Comparative Full Scale Trials of Two Fast Rescue Vessels

COMPARATIVE FULL SCALE TRIALS OF

TWO FAST RESCUE VESSELS

J. Ooms and J.A. Keuning

Report i 100-P

1997

International Conference SURV IV, Surveillance,

Pilot & Rescue Craft for the 21 st Century

13 & 14 May 1997, Gothenburg, Sweden

Proceedings Royal Institution of Naval

Architects, R.'I .N .A., Londôn, UK

TU Degft

Faculty of Mechanical Engineering and Marine Technology Ship Hydromechanice Laboratory

INTERNATIONAL CONFERENCE

SUR V .1V

SURVEILLANCE, PILOT & RESCUE CRAFT

FOR THE 21st CENTURY

13 & 14 MAY 1997 GOTUENBURG, SWEDEN

tAPERS;

TRE ROYAL INSTITUTION OF NAVAL ARCFÌITECTS

RINA

SMALL CRAFT GROUP

INTERNATIONAL CONFERENCE

SURV IV

SURVEILLANCE, PILOT & RESCUE CRAFT

FOR THE 21st CENTURY

GOTHENBURG, SWEDEN

13 & 14 May 1997

© 1997 The Royal Institution of Naval Architects

The Institution is not, as a body, responsible foriha opinions expressed by the individual authors or speakers

THE ROYAL INSTITUTION OF NAVAL ARCHITECTS 10 Upper Beigrave Street,

London, SW1X 880 Telephone: 0171 -235-4622 Fax: 0171-245-6959

Thé Royal Institution

_{of Naval ArchItects}

International Conference

SURV IV

Surveillance, Pilot & Rescue

Craft for the 21st Century

Programm

e

13 & 14 May 1997

Carnegie Suite

DAY ONE

08.55

- 09.Ó0

SESSION I

09.Ó0

- 0935

0933

10.10

10.10 - 10.45

10.45 - 11.15

Coffee

11.15 - 11.50

11.50 - 12.25

12.25 - 13.00

13.00 - 14.00

Lunch

16.40

17.15

17.15- 17.45

18.00

Opening Address by Captain Rolf Westerström, Swedish Sea Rescue Institution, Sweden

SEARCH & RESCUE Chaired by D Cannel!, David M Cannell Naval Architects UK

The Design & Development of Modern SAR Craft

- A Personal View

F D Hudson

- Independent Naval Architect, UK

New Design of 23m Rescue Cruiser

Capt U Klein

- German Sea Rescue Service, Germany

New Designs for the Swedish Sea Rescue Institute

Captain R Westerström, A Waligren, R Eliasson & A Jönsson - Swedish Sea Rescue

Institutiôn,Sweden

Latest Development and Experience of Danish Rescue Vessels

Lars Møller - The Royal Danish Administration of Navigation and Hydrography, Mr B

Moving - Carl Bra AS, Denmark

Comparative Full Scale Trials of Two Fast Rescue Vessels

J Ooms & J A Keuning - Deift University of Technology, Netherlands

What Scope is there for Aircraft?

Dr D Stinton - International Aero-Marine Consultant, UK

Preliminary viewing of attending vessels

DESIGN & HYDRODYNAMICS Chaired by Dr D Stinton, International

Aero-Marine Consultant, UK

Integration of Formal Safety Evaluation into the Design of Lifeboats

Dr R Birmingham & Mr C Cain

- University of Newcastle-upon-Tyne, UK

The Use of Model Tests in the Development of the Next Generation of RNLI Slipway

Launched Lifeboats

I Campbell - Wolfson Unit MTIA, UK

Dynamic Roll Instability for High Speed Mono-hull Craft

R Pederson - Marintek, Norway

General Discussion on Day I Papers

SCANIA Evening Reception

14.00 - 1530

SESSION II

15.30 - 16.05

16.05 - 16.40

DAY TWO

SESSION III PILOT & PATROL CRAFT Chaired by R Westerströrn,

_{Swedish Sea Rescue}

Institution, Sweden

09.00 - 09.35

_{The FRCM Huliform for Pilot, Patrol and Rescue Craft}

D Cannell, David M. Cannell Naval Architects, UK.

0935

_{- 10.10 New SWATH Generation of Pilot System for the German North Sea Coast}

K Spethmann - Abeldng & Rasmussen, Germany

1:0.10 - 1045

_{Sea Trials with Regard to Design and Operational Limits of}

_{Fast Pilot Vessel ms}

Voyager

J .J Journée

= teift University of Technology, Netherlands

10.45

11.15

Coffee

SESSION IV MACHINERY Chaired by K Spethmann, Abeking & Rasmussen, Germany

11.15 - 1130 Machineiy Developments With

_{a View to Matters of Specific Current Interest for Small}

Marine Craft

K Olsson - SCANIA, Sweden

11.50 - 12.25

_{Experience with Electronic Remote Control Systems}

_{on Modern Propulsion Plants for}

Fast Vessels

F 'Brekke - Scana Mar-El AS, Norway

12.25 - 1100 CP Propulsion Systems for Multiple Engine Applications

M Møklebust - Servogear AS, Norway

13.00 - .13 30

General Discussion on Day 2 Papers and the following written contribution:

Modern Rescue Boat Handling Techniques

Cdr J Hurlbatt - Caley Ocean Systems, UK

CONTENTS

SEÄRCH& RESCUE

THE DESIGN .& DEVELOPMENT OF MODERN S A R

CRAFT-A PERSONCRAFT-AL VIEW

by F D Hudson, Independent Naval Architect

NEW DESiGN FOR A23MRESCUE CRUiSER

by Capt U Klein, Deutsche Ge&ellschaft Zur Rettung Schiffbrüchiger, and

Dipling. B Bartels, Schweers Yard,. Bardent leth/Wèser

NEW DESiGN FOR THE SWEDISH SEA RESCUE INSTITUTION

by Capt Rolf 'Westerström, Swedish Sea Rescue Institution

(Swedén)

LATEST DEVELOPMENT AND EXPERIENCE OF DANISH RESCUE VESSELS

by L MølIer The Royal Danish Administration of Navigation and Hydrograph,

and B Moving, N C Engbjerg and P M Jorgensen Carl Bro als,

Dwinger Manneconsuit

_(Denmark)

5*

_{THE VON KOSS CLASS, HULL DESIGN PHILOSOPHY}

by T Stokke, Amble & Stokke AiS

_(Norway)

COMPARATIVE FULL SCALE TRIALS OF TWO FAST RESCUE VESSELS

by J Ooms and J A Keuning, Deift University of Technology

(Netherlands)

DESiGN & HYDRODYNAMICS

THE APPLICATION OF FORMAL SAFETY EVALUATiON INTO THE DESIGN

OF LIFEBOATS

by R Birmingham, P Sòn and :C Cain, Department of Marine Technology,

Newcastle University and R M Cripps, Royai National Lifeboat Institution, Poole

(UK)

8..

THE USE OF MODEL TESTS IN: ThE DEVELOPMENT OF THE NEXT

GENERATION OF RNLISUPWÄY LAUNCHED LIFEBOATS

by B Deakin and I Campbell, Wolf son Unit MTIA,

University of Southampton

(UK)

(UK)

(Germany)

9.

_{DYNAMIC ROLL INSTABILITY FOR HIGH SPEED MONO HULL CRAFT}

by R Pedersen and P Werenskiold, MARINTEK Norwegian Marine Technology

PILOT & PATROL CRAFT

THE DEVELOPMENT OF THE FRCV HULL FORM FOR PILOT PATROL

AND RESCUE CRAFT

by D;M Cannell, .DavidM Canhell NavalArchitects

NEW SWATH GENERATION OF PILOT SYSTEM FOR THE GERMAN

NORTH SEA COAS1

by Dr.-lng. K Spethmann, Abeking& Rasmussen, Lemwerder near Bremen

and Capt. W, Leue, Pilot Brotherhood Elbe, Brunsbüttel near Hamburg

SEA TRIALS WITH REGARD TO DESIGN AND OPERATIONAL LIMITS OF

FAST PILOT VESSEL MS VOYAGER

by. Jakob Pinkster and Johan M J Journée, Deift University of Technology

Department of Marine Technology

(Nethêrlands)

MACHINERY

(UK)

(Gemiany)

13.

MACHINERY DEVELOPMENTS, WITH A VIEW TO MATTERS OF SPECIFIC

CURRENT INTEREST FOR SMALL MARINE CRAFT

by K V Olsson, Scania Industrial & Marine Diesel Engines

(Sweden) 14.

EXPERIENCE WITH ELECTRON!C. REMOTE CONTROL SYSTEMS ON

MODERN PROPULSION PLANTS FOR FAST VESSELS

by F Brekke, Scana Mar-El AS

(Norway) 15.

CP PROPULSION SYSTEM FOR MULTIPLE

ENGINE APPLICATION

by M MøIebust, Servogear A'S

(Norway)

16.

WHAT SCOPE IS THERE FOR AIRCRAFT?

by D. Stinton, Darrol Stinton Limited, Fambam

. (UK)

1.7.

MODERN RESCUE BOAT HANDLING: TECHNIQUES

by Cdr J: HurIbatt OBE, MNI, Royal: Navy,

Naval Consultant to Caley Ocean Systems

(UK)

AUTHORS' NAMES AND ADDRESSES

*

PAPER NO.6.

COMPARATIVE FULL SCALE TRIALS OF TWO FAST RESCUE VESSELS

by J Ooms and J A Keuning Deitt University of Technology, Netherlands

Paper presented at the

International Conference

SURV IV

SURVEILLANCE; PILOT & RESCUE CRAFT

FOR THE 21st CENTURY

COMPARATiVE FULL SCALE TRIALS OF TWO FAST RESCUE VESSELS

Mr J Ooms is a researcher in the Ship Hydrornechanics Laboratory of Deift University of Technology (DUT). While working at the laboratory as an electronic technician he graduated in 1989 at the Electrical Engineering Faculty of DUT after which he obtained his present position. His

interests are in instrumentation, digital signal'processing, Wave generation in model tanks, and control systems.

Mr J .A

Keuning is an associate professor in the Ship Hydromechanics Laboratory of Delft University of Technology (DUT).

He graduated at the Faculty of

Maritime Technology of DUT in

1977.

In 1994 he

obtained

his PhD at

the same University on the

subject of' non-linear motions of high speed vessels in head seas.

1. INTRODUCTION

Many fast vessels must be able to'operate under adverse conditions. Examples of this category are pilot boats and rescue vessels.

Not only do they have to continue

operating but often they have to do so at 'high speeds. The demands of high speed and acceptable behaviour in rough seas are hard to combine in a design which always is' a compromise between conflicting reqúirements. New and radically different ship concepts may hold a promise for the future but at present the standard' planing ship is' the 'most common type of small fast vessel.

As a consequence of their 'basic design these ships are notorious for their high motion levels, in particular the vertical accelerations. The acceleration levels can

become so high that they form a danger for crew

members or ship.

Also the reduction

in operability

J Oorns and J A Keuning

Delft University of Technology, The Netherlands

SUMMARY

There is much argument amongst users about the relative virtues of small fast vessels. Their experiences with ships are used in the design of new generations in a continuous process of, often piecemeal, improvement. To verify the improvements in seàkeeping behaviour between two fast rescue vessels of the Dutch Lifeboat Institution (KNRM), full scale experiments were carried out n waveheights of between 2 and 2 5m on the North Sea in February 1996 Ship 1

(the Graaf van Bylandt) is slightly longer and has a larger displacement than (the older) Ship 2 (the Chnstien) In addrtion

it has à steeper deadrise ñear the bow. Measurements on both ships werecarried out simultaneously and concentrated on the accelerations. It was shown that the vertical and longitudinal accelerations in the wheelhouse of Ship i were between 5 and 10% lower than on Ship 2.

At higher acceleration levels strong asymmetry showed up 'in the positive and negative vertical accelerations resulting in very high peak values, mainly in the bow area. In the wheelhouse this effect was much less visible. Peaks of a similar character, but lower amplitude, were also present in the longitudinal acceleration of the ship. The Root Mean Square (RMS) value of the Iongitudinal.acceleration was about 35% of the vertical acceleration in the wheelhouse. However the peak values of the longitudinal accelerations were up to 65% of those of the vertical accelerations in the wheelhouse. They are therefore not negligible. Depending on speed and ship under consideration the minimum vertical acceleration level was found to be at between 20 and 30% of the ship s I w' at zero speed measured forward of the stern (as shown in the lineplan).

AUTHORS' BIOGRAPHIES

because of speed reduction and tired crew members can be considerable. It is therefore only natural that there is a continuous quest for improved seakeeping behaviour.

The Dutch Lifeboat Institution KNRM has in recent years commissioned several new rescue vessels Experiences with earlier vessels were incorporated into the newer designs. However, it was felt necessary to get a more' quantified measure of the relative peiformance of ships than just opinions of crew members, valuable as these

are Therefore 'measurements were carried out in

conjunction with the Ship Hydromechanics Laboratory of Delft University of Technology.

The results will be

presented here.

2. ThE SHIPS

Both ships are fast rescue vessels of the Rigid Inflatable Bottom (RIB) type. The general arrangement of Ship i and the lineplans of'both ships are shown in figure 1 and

2. Main particulars are listed. 'in Table 1. The principal differences of Ship 1 as compared'to Ship 2 are the:

steeper deadrise of the bow section;

larger displacement (05 tonnes);

larger waterline length (0.57m longer);

larger beam (0.4m wider).

During the test Ship i was slightly trimmed forward due to an extra 200 kg of batteries in the bow section.

LEP V. LAAC

It

1.flL

Fig. i

Top to bottom: general arrangement of Ship 1; lineplan of Ship 1; lineplan of Ship 2

Fig. 2 From top to botlom; lineplans of both ships superimposed, fender a'so shown; lineplan of Ship 1; lineplan of

Ship2

3

A2

L-The ships -are driven by water jets The, engine instal-lations -are identical and in calm water both ships can reach -a-speed-of 31 knots. The changes in the design of

Ship I were thought to contribute to. a ;better motion

behaviour in rough weather. :To.verify thisassumption -a measurement programme was planned and executed.

3. THE MEASUREMENT PROGRAMME

When tests with different ships are carried out in a model

tank test for comparison, conditions can be exactly

ontrolled and replicated for each ship to be-tested. This greatly facilitates the comparison of the test outcomes. For full scale measurements this situation does not exist.

lt is impossible to

test two ships under

identical

conditions. The closest one can get is-to test the ships simultaneously by sailing side by side. The present test programme- was set- up accordingly. Two series of tests

were planned, each consisting of 10 test runs at 5

different courses with respect to the wave direction.

During the first series the- ships sailed at -a constant speed. This speed was obtained from a test run in head seas with no helmsman interference and it turnedtout that 12 knots was the maximum speed possible considering

the conditions and motions.

During this series the

helmsmen were not allowed to-change course or reduce speed to evadé large waves. This is- -a situation which does not reflect real world conditions but- emulates tank test conditions and the assumptions in -many ship motion programs. For checking the results this is therefore -a useful condition. This series consisted of 1-_-0 runs.

Realworld situations werestrivenfor-in the second series. Here the helmsmen were asked to sail as fast as they considered jUstified at each particular heading. A proviso was that the ship with the slowest speed dictated the speed. The other ship had if necessary1 to reduce speed to stay side by side with the slowest one. Moreover, the helmsmen were allowed -to momentarily change course or reduceiíncrease speed toavoid large-wave irnpactson the ships. As a consequence of these conditions average speeds varied considerably at different courses, from

15-TABLE i Main particulars of the ships

4

-17 knots in head waves.- to 28-30 knots in beam and following waves; This series comprised of 12 runs-.

Mostofthe -speed/course combinationswere -testedtwice. Each of -those test runs was 5 minUtes long.-.

Duringthe tests the following pararneterswere measured:

vertical acceleration at the bow;

-vertical acceleration aft at the wheelhouse;

longitudinal (fore-aft-) acceleration;

pitch angle and trim;

speed, -course and position;

wave height, period and direction.

The wave data was obtained from a WAVEC directional wavebuoy from the Dutch Directorate, of Public Works

(Rijkswaterstaat).

Apart from the wave data-all-data-was-recorded with PC's on :boàrd of -the ships. Before recording frequencies above 10 Hz- were filtered away. All the signals were sampled at a rate of 30 samples/s.

4. MEASUREMENT -RESULTS

The tests were carried out in waves with a significant waveheight fluctuating -between 2.0 and 2.5m -and an average wave period between 7 and 8 s. The waves arrived from direction 010°.

An overview- of the test runs

is given in Table 2.

Unfortunately, due to some hardware problems, the data sets were not complete. After run 6 the sensors in the wheelhouse in Ship 2 went out of order. Furthermore, after test 17 the computer on Ship 1- developed problems, rendering the data of further tests unusable. -However, sufficient data remained available to obtain useful results. Graaf van Bylandt (Ship 1) Christlen (ShIp

2)-Length on waterline (m) 11.7 11..1

Length over all (m). 15.2 - 14.4

Beam (m) 5.4 - 5.0

Draught CWL (m) 0;75 0.75

Displacement (m3) 13.7 - 13.2

Max. Speed at r.p.m. (kn / rp.m.) 31- / 2300 31 / 2200

Deadnise at station 5 (deg) 27 25

As mentioned before attention was mainly focused on the vertical accelerations. These are the ones that Ultimately limit speed under rough conditions. For the well-being of the crew the accelerationsin the wheelhouse arethe most relevant. For loads on the ship the accelerations in the foreship are of more importance. Figure 3 shows the Root Mean Square (RMS) values of the vertical accelera-tions at 2.Om (Ship 1) resp i .Bm (Ship 2) forward of the stern (as shown in the lineplans) of the, ships forthe first senes of tests (12 knots). These positions will hereafter be referred to as vertical acceleration aft.

Also shown is the same information of the forward accel-erometers located at 10.8 (Ship 1) andt 1O.7m (Ship 2) forward of the stern Thisposition will be called the vertical acceleration fore The figure clearly shows the influence of wave direction on the vertical acceleration levels at a constant speed. Moreover,

ft can be seen that the

acceleration levels fore are 1.5 to 1.8 times as high as aft.

Figure4 gives the same information for the secondseries of tests (15 - 30 knots). In additionthis figure contains a line which shows the speed during these tests. As a consequence of the higher speeds at beam and following

seas the

acceleration

level does not decrease as

markedly as in ligure 3. This suggests that the vertical acceleration serves as a criterion forthe helmsmen when they decide on their speed for a given course.

TABLE 2 Overview of test runs

5

COmparing the acceleration levels of the two ships shows that there is very little difference between them During

series 2 the levels aboard of Ship 2 are on average

slightly higher.

The RMS value of the accelerations is an important

measure. Safety norms specifying what vibration and motion levels people may be subjected use this as their basic parameter together with frequency However often the basic assumption behind this is that the acceleration

signals are normally (Gaussian), distributed. When this is true it is easy to calculate the probability that a certain acceleration level wilibe exceeded However, aboard fast vesselsthe vertical and longitudinal accelerations may not automatically be considered as normally distributed. This is clearly visible in the vertical accelerations fore.

Figure 5 shows part of a time registration of such an

acceleration.

lt can be clearly seen that the positive

peaks (ship accelerating upwards), are much larger than the negativepeaks. With a normallydistributed signal the positive and negative peaks would have been equally distributed around the mean (zero in this case).

Figures 6 and 7 show for both ships the maximum

measured momentary magnitude of the positive and negative accelerations of all runs as a function of their ANIS value. So each plotted point is the absolute

maxi-Serles i tests

Runs Ship-heading (deg)

Rel. wave direction (deg)

1 Speed (kn)

1-2 15 185 11.7

34

321 131 12.2 5-6 272 82 12.4 7-8 227 37 12.0 9-10 t93 3 11.8 Series 2 tests

Runs Ship-heading (deg)

Rel. wave direction (deg)

1 Speed (kn)

11-12 192 2 28.7 13-14-15 225 35 27.6 16 260 70 - 30.0 17 310 120 19.3 18-1 9-21 29 199 16.0 20-22

i6

186 21.0

1 Relative direction of waves to ship. Head waves = 1800, beam port =270°, beam SB t=90°, following waves i=0°. Waves were coming from direction 010.

o

6

4

No data from wheelhouse of ship 2

D 6

o

Shipi fore

y

Ship 2, fOre :0 Ship 1., aft

- Speed curve (fifled)

V

30

10

o

90 180

Relative wave direction (dey)

Fig 4

RMS values of the vertical acceleration fore and aft as function of the relative wave direction Series 2 of tests Speed varied as shown by drawn line.

6 Ship 1,, fore

V

Ship 2, fore

o

Shipiaft

o

Ship 2, aft

N E 4 C) C)

s

Rel. wave direction:

_{O = waves coming in from behind:}

90 = beam waves from SB

V

>

u,

180 = waves coming in from ahead

90 180

Relative wave direction (deg)

Fig. 3 RMS values of vertical accelerations fore and aft as function of relative wave direction. First series of experiments. Speed was 12 knots.

.

8

V

V

30 20

lo

-10 -20 -30 60 (j) E u, (5 (5 o 40 0) C

0

u, (5 '5 > o 20 (J CS w -20

Run 2, ship 2, speed 11.7 kn, head waves RMS value 3.82 rn/s2

l/1000th level for normai distribution pos. i/l 000-th level of simulations V neg. l/1000-th Ivel of simulations

o

Negative max during run

D

Positive max during run Ship i (Graafvan Sy$andt)

each run lasted 300 s, simulations 400 s.

U

D D _D D

D

s

7 Time (s)

Fig. 5 Part of vertical acceleration signal near bow of ship 2

> (5 C

_s

-(I) o o. u, w 0)

QQO 9

V o _{2.5 5.0}

₇₅

2

RMS value of vert. acceleratIon near bow (rn/s

Fig. 6 _{Maximum positive and negative values during run of vertical acceleration in the bow of Ship 1. Also shown are} a few results of computer simulations

mum or minimum value measured during a 300 s long test run. For low RMS values the positive and negative maxima are approximately the same. However, at higher RMS values the negative maximum accelerations taper off at approximately 10 rn/s2. This is the free fall condition. On the other hand, the maxima start to increase almost quadratically with increasing RMS values. This is due to the impact on the water and the strong non-linearilies when the wedge-shaped bow is immersed into the water

The deadrise of the bow section

is considered an important parameter in this respect. Of the two ships Ship I had a larger deadrise. However, from the comparison

of the data of the ships

it is clear that the dIfference between the them is not very large. Ori average the peak values aboard Ship 1 were around' loe/o lower than those aboard Ship 2.

A similar trend was found using the non-linear time

domain simulation programme FASTSHIP (3) developed at the Laboratory for predicting the vertical motions of fast (planing) mono-hulls in head waves.

A number of

simulations were run, each having a length of 400's. The ships had aspeed of. 12 knots', hence corresponding with. the series 1 tests. Waveheight was varied .butthemean wave period -was kept at 7$ s. A. few' results 'of .lhese calculations for ship 2 are plotted in figures 6 and 7. The values, presented there are the mean value of the i % highest vertical acceleration peaks of. each run, The

correlation between measurements and simulations is quite good, in particular when considering that in the experimental results the (somewhat higher) highest peak values measured were plotted.

The correlation is even better when comparing the 1% exceedance levels as measured aboard ship 2 and the mean value of the 1% highest peak values found in the simulations (figure 8). The 1% exceedance level is the value that was exceeded 1% of the time during a test or simulation. The mean of the highest 1% peaks of the simulations is somewhat larger than the 1% level of the test runs which can also be seen in the figure.

The computational comparison between ships 1 and 2 again showed the slight superiority of ship 1 with respect to the'vertical accelerations whencomparedwith ship 2 in several spectra.

For proper interpretation of the results' a few things must be kept in mind. First there Is the fact that during the second series the helmsmen were free to evade waves by momentarily lowering speed and briefly changing course. Ditferences in handling of the ships may have affected the results somewhat. Secondly, because the tests were

conducted in one condition only, namely rather long

unidirectional waves, the results might not be

representative for other operating conditions.

From figures 3 and 4 it is clear that the vertical acceler-ations aft were considerably lower, approximately 50%, than in the bow section. This also resulted in much lower positive and negative maxima there (figure 9). lt is also

evident that the maxima remain almost symmetric at

higher RMS values of the accelerations.

8

On average the accelerations aft were between 5 and 10% lower aboard of Ship 1. Once again the difference between the two ships was small.

For optimum safety on board it is advantageous to locate the wheelhouse near the longitudinal position where

minimum vertical accelerations can be expected. There-fore, using the data from the two vertical accelerometers

on board the longitudinal point of minimum vertical

acceleration was determined for each of the

runs.

Because the data set for Ship 2 was incomplete this was only possible for a limited number of runs. Figure 10 shows the points of minimal vertical acceleration as a percentage of l.w.l. (forward of the att'point of the l.wW) as a functiOn of the relative wave direction for Ship 1 and 2.

For Ship 2 only data for the 12 knots condition was

available. lt is noticeable that the minima are almost independent of the relative wave direction. Furthermore the minimum level for Ship 1 is shifting aft 'at high speeds. This is thoughttobe due to the changefrom displacement to planing mode of the ships. Furthermore on board of

Ship 2 the minimum is slightly, more forward as' a

percentage of the l.w.l.

Besides acceleration level, the average period of the verticalaeleration is an important parameter'in national and international standards for allowable acceleration levels. The most remarkable finding during the tests was that this period changed litile with relative wavedirection. Furthermore the period times are short mpared'with the natural heave and pitch periods of this type of ships. The reason for this is the quadratic increase of acceleration with frequency. A small pitch angle component with a high frequency therefore gives rise to a large acceleration component with short period. Moreover, the strong nOn-linearities also generate motion components with high frequencies which also result in high frequency

acceleration contributions.

Table 3 summarizes the measured average periods for both ships. Average period is defined here as' the period of the mean frequency obtained by determining the centre of the spectrum. of' the signal under consideration. Also included in thetable isperiod dataof the pitch angle. The much larger range of periods for the pitch is caused by the, here clearly visible, influence of the relative wave direction. From the table it is also clear that the period times aboard Ship 2 were approximately 10% shorter than

those aboard Ship 1.

Table 3 also shows period times of the accelerations along the longitudinal axis of the ships. The magnitude of these accelerations is significantly smaller than that of the vertical accelerations. However, it can

still be

considerable and also shows a strong asymmetry like the vertical acceleration fore This is shown in figure .11 were the maxima and minima of the longitudinal acceleration are shown as a function of their RMS value. The largest excursions are here the negative ones, negative meaning that the ship is decelerating.

Unlike the

vertical acceleration the longitudinal acceleration is the same all over the ship. Its RMS value is aboût 35% of that of the

60 (O E w e o 40 C w e > j. 20 o e > -20

1/1000-th level for normal distr. pos. 1/1000-th level of sim ulations

y

neg. 1/1000-th level of simulations

o

Negative max during run D Positive max during run

Ship 2 (Christien) each run lasted 300 s, simulations 400 s.

-

D L.J 20 n u) C (n C 10 C

0

e > u) e o C '° _o

0

w w I.) X, u) -:1.0

D pos. 1% signaI level

0

neg. i % signal' lével'

pos. i % levelof simulations

neg. i % level of simulations i % level of normal distr.

Ship 2

D

-O

0 000

o

Speed in simulation runs was 12 knots.

9

Duration of each'run was 300 s,

of simulations 400s.

D D

D

DD-w D C -o

.

C (C u, e e. u, w o)

----_v0 oo0

o

(C O _{2.5 .5.0 7.5}

RMS value ofverj. acceleration near bow (mIs)

FIg. 7 Maximum positive and negative values during run of vertical acceleration in the bow of Ship 2. Also shown are a few results of computer simulations

.2

4 6

RMS value olvert. accelerationfore (mis2)

FIg. 8 1% exceedance levelsof positive and negative vertical accelerations in the bow of 'Ship 2 during each run. Also shown are some results of computer simulations

D D

.

D D D D

vertical acceleration aft but the peak values can be up to 60% of the peak values from the aft vertical acceleration. They are therefore not negligible in the wheelhouse.

Despite the fact that,. there

is only little longitudinal acceleration data available of Ship::2 figure. .11 suggests

that the acceleration level aboard. Ship 2 is

(again)

soniewhatlargerthanon.Ship1:'

5. CONCLUSIONS

From the test results a number of conclusions may be drawn.

Differences in acceleration levels between the ships were small However, there was a consistency in them in that acceleration levels aboard Ship.2 almost always were approximately 5 to 10% higher than on Ship 1. Moreover the average period time of the motions was lOto 15% shorter for Ship 2.

Therefore, for as f ar as the level of accelerations on board is concerned the motion behaviour of Ship I under the tested conditions is somewhat better than that of Ship 2.

There is a clear minimum in the vertical acceleration

along the length of the ships.

At 12 knots the

minimum on Ship 1 was at 25% of the.l.w.l forward of the stern. For Ship 2 this was 30%. At speeds between 15 and 30 knots the minimum shifted to 18% of the .l.w.I. on Ship 1 (no data for Ship 2). The minimum did not change measurably for different courses relative to the waves.

At high. RMS acceleration levels strong asymmetries

were found in the vertical fore and

longitudinal

accelerations. It may therefore not be assumed that the accelerations are normally distributed. At high RMS levels peak values are significantly larger than predicted with a normal distribution. Also the 1% level, the level that is exceeded 1% of the time, is significantly higher than that of a normal distribution. The high peak accelerations can be dangerous to crew members.

TABLE 3 Average periods of accelerations and pitch

10

In the att

side of the wheelhouse the vertical

accelerations are much less' asymmetricand peaked than in the bow section. Near the seats of the crew

members (located about 2m more. forward) the

accelerations Will . lie somewhere between the

situationsat the bow and the rear of the. wheelhouse.

Thelongitudinalaccelerations haveRMS values.of on average 35% of the vertical accelerations aft. Peak values however reach t upto 60%.of those of the aft vertical acceleration. They are therefore not negli-gible Here too the acceleration levels on Ship 2 were somewhat larger than those of Ship 1.

At high speeds evasive actions like speed reduction and temporary heading changes can be used to re-duce extreme accelerationpeaks toa certain degree. But even then peaks remain high as the results of the second series of measurements showed. .The

corrective actions depend heavily on anticiption

based on what the helmsman sees coming his way.

When he does have insufficient information, for

instance when sailing at night, even higher peaks then measured may be expected. The only way to prevent those will be an extra speed reduction. The degree to which timely actions are possible is also dependent on the manoeuvrability and achievable speed accelerations and .decelerations of the ships. Water jets as used on board'of'both vessels facilitate both these evasive measures.

Themeasurements:provided.a vast amount of ship motion data aboard Rigid Inflatable Bottom craft. This supplements .data about other types of small fast vessels [1] [2] that was already available. It should however be

kept in mind that the results are valid for the tested

conditions only. A limitation of full scale tests is that these conditions are to .a large extent dictated by the weather. lt would for instance have been interesting to extend the tests to include shorter waves as well. However, for the conditions on the test day, testing the two ships side by side made it possible to get a fair comparison between them. The differences were small and could not have been found had the ships been tested one after the other. Had that been the case the differences could have easily been caused by or attributed to changes in the weather conditions.

Speed 12 knots Variable speed 15 - 29 knots

Ship i

ShIp 2 ShIp i 2 ShIp 2

Vert. acc.

fore 1.5 - 2.25 s 1.25 - 1.9 s 1.0- 1.2 s 0.9 - 1.1 s Vert. aco. att 2 - 2.4 s 1.8- 2.2 s .1 1.45 - 16 s nO data

Long. acc. 1.3- 2.7 s 1.05 - 1.35 s .1 1.4 2.3 s no data

Pitch 2.5 - 7.4 s no data 2.85 - 5.45 s no data

1 Head to beam wave conditions. No data for following seas.

60 u, C Q 50 o 40 g

>e

E 30,

E o

c :2 20

o.

-C -c 10 C o _J -o

maximum of run, ship 2

y

minimum of run, ship 2 D minimum of run, ship i

o

maxim:um of run, ship 1

Each run lasted 300 s-.

qOO O

D O

0.0

cÇO..

V 11

cc . c.

D-

D LJ

_V

y yy

O 0.5 1.0 1.5 2.0 2 5

RMS value-of vert, acceleration aft (mis.2)

Fig. 9 Maximum positive and negative valuesof the vertical accelerationaft during each rUn. Values shown as functiôn of RMS value of the acceleration during the run

o

Ship 1, speed 12 kn.

y

Ship 1, speed between 19 and 30 kn Ship 2, speed 12 kn V V

o

o

V

o -- 90 1 80

Relative wave direction (deg)

-Fig. 10- LongitudinaIposition of minimum vertical acceleration in of l.w.L starting from stern as afunctionof the relative

wave direction .

-o

o

o

For a fuller picture over a range of conditions it would

have been necessary to set up a moro extensive

measurement programme. However, given the complica-tions and expense of such a programme model testing could be a better alternative. This has the advantage that the conditions can be precisely controlled, allowing even small differences to be detected. Another advantage is that it is possible to test several different ship concepts.

even extreme ones

if desired1 quickly and cheaply

Furthermore., extreme conditions that at fil scale are best avoided can be tested in a model basin Without danger.

A disadvantage is that model experiments with fast

vessels can only be carried: out in head and following waves. But as the head wave condition is in general the most severe one this is otten.not a serious limitation., At the Ship hydromechanics Laboratory a series attests is currently being conducted with four ships with different lengths and bow shapes. These are being tested inthree different wave conditions. Having data from full scale tests which include real world effects such as evasive

course and speed changes and wave directions not

covered in model'tests is a very 'valuable addition 'to the

developing body of data and. knowledge about the

seakeeping behaviour of' srnallfast,vessels.'':-.

6. ACKNOWLEDGEMENTS

The authors would like

to thank, the Dutch Lifeboat Institution for 'its kind' permission to use data from the

N 2 0 -o

8

VV

V

o

VV

o

00

00

o

o

RMS value of !ongitudinal acc, (mis2)

Fig. 11 Maximum positiveandnegative values ofithe longitudinal acceleration during eachrun. Values shown as function of the RMS value of the acceleration during the run

12

measurements for this paper. Furthermore, our thanks are also extended to the crews of the Graaf van Bylandt and Christien. lt has been a great pleasure to co-operate with the enthusiastic and capable individuals that we found aboard of these ships.

Finally1 the name of, ir. E Vossnack must be mentioned for the, assistance providéd in preparing this paper and the fruitful discussions 'throughout the project

7. , REFERENCES

PINKSTER, JAKOB and.JOHAN, JOURNÉE, J M J: 'Sea trialS With Regard to Design and Operational Limits of Fast Pilot Vessel MS Voyager'. Proceedings International Symposium SURV IV, Surveillance, Pilot and Rescue Craft IV, May 1997, Gothenburg.

OOMS, J and JOURNÉE, J. M J:

'Pilot Vessel Voyager, Part lia: Hydromechanic Behaviour' (in

DUtch), Technical Report No. 1004-0, October 1,994, Shiphydromechanics Laboratory, Delfl University. of Tèchnology, The Netherlands.

KEUNING, J' A: 'The Nonlinear Behaviour of Fast Monohulls in Head Waves'. PhD thesis, September

1994, Ship hydromechanics Laboratory. Delft

University of Technology, The Netherlànds.

E C u C 0) C 6 4

y

D

o

Positive rnax,, ship 2 Negative max, ship 2

Positive max, ship I

Negative max, ship 1

08

O