some lessons from a design

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Some lessons from a designVictor Dubrovsky

First published on: 20 July 2010

To cite this Article Dubrovsky, Victor(2010) 'Some lessons from a design', Ships and Offshore Structures, 5: 4, 371 — 375,First published on: 20 July 2010 (iFirst)To link to this Article: DOI: 10.1080/17445302.2010.485467URL: http://dx.doi.org/10.1080/17445302.2010.485467

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Ships and Offshore StructuresVol. 5, No. 4, 2010, 371–375

Some lessons from a design

Victor Dubrovsky∗

Independent designer, St. Petersburg, Russia

(Received 16 January 2010; final version received 27 April 2010)

This study describes results of a collaboration from the design work on a new type of ship. A passenger ship with asmall water-plane area was designed but without a deep understanding between the designers and the consultant. The maindifferences between the two options for the ship were the wet deck structure, the vertical clearance, the number of shafts,the number and placement of motion mitigation foils, and the possibilities for other forms of motion mitigation. Variousconsequences, such as displacement (empty and full), the propulsive coefficient, and the seaworthiness, are the result ofvarious technical decisions. The failure of the design was attributed to the lack of discussion and collaboration.

Keywords: design; ship with small water-plane area; vertical clearance; wet deck structure; motion mitigation; foils; ballasttanks

Introduction

Ships have special characteristics in terms of advantagesand disadvantages for the required purposes. However, thefinal positive result – the development of technical andeconomical characteristics in comparison with the samecharacteristics of existing ships – can be achieved only bya rational design, making full use of the advantages andminimising any disadvantages. Technically, if the designprocess is not successful, even very progressive ship typescan be technically and economically unsuccessful. There-fore, a discussion on the design of a new ship type can beuseful even if the design was not implemented.

Earlier, as a science consultant in Europe, the authorwas part of the design of a passenger ship intended forcruise lines in severe seas.

Unfortunately, the design was not realised, because ofboth different experiences of the designers and the consul-tant and the absence of an effective collaboration. However,some description of the differences can help promote a moresuccessful collaboration in the future.

1. The purpose and type of ship

A full description of the purpose is a commercial secret;therefore, only the main characteristics are noted to showthe problems associated with the specialist collaboration.

The passenger ship was intended for cruise sailing inthe severe seas of Western Europe. Hence, it was requiredthat the ship has high seaworthiness with a high level ofcomfort.

∗Email: multi-hulls@yandex.ru

Ships with a small water-plane area (SWA) have thehighest seaworthiness of all displacement vessels and thedesigners decided to conceive an SWA ship for this purpose.

An outrigger SWA ship differs from other SWA shipsin terms of the minimum relative mass of the hull struc-tures, i.e. the minimum relative price of the hull structureconstruction. This was the main reason for the selection ofthe type of passenger ship: a main SWA hull and two sidehulls (outriggers) of conventional shape and large aspectratio. The technical details of the SWA ship design givenby designers and the consultant are given below.

The designers had no previous experience with the SWAship design. Consequently, their experience with mono-hulldesign was applied – mainly trade one-shaft ships withmoderate speeds.

The design process began with a small series of consul-tant lectures on multi-hull specificity – mainly on the speci-ficity of outrigger ships. The overview was based on thepublished literature (Dubrovsky and Lyakhovitsky 2001;Dubrovsky 2004; Dubrovsky et al. 2007). Unfortunately,referring to the decisions made by the designers, a largepart of the initial information was not applied to the design.On the other hand, the rights and duties of the consultantwere not defined previously, so he was not in a positionto make technical decisions, which was not evident to thedesigners.

2. Initial design data

The characteristics of the initial design were (a) an innerdeck area of the above-water platform (about 6500 sq m),

ISSN: 1744-5302 print / 1754-212X onlineCopyright C© 2010 Taylor & FrancisDOI: 10.1080/17445302.2010.485467http://www.informaworld.com

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372 V. Dubrovsky

Figure 1. Ship cross section as proposed by the designers.

(b) restrictions on design draft (6 m) and overall beam (32m), and service speed 16–18 knots.

Unfortunately, the basic requirement of the initial trans-verse stability was not discussed and stated. As a result, theproposed options for the ship cannot be compared becauseof their sufficiently varied initial stability. This was one ofthe mistakes of the collaboration.

3. General arrangement and hull structure design

General arrangement, overall dimensions and hull structureof multi-hull ships must be designed together in order toensure minimal structural mass. The diverging understand-ing of these problems (by the designers and the consultant)was the second major difference between the design op-tions. The ship cross section as proposed by the designersis shown in Figure 1.

The main characteristics of the cross section are asfollows:

– distance between the platform bottom and design waterlevel (vertical clearance) equal to 6 m;

– a ‘doubled bottom’ between the platform bottom (wetdeck) and platform lower deck. The inner volume con-tains structural elements such as floors, longitudinal stiff-eners, and parts of ship systems. The ‘second bottom’height is 2 m.

The dimension shown in the cross section mean of thehull depth is 20 m; see Figure 1.

The length (140 m), main engine type (diesel-electric)and the hull depth selected by the designers defined the fulldisplacement of the ship as 6000 t.

The consultant proposed another scheme for the struc-ture cross section; see Figure 2.

It is also evident that the alternative section has twoinner decks. Differences from the previous option are (a) a

Figure 2. Structure cross section as proposed by the consultant(the dotted area is a section through the water-tight volume).

smaller vertical clearance (4 m instead of 6 m; Dubrovskyand Lyakhovitsky 2001) and (b) the absence of a ‘doubledbottom’ platform.

The platform bottom consists of plating and externallongitudinal stiffeners only, without transverse frames. Thegeneral transverse strength is ensured by the transversebulkheads between the platform bottom and the inner deckof the platform. The external stiffeners intersect the trans-verse bulkheads.

The volume between the platform bottom and innerdeck is convenient enough for the arrangement of passengercabins (in some water-tight compartments).

Of course, the smaller vertical clearance implies thereis a somewhat greater possibility of platform bottom slam-ming in waves. However, the difference is not very large,and instead, sufficient restriction of damage heel is ensuredby the upper water platform immersion in water at a heelof about 10◦ (a 6 m vertical clearance means the damageheel is 15◦, which seems too large a value for a passengership).

The smaller depth of the hull implies a reduction inthe requirement for initial stability and a smaller structuralmass because of the lower value of the transverse load armand corresponding bending moment.

Besides, a smaller vertical clearance means there is asufficient growth of water-plane area at a heel of 15◦, i.e. astrong restriction of damage heel.

It seems evident that inexperienced (in SWA ship de-sign) engineers are unable to observe the complex linksbetween ship dimensions and main characteristics.

The alternative hull depth (16 m), overall length (120m) and diesel generators as main engines give the full dis-placement for this ship option of about 4500 t – for neededdeck area.

This means there is a smaller area of outrigger water-plane, outrigger dimensions and mass, and a lower self-towing resistance of outriggers.

The consultant proposed the criterion for the initialtransverse stability selection, as is usual in US combat ships:the heel must not be more than 10◦ at rest and side wind

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Ships and Offshore Structures 373

speed of 100 knots. This means there is an outrigger water-plane area of about 2 × 100 sq m and a length of 65 m as aresult of such a demand for initial stability.

Unfortunately, the designers did not discuss the op-tions before the tests, and a model of a 6000 t ship wasmanufactured.

4. Propulsors

The option proposed by the designer for the arrangement ofequipment in the hull gondola stern is shown in Figure 3.

The selected diesel–electric engine includes two electricmotors and a coupling gear for one propeller; see Figure 3.

It seems clear that the simplest cheap and survivableoption is two shafts without any coupling gear. Each of thetwo propellers can have the same diameter as the single pro-peller, because the side propeller diameter is not restrictedby the thin strut of an SWA main hull. Furthermore, theadded propeller is evidently simpler and cheaper than thecoupling gear. Moreover, from the motion damping point ofview, a flat gondola stern for two shafts is a more effectiveshape.

However, the main advantage of two propellers versusone is a higher propulsive coefficient because of the samepower and twice the area of the propeller, i.e. a decreasedload on it.

Here the consultant was surprised by the different ex-perience of the designers. The usual Russian experienceis with graphical diagrams of the propeller action. Theseare indications of the possible development of propellereffectiveness (mainly by bigger diameter for decreasingthe relative load). In contrast, computerised propeller dia-grams are usual for Western specialists. Only initial dataare needed for the propeller element calculation (usually,diameter and/or rate), and no intermediate data are shownin the process of calculation.

To demonstrate (in a way evident to the Russian special-ists) the advantage of two propellers compared with one,

the consultant was forced to calculate two options of pro-peller elements together with one for the designers. It wasshown that changing the number of propellers can increasethe propulsive coefficient by up to 0.75–0.8. Unfortunately,the designers finally understood the advantage of two pro-pellers only after a one-shaft self-propelled model had beenmanufactured.

5. Motion mitigation

For a deeper understanding of motion mitigation problemswith the design, some characteristics of SWA ship seawor-thiness are mentioned below.

First, the larger relative inertial moments with decreasedrestoring moments mean an increase in the period of mo-tion (about twice as much in comparison with mono-hullsof the same displacement). This means motion resonanceconditions exist in following waves – not in head waves, asfor the majority of mono-hulls.

Far enough from resonance, a small water-plane areaensures small enough disturbing forces and moments – andmotion amplitudes are smaller, than those for compara-ble mono-hulls, approximately proportional to the relativewater-plane area.

Nevertheless, the defining characteristic of SWA shipsis a decreased damping of all kinds of motion. This meanslarge amplitudes of resonant motions exist in a narrow rangeof wave periods. However, large amplitudes of motion donot mean large accelerations, i.e. resonance motions aresmooth.

Besides, even near resonance, the disturbing forces andmoments are not very large, and mitigation forces and mo-ments from the usual motion stabilisers can be comparableto the disturbing forces. This means there is a high effec-tiveness of any motion mitigation device on SWA ships –but only for high absolute speeds.

The designers had proposed two pairs of active horizon-tal rudders: on the inner boards of the outriggers (for roll

Figure 3. Gondola stern arrangement proposed by designers.

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374 V. Dubrovsky

Figure 4. Tested model of outrigger ship with one pair of rudders for longitudinal motion mitigation.

mitigation) and on the main hull gondola; see Figure 4. Thenumber and placement of the foils and control laws werenot discussed with the consultant.

It can be supposed that the placement of hull ruddersthat is shown was selected for the lower risk of their bar-ing in waves. But this placement means that there is, first,a significant reduction of the damping moment arm anda significant asymmetry of mitigating foils relative to thetransverse axis – and relative overmitigation of bow in com-parison with stern. Besides, the area of the control surfaceson the gondola was very small.

Gulijev et al. (1972) showed that by foiled catamaranmotion mitigation, bow placement of very large foils im-plies overmitigation of bow vertical displacement, and thestern displacement can be even larger than on a ship withoutfoils. The same effect was realised for the described model.

The result of the examined model tests was the same:the vertical displacement of the stern was larger than thatof the bow – with bad results for the engines, propeller andshaft.

Referring to the recommendation of the mitigation rud-der area, this must be about 10% of water-plane area andthe stern foil area must be about twice as large as the bowarea.

An example of the better placement of mitigation foilsis shown in Figure 5, where two pairs of foils are at the endof the main hull gondola and one pair is near the middle ofthe ship on the outriggers. (If outriggers are near the middleof the ship, the roll mitigation foils must also be near themiddle of the outrigger.) Main hull foils must mitigate pitchand heave, while side foils counteract roll.

Besides, Figure 5 shows that a flat gondola with twoshafts and propellers is very convenient for stern mitigationfoils with twice the area of bow foils. For greater immersionof bow foils, they must be placed near the base plane of thegondola, or their axes can be inclined relative to the baseplane.

However, the modest speed of the ship means that thefoils are less effective for motion mitigation, and the cir-cumstance cannot be compensated for by the foil area. For-tunately, the special characteristics of SWA ships allowanother method of motion mitigation, which can also beapplied to stops that are activated by air tanks.

The modest inner volume of the struts ensures that thevolume of the ballast tanks is equal to the strut volume(over their entire height). This allows compensation of theimmersed volume by changing the strut height via the waterlevel, controlled by varying the air pressure.

Figure 5. Placement of mitigation foils: two pairs at the gondola ends and one pair at the middle of the ship on the outriggers.

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Ships and Offshore Structures 375

Besides motion mitigation, active tanks ensure thatSWA ship design for minimal draft is set equal to gondolaheight, and with design draft at sea equal to half the heightof the struts. This means there is a wider access to shallowharbours and does not mean any loss of seaworthiness.

The tests carried out were defined by varied experi-ences of the designers and the consultant. As is usual fortraditional ships, the model was tested with all rudder sta-bilisers. However, SWA ship models are usually tested withand without stabilisers, because the required area of ruddersis defined after bare hull tests.

After the definition of large amplitudes in a followingsea (and small enough accelerations), the designers wereso concerned about the results that they decided to stopthe process of designing of such a ship. No questions wereasked to the consultant and the possibility of applying activetanks was unfortunately ignored.

Because the results of the designed ship did not pro-vide better solutions and corresponding characteristics, thedesign was not accepted.

Conclusions

The main results of the study are given below:

1. The details of the options presented show a strong depen-dence of the final technical Characteristics on technicaldesign solutions. This means SWA ships have sufficientreserves for development – in contrast with traditional

mono-hulls, whose characteristics do not allow a possi-ble further development.

2. The example shows that optimal cooperation of design-ers, who are not so experienced, but have the right toselect technical solutions, with a more experienced con-sultant, who is not in a position to decide, is the key to thesuccess of the design. A more deep collaboration couldensure more detailed discussions and a wide applicationof existing knowledge.

3. Therefore, wide-ranging and thorough preliminary lec-tures were given by the consultant to different designers,and a preliminary agreement was reached on volume andorder of preliminary discussion on technical solutionsat all design stages.

4. Initial results that are not favourable should not be areason for stopping the design process, and they couldbe compensated by additional, more effective, technicalsolutions.

ReferencesDubrovsky V. 2004. Ships with outriggers. Fair Lawn (NJ):

Backbone, p. 88.Dubrovsky V, Lyakhovitsky A. 2001. Multi hull ships. Fair Lawn

(NJ): Backbone, p. 495.Dubrovsky V, Matveev K, Sutulo S. 2007. Small water-plane area

ships. Fair Lawn (NJ): Backbone, p. 256.Gulijev Ju, et al. 1972. Experimental researching of performance

and seaworthiness of a passenger catamaran. Transactions ofNikolajev Shipbuilding Institute, “Shipbuilding & Shiprepair-ing”, vol. 5, pp. 43–53 (in Russian).

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