realized corresponding to Froude Number ranges from 0.14 to 0.75 and the results were

compared with the experimental data. Son [7] performed CFD computations of a systematic series of

trimaran hull forms. The center-hull form of the trimaran was developed based on the NPL (National

Physical Laboratory) systematic series of round bilge hulls and the side-hulls were created by scaling

the center-hull to one-third size.

Further, the development for hull optimalization was using Axe-Bow. It uses straight vertical sides

to dampen waves from the bow, this can result in a smooth pitching motion. Basically, the Axe-bow in

the extended section is empty space. The study of Axe-bow shows an increase in efficiency and a

reduction in pitch acceleration, because of which ships with Axe-bow have less resistance in

conventional models and reduce fuel use [8].

The Axe-Bow developed by Damen Shipyard has better efficiency as well as better head sea

performance with less slamming and higher speeds [9]. Damen Shipyard [10], made the delivery of

the first ship, the Patrol Boat with axe-bow. The ship exhibits effective movement behavior and

significantly lower drag while sailing. This provides a 20% reduction in fuel use and consequently

lower emissions.

Romadhoni and Utama [11], conducted a study specifically discussing the use of Axe-bow using

CFD. The results of the study based on numerical analysis (Maxsuft–Hullspeed) and CFD showed that

at speeds of 17 knots to 25 knots the Axe-Bow hull form has a smaller resistance value than the

planning hull chine (HPC) and rounded hull (RH) ship models. The results of numerical calculations

and CFD have almost the same value for each variation of the model. The results of the comparison of

the total resistance on the use of axe-bow and conventional bow models obtained a difference of 4-8%.

In this study, the main objective is to analyze the resistance configuration on a trimaran resistance

with Axe-Bow modification at main hull by utilizing CFD method based on RANS (Reynolds-

averaged Navier-Stokes). a brief introduction of the CFD solver is presented, followed by the

description of the numerical setup consisting of mesh generation and boundary conditions. The

interference effect is calculated to define the best configuration.

2. Method

2.1. Ship Model

As shown in Figures 1 and 2, the hull geometry of a trimaran with conventional NPL Hull dan Axe-

Bow modification. The main hull provides most of the buoyancy during forward motion, while the

demi-hull is designed to keep the directional stability. The main dimensions of the model are listed in

Table 1.

Table 1 Principal Dimension of Model

Parameter Unit Mainhul NPL Mainhull Axe-Bow Sidehull

Lmodel m 1.252 1.252 1.058

Lwl Model m 1.218 1.252 0.990

Bmodel m 0.168 0.168 0.096

Tmodel m 0.067 0.096 0.058

WSA m 0.17 0.19 0.114

Displacement kg 3.119 3.119 1.560

(a) (b)

Figure 1 Mainhull of Trimaran (a) NPL 4a; (b) NPL 4a with Axe-bow Modification

(a)

(b)

Figure 2 Trimaran configuration; a. S/L=0.3; b. S/L=0.4

2.2. Numerical Simulation

During simulation, the governing equation of incompressible viscous flow is described by the

Reynold–Averaged Navier–Stokes Equations (RANSE) which is the most widely used method in

engineering. The equation is described as following,

(1)

where 𝑢𝑖, 𝑢𝑗 denote the time averaged velocity components, u′i, u′j are the fluctuations of the

velocity components, 𝑝 is the time averaged pressure, ρ is dynamic viscosity coefficient, t is time and

xi, xj are unit vectors in directions of i and j. To close this set of equations, the Shear Stress Transport

(SST) turbulence model is used.

The transported variable of turbulent kinetic energy, k, is defined as following,

(2)

The second transported variable ω is defined as following,

(3)

Centreline

Centreline Mainhull

Sidehull

S = 0.376m Centreline

Centreline

Sidehull

Mainhull

S = 0.501m

Gk, Gω represent the turbulent kinetic energies due to the average velocity gradient, Yk, Yω are

turbulent dissipation terms, Γk, Γω denote the effective diffusion terms, Dω is the orthogonal

divergence term.

The volume of fluid (VOF) model is applied to track the location and evolution of the free surface

[12]. The basic idea of the VOF is to define the marking function α in the discrete domain and to

determine the value of the volume function in one grid according to the volume of the fluid in it, when

the value of α is 1 or 0, there is only one fluid in the grid. When the value of α is between 0 and 1, it is

occupied by two kinds of fluids, that means there is a free surface in the grid. α satisfies the following

transport equation,

(4)

2.3. Domain and Boundary Conditions

The domain dimensions and boundary conditions are specified in Figure 3. Considering the

symmetrical characteristic of the flow field, only half of the hull body is modelled. The boundary

conditions are specified as follows: the hull body is a moving boundary and a no-slip condition is

imposed on the hull surface; free-slip condition is applied to the top, side and bottom walls; symmetry

condition is used for the hull centre plane; the flow velocity at inlet is defined as the tested speed; at

the outlet, the hydrostatic pressure defined as a function of water level height is applied; furthermore,

the initial location of the free surface is determined by defining the volume fraction function of water

and air at the inlet and outlet.

Figure 3 Boundary Condition

2.4. Meshing and Grid Independence Study

The mesh generation in this study is accomplished using Design Modeler. The calculation domain is

discretized using structured and unstructured meshes. Considering the complex geometrical

characteristics of the hull, a mesh with triangular elements is generated on the hull surface and the

boundary layer is refined with prism elements created through extending the surface mesh node. The

region around the boat is filled with tetrahedral elements with inflation, while in the far field,

unstructured mesh with grid generation is generated for reducing the number of elements.

The mesh size plays an important role in the calculation procedure, a fine mesh can always bring

credible results in ANSYS CFX but at the same time increases the computational cost and time

consumption due to the large element number. Therefore, to determine the mesh size with acceptable

numerical accuracy and element number, mesh convergence studies are carried out for the trimaran

model of NPL hull with S/L=0.3 at Froude number of 0.4. Grid independence study is shown at Figure

4.

Table 2 Grid Independence Study

Total Element (x1000) 192 398 823 1,583 2,876

Resistance Total Coefficient (CT)

(x10-3) 13.365 9.6136 8.1033 7.2356 7.1153

Different (%) 28.07 15.71 10.71 1.66

Figure 4 Grid Independence Study

3. Result and Discussion

3.1. Resistance Total Coefficient

The calculation of the trimaran vessel with axe-bow modification shows a smaller value than the

trimaran NPL hull vessel, as shown in the Figure 5 and Table 3. This is due to the modification of the

axe-bow which is able to reduce wave generation around the ship. Modification of Axe-bow without

hull interaction can reduce the drag of the trimaran NPL hull by an average of 3.71%.

Figure 5 Resistance Total Coefficient of Trimaran

hull independently

Figure 6 Resistance Total Coefficient of Trimaran

hull with variation configuration

Table 3 Total Resistance Coefficient (CT)

Fr

Individual

Trimaran

difference Trimaran with

S/L=0.3

difference Trimaran with

S/L=0.4

difference

NPL

hull

Axe-

Bow

(%) NPL

Hull

Axe-

Bow

(%) NPL

Hull

Axe-

Bow

(%)

(x 10-3) (x 10-3) (x 10-3)

0.15 4.2636 4.0986 3.87 4.5653 4.4586 3.21 4.4187 4.3956 1.41

0.2 4.5563 4.3235 5.11 4.8860 4.5683 4.70 4.6563 4.4513 2.56

0.25 4.9321 4.8692 1.28 5.5998 4.9683 8.45 5.1268 4.9156 1.06

0.3 4.5365 4.3256 4.65 5.0657 4.7569 6.16 4.7536 4.6854 1.50

0.4 6.5981 6.2351 5.50 7.2356 6.6865 4.42 6.9156 6.5346 2.27

0.5 5.5265 5.4253 1.83 6.2353 5.8683 8.01 5.7356 5.6769 3.26

Average 3.71 Average 5.83 Average 2.01

The interaction effect between hulls in the transverse direction (S/L) greatly affects the total drag

coefficient (CT), both trimaran NPL hull and trimaran with Axe-Bow modification, where for hull

S/L= 0.3 greater than S/L = 0.4, as shown in Figure 6. CFD results show that the smaller the distance

between the catamaran hulls (S/L), the greater the resistance that occurs. This phenomenon arises

because of the effect of viscous and wave interaction between the hulls [13].

(a) (b)

Figure 7 Velocity Distribution of Trimaran with NPL Hull at Fn=0.4; a.S/L=0.3; b.S/L=0.4

(a) (b)

Figure 8 Velocity Distribution of Trimaran with Axe-Bow Modification NPL Hull at Fr = 0.4

a. S/L=0.3; b. S/L=0.4

The CFD calculation shows that the difference between trimaran NPL hull and Ax-Bow

modification is 23.94% at S/L = 0.3 and 30.92% at S/L = 0.4 as shown in table 3. The trimaran hull

resistance with the Axe-Bow modification has a lower total drag coefficient value than the trimaran

NPL hull with an average difference about 26.59% because in this condition it appears that the

interaction between hull of NPL trimaran are higher than the trimaran Axe-bow, as shown at figures 7

and 8. This shows that the separation distance between the hulls (S/L) is very crucial for the

interaction of wave making opposing each other from the front (bow) and propagating to the back

(stern) of the ship.

l

(a) (b)

Figure 9 Water Volume Fraction NPL Trimaran Hull at Fr=0.4; a. S/L=0.3; b. S/L=0.4

(a) (b)

Figure 10 Water Volume Fraction Trimaran with Axe-Bow Modification of NPL Hull at Fr=0.4

a. S/L=0.3; b. S/L=0.4

The CFD simulation shows that the speed distribution between the NPL trimaran hull is more rapid

than that of a trimaran ship with a trimaran ship with Axe-Bow modification as shown in Figures 7

and 8 this causes the resistance of conventional trimaran NPL hull become bigger than trimaran with

Axe-Bow modification. The bow shape of a trimaran with Axe-bow modification is able to reduce

wave-making than trimaran vessels with conventional NPL hull, as shown in Figures 8 and 9. This is

also reinforced by research conducted by Damen Shipyard [14], which states that the Axe-Bow

modification is able to significantly lower resistance through the water.

4. Conclusion

This study explains that CFD provides a very good contribution related to the calculation of resistance

on trimaran ships, both conventional NPL and with Axe-bow modification. Ships with Axe-Bow

modifications have a positive effect on trimaran resistance at S/L = 0.3 and S / L = 0.4 with an average

drag reduction of 26.6%. This can occur because the Axe-bow modification is able to reduce wave

generation to reduce the interaction between the hulls. The positive value of Axe-bow modification

can be inputted and considered for the initial design of the ship

Acknowledgment

The authors would like to thank the Directorate of Research and Community Services (DRPM) ITS

for supporting the research financially under a scheme called "Postgraduate Research Grant (Hibah

Penelitian Pascasarjana)" with the contract number: 920/PKS/ITS/2020.

References

[1] Z. Elcin, “WAVE MAKING RESISTANCE CHARACTERISTICS OF TRIMARAN

HULLS,” Naval Postgraduate School, Canada, 2003.

[2] B. Yildiz, B. Sener, S. Duman, and R. Datla, “A numerical and experimental study on the

outrigger positioning of a trimaran hull in terms of resistance,” Ocean Eng., 2020, doi:

10.1016/j.oceaneng.2020.106938.

[3] Y. Chen, L. Yang, Y. Xie, and S. Yu, “The Research on Characteristic Parameters and

Resistance Chart of Operation and Maintenance Trimaran in the Sea,” Polish Marit. Res., 2016,

doi: 10.1515/pomr-2016-0041.

[4] Alexander W. Gray, “A Preliminary Study of Trimarans,” West Virginia Univ., 2003.

[5] M. Javanmardi, E. Jahanbakhsh, M. Seif, and H. Sayyaadi, “Hydrodynamic Analysis of

Trimaran Vessels,” Polish Marit. Res., vol. 15, no. 1, pp. 11–18, Jan. 2008, doi:

10.2478/v10012-007-0046-5.

[6] S. Mahmood and H. De-Bo, “Resistance calculations of trimaran hull form using

computational fluid dynamics,” in Proceedings - 4th International Joint Conference on

Computational Sciences and Optimization, CSO 2011, 2011, doi: 10.1109/CSO.2011.225.

[7] C. H. Son, “CFD Investigation of Resistance of High-Speed Trimaran Hull Forms,” Florida

Institute of Technology Melbourne, Florida, 2015.

[8] J. L. Gelling, “the Axe Bow: the shape of Ships to Come,” in International HISWA Symposium

on Yacht Design and Yacht Construction, 19th. Amsterdam, NL, 13-14 November 2006, 2006.

[9] T. Buckley, “The Axe Factor : Damen dan Amels Take a Bow,” The Yacth Report, no. 111,

2010.

[10] Damen Shipyard, “P511-Guardião: Damen Shipyard’s first full axe-bow patrol vessel delivered

to Cape Uerdean coast guard,” Marit. by Holl., 2012.

[11] Romadhoni and I.K.A.P. Utama, “Analisa Pengaruh Bentuk Lambung Axe Bow Pada Kapal

High Speed Craft Terhadap Hambatan Total,” Kapal, vol. 12, no. 2, pp. 78–87, 2015, doi:

10.12777/kpl.12.2.78-87.

[12] A. Caboussat, “Numerical simulation of two-phase free surface flows,” Arch. Comput.

Methods Eng., 2005, doi: 10.1007/BF03044518.

[13] R. B. Luhulima, Sutiyo, and I. Utama, “An Investigation into The Correlation Between

Resistance and Seakeeping Characteristics of Trimaran at Various Configuration and with

Particular Case in Connection with Energy Efficiency,” Proc. Int. Symp. Mar. Eng. Oct. 15-19,

2017, Tokyo, Japan, 2017.

[14] Damen Shipyard, “DAMEN TAKES A BOW,” 2020. [Online]. Available:

https://www.damen.com/en/innovation/some-key-projects/sea-axe-design.

Experimental and Numerical Investigation into the Effect of Axe-Bow on Drag Reduction of Trimaran Configuration I K A P Utama1i, Sutiyo, I K Suastika

Abstract.

The use of axe-bow to reduce total ship resistance on monohull ship has been well known. This advantage has been further applied on trimaran configuration together with its space to length (S/L) ratio differences. The investigation was carried out experimentally using an ITTC standard towing tank and numerically using computational fluid dynamics (CFD) analysis. The base model for the study uses an NPL 4a both for the main-hull and side-hulls of the trimaran and later the main-hull is modified by attaching a front bulb known as axe-bow. The resistance analysis of trimaran was conducted without and with axe-bow on the main-hull together with S/L ratios of S/L = 0.3 and S/L=0.4 and at various Froude (Fr) numbers: 0.15, 0.2, 0.25, 0.3, 0.4 and 0.5. The results showed that the trimaran hull with the axe-bow had smaller drag than that without axe-bow of the order up to 20%. This is an indication of a positive influence of the use of axe-bow on the total resistance of trimaran configuration. In addition, both experimental and CFD methods showed such a good agreement of the order 5% error. Keywords: trimaran; resistance; experiment; CFD; NPL; axe-bow.

1. Introduction

There is an increase interest in trimaran vessels due to its advantages and applications [1]. Because of the stability gained from the side hulls, the trimaran can use slender hulls that reduce residuary resistance. Therefore, the trimaran reduces fuel consumption compared to an equivalent monohull. The trimaran’s three hulls have the flexibility to accommodate many propulsion plant arrangements. An important feature of the trimaran form is the additional upper deck and upper ship space that is created. For the same displacement or volume as a monohull, the trimaran form will generate a ship with a greater length and, in the useful central region, greater upper deck beam that is extreme breadth between the two side hulls, giving the possibility for many potential uses. Considering the advantages of the trimaran concept, a lot of research has been done during the last decades. When the literature on trimarans is examined in general, it can be clearly seen that the most important parameter in resistance optimization is the configuration of the out-riggers because of the flow interference effect between center-hull and outriggers [2]. Optimum placement of them will result in an interaction between the wave train produced by the center-hull and the wave trains produced by the outriggers that ideally counteract each other at the primary speed(s) of interest [3]. Preliminary researches of trimaran was carried out [4]. In this study, resistance characteristics of a trimaran hull form with different arrangements were investigated to verify the theoretical prediction with comparing towing test. CFD method was utilized by [5] to analyze the hydrodynamic performance of a trimaran hull form with small-sized outriggers to determine optimum outrigger positions for minimum wave resistance performance. They also considered the wave interactions between the center-hull and outriggers to predict the total wave-making resistance. Mahmood and De-bo [6] investigated the prediction of wave resistance on the trimaran hull forms using a CFD software. Three different mesh sizes and two different turbulence models were used to investigate the effect of mesh structure and turbulence models on the

prediction of the resistance. CFD analyses were realized corresponding to Froude Number ranges from 0.14 to 0.75 and the results were compared with the experimental data. Son [7] performed CFD computations of a systematic series of trimaran hull forms. The center-hull form of the trimaran was developed based on the NPL (National Physical Laboratory) systematic series of round bilge hulls and the side-hulls were created by scaling the center-hull to one-third size. Further, the development for hull optimalization was using Axe-Bow. It uses straight vertical sides to dampen waves from the bow, this can result in a smooth pitching motion. Basically, the Axe-bow in the extended section is empty space. The study of Axe-bow shows an increase in efficiency and a reduction in pitch acceleration, because of which ships with Axe-bow have less resistance in conventional models and reduce fuel use [8]. The Axe-Bow developed by Damen Shipyard has better efficiency as well as better head sea performance with less slamming and higher speeds [9]. Damen Shipyard [10], made the delivery of the first ship, the Patrol Boat with axe-bow. The ship exhibits effective movement behavior and significantly lower drag while sailing. This provides a 20% reduction in fuel use and consequently lower emissions. Romadhoni and Utama [11], conducted a study specifically discussing the use of Axe-bow using CFD. The results of the study based on numerical analysis (Maxsuft–Savitsky) and CFD showed that at speeds of 17 knots to 25 knots the Axe-Bow hull form has a smaller resistance value than the planning hull chine (HPC) and rounded hull (RH) ship models. The results of numerical calculations and CFD have almost the same value for each variation of the model. The results of the comparison of the total resistance on the use of axe-bow and conventional bow models obtained a difference of 4-8%. In this study, the main objective is to analyze the resistance configuration on a trimaran resistance with Axe-Bow modification at main hull by utilizing CFD method based on RANS (Reynolds-averaged Navier-Stokes). a brief introduction of the CFD solver is presented, followed by the description of the numerical setup consisting of mesh generation and boundary conditions. The interference effect is calculated to define the best configuration. 2. Methods 2.1 Trimaran Model The investigation used an NPL4a model, without and with axe-bow as shown in Figures 1 and 2, and the principal particular can be seen in Table 1.

Figure 1 NPL 4a

Figure 2 NPL 4a with axe-bow

Table 1 Principal Particular of the model

Parameter Unit Mainhull

NPL 4a Mainhull Axe-Bow

Sidehull NPL 4a

Lmodel M 1.252 1.252 1.058 Lwl Model m 1.218 1.252 0.990

Bmodel m 0.168 0.168 0.096 Tmodel m 0.067 0.096 0.058

WSA m 0.17 0.19 0.114 Displacement kg 3.119 3.119 1.560

2.2 Prediction of Resistance The enhanced design features of a trimaran leads to reducing the residual resistance, however the consequence is a new form of resistance: the close positioning of the separate hulls leads to interaction in both the total resistance.

This means that the following may be constructed. Consider a hull of beam B split into two equivalent hulls each having a beam of B/2 and Main hull. The Total resistance for the original hull was RT however this has now been divided into two equal resistances RTSidehull and RTMainhull. TSidehullTMainhullT RRR 2+=

(1)

As mentioned, the interaction of the waves is due to the position of the various hulls with reference to separation, implying that if the hulls are positioned in such a way that there is no interaction between the hulls, then no interference resistance would be experienced[10]. By investigating the variations in separation, this interference resistance can be reduced, eliminated and even taken advantage of. An interesting point is that although an interference would cause the hull to be inefficient, there are some positions when the interference produce favourable situations when and the complete vessel would experience less resistance than that addition of the individual hulls acting separately. This interference resistance can be calculated, such that:

WVTHulllT RRRR ++= 3 (2)

ceInterverenTHulllT RRR += 3 (3)

where ∆RTV and ∆RTW can be grouped as the interference resistance due to the trimaran effect”.

Empirical formulation to estimate the total resistance of trimaran is so far not known and depends highly on the experimental results [11]. This is also attributed to the minimum publications of trimaran resistance both experimentally and numerically.

Computational Fluid Dynamics (CFD) technique, of a varying degree of complexity, may be used to predict various resistance components. The method would provide some insight into the pressure form drag. Full Reynolds-Averaged Navier-Stokes (RANS) codes may be used to predict the flow where separation and circulation occur, thus potentially providing good estimates of form factor and possible scale effect. However, these methods

are extremely computationally intensive, particularly for the computation of high Reynolds number flow [11].

The choice of turbulence models is found to be very crucial in the simulation of wake fields. The turbulence model used in the current study is the SST (Shear Stress Transport) model developed by [12] and the SST model has been used and validated by several researchers including [13] with successful results. The viscous flow field is solved using RANS (Reynolds Averaged Navier-Stokes) solver implemented in ANSYS CFX. The RANS, turbulence k-ω and turbulence SST equations are shown in Equations (5), (6) and (7). RANS Equation

( ) 0=

+

j

j

Uxt

(4)

( ) ( ) Mjiij

jj

ji

j

Suuxx

pUU

xt+−

+

−=

+

(5)

The left side represents the change in mean momentum of fluid element to the unsteadiness in the mean flow. This change is balanced by the mean body force. The mean pressure field, the viscous stresses, and apparent stress to the fluctuating velocity field. k-ω, equation

( ) ( )( )

+

+−=

+

j

tk

jj

j

x

k

xkP

x

ku

t

k

*

(6)

Menter’s SST Equation

( ) ( )( ) ( )

jjj

t

jtj

j

xx

kF

xxP

vx

u

t

−+

+

+−=

+

1212 21

2 (7)

2.3 CFD 2.3.1 Numerical Domain The domain dimensions and boundary conditions are specified in Figure 3. Considering the symmetrical characteristic of the flow field, only half of the hull body is modelled. The boundary conditions are specified as follows: the hull body is a moving boundary and a no-slip condition is imposed on the hull surface; free-slip condition is applied to the top, side and bottom walls; symmetry condition is used for the hull centre plane; the flow velocity at inlet is defined as the tested speed; at the outlet, the hydrostatic pressure defined as a function of water level height is applied; furthermore, the initial location of the free surface is determined by defining the volume fraction function of water and air at the inlet and outlet.

Figure 3 Boundary Condition

2.3.2 Grid Independence The mesh generation in this study is accomplished using Design Modeler. The calculation domain is discretized using structured and unstructured meshes. Considering the complex geometrical characteristics of the hull, a mesh with triangular elements is generated on the hull surface and the boundary layer is refined with prism elements created through extending the surface mesh node. The region around the boat is filled with tetrahedral elements with inflation, while in the far field, unstructured mesh with grid generation is generated for reducing the number of elements.

The mesh size plays an important role in the calculation procedure, a fine mesh can always bring credible results in ANSYS CFX but at the same time increases the computational cost and time consumption due to the large element number. Therefore, to determine the mesh size with acceptable numerical accuracy and element number, mesh convergence studies are carried out for the trimaran model of NPL hull with S/L=0.3 at Froude number of 0.4. Grid independence study is shown at Figure 4.

Table 2 Grid Independence Study

Total Element (x1000) 192 398 823 1,583 2,876

CT (x10-3) 13.365 9.6136 8.1033 7.2356 7.1153

Difference (%) 28.07 15.71 10.71 1.66

Figure 4 Grid Independence Study

2.4 Experiment The test model creation procedure followed ITTC recommendations 7.5-02-05-01 [21]. The size of the model is 1: 11.3 from the actual ship size, λ = 1/7 [22]. The size of this winged ship test model adjusts the size of the towing tank test facility at the Hydrodynamic Laboratory of Naval Architecture Department at the ITS with a length of 50 m of pulling pool; width 3 m; and a depth of 2 m. The maximum towing carriage speed is 4.5 m/s. The mass density of the air towing tank is 999.1 kg / m3, and the mass density of the air is + 1.164 kg/m3, (for temperatures of 30 0C), while the water temperature ranges from 16 - 18˚C. The test model is attached to the towing carriage through the towing guide (see Figure 3) with the setup that the test model can only move heave and pitch freely, cannot move yawing or swaying so that no rolling or heeling moments arise. The trim meter was installed in an upright position at the front and at the rear of the test model in a position that did not interfere with the towing guide. The test model is installed with the center line of the model. The instrumentation cable is laid in such a way that at the time of measurement it does not interfere with the motion of the test model.

Figure 3 Experimental Setting Trimaran Model

3. Results and Discussion 3.1 Effect of Axe-Bow

Table 2 Experiment and CFD Calculation for Mainhull Trimaran Model

Fr Experiment CFD

NPL 4a Axe-Bow NPL 4a Axe-Bow

0.15 5.2294 4.6416 5.3656 4.9688 0.20 5.6972 4.8050 5.7569 5.0326 0.25 6.4407 5.6068 6.6326 5.9866 0.30 6.1432 5.3373 6.5357 5.2356

0.40 7.5782 6.9713 7.6592 7.3569

0.50 5.8830 5.4595 6.3256 5.6483

Figure 3 Comparison of experiment and CFD of trimaran main hull

Figure 4 Experiment of wave making of main hull at Fr=0.4

a. NPL 4a without axe-bow; b. NPL with axe-bow

Figure 5 CFD simulation on water volume fraction of main hull at Fr=0.4

3.2 Hull Interaction The interaction effect between hulls in the transverse direction (S/L) greatly affects the total drag coefficient (CT), both trimaran NPL hull and trimaran with Axe-Bow modification, where for hull S/L= 0.3 greater than S/L = 0.4, as shown in Figure 6. CFD results show that the smaller the distance between the catamaran hulls (S/L), the greater the resistance that occurs. This phenomenon arises because of the effect of viscous and wave interaction between the hulls [12].

Table 3 CFD simulation result for total resistance coefficient (CT) (x10-3)

Fr Individual Trimaran Trimaran S/L=0.3 Trimaran S/L=0.4

NPL 4a Axe-Bow NPL 4a Axe-Bow NPL 4a Axe-Bow 0.15 4.2636 4.0986 4.5653 4.4586 4.4187 4.3956 0.20 4.5563 4.3235 4.886 4.5683 4.6563 4.4513 0.25 4.9321 4.8692 5.5998 4.9683 5.1268 4.9156 0.30 4.5365 4.3256 5.0657 4.7569 4.7536 4.6854 0.40 6.5981 6.2351 7.2356 6.6865 6.9156 6.5346 0.50 5.5265 5.4253 6.2353 5.8683 5.7356 5.6769

Table 4 Experimental result for total resistance coefficient (CT) (x10-3)

Fr Individual Trimaran Trimaran S/L=0.3 Trimaran S/L=0.4 NPL 4a Axe-Bow NPL 4a Axe-Bow NPL 4a Axe-Bow

0.15 4.2381 3.9680 4.4768 4.3235 4.2356 4.1256 0.20 4.4435 4.1334 4.6368 4.5266 4.5356 4.4365 0.25 4.7724 4.5367 4.9865 4.6366 4.7869 4.6358 0.30 4.4352 4.2356 4.7365 4.4124 4.5369 4.3668 0.40 6.3392 6.0350 6.7562 6.5236 6.6367 6.3546 0.50 5.4530 5.3256 5.5698 6.0325 5.7598 5.6998

Figure 6 CFD simulation for trimaran model

(x10-3) Figure 7 Experiment for trimaran model

(x10-3)

(a) (b)

Figure 8 Wave elevation of trimaran without axe-bow at Fr=0.4 for S/L=0.4 b. experiment; b. CFD simulation

(a) (b)

Figure 9 Wave elevation of trimaran with axe-bow at Fr=0.4 for S/L=0.4 a. experiment; b. CFD simulation

(a) (b)

Figure 10 Velocity distribution of trimaran at Fr=0.4 with S/L=0.4 for main hull a. without axe-bow; b. with axe-bow

The CFD calculation shows that the difference between trimaran NPL hull and Axe-Bow modification is 23.94% at S/L = 0.3 and 30.92% at S/L = 0.4 as shown in table 3. The trimaran hull resistance with the Axe-Bow modification has a lower total drag coefficient value than the trimaran NPL hull with an average difference about 26.59% because in this condition it appears that the interaction between hull of NPL trimaran are higher than the trimaran Axe-bow, as shown at figures 7 and 8. This shows that the separation distance between the hulls (S/L) is very crucial for the interaction of wave making opposing each other from the front (bow) and propagating to the back (stern) of the ship. 4. Conclusions

This study explains that CFD provides a very good contribution related to the calculation of resistance on trimaran ships, both conventional NPL and with Axe-bow modification. Ships with Axe-Bow modifications have a positive effect on trimaran resistance at S/L = 0.3 and S / L = 0.4 with an average drag reduction of 26.6%. This can occur because the Axe-bow modification is able to reduce wave generation to reduce the interaction between the hulls. The positive value of Axe-bow modification can be inputted and considered for the initial design of the ship Acknowledgment

The authors wished to thank the Directorate of Research and Community Services (DRPM) ITS for financing the research under a research scheme called "Postgraduate Research Grant" with the contract number: 920/PKS/ITS/2020. References

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[9] T. Buckley, “The Axe Factor : Damen dan Amels Take a Bow,” The Yacth Report, no. 111, 2010.

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iCorresponding author’s email: name@ai.ue.oa, Tel.: +00-00-000000; fax: +00-00-000000 doi: 10.14716/ijtech.v0i0.0000