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Marine engineering the trimaran hull form –opportunities and constraints

Dr A R Greig and Dr R W G BucknallDepartment of Mechanical Engineering , University College London

The trimaran hull form has been gaining a great deal of attention from Naval Architectsrecently, both in Europe and the USA. A 100m trimaran is planned for completion in 2000as a technology demonstrator for the Royal Navy’s Future Escort Frigate programme and theUS Navy is taking an active part in this project; other NATO navies and the US Coast Guardhave also shown a keen interest.1 In Finland, Kvaerner-Masa have put forward a conceptdesign for a 35kt trimaran cruise liner. The trimaran hull form offers a number of advantagesover a monohull and for many roles it is seen to be superior. The aim of this paper is tointroduce the trimaran to a marine engineering audience, highlighting its benefits andconstraints. It will then concentrate on how these features impinge on the marine engineer-ing design of the vessel. It will show that for the trimaran concept to be a success, the marineengineering of the vessel must be considered from the outset of the design and that the threelong thin hulls of the vessel make it very sensitive to alterations in machinery fit.

Authors’ biographiesDr Alistair Greig studied at University College London where he gained his PhD in 1992. He has been a lecturer in the department ofMechanical Engineering at UCL since 1989. Prior to that he was a marine engineer with the Royal Corps of Naval Constructors. His currentresearch interests include marine engineering, mathematical modelling and underwater robotics. He co-ordinates the marine engineeringMSc students on their ship design exercise.Dr Richard Bucknall began his marine engineering career with BP Shipping Ltd. He left the sea to study for a degree in electrical andelectronic engineering and after spells with British Rail and Becton Dickinson UK, he joined the teaching staff at the Royal NavalEngineering College where he took the opportunity to study for his PhD. Upon closure of the RNEC in 1995 he joined the staff at UCL.He is now course co-ordinator of the marine engineering MSc and is undertaking research into the transient behaviour of electrical systems.

INTRODUCTION

A brief history of the trimaran

It would be incorrect to call the trimaran hull form new sinceit has been in existence for hundreds, if not thousands, ofyears as witnessed by the outrigger canoes used in Indone-sia. More recently, trimarans have been used successfully asoffshore racing yachts and Robin Knox Johnson sailed onearound the world, setting a world speed record. What is newis the extension of the idea from small unpowered craft tolarge, powered vessels of a few thousand tonnes or more.

The record breaking around Britain voyage of the powerboat Ilan Voyager in 1990 demonstrated the potential ofpowered trimaran craft. This prompted D R Pattison (at thattime the Professor of Naval Architecture at University Col-lege London) to investigate the idea further. The result wasthe concept design of a future frigate with a trimaran hullconfiguration as one of the MSc ship design exercises in1990,2,3 (see Fig 1). Subsequent MSc concept designs on othertrimaran vessels of various types indicated the viability ofthe idea and are summarised in the 1994 paper by Pattisonand Zhang.4 Since then the concept has been taken further bythe UK Ministry of Defence and has been considered as onepossibility for the future escort frigate.5 Model tests havebeen conducted at DERA, Haslar. Commencement of con-struction of a 100m trimaran technology demonstrator is dueto start soon and it has a planned completion date of 2000.6

Background

One of the most significant operating costs of vessels is the fuelrequired for propulsion so there is a strong desire to design vesselswith hydrodynamically efficient hulls. In ship design, there is acompeting requirement to build vessels which have useful andfunctional layouts for their payload, for example to optimise thecar deck in a ro-ro ferry or maximise the hangar space in an aircraftcarrier. These two design requirements when applied to amonohull, drive the hull form in opposing directions. For lowresistance a vessel needs to have a long thin, tapering hull, ie alowblock coefficient ( CB) and a high length-to-beam ratio (L/B).Unfortunately, in such vessels the proportion of the total volumethat is usable is low and there will be severe layout constraints.Seakeeping, especially roll will also be a problem. In contrast a‘short/fat’ ship with a blunt bow, where the block coefficient isapproaching unity, will offer a high proportion of usable volumeand the layout can be easily adapted to the vessel’s role. This hasbeen gained at the expense of performance, with resistance beinghigher. This conflict between form and function is the paradox ofship design.

One of the main attractions of a trimaran is that it offersa solution to this paradox. It provides a large box volumewith broad deck areas which are ideally suited for mostpayload layout requirements and it has hydrodynamicallyefficient hull forms with good seakeeping.6 It has beenestimated that compared to a similar displacement monohulla trimaran will require less effective power at high speeds,but there are penalties to pay.4 The hull is more complex and

Paper read on 10.03.1998

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more expensive to build and, while the broad, well propor-tioned cross structure offers the opportunity for an efficientinternal layout for the payload, the machinery spaces in thelong slender hulls become very constricted.

The design problem

All the trimaran designs to date have taken the basic form asshown in Fig2. Hydrodynamics dictate that for an efficientvessel there should be a large centre hull (85–95% of totaldisplacement) with a pair of small side hulls. If the propor-tion of the total displacement of the side hulls is increased,the total resistance of the vessel will increase rapidly. Thethree hulls are connected by a cross structure.

In a monohull of a given displacement the main param-eters which decide the vessel’s hull shape can, in simplisticterms, be considered as its length, beam and draught. For atrimaran additional parameters are required because nowthere are three hulls instead of one, so not only does the formof each hull have to be considered but also their relativepositions. Extending this simplistic model would suggestnine parameters are needed to describe the three hull formsand a further four for their relative positions. It is a reason-able assumption to make that the vessel will be symmetricalabout its longitudinal centre line, in which case the thirteenbasic parameters can be reduced to eight to describe the basicshape of the vessel, these being: the main hull length, beamand draught; the side hull length, beam and draught; therelative longitudinal position of a side hull’s amidships tothe main hull amidship position; and overall beam (fromwhich the main hull to side hull separation can be deduced).It is important to appreciate that, unlike the main hull, theside hulls do not necessarily need to be symmetrical abouttheir longitudinal axes.

Compared to a monohull the ship designer has manymore decisions to make to achieve the ‘ideal’ vessel and mostof these decisions must be made much earlier on in the

design process. This is one of the challenges of designing atrimaran. Certain design solutions are imposed by physicalconstraints and these have a significant impact on the sidehull design in particular. The task is not aided by the lack ofdesign and experimental data available on the hull formsrequired for trimarans. The high L/B ratios can take the hullforms beyond the limits of existing design and experimentaldata; the asymmetric form of the side hulls add furthercomplications.

The length and the fore and aft location of the side hullsare dictated by the wave pattern produced by the hulls, aswell as by stability considerations. When considering thewave patterns, the goal is to produce destructive interfer-ence between the patterns produced between the centre andside hulls and reduce the wave making resistance of thevessel when operating at its design speed. The wave pat-terns are a function of the Froude number (Fr = V/(g.L)0.5), sothe location and length of the side hulls is now linked to theoperating profile of the vessel. The interference effects of thetrimaran’s three hulls are much more marked than the bowand stern wave interactions of a monohull, but even for a

Fig 1 The advanced technology ASW frigate 2

Fig 2 View of generic trimaran from underneath

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monohull the wave making resistance is of the order of 10%of the total resistance for a tanker and up to 30% of that fora frigate.7 To minimise resistance and to gain maximumbenefit from wave cancellation the side hulls need to belocated as far aft as possible. The more demanding damagedstability requirements of a warship, and the desire to protectthe high value compartments which need to be locatedamidships in the centre hull, will mean that warship sidehulls are further forward (usually centred at 10–20% aft ofamidships of the main hull) than on civilian designs. Themore stringent damaged stability requirements means alarger water plane area, hence the side hulls also tend to belonger for naval vessels. Studies at UCL and DERA, Haslar,suggest that a side hull length up to 40% of the main hulllength is required to meet MoD standards.8

The transverse location of the side hulls is dictated bystability requirements (adequate metacentric height to givea good roll performance), wave interference and total beamlimitations (construction, docking, harbour and canal re-

strictions). To avoid undue slamming on the underside ofthe cross structure the air gap should be at least 3.2m forseagoing vessels (see Fig 3).

By comparison the main hull is more easy to design; it hasa long, slender, hydrodynamically efficient form to mini-mise resistance. From a naval architecture view point theupper limit for L/B is about 16–18 (depending on hull depthand role), beyond which the hull becomes structurally inef-ficient and the increased wetted surface area drives up theviscous resistance. However, the controlling factor is usu-ally the minimum beam that is acceptable to the marineengineer for location of machinery. Thus the marine engi-neering of trimarans is beam driven rather than lengthdriven on most monohulls.

What is not so obvious is that not only is the beam of thecentre hull very sensitive to the machinery fit but so is thelength. Because of the high L/B ratio, a small change in beamwill result in a significant change in length (if displacementis to be maintained). This will, in turn, affect the resistanceof the vessel in two different ways. Firstly, the change in hullform of the centre hull will increase its resistance and,secondly, it will alter the wave pattern set up by the centrehull, with the result that the side hull geometry may requireadjustment.

For a given operating profile, stability, seakeeping andhydrodynamic considerations will restrict the design op-tions for the hull configuration. Practical considerationssuch as launching, docking and canal passage will imposefurther constraints. Into this envelope the marine engineermust fit propulsion machinery and the propulsors.

COMPARISON OF TRIMARAN AND MONOHULLS

Table I compares a range of civilian and military trimaranconcept designs with similar monohulls; the monohull val-ues are in italics. With the exception of the ASW frigate, thetrimaran concept designs are the results from MSc shipdesign exercises undertaken at UCL. The ASW trimaran istaken from a concept design conducted by Future Projects(Naval), UK MoD.5 The values for the monohulls have been

Vessel type Fast ferry Large car ferry Cruise liner ASW frigate Small aircraft carrier

Reference [10] [11] [12] [13] [14] [15] [5] [16] [17] [18]Deep disp ∆ 1130 1130 7850 7850 27650 27650 5830 5830 16660 16660Beam overall 19.2 15.4 35.0 22.4 45.0 31.1 25.3 18.0 43.0 25.6

Main hullLWL 99.0 78.8 173.0 117.3 257.3 215.6 160.0 148.4 220.0 179.4BWL 6.8 15.4 12.8 22.4 17.6 31.1 11.1 18.0 14.5 25.6T 3.4 2.8 6.0 5.1 8.8 7.4 5.9 6.1 8.0 7.5L/B 14.6 5.1 13.5 5.2 14.6 6.9 14.4 8.3 15.2 7.0

One side hull% total ∆ 4.0 4.8 7.0 2.6 6.8LWL 35.0 70.0 72.1 59.2 82.0BWL 1.5 3.4 4.7 2.4 4.0T 2.0 3.0 5.0 2.7 4.5

Monohull values are in italics.

Table I Comparison of principal dimensions of trimarans with monohulls of similar displacement

Fig 3 Generic trimaran cross section

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taken from vessels of similar displacements and roles to thetrimarans and then scaled to give the same deep displace-ment, maintaining the same CB. The intention here is toindicate the relative principal dimensions of a range ofvessels. Direct comparisons between monohulls and trima-rans must always be treated with caution because they arenot creatures of the same species. The vessels have beendesigned to exploit the features each hull form offers. Forthis reason powering and speed comparisons have not beenmade.

It is apparent from Table I that when comparing thetrimarans with their similar monohulls that:

1. the overall length and beam of the trimarans are greater;

2. the extreme L/B ratio for the main hulls of the trimaransis 13.5–15.2 compared to 5.1 –8.3 for the monohulls;

3. the trimaran main hull beams are between 44% and 62%of that for the monohulls.

It is the beams of the main hulls and side hulls of thetrimarans that bear close investigation from the marineengineer. On the smaller vessels the beams of the side hullsare very low. In two instances, the fast ferry and ASWfrigate, they are so low that the space has very limited use.The side hulls have been reduced to an absolute minimumstill to satisfy stability requirements. As a consequence, thehulls are kept as small as possible and have a large numberof watertight subdivisions. The two larger vessels, an air-craft carrier and a cruise liner, have adapted a differentapproach; it should be noted how the proportion of totaldisplacement taken up by their side hulls is larger, about 7%each compared to the 3–5% of the other vessels. The sidehulls have now approached a useful size and it is worthconsidering increasing their beam to use them functionallyto house machinery and accepting the extra penalty inincreased resistance.

The most striking feature is the beam of the main hulls.Figure 4 compares the cross sections of the monohull andtrimaran frigates, scaled to the same displacement. The extra

overall beam of the trimaran is also obvious and, while thismay pose problems for construction and docking, it is a greatadvantage to an ASW vessel, where the trimaran can offer alarger more stable flight deck for helicopter operations thana similar displacement monohull. A more dramatic exampleis a comparison of a trimaran aircraft carrier with the samemonohull frigate (see Fig5). The main hull has a beam whichis narrower than the monohull frigate yet its displacement isnearly three times greater, and this is the real challenge to themarine engineer.

MARINE ENGINEERING CONSIDERATIONS

Introduction

The design of any vessel is the result of the close workingrelationships developed between naval architects, marineengineers and other interested parties such as weapon engi-neers (for naval vessels). Each of these professional groupshave conflicting interests, so inevitably the final ‘balanced’design will be the result of suitably optimised solutions fromeach of these professional’s points of view.

For the marine engineer, the selection of the main propul-sion machinery for a vessel is largely influenced by thepower speed curve and the expected operating profile of thevessel. Other important criteria that need to be consideredare: the available space in which to locate the machinery; themachinery weight, signatures and emissions, shock require-ment; availability, reliability and maintainability (ARM);and cost. It is the marine engineer’s task to provide a costeffective propulsion system solution within such designconstraints.

This applies to trimarans in the same way as it does tomonohull vessels, but there are some notable differences inthe design constraints. Firstly, the resistance of a trimaran isless than that of an equivalent monohull, particularly at highspeeds, so the trimaran will require less installed propulsive

Fig 4 Comparison of midship cross sections of 5830t displacement trimaran and monohull frigates

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power for the same operating speed. On the other hand, theopportunity to exploit the lower resistance of the trimaranand employ the same propulsive power to increase thevessel’s maximum speed presents itself. The effective power(Peffective) to overcome the resistance of the trimaran may beexpressed by the equation:

Peffective = (RR(Main) + RF(Main)+ 2(RR(Side)+RF(Side))+Rinterference).V

The propulsive power requirement depends upon thefriction (RF) and residuary (RR) resistances of the main andside hulls, and the interaction between them. Investigationsusing analytical results and observations of practical inves-tigations have shown that at lower speeds the effectivepower requirement of the trimaran is similar or slightlygreater than that of an equivalent monohull, but at higherspeeds its slender hull forms result in a lower total resist-ance, with the actual value depending upon the dimensionsof the trimaran. At high speed, the power required to drivethe vessel is some 10–15% less, as shown in Fig 6. The curvesshow the power requirement for vessels scaled to 2600tdisplacement. The monohull data is based on results from aLloyd’s Register of Shipping enquiry for high speed vessels,9

and these are overlaid with trimaran data obtained fromTaylor data predictions provided by Pattison and Zhang, 4

for trimarans of similar displacement. The lower case is thelimiting case, where the side hulls just touch the water andin calm seas provide negligible displacement, as in the IlanVoyager. The upper case is for side hulls of 5% of totaldisplacement each.

Another important consideration for trimaran propul-sion is the location, size and weight of the propulsion ma-chinery. In the early days of ship design, propulsion ma-chinery tended to be located in the centre of ships, mainlybecause it was heavy and large volumes were needed toaccommodate the large boilers and bulky propulsion ma-chinery. As marine engineering advanced, propulsion ma-chinery became more efficient and power densities im-proved, allowing the propulsion machinery to be moved aftto occupy the stern region, thus increasing the volume

available for other uses. Most modern monohull vesselshave their propulsion machinery located in the stern. Withthe trimaran hull form the marine engineer is presented withnew opportunities and questions to answer, such as: howshould the propulsion machinery be distributed in the tri-maran’s three hulls?

Distribution of the propulsion machinery in atrimaran hull form

The distribution of the propulsion machinery in the trima-ran hull form may be in one of the following ways:

1. split evenly between the side hulls, with nothing in themain hull;

2. located in the main hull alone;

3. divided between the main hull and the side hulls.

The exact distribution of the propulsion machinery willdepend upon many diverse factors and these are likely to be

Fig 5 Comparison of midship cross sections of 16660t trimaran aircraft carrier and 5830t monohull frigate

Fig 6 Resistance of trimarans compared to monohulls, scaled to2600t D

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different for each type of vessel. Such factors must includethe need of the vessel to meet its intended role, the availablespace for machinery in the main and side hulls, the requiredlevel of redundancy and cost. Studies have shown that thefirst option is difficult to achieve in the available space whilstmaintaining satisfactory resistance characteristics.6 Otherstudies,19,20 have shown that the second and third options aremore suitable, with the third option being practicable inlarger vessels.

Side hull propulsion only

This option is attractive if the machinery and propulsors canbe squeezed into the limited space of the side hulls. Such anarrangement would usefully employ the limited volume ofthe side hulls and free the useful volume of the centre hull,which could then be exploited for other purposes such aspassenger accommodation and vehicle garages in ferries, orfor weapon systems in warships. The manoeuvring charac-teristics of the vessel would also be superior because thelines of propulsive force will lie parallel to, and a consider-able distance from, the fore and aft centre line of the vessel.There is also the opportunity to employ shorter trunkingroutes, with the exhaust gases being vented underneath thebox structure. For warships, such an arrangement wouldhelp reduce the infrared signature and improve the stealthcapability of the vessel. For passenger vessels, the locationof machinery in the side hulls will provide a remote, lownoise and low vibration drive system. The machinery wouldnot be located deep in the vessel so removal routes for themachinery could easily be provided.

Unfortunately, the marine engineer is faced with one ofthe main design constraints of the trimaran. The propulsionmachinery will need to be fitted into the two side hullswithout increasing the side hull beams to a point where thetotal resistance becomes untenable. The combined volumesof the two side hulls of a trimaran will be much smaller thanthe main hull volume in trimarans with low resistancecharacteristics. The space is also restricted in length due tothe number of watertight bulkheads required in the sidehulls to meet damaged stability requirements. Furthermore,the outer hulls are more vulnerable than the centre hull to

collision damage, so consequently the machinery containedwithin them is also more vulnerable. The marine engineeralso needs to consider the operation of the vessel aftercollision damage has occurred, since the subsequent flood-ing may produce sufficient heel to lift the opposite side hullclear of the water, leaving the vessel without any propulsiveforce available.

The bulk of the propulsion power will therefore need tobe located in the main hull and this brings the marineengineer to consider the second option.

Centre hull propulsion only

Modern monohull vessels normally have one or two shafts.Smaller or low speed vessels are designed with a single shaftsystem, but for larger vessels or those vessels requiringhigher speeds two shafts are the norm. Marine engineershave substantial experience in both of these designs and thisknowledge may be applied to the trimaran vessel.

Single propulsor arrangement

A single shaft propulsion system in the main hull gives thecheapest engineered solution of all available options butprovides the greatest risk, with little redundancy. The mainhull has a greater beam than the side hulls and, although thisis less than for a monohull for small or low powered vesselswhere one shaft is the norm, this solution should also beacceptable for the trimaran. The floor of the cross structureis the main structural member which holds the three hullstogether and large openings in it need to be avoided. Thisdesign constraint needs to be considered at the outset. Thelocation of the machinery in the centre hull, positioneddirectly between the two side hulls, is attractive for warshipsbecause it gives added protection, but it provides greaterchallenges in providing removal routes and the routeing ofthe inlet and exhaust trunking. This will also lead to longshaft runs, especially because of the greater length on themain hull.

Two main hull propulsors

Two main hull propulsors will be better suited for higherspeed operation and this configuration will give greaterredundancy, but the design constraints mentioned whenconsidering the single shaft system arrangement will applyto this configuration, together with some additional con-straints. In monohulls, two propulsion trains are easilyaccommodated due to the wide beams available in themachinery rooms. As mentioned earlier, the centre hull of atrimaran has a much narrower beam so, consequently, twoshaft trains are more difficult to accomplish as the propul-sion machinery cannot easily be placed side by side, sincevery little room remains to carry out maintenance proce-dures and provide for suitable removal routes. The arrange-ment of the shaft lines and the location of the propulsors isalso more difficult; conventional propulsors may operatecloser together than is usually the case in a monohull.Furthermore, the location of the machinery in the main hullmay need to be angled in respect of the usual fore aft

Fig 7 Side hull propulsion: the arrangement shown demonstrateshow high speed electric motors can be used with translation gear-

boxes to minimise electric motor size

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centreline arrangement, thereby impacting upon bearingarrangements and the flow patterns over the propellers. Ifthe use of podded propulsors is preferred to reduce themachinery space in the vessel, then these may require loca-tion at the side of the vessel, since room for two pods at thestern of such a slim hull is not practicable. In such anarrangement fixed podded propulsors mounted at an angleare likely to operate between the hulls; it is not clear how rollmotion will affect their operation.

Combined main hull and side hull propulsion

Distribution of the propulsion machinery in all three hullsprobably offers the most suitable solution for larger vessels,since this arrangement improves the vessel’s manoeuvringcapability and provides improved redundancy. The ‘move’capability of the vessel is improved simply because there arethree well separated engine rooms and shaft systems. Thedistribution of machinery in such a way inevitably means agreater number of machinery rooms and machinery sys-tems, which implies greater cost, so this would appear to bethe most expensive option. The exact split of propulsivepower will depend upon the available space in each of thehulls and upon the vessel’s operating profile.

Operating arrangements of distributed propulsion

In designs which favour the distribution of the machinery inthe main and side hulls several modes of operation arepossible, each having their own advantages and disadvan-tages. The propulsive power fitted to the trimaran’s sidehulls is likely to be less than that fitted in the centre hull. Theside hulls could, for example, be designed to provide for lowpower cruise propulsion and for manoeuvring, with thecentre hull providing the boost power propulsion in a sidehull AND centre hull propulsion arrangement. Alterna-tively, the side hull propulsion systems could be designed toprovide low power for cruise and manoeuvring only, withthe centre hull providing the higher propulsion powers in aside hull OR centre hull arrangement.

Consider the AND arrangement. The total installedpower in the vessel will be less than that which is needed inthe OR arrangement, therefore this arrangement will be themost economical from a weight and space point of view.Furthermore, propulsors in the side hulls are ideally suitedfor manoeuvring, as they are at some distance from thecentreline and can provide a large turning moment. It alsoprovides the opportunity for dispensing with any steeringarrangement for the main hull. The AND arrangement,however, requires some very careful design and matchingsince the side hull propulsors will need to be designed tooperate efficiently over the full range of vessel speeds.

The OR arrangement overcomes some of the propulsordesign and matching difficulties, but it will mean that someadditional power needs to be installed into the centre hull ofthe vessel if the maximum vessel speed is to be maintained.This may only be achieved at the expense of reducing thevessel’s endurance or increasing the vessel’s size. Addition-ally, if the side hull propulsors are not retractable the addi-tional drag imposed by these must also be overcome. Steer-

ing gear must now also be fitted to the main hull and the sidehulls.

The choice between the AND and OR configurations isnot clear cut and depends very much on the role, size andoperating profile of the vessel.

The propulsor problem

The choice of propulsor(s) for a trimaran is less clear. Con-ventional propellers, waterjets and podded drives may beconsidered for propulsion in the side hulls and main hulls.For trimarans, the propulsor problem is twofold. Firstly, inarrangements where there are two propulsors in the mainhull these will operate much closer together than is usual ina monohull vessel and, secondly, in the side hull AND centrehull configuration there is a need to design the side hullpropulsors to operate at near constant power over the speedrange of the vessel.

Whatever the propulsive power, the locating of twinscrews at the stern of a trimaran centre hull is a considerablechallenge. The usual ‘rules of thumb’ for locating a propellerare:

1. Blade tip clearance of at least D/n from the hull (whereD is the propeller diameter and n is the number ofblades) to avoid excessive transmission of vibrations tothe hull.

2. Top of propeller disc at least 0.3D below deep water lineto ensure immersion and prevent ingestion of air fromthe surface.

3. A maximum 1.0m projection of propeller disc below thekeel, to maintain propeller clear of dock bottom whendocked down.

4. The propeller disc must not extend beyond the maxi-mum beam of the hull at amidships, to prevent damageto the propeller when berthing.

Interestingly, there appears to be no rule for the mini-mum separation between a pair of propellers and this is anarea which has received little attention, mainly because it isnever the limiting case for a monohull. A minimum value of0.2–0.3D appears to be an acceptable limit. For a trimaranthis is often the governing factor because the constraints ofthe hull make a large separation of the shafts difficult. Theclose proximity of two propellers and the narrow hull willmake the flow patterns into the propellers disadvantageous.The acceptable projection of the propeller disc below the keelcan be increased by using higher dock blocks or using a pit;with the advent of podded drive systems these facilities willbecome more common. It may also be acceptable to let thepropeller disc extend beyond the beam of the main hullbecause it will still be within the total beam of the vessel, butthis will make it more vulnerable, especially on warshipswhere the side hulls are further forward. For propellers onside hulls this limit cannot be relaxed. The large roll lever onthe side hulls also suggests that the top of the propeller discshould be more than 0.3D below the deep water line toensure immersion.

A further propeller design constraint is encountered whenthe total propulsive power is provided by the side hulls and

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(a) Twin shaft arrangement using two 18 MW gas turbine prime-movers: note the limited space around the gas turbines, the long shaftlines and the close location of the propellers

(b) Single shaft arrangement using two 18 MW gas turbine prime-movers: note the limited space around the gas turbines, the long shaftlines and the large single propeller

(c) Single shaft arrangement using one 18 MW gas turbine and a 4 MW diesel: note that increased space is made available around theprime-movers but the consequence is a reduction in power and vessel speed

(d) Single shaft arrangement with gas turbine alternators and induction motor: note the electrical machinery contained within the hullwhich is employed at the expense of the gearbox

Fig 8 Layout of machinery in the centre hull, showing how the limited space provides its own problems

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the main hull. Normal design practice dictates that thepropulsor should be designed to fit within the availableappendage space and that maximum propeller efficiencyshould coincide with maximum shaft power. For the trima-ran’s centre hull this continues to be the case but the designof the side propulsors poses a greater problem.

With the design of the side hull propulsors it is importantto realise that the speed of advance and wake fraction willchange considerably as the vessel moves from slow speedoperation (side hull propulsors only) to high speed opera-tion (side hull and main hull propulsors). The shaft powersupplied to the side hull propulsors will be at its maximumvalue for a wide range of vessel speeds, so careful design ofthe propulsor is needed if its efficiency is to be maintainedacross the vessel’s speed range. No standard design proce-dure exists for operating propulsors in this fashion but asolution may be found by providing for changes to thepropulsor’s pitch, shaft torque and speed. The combineduse of a cp propeller (allowing variations in pitch to be made)and an electric drive (allowing variations in speed andtorque) may help in providing a suitable solution.

Side hull propulsion system options

The limited space in the side hulls implies that it is onlypractical to place one, small propulsive system in each. Evensmall prime-mover systems require some ancillary equip-ment, such as trunking for air intakes and exhaust uptakes.As the side hull mounted propulsors are most likely to beused for manoeuvring, some form of reversing capabilitywill be required. Sufficient space around the prime-moversis also required for maintenance, and facilities to operate theengine at a local control unit are also needed. Alternatively,electric motors could be used and these could be designed tofit in the space available. Electric drives are also morereliable, are virtually maintenance free, require minimalancillary equipment and the electrical power may be fedeasily to the motors from generator sets which may belocated in other parts of the vessel. Electric motors can beused to drive the propulsor directly or via a smaller gearbox.The latter method offers reduced motor size and weight andpermits translation to the propulsive line, but this will needto be balanced against the need for a gearbox and theincrease in maintenance this will bring. Electric motors aresimple to control and they may easily be operated in both theforward and reverse directions, providing forward andreverse thrusts. Figure 7 shows how small electric motorscan be accommodated in the narrow side hulls.

The location of the propulsor could be at the front or rearof the side hulls. Front mounted propulsors would operatein clearer water, but they will inevitably be more vulnerableand in warships this position may be unacceptable due totheir close location to bow mounted sensors. Conventionalpropulsors may also be considered and at low powers theconventional propulsor option will be more economical andfar less complicated. Waterjets are not an option as theirefficiency is poor at part loads and speeds below about 20kn.They would also be more susceptible to air ingestion due tothe large roll lever. Podded propulsors would operate indeeper water and provide for additional manoeuvrability,

but the downside of using these is that further strengtheningof the side hulls will be necessary to transmit the forces andthe requirement to fit control systems and inspection hatchesinside the side hull. Locating a pod beneath a side hullwould also place it close to the extreme beam of the vesseland this would make it vulnerable to damage when berthingand limit its angle of rotation.

Main hull propulsion system options

The main hull will need to provide the bulk of the propulsionpower and gas turbine or diesel prime-movers may beconsidered. Prime-movers of this type are connected to thepropulsion shaft via gearboxes and they may be connectedin various standard arrangements as commonly found inmonohull vessels. The main difficulty encountered is therestricted space and, although single shaft arrangements areachieved reasonably well, two shaft arrangements are muchmore difficult to arrange. In this case gas turbines offer anattractive solution because of their small size and superiorpower-to-weight ratio, but the box structure and the require-ment for a gearbox restrict the available space for mainte-nance and removal routes. Diesel or gas electric propulsionoffers additional flexibility as to the location of prime-moversystems, improves redundancy and lowers noise and vibra-tion, but it has the consequence of being a heavier, larger andmore costly propulsion system. Owing to the large size oftraditional electric motors, a traditional electric solution isonly viable for a single shaft arrangement. Figure 8 showsvarious proposed configurations that may be considered inthe main hull.

Waterjets are an attractive proposition for the main hullas they have been used successfully on high speed catama-rans, where they have been fitted in very narrow beams; theyalso have the advantage of offering a horizontal shaft, thusreducing the required deck height. A waterjet will allow areduction in the size of the gearbox and this will need to beconsidered against the requirement for ducting and thehydraulic systems for controlling the manoeuvring ‘bucket’.Alternatively, podded drives may be used. As mentionedearlier, a podded drive may be fitted to the stern of the vesseland it is very attractive from the point of view that the electricmotor is placed outside of the hull. In both cases, there willbe a requirement to strengthen the rear section of the vesselto cater for the propulsive forces; additional hydraulic sys-tems will be required and inspection access provided. Thepodded drive will remove the need for steering gear and islikely to give some additional manoeuvring capability. Fig-ure 9 shows a possible propulsion system solution using apodded drive.

IMPACT OF FUTURE TECHNOLOGIES

The equipment developments of the future are likely to havea major impact on the marine engineering of trimaran hullforms. The development of marine recuperated gas turbineswill provide the marine engineer with prime-movers whichwill have a good power-to-weight ratio and fuel consump-

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A R Greig and R W G Bucknall

Fig 9 Use of a podded drive fitted to the centre hull

Fig 10 Possible electric propulsion scheme for a trimaran using advanced gas turbines, electronic converters and propulsion motors

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tion across the speed range. Such machines could be used todrive the propeller in a mechanical propulsion arrangement,or they may be used to drive electrical generators to produceelectrical power in an electrical propulsion arrangement.The development of efficient, high power, low weight, smallsized electric propulsion motors from rare earth magneticmaterials will also provide new opportunities for the pro-pulsion design engineer, as has been discussed elsewhere.21,22

These developments are improving the power density ofmarine engineering plant and, consequently, they will en-able greater powers to be fitted to improve the vessel’s speedand reduce the space required for machinery. Figure 10shows how the use of new equipments can help to reducesize.

CONCLUSIONS

The basic form of the trimaran has been established butnaval architects are still developing their ideas and conduct-ing research to provide greater insight, as they extend theirknowledge and experience in the areas required to cover theunique features of a trimaran, including: the behaviour ofthe hull forms with unusually high L/B ratios; the interfer-ence effects between the hulls; and the structural problemsof tying the three hulls.

Marine engineers have an important part to play in trimarandevelopment and they face significant challenges to provide themost appropriate solutions to very difficult propulsion problems.The location of the prime-movers and propulsors is a considerableproblem not only due to the constraints imposed by the narrowbeams of the hulls, but also by the choice of locations and combi-nations: ie what, in which hull?

Originally the location of main machinery was volume drivenand more recently it has been length driven; for the trimaran themain driver is width. The beam of a trimaran main hull is typicallyonly 45–60% of that of a monohull of similar displacement. Thecurrent favoured option for machinery fit is with main propulsionin the centre hull and smaller power plants and propulsors in theside hulls. The exact fit has to be tailored to the role of the vesseland the size of the side hulls. The design is very sensitive tovariations in side hull size. There are also considerable matchingproblems between the side hull and main hull propulsors. In anAND configuration the side hull propulsion plant must operateefficiently over a wide range of speeds which may be very difficultif not impossible to achieve. In the OR configuration the matchingproblem is eased but with the penalty of extra machinery andhence cost.

New technology such as pods and advances in electricmotors will offer solutions to some of the many challengesposed by the trimaran hull form. To achieve the full poten-tial of trimarans the marine engineering , as well as the navalarchitecture, challenges must be met.

ACKNOWLEDGEMENTS

The authors would like to thank Professor David Andrewsand Commander John Newell for their comments on the textand Christopher Broadbent for the loan of images. Thanksgo also to the staff and students of UCL, both present and

past, whose ongoing contribution to the science of trimaransis increasing the knowledge base.

REFERENCES

1. C Broadbent and B Short, ‘DERA trimaran demonstrator’, in IntlMaritime Defence: A Total Systems Approach to Naval Warfare, ProcIMDEX 97, Vol 1, Part 2, pp 183-191, Spearhead Exhibitions Ltd,Greenwich, London, UK (October 1997).

2. C Bastisch, T Peters and J W Johnson, Advanced Technology Frigate- Mk II, MSc Thesis, Ship Design Exercise Report, Mech EngDept, UCL (1990).

3. C Bastisch, ‘An advanced technology ASW frigate for the year2000’, Proc RINA Int Symp on Affordable Warships (1992).

4. D R Pattison and J-W Zhang, ‘Trimaran ships’, RINA SpringMeeting, No1, RINA (April 1994).

5. A B Summers and J F P Eddison, ‘Future ASW frigate concept studyof a trimaran variant’, Proc Intl Maritime Defence, Vol 1, pp 1-36,Spearhead Exhibitions Ltd, Greenwich, London, UK (March 1995).

6. D J Andrews and J-W Zhang, ‘Considerations in the design of atrimaran ship: the configuration for the frigate of the future’,Naval Engineers Journal, Vol 107, No3, pp77-94 (May 1995).

7. B R Clayton and R E D Bishop, Mechanics of Marine Vehicles, p 233,E & F N Spon Ltd, London (1982).

8. D J Andrews and J-W Zhang, ‘A novel design solution to stability– the trimaran ship’, Intl Conf on Watertight Integrity and ShipSurvivability, No 6, London, UK (November 1996).

9. Warship Hull Enquiry, pp 7-27, Lloyd’s Register of Shipping,HMSO (1988).

10. A Merchant and C Hill, Trimaran Fast Ferry, MSc Thesis, ShipDesign Exercise Report, Mech Eng Dept, UCL (1993).

11. ‘Pegasus 1’, in Significant Ships of 1996, pp 83-84, RINA, London, UK.12. G Koulikis and H Soh, Large Trimaran Car Ferry, MSc Thesis,

Ship Design Exercise Report, Mech Eng Dept, UCL (1995).13.‘Maren Mols’, in Significant Ships of 1996, pp 67-68, RINA, Lon-

don, UK.14. P Pentland, H Corvalan and J Zoulakis, Trimaran Cruise Liner, MSc

Thesis, Ship Design Exercise Report, Mech Eng Dept, UCL (1997).15. ‘Legend of the Seas’, in Significant Ships of 1995, pp 74-75, RINA,

London, UK.16. ‘Type 23’, in Jane’s Fighting Ships 1995-96, 98th Ed, p 768, Jane’s

Defence Data, London, UK.17. A Cudmore, G Best and G Livanos, Small Aircraft Carrier, MSc

Thesis, Ship Design Exercise Report, Mech Eng Dept, UCL (1992).18. ‘Invincible Class’, in Jane’s Fighting Ships 1995-96, 98th Ed, p 762,

Jane’s Defence Data, London, UK.19.S Smith, Trimaran Propulsion Options, MSc Thesis, Naval Archi-

tecture, Mech Eng Dept, UCL (1996).20. A Moody, Trimaran Propulsion Options: A Parametric Study, MSc

Thesis, Marine Engineering, Mech Eng Dept, UCL (1997).21. R W G Bucknall and A R Greig, ‘An electric AAW frigate’, Proc

1st Intl Symp Civil or Military all Electric Ship AES 97, SEESociété des Electriciens et des Electronciens, pp 75-80, Paris,France (March 1997).

22. C G Hodge and D J Mattick, ‘The Electric Warship III’, TransIMarE, Vol 110, Part 2 (1998).

NOMENCLATURE

B Beam (m)CB Block coefficient = ∆/LBTD Propeller diameter (m)Fr Froude number = V/√(gL)L Length (between perpendiculars) (m)P PowerR ResistanceT Draught (m)V Velocity ∆ Displacement (t)

A R Greig and R W G Bucknall

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Discussion

Dr G Armstrong (Three Quays Marine Services) Manythanks to the authors for a very interesting paper. Could theauthors comment on the structural implications of placingthe exhaust outlets between the hulls? They are likely to bequite large for the powers under consideration. Are thereany figures for costs of machinery alternatives? What con-figuration is proposed for the trimaran technology demon-strator?

Why is twin screw in the centre hull being considered? Isit because of propeller diameter/blade loading/immersionproblems? If so, would a contra rotating arrangement beattractive?

Dr A R Greig and Dr RWG Bucknall (University CollegeLondon) Placing the exhaust between the hulls would havestructural implications. The most significant of these is theheight of the air gap to prevent water entering the exhaust.This would be a significant design driver on smaller trima-rans. The structure surrounding the exhaust would have tobe strengthened to accommodate the large holes and thethermal effects of the efflux would also have to be consid-ered. Obviously these effects depend on the type of enginesused: diesel, simple cycle gas turbine or ICR gas turbine. Thiswould all come at extra cost, but the additional cost wouldhave to be weighed against the possible benefits in reducinginfra signature. This is another area for investigation thatstill needs to be explored for trimarans; provision has beenmade for this in the DERA demonstrator specifications.

We have no detailed costings for machinery fits for trima-rans at present.

The trimaran demonstrator is still under review but theoriginal full specification included the following points,summarised from Ref 1 in the paper:

1. Diesel electric propulsion:

a. single screw in main hull;

b. conventional technology geared industrialmotor and box for first two years;

c. option of water jets, propellers or ‘Z’ drive inside hulls.

2. Provision for future electric propulsion trials:

a. US IPS component wiring;

b. installation arrangements to allow replacementof main motors with permanent magnetmotors;

c. potential battery / fuel cell compartment;

d. flight deck and engine room sites for at seatesting of future gas turbine alternators.

3. Exhausting between the hulls’ test facilities.

The main reason for considering twin screws for the mainhull is redundancy; virtually all warships of frigate size orlarger have two propulsors. For commercial vessels a single

propulsor could be acceptable but for warships twopropulsors are the norm. If small cruise or manoeuvringthrusters were provided in the side hulls, these could beused in a backup or ‘get you home’ mode. The large physicalseparation would give excellent redundancy.

N Yeaman (MoD)

1. In view of the reduced height between hulls, ie betweenwaterline and box section, what are your views onproblems with exhaust discharge?

2. In view of rolling action of ship, what are your views offitting pods into sidehulls, particularly as conventionalpropellers may breach the waterline in rolling actions?

Dr A R Greig and Dr R W G Bucknall (University CollegeLondon) Exhaust discharge between the hulls will requirecareful design to avoid entry of water into the exhaust.Diesel exhausts are regularly placed close to the waterline sothese should not pose a problem; gas turbines with theirlarger air flow rates will be more of a challenge. Carefuldesign of the surrounding structure will be required. Forsmaller vessels the size of the air gap will become a signifi-cant design driver, though for larger vessels such as aircraftcarriers it should be possible to achieve a satisfactorily largeair gap.

There are two main reasons why the exhausts are beingdirected between the hulls: to reduce the vessel’s infra-redsignature for stealth operations; or because pushing theexhausts up through a conventional funnel placed along theship’s centreline would not be convenient. Generally whena vessel is operating stealthily it is at low power, therefore itmay only be necessary to place a small exhaust between thehulls of a few megawatts for a frigate sized vessel. On aircraftcarriers, keeeping exhaust trunking away from the hangarvolume and flight deck is very advantageous; this can beachieved by exhausting between the hulls. The larger thevessel the greater the possible air gap and the less of aproblem this becomes.

Pods fitted to side hulls would have to be deep enough toavoid breaching in roll. The pods can extend below the keelof the side hull because docking depth is dictated by thedraught of the main hull which is deeper. The other restric-tion on pods attached to the side hulls is the beam of the sidehulls. The narrow beam restricts the amount of structure forthe pod and access to the pod from the vessel could also bedifficult. Only very small azimuthing pods could be fitted tothe side hulls again because of the narrow beam. They wouldbe very vulnerable to damage when coming alongside ifthey protruded beyond the side of the vessel.

N L P Bernier (DERA) Although the siting of rudders andsteering gear have been mentioned, this excellent presenta-tion has primarily concentrated on the many permutationsof machinery and propulsor layouts the trimaran hull formoffers. However, manoeuvrability is a significant issue for

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trimarans, and this is highlighted by the fact that a simpletank model requires twice the rudder area to achieve thesame turning radius as the equivalent monohull. Each po-tential machinery layout for a trimaran generates a numberof rudder/steering gear options which will require carefulassessment. Could the authors please comment on whetherthey see manoeuvrability requirements prohibiting somemachinery configurations for a frigate sized warship?

Dr A R Greig and Dr RWGBucknall (University CollegeLondon) Early on in the concept design of the trimaran,manoeuvrability was identified as a cause for concern. Thelong centre hull gives the trimaran a high directional stabil-ity, which will make manoeuvring more difficult; the rela-tively small and shallow side hulls will have little effect bycomparison. The wide separation of the side hulls can beused to advantage, any force applied at these providing agreater leverage than could be obtained on a monohull.Tests conducted at Haslar and UCL (see Refs 1and 2 below),showed that the trimaran could be steered satisfactorily witha variety of combinations of thrusters and control surfaces.It would not be wise to design a frigate sized trimaran whichonly had a single (or even two) propellers for the main hulland no other propulsors. A bow thruster or small side hullpropulsors should be fitted to assist with manoeuvringwhen coming alongside – in open waters its characteristicswill be similar to that of an equivalent monohull.

References

1. J Bowman, An Investigation of the Manoeuvrability of Trimaran HullForms, BSc Thesis, Naval Architecture, Mechanical EngineeringDepartment, UCL (1992).

2. M Alder, Manoeuvring Tests on a Trimaran with Outrigger

Propulsors, BSc Thesis, Naval Architecture, Mechanical Engi-neering Department, UCL (1996).

L D Ferreiro (US Coast Guard) Drs Greig and Bucknall havepresented a concise overview of most of the propulsionoptions available to the marine engineer who may be facedwith the task of powering this new hull form. I wouldsuggest that one of the most interesting options has been leftunexplored: the use of electric drive to locate prime-moversin the cross structure or in the island. This arrangement isgenerally more feasible for a trimaran than for a monohull,because the naval architect can control the stability by ad-justing the separation of the side hulls, without grosslydistorting the internal volume; thus heavy engines can beplaced higher in the ship with a smaller overall ship impact.This arrangement presents the following advantages: abilityto install engines at a later date in the construction schedule,so that becomes less of a long lead-time burden to theshipyard; generally shorter intake/exhaust runs; the possi-bility of easier access for engine overhaul and replacement;and for warships, a reduced radiated noise signature due tothe increased separation of a major noise source from thewater. These benefits must, of course, be weighed against theuse of ‘prime’ real estate in the ship for machinery instead ofpayload, and for warships, the possible increased vulner-ability to cruise missile impacts.

Dr A R Greig and Dr R W G Bucknall (University CollegeLondon) The authors agree with Mr Ferreiro that thetrimaran is an ideal candidate for IFEP. It is intended toincorporate components of this technology into the DERAdemonstrator, with the option to fit other items at a laterdate. These have been summarised in an earlier answer.