Seakeeping Comparison

Seakeeping Comparison:

Nigel Gee and Associates X - c r a f t ( M o d C A T ) versus A M D W a v e Piercing C a t

Introduction

A f e w years ago B M T N i g e l Gee and Associates

( N G A ) were engaged by the U S N a v y O f f i c e o f Naval Research to develop a catamaran hull f o r m f o r their Littoral Surface Craft concept, code named X-craft. N G A developed a seakeeping hull f o r m w h i c h claimed to have excellent seakeeping capability but w h i c h suffered a small resistance penalty. The h u l l f o r m is being promoted by N G A as the M o d C A T .

A M D was recently requested by a client to compare the seakeeping capability o f the A M D wave piercing catamaran w i t h the X-craft. N G A

has published a number o f papers on the design PiiiHie 27 -Lsc(x)Crart

program f o r the X - c r a f t w h i c h contain the resuhs o f computer and tank test seakeeping investigations. The N G A papers also include the seakeeping o f a conventional catamaran, called "Vanilla", as a comparison to the X - c r a f t vessel. These papers provide seakeeping' information f o r comparison.

A M D has scaled the resuhs o f our o w n wave piercmg catamaran tank test program to match the size o f the X - c r a f t prototype and the V a n i l l a catamaran, and have overlaid the A M D seakeeping data on the graphs provided in the

N G A papers. A M D has also scaled the actual seakeeping trials resuhs o f the A M D 3 5 0 wave piercing catamaran prototype and plotted these results on the N G A graphs as confirmation o f the A M D tank test results.

Sea keeping Comparison

The f o l l o w i n g figures are f r o m the paper "Ocean Transits in a 50m, 45 knot Catamaran -The Minimisation o f Motions and Speed Loss" by Dudson and R o y o f N i g e l Gee and Associates. The figures represent seakeeping expectations f o r the X - c r a f t / M o d C A T design. The figures have been overlaid w i t h the results o f the A M D wave piercing catamaran tank test program, and also the trials carried out by M a r i t i m e Dynamics Inc on the A M D 3 5 0 wave piercing catamaran "Marie-Helen" during the commissioning o f t h e M D I ride control system on that vessel. The A M D data has been scaled to the size o f t h e X craft using industry standard scaling methods used f o r all sea keeping tank test scaling.

The A M D W P C tank test data is f o r a vessel W I T H O U T ride control and is shown by the red line. Points A and B are trials points f o r the A M D 3 5 0 , 42m wave piercing catamaran, w h i c h is fitted w i t h an M D I T - f o i l ride control system. Point A is w i t h the Ride Control ( R C ) switched o f f . Point B is w i t h the Ride Control switched on.

The particulars o f the vessels when scaled are:

X craft A M D WPC tank test length between perpendiculars 54m 54m Displacement 5501 487 1

A M D 3 5 0 W s

54m

X

5 3 8 t ^ \ A M D also carried out one wave piercing tank test at an equivalent o f 551 tonnes, to test the effect o f increased displacement on sea keeping, however only one test run was made at that displacement. This point is shown on the graphs as a blue chcle.

A M D W P C tank test without ride control 487 tonne S I G N I F I C A N C W A V E H E l G H r, I l3 [ m ] ' 1 0 . 0 0 1 : n 0 . 0 ' — B - - V f l n i l J s ; 4 O . O O k n 0 . 0 ' ModCAT i RC<! 4 0 . O O k n 0 . 0 ' l / s i n i U a 4 RCS ,• 4 0 . O O k n 0 . 0 '

Figure 4 - RMS Vertical Acceleration @ FP

The above f i g u r e shows vertical acceleration levels at the f o r w a r d end o f t h e vessels. The acceleration f o r the X - c r a f t is calculated at the f o r w a r d end o f the waterline (Forward Perpendicular or FP), whereas the acceleration on the A M D WPC was measured at the

forward most passenger seat location. The dotted brown line and dotted light blue line show the N G A prediction f o r the Vanilla Cat and the X - c r a f t w i t h ride control systems active. A M D suspects that these predictions are somewhat optimistic. The reason is that Maritime Dynamics, the w o r l d leader in ride control f o r high speed craft, consistently predict acceleration reductions w i t h their state o f the art ride control o f around 50 to 55% o f uncontrolled accelerations. The N G A results on the previous page predict reductions o f 62% f o r the X - c r a f t and 70% f o r the conventional catamaran. To our knowledge such reductions have not been achieved on vessels in practice.

A M D W P C Si tank test without ride control 0.4 0.3

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^'Po nt A RC ( rf*' I-» — 1 a i r AP t551 1 D \ tonr SVC es ,1 i f r. l ' ^ P( i n t B : RC 6 1.0 1.2 1.4 1.6 1,8 2 0 ! 2 2 2 4 . :!2 — H .6 2 1 Ö 3. —(— _{0 3} • " t ~ -2 OFF O N S I G N I F I C A N T W A V E HEIGHT, Hs (m) - • WotiG.W ; 4 U . O 0 l ; r i I J , o " ModCAT c KO^ ƒ ' l O . O O k n 0 . 0 * ~ • > - V a i i L l l a I . l O . o a k n O.O"' V i l t l i l l a i R C 3 ; 4 0 , O O k n D . O "

Figure 4 - RMS Vertical Acceleration @ CG

Acceleration levels at the Longitudinal Centre o f Gravity ( L C G or CG) Note that a T f o i l ride control system provides some ride control even when turned o f f (the inactive f o i l p r o v i d i n g some damping o f pitch m o t i o n ) w h i c h w o u l d partly explain why the M D I trials point w i t h ride control turned o f f shows lower acceleration than the A M D tank test prediction. Please note that this ride control trial was carried out not by A M D personnel but by M D I personnel as part o f their commissioning o f t h e f o i l system.

Also note that the A M D 3 5 0 vessel was not fitted w i t h a stern ride control ( t r i m tab or interceptor), and hence i f a t r i m tab system was fitted then accelerations in the A M D S 50 ride control trial would have been even lower.

A M D W P C ^ tank test without ride control S I G N I F I C A N T W A V E HEIGHT.JHs [ m ] — ' j s i - — — - * tlodqAT / ilU.OUki-i O . U " Madt;AÏ £ llCii J - l O . O O k n 0 . 0 ° Van.il ifl V a r . i l l a t Kfji i. -to.OOkn 0 . 0 ' J 4 & . 0 0 k n 0 . 0 "

Figure 4 - RMS Vertical Acceleration @ AP

Accelerations at the aft end o f the vessels. Note that on the A M D 3 5 0 prototype there is no accelerometer at the a f t end so there are no readings to put on this chart f o r the M D I ride control trial.

It can be seen that on a l l three diagrams, that is at the forward, centre o f gravity and aft location, the A M D W P C without Ride Control (red line) has lower accelerations than the X - c r a f t / M o d C A T also without Ride Control (blue line). The A M D WPC tank test data is supported by the trials data f r o m the M D I test o f the ride control system on the A M D 3 5 0 .

Ride Control

Computer simulations carried out by M a r i t i m e Dynamics (plotted on the graph on the next page) also show that the A M D wave piercing catamaran w i t h ride control has lower

accelerations than the X - c r a f t / M o d C A T w i t h ride control, in all passenger locations on the vessel. This computer simulation w o r k is supported by actual trials results collected by M a r i t i m e Dynamics on trials w i t h the ride control system on the A M D 3 5 0 wave piercing prototype "Marie-Helene" .

The M D I predictions f o r accelerations on the A M D W P C are shown by the bright red line on the adjacent figure, about 50% o f the acceleration levels on the A M D W P C without ride control. The position o f

the line is supported by the M D I ride control trial carried out on the A M D 3 5 0 vessel. This line shows that the A M D W P C w i t h ride control has lower accelerations than the N G A computer predictions o f the X - c r a f t w i t h ride control.

It should be noted that the lines on the acceleration graph f o r the N G A " V a n i l l a " conventional catamaran and the X - c r a f t were not produced by a ride control manufacturer, but by a consultant. These lines show a reduction i n accelerations o f 70% f o r the " V a n i l l a " conventional catamaran.

3.0 t j j ' ^ . i SiQfif JOi^JT VMVft f^",*5i(r. (nij

I ^ D I trial point, -MdeJcontrol on

Figtiie 4 - RMS Vci-tic:il Acceleration @ CG This is an acceleration reduction so far never experienced i n practice, and never claimed by any ride control manufacturer. A reduction o f 57% is shown f o r the X - c r a f t / M o d C A T , w h i c h is more realistic but still optimistic given that 50, maybe 55% reduction is claimed by the ride control manufacturers themselves.

The above comparisons demonstrate that the A M D wave piercing catamaran is superior in its seakeeping capability to the s e m i - S W A T H hull f o r m o f t h e X - c r a f t / M o d C A T , and much better than the conventional " V a n i l l a " catamaran. For this reason the A M D Wave Piercing catamaran h u l l f o r m has been chosen f o r a number o f roles where seakeeping is paramount.

A recent example is the selection o f t h e A M D wave piercing hull f o r m by the Chinese Salvage and Rescue Bureau f o r an offshore rescue vessel. The ability to operate in severe sea conditions was a major consideration o f the Bureau. The vessel is currently being designed and an order f o r three vessels w i t h an option for six more w i l l soon be placed w i t h a Chinese shipyard.