Optimisation of the seakeeping behaviour of a fast monohull

TU Deift

DeIft University of Technology

Optimisation of thé

sea-keeping behaviour of a

fast monohull

J.A. Keuning and Jakob Pinkster

Report 1035-P 1995

FAST95 Third mt. Conference on Fast Sea Transportation, Lübeck, Sept. 25-27, '95

Faculty of Meohanical Engineering end Marine Technology Ship Hydromechenica Laboratory

FAST'95

r

VOLUME i

ProceedIngs of the

Third International Conference

on

Fast Sea TransportatIon

Lübeck-Travemünde

Germany

September 25-27 1995

FAST95

£A V99

Third international Conference

on Fast Sea Transportation

Lübeck-Travemünde, Germany

September 25-27, 1995

Editor:

C.EL. Kruppa

Institut für Schiffs- und Meerestechnik

Technische Universität Berlin

V®LLUI1t

OPTIMISATION OF THE SEAKEEPING BEHAVIOUR OF. A FAST MONOHULL.

J. A Keuning Deift University of Technology The Netherlands

Jakob Pinkster DeiftUniversity of Technology The Netherlands

ABSTRACT

The behaviour of a monohull at high forward speed in head waves may lead to an

unacceptable level of Vertical accelerations which may hamper the. safe operability of the craft In general this has leadto the development of alikinds of "advanced" concepts like the

SWATH, the Hydrofoil or SES. All these concepts, however, may tend to. be.. considerably

more complex and therefpre more expensive to build, maintain and to use. The qUestion

therèfore arises to which extend the seakeeping. behaviour of a fast monohull may be

improved upon..

One of the possibilities to improve the seakeeping behaviour in waves of a fast monohull at

reasonable cost; apart from raising the deadrise and changing the bow shape, is to make use of the socalled "enlarged ship" concept.

In this concept the length of the hull is increased, considerably, in particular forward of the the

accommodation, and no changes are made to the "payload" functions and the layout and

interior of the ship.

As a. result the length, the length to beam ratio and the length to

displacement ratio increase, all benificial for seakéeping.

In addition this enlarged ship concept may be further improved by modifying, the bow sections

which may be shaped in such a way as to minimise motions and wave-impacts in head wa\es.

To optimise the seakeeping behaviour for one particuiar.design by using this "enlarged" Ship

concept, motion calculations and operability analyses with the codes SEAWAY and

FASTSHIP of the Deift Shiphyromechanics Laboratory have been performed fòr both the

original and the enlarged concepts. An assessment of the possible changes in resistance &

propulsion, weight building cost and operation cost has been made in order to be able to

quantify and qualify the increase in operability against the increase in cost. By doing so a

1. INTRODUCTION

The behaviour of a monohull at high forward speed in head waves is the focus of the work

presented in this paper.

When looking at vessel speed we often use the well known

definition of Froude number, v/sqr(.LM) ( V in rn/s1 g = 9.81 rn/s2 and Lv4 in rn). For a

medium-sized vessel of approx. I 50 rn. vessel wiith a speed of about 30 knots the Froude

number is approx. 0.4. It is at this Froude number (i.e. approx.

Fn = 4) that the wave

resistance increases with a relatively high power of the ship speed. Above this boundary the necessary power will increase disproportionally with the ship speed thus resulting in high first

costinvestment (building costs) and high operational costs (fuels bills etc.).

In order to decrease these negative effects hard chine (semi-displacement) monohulls have been developed which are well suited for higher speeds (i.e. Froude number of approx.. 0.5

and higher). However tearing around at high speeds at sea with rough weather conditions

may well lead to an unacceptable level of vertical accelerations which, in turn, may hamper the safe operability of the craft. lt is true that this problem itself has lead to the development

of all kinds of "advanced" concepts like the SWATH, the Hydrofoil or SES, however these

solutions may tend to be considerably more complex and therefore more expensive to build,

maintain and to operate.

On the other hand, the latter problems raise the interesting

question (or indeed challenge!) as to what must be changed in the design of a fast monohull

so that thé seakeeping behaviour may be substantially improved upon with little or no

detrimental consequences with regard to the aforementloed first investment costs and/or operational costs.

One of the possibilities to improve the seakeeping behaviour in waves of a fast monohull at reasonable cost, apart from raising the deadrise, is to make use of the socalled "enlarged

ship" concept. In this concept the length of the hull is increased considerably (i.e. approx. 25 - 50%) in particular forward of the accommodation and no changes are made to the "payload" functions and the layout and interior of the ship. In addition the bow sections are shaped in such a way as to minimise motions and wave-impacts in head waves. As a result the length, the length to beam ratio and the length to displacement ratio increase, all benificial for

seakeeping.

In this paper the authors, have appíied this "enlarged" ship concept on an existing semi-planning fast patrol boat (Royal Hong Kong Police 11King Class" 26 m., speed 25 knots). Along with the basic design with .a length of 26 m. (100%) another two (enlarged) design.

alternatives have been introduced with respectively 33 m. (125%) and 40m: (150%).

Each of these three designs have been evaluated with regard to vessel motion in a North Sea environment. (JO.NSWAP spectrum). This was done usingthe_S.EAWAY_shipmtbns

designs at this stage results in a first optimisation estimation of the designs with regard to

shipmOtiOflS.

To further optimise the seakeeping behaviour for the most favourable of the aforementioned three design alternatives more design alternatives may be brought forward whereby the bow sections are redesigned in an attempt find yet a better form so as to minimise vessel motions and wave-impacts in head waves even more. This may be done in a later stage.

Finally the best design alternative is chosen based after an assessment of the possible thanges in resistance & propulsion weight, building cost and operational' cost has been

made in order to be able to quantify and qualify the increase

in operabHity against the

increase in cost. By doing so , 'realistic and relevant conclusions are reached about the

feasability of the concept compared with other possible solution which may make use of

different advanced marine vehicle concepts.

2. BASIC FAST MONOHULL DESIGN (STAN PATROL. 2600)

As basic design the authors have chosen an existing semi-planning fast patrol boat (Royal

Hong Kong Police EKing Class" 26 rn., speed 25 knots) which is a well proven design from the Damen Shipyard Group of the Netherlands.

The Damen Stan Patrol 2600 'has been specially designed for:

surveillance in coastal waters of sea-areas vital to the nation's security

protection of economically important a ivities or installations against sabotage prevention or repression of illegal actMties

fishery 'protection - ecological protection

convoying and control of traffic in coastal areas law enforcemeñt

pilot services

Relevant design information regarding hull form, stability and trim, weights,, building costs etc. were kindly made especially available 'to the authors for. the work carried out here. The main vessel design particulars are listed in Table i. The relevant General Arrangement'Plan

is shown in Figure 1.

The Damen Stan Patrol 2600 is a modern fast steel patrol boat with an aluminium

PROFILE

Fig. 1. General Arrangement Plan Stan Patrol 2600

Damen Stan Patrol 2

,, -n--t- ---

t-t-

.

; --.---;-. ,;

Table i

Vessel design particulars

Stan Patrol 2600

Length o.a. 26.70

Length w.l.. 24.85 m

Beam mid. 5.80 m

Depth mid at half length 3.35 m Draught midships approx. 1.60 m Draught aft approx (at skeg) 1.95 m

Fuel oil (mcl. daytank) 11.1 m3

Fresh water capacity 4.0 m3

Waste water capacity 1.2 m3

Displacement 970 kN

LightshipWeht 810 kN

Main Engines

2xi000

kW

3 . ENLARGED SHIP DESIGNS (3300 & 4000)

In principle the basic Stan Patrol 2600 design is eriariged in length only. Two suth designs

(alternatives 3300 and 4000) are made each having a length of respectively 33 m., and 40 m. Each vessel has been increased in length by respectively 25% and 50% with respect to the

basic design .

With regard to engineering of these two alternatives the starting, point was relative data related to the basic design. The increase in length: was created by stretching the original

body plan using the respective length factors of 1.25 and i .50. The body plan remains

practically speaking the same for aH three designs with the exceptión hoVer regarding the

number of frames (frarnespacing

I

rn. för all designs computed.) and their longitudinal

positions. Subsequently hydrostatic particulars re computed for the new (thus lengthened)

body plans. The increase in structural weights of these o alternatives was also computed

via the original weight data Wiiich was augrnented'with extra frames and hUll plating 'ile, at

'e same time, taking into, account the relevant positions of the centres of gravity 'of all'

components of the designs. 'The resistance and propulsión calculations were also made' for

each alternative and the. position of the system., centre of gravity of each of the two

alternatives were optimised with respect to minimising of .the required 'installed horsepower for the given speed of 25.00 knots.

Since the idea behind the enlarged ship concept is equal payload for all possible alternatives

it stands to reason that the vessel configuration (i.e. also position of accommodations etc.)

remains unchanged 'to that of the basic design. for each design alternative concerned.

Both enlarged alternatives are shown' in Fig. 2 along with the basic vessel configUratiôn. The main design particulars for the altennative designs are shown 'in Table 2 along With: the basic vessel configuration.

iviost interesting are the conclusions one may mak from the results shown in Table 2, namely that the larger the design the relatively lighter the ship becomes and to a lesser degree the lower the engine power becomes to propell the vessel at a constant speed of 25

knots. lt should be noted, however,, 'that the basic design is rather over-dimensioned with regard to scantlings in view .of the working boat philosophy of the designing. yard.

ENLARGED SHIP CONCEPT

Fig. 2 The two enlarged alternatives along With the basic vessel configuration.

Table 2 Main vessel design particulars for basic ship and alternatives

GM . m 1.62 - Tht.HEngine Power kW 2000

1MOOxL

(26MO mR)

i.N25

x 'L

33MO rn1)

1.50 x L

@0.00 m.?

3300 4000 33.70 40.70 31.85

3ft85

5.80 5.80 3.35 .3.35 1.47 1.38 1.82 1.61 1040 111.0 170 . 170 1.93. 2.19 1300 1200 St. Patrol 2600 lèngth o.a. rn 26.70. Length w.l. rn 24.85 Beam mid. m 5.80 Depth mid. (112L) rn 35 Draught midships rn 1.60

Draught aft approx. rn 1.95

Displacement kN 970

One of the interesting effects of the enlarged ship concept turns out to be that all the

parameters which are important for the determination of the ships 'resistance are improved

considerably yielding; a far lower specific resistance, Lesesistance per ton of displacement,

of the enlarged condepts. The usual design trend, i.e to cramp ali the functions of the ship

into the shortest overall possible length ( driven by the supposed direct relationship between.

building. cost and length) yields a relative low lèngth:tobeam and high loading factor of the planing hull. The resistance calculations of the three designs. have-been carried out using

the code FASTSHIP Of the ;Dèlft Shiphydromechanics Laboratory (Keüning 1994),

This

program approximates the resistance, the sinkage and' the. turn using polynomial expressions derived from, the results. of the Deift Systematic Deadrise Series (Keuning et al. 1993).

The results of the resistance calculations for the three design are presented in FigUre 3.

150_

¶100

50

.15

4

RES1S iTA NCE VERSUS SPEED

20

vs (')

25

Figure 3 Resistance of the three designs in relation to forward. speed.

The enlarged designs show a considerable decrease 'in actual 'total resistance, largely due to the high UB ratio and in particular higher loading factor Ap /(DlSP*2/3), when compared with the original design.

¿ 97J1 103.9 ff0.5

L,5 _{440 58I 7.03}

5.61 19 787 Lcç 2

X Y. 6.. 7X

4. ESTIMATION OF OPERABILITY

In order to be able to assess the operability of the three designs in a realistic environment, which is the basis for conclusion with regard to 'possible improvement in economics of the

advanced ship, use has been made of the calculation method (Beukelman 1988).

In the framework of this present study only the behaviour of three ships :jfl head seas will 'be considered, since it is known from the literature and from real world experience that this ' condition, generally spoken, imposes the largest- restrictiòns on the safe (and comfortable)

use of the ship. A comparison of the three designs in this condition will yield a clear insight

in any possible improvement.

As operational area of:the ships, the Southern Part of the North Sea has been chosen. The

scatter diagram presenting the wave statistics of this area has been obtained from the well known statistical data (Hogben and Lumb 1 967). To limit the amount of work, the all year

statistics have behn used and only one forward speed, of the ships, i.e. 25 knots, has been considered., No attempt has been made to incorporate. different mission profiles of the ships into the calculations. The wave scatter diagram used is presented in Table 3.

The large number of ship rrotibn calculations necessary to perform the operability calculation

makes the use of a linear superposition approach quite attractive Therefore, as a first

approach, the necessary ship motion, calculations have been carried out using the

computercodé SEAWAY. This code is based on the well known linear strip theory approach. Although the behaviour of fast planing boats in head waves may be considerable nonlinear, in particular when: the vertical accelerations are concerned, thé use of such. a linear theory

for high deadrise boats, as long as only significant, values are being used, i.e. significant motion and acceleration amplitudes, may be justified for the sake of comparison Keuning

1994)

The limiting criteria with respect to the safe operation of the ships in waves are derived from the egulations of the Dutch National Authority. For patrol 'boats (and similar craft) on the

North Sea these regulations state that the maximum significant vertical acceleration in the

wheelhouse must be less than 0.35 times the acceleration of gravity.

The results of the calculations are presented in Figure 4 presenting the heave and pitch response functions for the three craft at a speed of 25 knots in head waves. The plots are presented on a basis of wave frequency for the sake of direct comparison. This is possible

since the forward speed and the waves encountered are the same for all three craft

considered.

From this figure the improvement in seakeeping behaviour with increasing length ¡s obvipus. This ofcourse-is-a-weII-known-phenornena-in-shipmotion-analysis.

1.0 0.5 0.5 g 8 u Io 5 4 3 2

i

1.0

(A)p (radIs)

HEAVE RAO

25 Kri

1.5

Figure 4 Heave and Pitch. Response Functions for all three designs in

-PITCH

A

V525Kn

RAO

i'

0.5 i .0 15

(radii)

Table 3

Wave scatter diagram of Area 4 (Southern Sea) and

operability calculation results for alternative 4000.

010 No Tp sec. % freq. Q! H113 0.5 1.5 2.5 3.5 4.5 HI; 5.5 6.5 7.5 8.5 H 9.5 I: i 3.2 2.4 2.4 2 4.8 3.5 1.8 1.7 3 6.3 ILO 3.4 7.4 0.2

47.5

14.3

426636

5 8.8 13.3 3.2 3.6 5.6 0.9

-6 9.7 17.6

37345.34.60.5

7 10.9 10.1 2.1 2.2 1.6 1.8 2.0 0.4 8 124 98

21 2211 09 12 1606 01_

9 13.8 5.2 L3 1.3 0.7 05 0.3 0.1 0.3

.3 0.!

10 8.7 3.4 2.1 1.2 0.8 0.3 0.2 0.2 02 0.2 0.1 11 16.4 3.7 0.8 0,8 0.7 0.5 0.3 0.2 0.1 01 0.05 0.1 tot -al 99.6 No T sec. % opera-biity 40 0.5 1.5 2.5 V1 =25 knots 111,3 3.5 4.5 5.5 Area 4 6.5 7.5 8.5 i 3.2

2.4O;05_

2 4.8 3.5 0.3 0.7 3 6.3 10.8 LI 3.2 5.4 4 7.5 10.8 1.0 3.1 5.2 5 8.8 6.8 0.9 2.5 4.2 5.9 6 9.7 12.4 0.7 2.2 3.5 5.2 6.7

-7 10.9 5.9 0.6 1.8 3.2 4.2 5.4 6.6 8 12.4 6.2 0.5 1.5 2.6 3.5 4.5 5.5 6.5 7.5 9 13.8 41 0.4 1.2 2.0 2.8 3.5 4.4 5.2

6.0 68

10 15.0 78 0.3 1.1 1.7 2.5 3.1 3.8 4.5 5.2 6.1 11 16.4 3.3 0.3 1.0 LS 2.3 2.7 3.3 3.9 4.5 5.1 tot -al 74%

Since the accommodation (paload) area and layout is identical for all three designs yet

another improvement with respect to seakeeping behaviour may be introduced in the

enlarged designs. The accommodatiOn can be moved aft into the area of minimum vertical

motion.

This relative movement aft of the wheelhouse is clearly visible from the side

elevation plans presented in Figure 2. The effect of this on the vertical accelerations in the

wheelhouse therefore yields a further improvement of the seakeeping behaviour of the

enlarged 'concept. This is clearly shown in Figure 5: showing the vertical accelerations in the wheelhouse as a function of the wave frequency.

L f.5 LO

O.5_

VERTICAL MOTION IN WHEELHLSE 0.5 IO (radis)

V525 K,

APP = 15

Figure 5 Vertical accelerations in the wheelhouse for the three designs in relation to wave frequency.

From this Figure the dramatic reduction in the level of vertical acceleration in the wheelhouse over the entire frequency range is obvious.

The effect of this on the operability calculation, using the scatter diagram, may be seen in Table 3 in which the results for the Vertical accelerations in the wheelhouse for the three different designs are presented. The operability increases from 44% for the original design

of the stan Patrol 2600 to 51% for the 3300 design and finalises at 74% for the 150%

enlarged design 4000. The differences in operability .are indexed with regard to the St8n Patrol 2600 in Table 4. Note the reasonable increase of operability of approx. 1:6% for the first 25% increase n vessel length. For a 50% increase in Vessel length the increase In operability is even more larger, ie 68%

5. ECONOMICEVALUATIQN OF THE DIFFERENT DESIGN ALTERNATIVES

In order to make an economical evaluation the building costs of the different design

alternatives have been estimated. These were estimated using the original building costs of

the Stan Patrol' 2600 (of which all costs components were known) and correcting this for òhanges ib steel weight of the hull and extra painting costs (i.e. cleaning, preparation and

painting). The main engine installation has been left unchanged as far as costs and weights are concerned as the authors seek the reduction in engine power via derating of the engines

in this stage of the design exercise; in doing so, however, the outcome of building costs is

more pessimrnistic The differences in building costs are indexed' with regard to the Stan:

Patrol 2600 in Table 4. Note the low increase in building costs of approx. 3% per 25%

increase in vessel length.

The operational costs of all the design alternatives are considered for a scenario of a ten

year economic life, sailing 6 hours per day at full speed, 7 days a week for 48 weeks per year

and crewed by 5 person (3 shifts per 24 hours). The differences in operational costs are indexed with regard to the Stan Patrol 2600 in Table 4. Note the relatively high decrease in

operational costs of approx. 6% for design alternative 3300. This 'decrease is less dramatic in the case of the 4000 design alternatie (i.e 7%).

The transport efficiency (TE) - defined as (paylóad(kN) * service speed(m/s)) I installed

power (k W) - has been calculated for all three designs.

The differences in TE are indexed with regard to the Stan Patrol 2600 in Table 4. Note the relatively high increase in TE of approx. 54% for 3300 design alternative. Again this increase is less dramatic in the case of the 4000 design alternative (i.e 67%).

a

CONCLUSIONS ENLARGED SHIP CONCEPTS

Given the three designs presented in this paper, alongwith the wave scatter environment, seakeeping criteria nd the patrol boat mission profile, the follówingconclusions are drawn with regard to the enlarged ship concept (see also table 4):

- The. enlarged ship concept is, for the case presented here, very attractive indeed

An increase.in vessel length of 50%,, LealternatiVe 4000, results in the best vessel in terms ofoperational costs, operability and' transport efficiency (i.e.

vessel resistance).

With regard to vessel resistance, an increase in. length of 25% or 50% does not lead to large differences. The sign of these differences vary with speed

domain. ln absolute sense these differences remain below approximately 10%. - With regard to operability, an increase in vessel length of 50% leads to a much larger increase (i.e, approximately 4 times as rnúch) than that compared to an increase in vessel.length of 25%.

- The building costs of the vessel, provided vessel speed and payload' capacity remain unchanged, increases remarkably little with length (i.e. approximately 3% for every' 25% increase in length).

- The operational costs of the vessel, provided vessel speed and payload capacity remain unchanged, initially Shows a large decrease of 6% for 25% increase in vesSel length, remaining finally however, almost at the same level for a 50% increase in vessel length.

7. ACKNOWLEDGEMENT

Although the results and views expressed in this paper are those entirely of the

authors, accurate design studies of this type are not possible without "real-time design

information" from the field itself.

Therefore special thanks are due to Damen

Shipyards of the Netherlands for providing detailed information concerning their Stan Table 4 Estimated costs for basic ship and design alternatives

St. Patrol 2600 3300 4000

Building costs index . E-] 1.00 1.03 1.06

Operational costs index _[-] 1.00 0.94 0.93

Transport efficiency index [-] 1.00 1.54 1.67

9 REFERENCES Jourñee, J.MJ., (1992)

"SEAWAY-Deift, User Manual and Technical Background of Release 4.00", Delft University

of Technology, Ship 1H ydromechanics Laboratory, report no 910

Keuning, JA. (1994)

rThe Nonlinear behaviour of fast monohülls in head waves", Doctor's thesis Deift University of Technology - withrefr. ISBN 90-370-0109-2

Keuning, J.A., Gerritsma,j., and Terwisga, P.F., (11993)

"Resistance Tests of a. Series Planing Hull Forms with 30 degrees :Deadrise and a. calculation

Method Based on this and Similar Seriès",. mt. Shipbuilding Progress,. December 1993

Beukelman, W., (1988)

"Prediction of Operability of: Fast .Sernipláning' Vessels in a Seaway",. Delft University .òf Technology, Ship Hydromechanics Laboratory, report no. 700

Hogben, N., and Eumb, F.E., (1967)

"Ocean Wave Statistics", National Physical Laboratory, .FWSO, 11967

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Contents

News Review 2

Longer is better?

The seakeeping of a monohull in head seas can be improved by an increase

in length

without altering other parameters, giving improved operational characteristics at an

affordable cost.

A research programme at Deift University of Technology in the Netherlands has been

test-ing the validity of the above assertion, and fmding it basically true. The results to date were presented in a paper by Jakob Pinkster

and J A Keuning at the Fast 95 conference. The vessel used as the yardstick in the study is the Damen Stan Patrol 2600, a 26 metre, 25 knot patrol boat which has proved an effective

package, with examples in service with the

Royal Hong Kong Police.

It combines a steel hull with aluminium 'iperstructure and has a moderate deep vee .ullform. The main particulars are given in

Table 1.

For the purposes of the study the boat was considered to be lengthened, without in prin-ciple changing other parameters, to give 33m

and 4Dm long craft. To do this the stations

were spaced 25% and 50% further apart than

the basic design, and the lines faired. For

weight calculation purposes the frame spacing

of 1m was retained. Apart from this the boat remains the same. The wheelhouse is

identi-cal,as is the machinery, and all are located at

the same distance from the transom as in the

standard design. The payload remains the

same, and internally the extra space generated by the radical length increase is considered to be unused void space.

Damen Shipyards provided design

infor-mation on huilform, stability and trim,

weights and building costs and the analysis

Fig 2. Resistance plotted against speed for the various boat lengths.

was carried out using DeIft

Ship-hydromechanics Laboratory SEAWAY and

FASTSHIP software. SEAWAY ship motions

program uses a linear strip theory approach

and a North Sea operation area was assumed,

using the JONS WAP sea spectrum.

Hydrostatic particulars were computed for

the new (thus lengthened) body plans. The increase in stnictural weights of these two

alternatives was also computed via the

origi-nal weight data which was augmented with extra frames and hull plating while, at the same time, taking into account the relevant

positions of the centres of gravity of all

corn-Table 1 Main vessel design particulars for basic ship and alternatives

'J length o/a m length w.l. m Beam mid. m Depth mid.(1/2) L) m Draught midships m

Draught aft. approx m

Displacement kN

Deadweight kN

GM m

Tot. Engine Power kW

WORKBOATS

ponents of the designs. The resistance and propulsion calculations were also made for

each alternative and the position of the system

centre of gravity of each of the two

alterna-tives were optimised with respect to

minimis-ing of the required installed horsepower for

the given speed of 25.00 knots.

Since the idea behind the enlarged ship con-cept is equal payload and speed for all possi-ble alternatives it stands to reason that the ves-sel configuration (i.e. also position of accom-modations etc.) remains unchanged to that of

the basic design for each design alternative concerned. Both enlarged alternatives are

Fig 1. The Damen Stan Patrol 2600 on which

Thudyisbasedisshow!TattheiOpwith the

33 and 40 metre theoretical versions below.

Table i gives the particulars.

4 - 97.11 f011 flO.ß 460 '58r -7es Api. _5.61, 7.I7-Lcç- '2'X 6.4%. 'Ï 6'-

zs'' 'zs'

25' St. Patrol 3300 4000 26.70 33.70 40.70 24.85 31.85 3885 5.80 5.80 5.80 3.35 3.35 3.35 1.60 1.47 1.38 1.95 1.82 1.16 970 1040 1110 170 170 170 1.62 1.93 2.19 2000 1300 1200

shown in Fig. 1 along with the basic vessel

configuration. The main design particulars for

the alternative designs are shown in Table i

along with the basic vessel configuration.

Most interesting are the conclusions one

may make from the results shown in Table 1, namely that the larger the design the relatively

lighter the ship becomes, and to a lesser

degree the lower the engine power becomes to

propel the vessel at a constant speed of 25 knots. It should be noted, however, that the

basic design is rather over-dimensioned with

regard to scantlings in view of the working

at philosophy of the designing yard.

One of the interesting effects of the

enlarged ship concept turns out to be that all

Fig 4. Heave behaviour for the three hulls.

VERrICN.MoTiccl W WHEELJII5E V5 25 Kn X*pp =15 'n 15 06 HEAVE RAO 4. '# ..25 Kit .VD V. Fig 3. Vertical motions in the wheelhouse at25

knots in head seas.

the parameters which are important for the

determination of the ship's resistance are

improved considerably yielding a far lower specific resistance, i.e. resistance per ton of

displacement, of the enlarged concepts. The usual design trend, i.e. to cramp all the func-tions of the ship into the shortest overall

pos-sible length (driven by the supposed direct

relationship between building cost and

length), yields a relative low length to beam

and high loading of the planing hull. The

resistance calculations of the three designs

have been carried out using the code

FAST-SHIP of the Deift Shiphydromechanics Laboratory (Keuning 1994). The results of the

resistance calculations for the three designs

Fig 5. Pitch behaviour for the three hulls.

WOR KBOATS

are presented in Figure 2.

The enlarged designs show a considerable decrease in actual total resistance, largely due to the high L/B ratio and in particular higher loading factor AP/(DISP**213), when com-pared with the original design.

In order to be able to assess the operability of the three designs in a realistic environment, which is the basis for conclusion with regard to possible improvement in economics of the advanced ship, use has been made of the cal-culation method (Beuke [man 1988).

In the framework of this present studyonly

the behaviour of three ships in head seas was considered, since it is known from the litera-ture and from real world experience that this condition generally imposes the largest restrictions on the safe (and comfortable) use of the ship. A comparison of the three designs in this condition wutyield a clear insight in any possible improvement.

As operational area of the ships, the south-ern part of the North Sea has been chosen. To limit the amount of work, the all year statistics were used, and only one forward speed of the ships, i.e. 25 knots, has been considered. No attempt has been made to incorporate different mission profiles of the ships into the

calcula-tions. V

The large number of ship motion calcula-tions necessary to perform the operability cal-culation makes use of a linear superposition approach quite attractive. Therefore, as a first approach, the necessary ship motion calcula-tions have been carried out using the software

SEAWAY. Although the behaviour of fast

planing boats in head waves may be consider-ably nonlinear, in particular when the vertical accelerations are concerned, the use of such a linear theory for high deadrise boats, as long as only significant values are being used, i.e.

significant motion and acceleration

ampli-tudes, may be justified for the sake of compar-ison (Keuning, 1994).

The limiting criteria with respect to the safe

operation of the ships in waves are derived from the regulations of the Dutch National

Authority. For patrol boats (and similar craft)

PITCH RAO V ..25 W,,

I-rn"

on the North Sea these regulations state that

the maximum significant vertical acceleration in the wheelhouse must be less than 0.35 times the acceleration of gravity.

The results of the calculations are presented in Figs 4 and 5 presenting the heave and pitch

response functions for the three craft at a

speed of 25 knots in head waves. The plots are presented on a basis of wave frequency for the

sake of direct comparison. This is possible

since the forward speed and the waves

encountered are the same for all three craft

considered.

From this figure the improvement in sea-keeping behaviour with increasing length is

obvious. This of course is a well known phe-nomena in ship motion analysis.

Since the accommodation (payload) area is

identical for all three designs yet another

improvement with respect to seakeeping behaviour may be introduced in the enlarged

designs. The accommodation can be moved

'ft into the area of minimum vertical motion. ihis relative movement aft of the wheelhouse is clearly visible from the side elevation plans presented in Figure 1. The effect of this on the vertical accelerations in the wheelhouse there-fore yields a further improvement of the

sea-keeping behaviour of the enlarged concept.

This is clearly shown in Fig 3 showing the

ver-tical accelerations in the wheelhouse as a

function of the wave frequency. From this graph the dramatic reduction in the level of vertical acceleration in the wheelhouse over

the encire frequency range is

obvious.

Economic case

-In order to make an economical evaluation the building costs of the different design alter-natives have been estimated. These were esti-mated using the original building costs of the

Stan Patrol 2600 (of which all costs

compo-nents were known) and correcting this for

changes in steel weight of the hull and extra painting costs (i.e. cleaning, preparation and painting). This main engine installation has

been left unchanged as far as costs and

weights are concerned as the authors seek the. reduction in engine power via derating of the. engines in this stage of the design exercise; in

.doing so, however, the outcome of building costs is more pessimistic. The differences in building costs are indexed with regard to the

Stan Patrol 2600 in the bar chart. Note the low

increase in building costs of approx. 3% per

25% increase in vessel length.

The operational costs of all the design alter-natives are considered for a scenario of a ten year economic life, sailing 6 hours per day at

full speed, 7 days a week for 48 weeks per

year and crewed by 5 persons (3 shifts per 24

hours). The differences in operational costs

are indexed with regard to the Stan Patrol

2600 in the bar charts. Note the relatively high

decrease in operational costs of

approxi-mately 6% for design alternative 3300. This decrease is less dramatic in the case of the

4000 design alternative (i.e. 7%).

The transport efficiency (TE) - defined as

(payload(kN) x service speed(m/s))/installed

power (kW) has been calculated for all three designs.

The differences in TE are indexed with

regard to the Stan Patrol 2600 in the bar chart.

Note the relatively high increase in TE of

approx 54% for 3300 design alternative.

Again this increase is less dramatic in the case of the 4000 design alternative (i.e. 67%).

Conclusions

Given the three designs presented in this paper, along with the wave scatter

environ-ment, seakeeping criteria and the patrol boat mission profile, the authors drew the follow-ing conclusions to the enlarged ship concept, also shown visually in the charts.

The enlarged ship concept is, for the

case presented here, very attractive indeed An increase in vessel length of 50%, i.e. alternative 4000, results in the best vessel in

terms of operational costs, operability and

transport efficiency (i.e. vessel resistance)

With regard to vessel resistance, an

increase in length of 25% or 50% does not

lead to large differences. The sign of these

dif-Fig 6. Intermediate design result weight indices.

.

I

4.11

-il

I Ij

Fig 7. Intermediate design results with displacement, lightship, deadweight and speed indices.

'L'I

e

I

I

s:

ferences vary with speed domain. In absolute sense these differences remain below approxi-mately 10%

With regard to operability, an increase in

vessel length of 50% leads to a much larger

increase (i.e. approximately 4 times as much)

than that compared to an increase in vessel

length of 25%

The building costs of the vessel, provid-ed vessel speprovid-ed and payload capacity remain

unchanged, increases remarkably little with

length (i.e. approximately 3% for every 25% increase in length)

The operational costs of the vessel,

pro-vided vessel speed and payload capacity

remain unchanged, initially shows a large decrease of 6% for 25% increase in vessel

length, remaining finally however, almost at

the same level for a 50% increase in vessel

length.

Optimisation of the seakeeping behaviour

of a fast monohull. By JA Keuning and Jakob

Pinkster, Deift Universily of Technology. Fast

95, Tra vemünde, Germany.

U

..p

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