International Conference on Marine Research and Transportation

ICMRT ’07, June 28-30, 2007, Ischia (Naples), Italy

ON THE DESIGN OF ICE-STRENGTHENED SUPERYACHTS

Vittorio BUCCI and Alberto MARINO

University of Trieste, Dipartimento di Ingegneria Navale, del Mare e per l’Ambiente,via Valerio 10, I-34127 Trieste, Italy

ABSTRACTThe paper deals with the design of ships strengthened for navigation in icy waters. The at-tention is focused, in particular, on the expedition yachts: innovative luxury vessels conceivedfor a service also in harsh climate regions. For such a special yacht the design must face theproblems related to the navigation in brash ice channels, and for this reason the vessel mustcomply with an ice class regulation.

The ice class rules of the Finnish and Swedish Maritime Administrations can be consideredas the main reference in this field; indeed these regulations have been adopted by most of theIACS members since 1985.

The design philosophy behind the ice class rules is analysed in relation to its main features,which are essentially based on both operative and safety aspects. A sensitivity analysis isperformed on the parameters affecting the structural scantling and the propulsion power re-quirements.

The ice class concepts have been applied for an expedition yacht 35 m long. Feasible structuralsolutions for the ice-strengthened hull are shown along with the verification of the poweringneeded for a safe navigation in ice.

1. INTRODUCTION

In the last years a new type of ship has appeared onthe superyacht market: the expedition yacht. It is aluxury vessel with specific construction and technicalpeculiarities, which enable a service with relativelyharsh climate.

On this basis, the design of an expedition yacht mayconsider also the possibility of navigating on routesin brash ice channels opened by icebreakers. Thepresence of ice floes, and consequently the possibil-ity of impacts at high speed against the hull, imposesadequate local hull reinforcements. Moreover, there

are also design consequences, that concern in particu-lar the minimum main engine power, the dimensionsof the propeller, shafting and gearing, as well as thescantlings of rudders and steering engine.

Pioneers in this field have been the Finnish MaritimeAdministration and the Swedish Maritime Adminis-tration, which in cooperation with some classificationsocieties have developed appropriate rules for naviga-tion in ice. The “Finnish-Swedish Ice Class Rules”[1] have been adopted by most of the IACS members,which have incorporated them in their own rules.

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As for the type of ice, it is necessary to consider thatits nature is not all the same, there are indeed threetypes of ice: the first-year ice, that only forms for afew months, is 30 to 200 cm thick and is quite easy tonavigate; the second-year ice, that survives the thaw-ing period as a continuous frozen layer; the multi-years ice, that survives several thaws, and is over 4 mthick.

The rules consider the first-year ice, with a max-imun thickness of 1.2 m, a bending strength (can-tilever beam test) of about 0.5 MPa and a compressivestrength between 2÷5 MPa.

The design philosophy behind the Finnish-Swedish IceClass Rules is based on both operative and safetyaspects. As a matter of fact, the minimum enginepower requirement for a ship sailing in icy water isintended to avoid traffic congestion due to decreasingspeed. Regulations for the minimum engine outputpower are essentially based on long-term experience,and are aimed at ensuring a minimum speed of 5 knotsin brash ice channels. The strengthening of the hull,rudder, propellers, shafts and gears is related to thesafety of navigation in ice.

Ice-classed ships have a particular local strength ofthe hull (plating, frames, stringers and web frames)able to withstand ice loads. A so-called ice belt, witha certain vertical extension all around the waterplane,is adopted to ensure proper strength. In the bow area,the ice belt height is increased if the vessel operates inopen sea, where there are both high swell and floatingice. Anyway, it is to be noted that the rules do notconsider extreme situations, such as large ice forcesacting on the parallel midbody or compressive forcesdeveloped when the ship is stuck in ice.

As for scantling of propellers, shafts and gears, a hi-erarchical strength principle is adopted: the propellerblades, although reinforced, must be the weakest ele-ments in the propulsion system.

2. THE PROJECT “BRAVE GOOSE”

The concept of expedition yacht has opened a newscenario in the yachting market. Expedition yachtsare quite different from traditional yachts, which aretypically understood as boats having fairly slendershapes. Conversely, the expedition yachts look likemerchant vessels as tugs, supply vessels or trawlers,that is working boats displaying quite full sections,square sterns and sometimes vertical stems.

In despite of the exterior, the expedition yachts arevery comfortable luxury ships able to perform greatchallenges of navigation. In other terms, compatiblywith its own dimensions, an expedition yacht is de-signed with the same reliability to work both in arcticand tropical regions, with quite long ranges compara-ble to bigger ships.

Because of its peculiar service profile, an expeditionyacht usually meets also the requirements of addi-tional class notations, such as the Ice Class for thenavigation in icy waters, the Green Star for sea andair pollution prevention, and the Comfort Class forlow levels of noise and vibration on board.

In this paper, as a design example in order to highlightthe implications of ice loads on the hull structures,the superyacht “Brave Goose” – a project partiallyelaborated by the authors – is considered. The mainparticulars of this vessel are:

length overall . . . . . . . . . . . . . . . . . . . . LOA = 35.00 m

length on waterline . . . . . . . . . . . . . . LWL = 32.50 m

length between perpendiculars . . . LBP = 31.50 m

breadth . . . . . . . . . . . . . . . . . . . . . . . . . . . . . B = 8.20 m

depth . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . D = 4.10 m

scantling draught . . . . . . . . . . . . . . . . . . . . T = 2.35 m

displacement . . . . . . . . . . . . . . . . . . . . . . . . ∆ = 320.0 t

main engine output . . . . . . . . . . . . . . . ≈ 2 × 500 kW

maximum speed . . . . . . . . . . . . . . . . . . . . . . 13.5 knots

cruise speed . . . . . . . . . . . . . . . . . . . . . . . . . . .11.5 knots

range at cruise speed . . . . . . . . . . . . . . . . . 4500 miles

passengers + crew . . . . . . . . . . . . . . . . . . . . . . . . 10+ 7

The external view of the “Brave Goose” is shown inFig. 1, whereas her general arrangement plans are il-lustrated in Fig. 2.

3. FOCUS ON ICE CLASS NOTATIONS

On the basis of the operative profile, different require-ments need to be considered. In fact, the naviga-tion in ice may impose more or less demanding de-sign pecularities according to the service reliabilitythat must be ensured to the ship. In this framework,the Finnish and Swedish Maritime Administrationshave issued rules for ships sailing in the Baltic Sea inwinter. Specifically, the “Finnish-Swedish Ice ClassRules” define six different levels concerning structuralstrength and main engine output power. The follow-ing ice class notations are stated:

– IA Super, for ships able to navigate in extreme iceconditions (floes 1.0 m thick), without traffic re-strictions;

– IA, for ships able to navigate in severe ice condi-tions (floes 0.8 m thick), with some size restrictions;

– IB, for ships able to navigate in medium ice con-ditions (floes 0.6 m thick), with limited access toports for part of the year;

– IC, for ships able to navigate in light ice conditions(floes 0.4 m thick), with analougous operative char-acteristics as IB;

– II, for ships structurally fit for navigation in highseas, but not in ice;

– III, for ships that do not belong to any of the pre-vious ice classes.

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property of Meccanonet

Fig. 1 – External view of the expedition yacht “Brave Goose”

Fig. 2 – General arrangement plans

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4. ICE LOAD DEFINITION

Navigation in channels opened in icy waters inducespatch loads on the hull as a consequence of the im-pacts of the ice floes.

There exist large uncertainties in defining a reliable iceload model. For design purposes, the Finnish-SwedishIce Class Rules propose a value for the ice pressurederived from probabilistic analyses of full-scale mea-surements collected on vessels sailing in winter in theBaltic Sea. For each ice class, the design ice pres-sure is defined as a function of the displacement ofthe ship, the engine output power and the location ofthe structure.

On transversely-framed vessels a non-uniform icepressure distribution with peaks in way of frames isassumed (Fig. 3). Such a model reflects the differentflexural stiffness of the supporting members and theshell plating between them.

Fig. 3 – Non-uniform ice pressure distribution

The scantling of the ice belt structures is different ac-cording to the hull location. For this end, the ship’shull is divided into three regions (forward, midshipand aft) having adequate longitudinal and vertical ex-tensions. Obviously, the forward region is the moststrengthened.

The ice belt of the “Brave Goose” is depicted in Fig. 4,where the shaded area includes both the shell plating(dashed contour) and the supporting members (solidcontour).

Fig. 4 – Ice belt extension

The design ice pressure to be considered for the struc-tural scantling is related to the mean value p of thenon-uniform distribution (see Fig. 3), and is differ-ent for shell plating, frames, ice stringers and webframes. The pressure p depends on the displacement∆ [t] and the engine output power P [kW] throughthe factor K =

√∆ · P/1000. For the design exam-

ple hereinafter considered the value of the factor K isequal to 0.57 [t · kW]0.5.

In Fig. 5, 6 and 7 the graphs of the design ice pres-sure for each structural element of the ice belt, inaccordance with the hull region, the ice class and theframing system adopted, are plotted.

Fig. 5 – Design ice pressure for shell plating

Fig. 6 – Design ice pressure for ordinary frames

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Fig. 7 – Design ice pressure for stringers and web frames

For all the structural elements of the ice belt the de-sign ice pressure decreases passing from the forwardto the aft region, and – as can be seen – only forthe midship and aft regions it depends also on the iceclass. For transverse framing, the ice pressure on shelland frames is constant with the spacing of the frames,because it is assumed that the full length of the areaconsidered is under pressure at the same time. Onthe contrary, for the reinforced members (ice stringersand web frames), and for shell and ordinary frames inlongitudinally-framed ships, the ice pressure decreaseslinearly with the spacing or span, accordingly. Suchan assumption is justified by the decreasing of boththe flexural stiffness and the probability of having theentire length of the considered element loaded at thesame time.

5. SENSITIVITY ANALYSIS

In the study performed in order to analyse differentstructural solutions for the “Brave Goose”, a high-strength steel with a yielding stress of 315 MPa hasbeen considered.

In Fig. 8 the shell plate thickness is stepped by 0.5 mmto meet the commercial sizes. To take into accountthe above-mentioned non-uniform load distribution, a75% reduction in ice pressure is assumed. As a matterof fact in ice-classed ships the frames are quite closeone to the other, so that the ice floes may bridge overthem. In fact, for longitudinal frames the Rules es-tablish a maximum spacing of 0.35 m for ice classesIA Super and IA, and 0.45 m for the other classes.

Fig. 8 – Thickness for shell plating

It should be pointed out that the shell plate size at-tained are somewhat thick for a vessel 35 m long (asthe superyacht considered), specially in the forwardregion and for the highest classes. Moreover, a thick-ness increment (normally equal to 2 mm) is addedagainst abrasion and corrosion.

As for the scantlings of transverse frames and webframes, first of all, some assumptions about icestringers, if present, should be done. In the designexample the height between the lower and the maindeck is ` = 2.70 m. At the ends, the beams arealways considered as clamped at the lower-deck andpinned at the main deck. If ice stringers are present,they are simulated either as simple supports (for ordi-nary frames) or as elastic supports (for web frames).Three cases have been examined: without stringers,with one stringer at mid span, and with two stringersdividing the span in three equal parts. As for theload patterns, different assumptions have been donefor sizing the frames or the web frames: for the framesthe load is always applied at mid span, so the modelis simpler, but more conservative, while for the webframes, for a more realistic picture, a concentratedload at the actual position is assumed.

In Fig. 9 the above-defined load models are sketched.

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Fig. 9 – Load models for the transverse frames

Fig. 10 reports, for the different hull regions and iceclasses, the required section modulus of the transverseframes. The load models adopted are those of Fig. 9(cases a, b, c), applying 100% of the design ice pres-sure as plotted in Fig. 6.

Fig. 10 – Section modulus for transverse frames

Fig. 11 is referred to both section modulus and sheararea of longitudinal stiffners. Calculations have beencarried out considering a spacing between longitudi-nals equal to 0.40 m. The stiffners are assumed ascontinuous beams attached to transverse supportingmembers by brackets.

Fig. 11 – Section modulus and shear area for

longitudinal frames

Fig. 12 – Section modulus and shear area for stringers

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Fig. 12 shows the graphs of section modulus and sheararea of stringers of the ice belt. In this case the num-ber of stringers has no influence.

In carrying out the sizing of web frames, direct stresscalculations have been performed. With reference tothe load models depicted in Fig. 9 (cases d, e, f) theconcentrated load is located at the waterline level.The stiffness of the elastic supports (springs) simu-lating the ice stringers has been determined consid-ering as a model for the stringer a continuous beamsupported on five equally-spaced primary supportingmembers (web frames and/or bulkheads), and evalu-ating the deflection at the central elastic support whena concentrated load is applied there (Fig. 13).

Fig. 13 – Structural model to evaluate the stringer

flexibility

The expression of the spring stiffness ks comes out tobe:

ks =9625

E Js

S3(1)

where E is the Young modulus of the steel, S is thespan of the stringer between two supporting members,and Js is the moment of inertia of the stringer.

The Js value has been determined considering a con-stant web height hs = 200 mm, and assuming a sec-tion modulus Zs as previously calculated and plottedversus span S in Fig. 12. With a symmetric cross sec-tion (as that considered below) the moment of inertiais simply given by: Js = Zs hs/2.

On these bases it is possible to determine the shearand moment diagrams for the web frames. The differ-ent values of the shearing force Q and of the bendingmoment M need to be considered along the lengthof the web frame in order to determine the sectionalproperties of the beam. An I-section beam (Fig. 14) isassumed, with a web area Aw, a flange area Af and aneffective area of the attached plate Ap = Af . More-over, the height hw of the web plate is fixed (in thedesign example, for habitability reasons, it has beenchosen equal to 200 mm), and the flange area is ex-pressed proportionally to the web area in accordancewith the relation: Af = k Aw.

That being stated, starting from the knowledge of thestresses Q and M acting in the section, it is possibleto determine the cross section dimensions of the webframe.

If only the shearing force Q is present, the shear areaA of the cross section (i.e., the area required for the

web plate to be at the onset of the yielding) can be de-rived imposing the condition based on the von Misescriterion:

τmax = αQ

A=

σy√3

(2)

where τmax is the maximum shearing stress at theneutral axis, α is the so-called shear factor and σy

is the yielding stress of the material. Therefore, theshear area A turns out to be:

A =√

3 α Q

σy(3)

For an I-section beam with Ap = Af = k Aw the shearfactor α referred to the stress at the neutral axis is:

α =k + 1/4k + 1/6

(4)

Fig. 14 – I-shaped cross section for primary supporting

members

To take into account the concomitant action of botha shearing force Q and a bending moment M thecondition to be imposed is that the von Mises stressσ∗ =

√σ2

o + 3 τ2o at the juncture of the web plate

with the flange is equal to σy.

The maximum normal stress σo is:

σo =M

Z(5)

where Z is the sectional modulus of the section. Inparticular, for the I-beam considered:

Z = Aw hw (k + 1/6) (6)

The shear stress τo is:

τo = αoQ

Aw(7)

with αo the shear factor at the web-flange junctureequal to:

αo =k

k + 1/6(8)

From equation:

σy =

√(M

Z

)2

+ 3(αo

Q

Aw

)2

(9)

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after some algebra, the web area Aw that shall betaken in presence of Q and M comes out to be:

Aw =

√M 2 + 3 (k hw Q) 2

σy hw (k + 1/6)(10)

In conclusion, the actual cross section area of the webplate shall be determined by formula (3) or (10), ac-cording to the case. Obviously, the most demandingstress situation along the length of the beam must beconsidered, namely:

Aweb = max(A, Aw) (11)

Within the design example focused on the “BraveGoose” project, the scantling analyses of the webframes have been carried out with reference to theice class notation IC.

Fig. 15 – Section area for web frames

Fig. 15, in particular, shows the results in terms ofcross section area of the web frames in the midshipregion, assuming either no stringer, one stringer ortwo stringers in accordance with the hypotheses for-mulated above. Similar charts have been drawn alsofor the forward and aft regions.

The section area Atot of the web frames:

Atot = Aw + Af = Aw (1 + k) (12)

is plotted versus the spacing of the web frames, withreference to different values of the ratio k = Af/Aw.

Fig. 16 is an elaboration of a part of Fig. 15, and showsthe section area Atot of the web frames when the so-lution with two stringers is adopted. In the sameFig. 16, besides the curves of the section area Atot

(solid lines), also the curves relating to the thicknesstw of the web plate of the section (dotted lines) havebeen traced. In deriving the curves of tw it has beenborne in mind that a constant height hw = 200 mmwas assumed for the web plate. It is then immediatlypossible to draw the relevant web thickness, startingfrom a given Atot :

tw =Atot

hw (1 + k)(13)

Thus, such graphs turn out to be very useful for thedesigner in establishing realistic size for the structuralelements.

Fig. 16 – Section area and web thickness for web frames

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5. ICE STRENGTHENING SOLUTION FORTHE “BRAVE GOOSE”

The just performed sensitivity analysis along with thedrawn charts have allowed a straightforward deter-mination of the scantlings of the different structuralelements of the ice belt.

In general, the service profile of yachts do not justifythe heavy ice strengthening necessary for the highestice class notations. For this reason, the project ofthe “Brave Goose” has been developed within the iceclass IC.A combined framing system has been adopted, withtransverse framing in the ice belt zone. Fig. 17, inparticular, shows the structural solution proposed for

the midship section: web frames are spaced 2500 mm,and ordinary frames 500 mm. Two ice stringers divideinto three equal parts the span of the transverse mem-bers between the lower and the main decks. With suchan arrangement, the thickness of the ice belt platingin the midship region ensues to be 11 mm (includinga 2 mm margin for abrasion and corrosion), whereasadequate ordinary transverse frames are offset bulbplates 100×6. As for the reinforced members, bothweb frames and stringers have wide-flange cross sec-tions: the former are 200×14/160×14, and the latter200×10/140×12.

The web height of the reinforced members, as alreadymentioned, has been taken equal to 200 mm for a bestexploitation of the below-deck volumes.

Fig. 17 – Midship section of the ice-classed-IC ship

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For a comparison, Fig. 18 shows an unstrengthenedversion of the “Brave Goose”, where a longitudinalframing system is adopted.

Fig. 18 – Midship section of unstrengthened ship

Clearly, ice strengthening involves an increase of thehull weight if compared to that of a not ice-class ship.In particular, for the studied design example, thereis an increment of about 15% of the hull weight inthe midship region. Different weight increments areregistered also for the other regions, with a markedincrease for the forward region.

6. MAIN ENGINE OUTPUT

For ice-classed ships an incresed output power of thepropulsion machinery is required in order to over-come the difficulties related to navigation in brashice channels. Specifically, the installed power needsto be sufficient to ensure a minimum speed of 5 knotsthrough the ice floes, so that traffic congestion may beavoided. Anyway, it is assumed that an icebreaker isalways available for assistance if an excessive decreaseof speed should occur.

The engine power required increases enormously withthe thickness of the ice, and therefore with the iceclass notation to which the ship is entitled.

According to the Rules, the engine output to be as-sumed shall be not less than the powering calculatedfor the two extreme draughts amidship: the maximumat the load waterline (LWL) and the minimum at theballast waterline (BWL).

The necessary output power is a function of the ship’sresistance in a channel with brash ice (only for ice

class IA Super a 0.1 m thick consolidated layer of iceis also considered), of the number, diameter and typeof the propellers (fixed or controllable pitch), and ofthe type of the propulsion machinery (conventional,electric or hydraulic engine). Moreover, the outputdepends greatly on the hull’s form in the bow area.

For the superyacht “Brave Goose” the required enginepower for the ice class IC turns out to be 327 kW, butthresholds of 1000 kW for ice classes IA, IB and IC,and of 2800 kW for IA Super are imposed. So that,with the installed power (2×500 kW) necessary for themaximum speed in open sea (13.5 knots) the powerrequirement is highly fulfilled, and no increase needsto be taken into account.

It is worth noting that in the examined case the powernecessary for the navigation in icy water is about 8times greater than that for normal trading in openwater at the same speed of 5 knots.

7. CONCLUSIONS

The paper deals with the strengthening of superyachtsable to navigate in icy waters. Different ice load con-ditions are considered in accordance with the ice classnotation to achieve.

In order to investigate on the influence of the variousparameters into the structural scantling, a sensitivityanalysis has been performed. From such an analysis afew charts very useful for a proper choice of the struc-tural elements in the ice belt zone have been drawn.

A design example has been thoroughly exposed, tak-ing into account the project of an expedition yacht35 m long. The structural configuration of the mid-ship section for the IC ice-class ship has been com-pared with the one of an unstrengthened version, andan evaluation of the hull weight increase has been car-ried out.

REFERENCE

[1] FMA, “Finnish-Swedish Ice Class Rules”,Finnish Maritime Administration BulletinNo.13/1.10.2002, 2002

[2] FMA, “Guidelines for the application of theFinnish-Swedish Ice Class Rules”, Finnish Mar-itime Administration, 2005

ACKNOWLEDGEMENTS

The authors are grateful to Meccano Engineering srlfor the opportunity offered by the cooperation in the“Brave Goose” project.

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