353
1 INTRODUCTION.
Ship hydromechanics and in particular ship
hydrodynamics constitutes the foundationof naval
architecture, the science of how to build safe and
efficient ships. It is the most important branch of
knowledgeneededtodesignandconstructshipsand
objectoftheoff‐shoreindustry.
Laws of ship hydromechanics are creating the
ba
sis for development of computer codes used
recently for the design purposes of ship hull,
propeller and appendages and for estimation of
hydrostatic data and for resistance,propulsion,
stability and manoeuvrability characteristics of the
shiptobeconstructed.
In old times ships were built without any
calculations and laws of hydromechanics were not
known, alt
hough insomewaytheywereintuitively
respected,otherwiseshipscouldnotsail.Shipswere
builtonthebasisofexperiencethatinturn,wasbased
on trial and error method. Naval architecture was
more art than science. With the development of in
industrialworldnavalarchit
ecturegraduallystarted
torespectbasiclawsofhydrostaticsand,lateron,also
hydrodynamics.Bytheendofthenineteenthcentury
owing to the work of William Froude naval
architecture started tobe based on model
experiments.Duringthesecondhalfofthetwentieth
century there was remarkable rapid development of
theoreti
calandexperimentalshiphydrodynamicsand
with the advent of computer technology pra ctical
applications of sophisticated methods of ship
hydrodynamicsarepossible.
Nowadaysshipdesignersunderstandwellthatin
order to design good ship they must utilize laws of
shiphydromechanics.
2 WORLDFLEETANDOFF‐SHOREINDUSTRYIN
THEXXICENTUR
Y
In the beginning the twenty first century we may
notice remarkable changes in ship types, in ways of
Challenges to ship hydrodynamics in the XXI century
L.Kobyliński
FoundationforSafetyofNavigationandEnvironmentProtection,Poland
ShipHandlingResearchandTrainingCentre,Ilawa,Poland
ABSTRACT:Thebeginningoftwenty‐firstcenturyischaracterized withimportantchangesinworldshipping
andexploitationofoceanresources.Threeimportanttrendsareclearlyvisible:environmentprotection,safety
andeconomy.Theymaterializeinimport
antchangesinthestructureofworldfleetwheresomeexistingship
types are going to disappear and new ship typesemerge. Increasingthe size of some ship types is another
visibletendency.Stressonenvironmentprotectionhasseriousimpactonthehydrodynamiccharacteristicsof
ships whetherwith regard to safety zero accident rate is the goal. Important challenges to ship
hydrodynamicscausedbythosetendenciesarediscussedinthepa
per.
http://www.transnav.eu
the International Journal
on Marine Navigation
and Safety of Sea Transportation
Volume 8
Number 3
September 2014
DOI:10.12716/1001.08.03.04
354
transporting goods by sea and in methods of
exploitationofsearesources.
Thisisobviouslytheresultofglobalwarming,and
changes in the world climatic conditions, of
developing economy, globalization of the world,
increase of world population, wide exploitation of
ocean resources. Also some poorly developed
countries started to
develop rapidly at astonishing
speed.
Thenegativeeffectofexpandingcivilisationonthe
environmentiswellknown.Internationalcommunity
is worried that further industrial development may
irreversibly destroy our present world. People
realizedthatinordertoprohibitfurther devastation
ofseaenvironmentshipsmustbeconstructedinsuch
a
way, that strict environment pollution limits must
be imposed on shipping. This resulted in proposing
EEDI(EnergyEfficiencyDesignIndex)valuesthat,if
finallymade a law, will considerably influence the
construction of future ships and off‐shore facilities.
Thisresultedintheconceptof“greenships”,i.e.ships
with extremely
low emission of noxious and toxic
gases and wastes, that make no harm to the
environment. Green ships must characterize also by
lowfuelconsumption,becauselowfuelconsumption
means not only economic gain, but also smaller
emissionofharmfulsubstances.
Thewayoftransportinggoodsbysea becamenow
entirely different because of containerisation. This
resultedinappearing new ship type, containership.
When about 50 years ago first small container ships
wereconstructed,currentlythesizeofcontainerships
increased enormously, as fig.1 shows, the largest
container vessel reaching 18000 TEU and length
about 400 m. Currently
majority of general cargo is
transportedincontainersandoldgeneralcargoships
almostdisappeared.
Figure1.Developmentofsizeofcontainerships.
During last twentyyears bunker prices increased
considerably. This tendency will last and looking
ahead to the year 2025 bunker pricesare likely
increaseby50%.Inordertocuttheoperatingcostand
at the same time to reduce the pollution, most
container companies adopted the slow steaming
policy,
reducingthespeedoftheircontainershipsto
17 or even 14 knots. This also may be a remedy for
existingovercapacityinparticularincontainersfleet.
Another remarkable change in shipping is
developmentofcruisepassengerships.Oldpassenger
ships almost disappeared and passenger transport
wasovertakenbyairplanes.But
passengershipswere
replaced by cruise ships which carry passengers in
luxury conditions. In those ships, according to
definition,passengers are boardinginthe same port
as disembarking. The size of cruise ships from the
time that first cruise ships emerged is doubling in
everydecadeasTable1shows,
andfinallymodern
cruise vessels are real monsters reaching length of
almost 380m and carrying more than 6000
holidaymakers.
Table1.Developmentofcruisevessels
_______________________________________________
Year NameNumberofcabins Length[m]
_______________________________________________
2010 Allure of the Seas 2712 360
2006 Freedom of the Seas 1816 339
1999 Voyager of the Seas 1565 311
1996 Grandeur of the Seas 975 279
1990 Fantasy 1028 260
1982 Song of America 707 214
1970 Song of Norway 377 168
_______________________________________________
Rapidly expanding off‐shore industry is another
remarkabledevelopmentoftherecenttimes.Itcaused
that several new types of ships emerged, many of
them designed to perform tasks that were never
beforeheard.
In particular, in off‐shore industry, apart from
traditional drilling platforms, many types of service
ships
as well as ships for specific tasks emerged.
Acquiring “green energy” from wind and some
oppositionto the installation of wind farms on land
causedfast development of wind farms on coastal
seaareas.Thisledtothedevelopmentofspecialtype
of ships for installation of wind towers in shallow

waters and also special type of ships for servicing
windtowers.Exampleofsuchshipforinstallationof
windtowersinwindfarms,isshowninfig.2.
Figure2.Windfarminstallationship[1]
In near future because of global warming and
consequential reduction of ice covered waters, arctic
waterswillbecomenavigableandalsoaccessiblefor
exploitation of gas and oil resources. Attention of
several countries is therefore focused on ship types
and oil rigs that may be used in these regions. An
example
ofoiltankerspeciallydesignedforoperation
inarcticwatersisshowninfig.3.Attentionisdrawn
tothepropulsionsystemconsistingoftwoazipodsof
conventional type with mechanical gear however.
Such propulsion system never would be used in
conventionaltankers.
355
Apparentlyazipodsreveal certain advantages for
ships sailing in ice covered waters because high
power azipods with mechanical gear have been
recentlydevelopedandadvertised.
Figure3.Tankerdesignedforarcticwaters.[2]
Another remarkable tendency in shipping in the
XXI century is the tendency to increase safety
standards, in particular safety standards for
passenger ships, either cruise ships or others, e.g.
passengercarferries.Importantcasualties, if only to
mentionEstoniaorCostaConcordiadisasters,stirred
public opinion and forced authorities and the
InternationalMaritimeOrganisation(IMO)toinclude
in their work programmes development of
enhancedsafetystandardsforpassengerships.
Theimportanceofsafetyatsea andprotectionof
sea environment from pollution from ships is now
well recognized and IMO is actually responsible for
development and for implementation of respective
requirements
in the form of regulations and
recommendations. For the first time in history of
seafaring international organization responsible for
safety at sea and protection of sea environment was
created. IMO work influences greatly the design of
futureshipsandthisinturn,createsnewchallenges
to the important element of design,
ship
hydrodynamics.
Allthesefactorscausedthatnavalarchitecture
and in particular ship hydrodynamics faces some
important challenges. There are at least three
important areas where this is particularly felt:
environmentpollution,safetyandeconomy.
3 SHORTHISTORICALDEVELOPMENTOFSHIP
HYDROMECHANICS
Shipisfloatingobject.Itisthereforesubjected
tolaws
ofhydromechanics.Iftheshipisatrestincalmwater,
Archimedes law is valid. The static pressures act on
the submerged external surface of ship hull. This is
comparativesimpleproblemofhydromechanics,well
known since Archimedes times, in fact it consists of
calculationofareasandvolumes.
Shipswerebuiltfor
centuries with only some hydrostatic calculations
made.
In old times this was only Archimedes law that
was intuitively observed and beca use of that before
Archimedes, people did not realize that when
building ships they obeyed Archimedes law, but
intuitivelytheymadeuseofit.
With progressing
time and development of
technology ship hydromechanics was studied, and
shipbuildersmademoreandmoreextensiveuseofit.
At first only hydrostatics was studied, floatation,
static stability, subdivision. Later on, ship
hydrodynamics was applied in ship design, firstly
shipresistanceandpropulsion,lateronshipmotions
in waves, manoeuvrability, dynamic
stability and
theory of propellers. Nowadays, with the advent of
computeriseddesignshiphydrodynamicsconstitutes
background of any computer code used in ship
designprocess.
Basic laws of ship hydrodynamics were known
quite a long time ago. Leonhard Euler and other
scientistsmadeimportantcontributioninthisrespect,
but naval
architecture was really more craft than
science. Only in the second half of the nineteenth
century hydrodynamics began to influence naval
architecture appreciably. Bouguer, however, laid
down the fundamentals of naval architecture based
on hydrodynamics little earlier. Still, even in the
twentiethcentury,beforethesecondworldwar, only
laws of
hydrostatics were applied,ship
hydrodynamics pla y ed small part in naval
architecture and its application was limited to the
extensive use of someempirical formulae. Only
afterthe second worldwar we may observe
development of calculation methods based on
accuratetheoreticalbackground.
Emerging computer technologyenabled
transformation of theoretical models
into pra ctical
calculation methods that are used in ship design
process. In the advent of twenty first century we
observe rapid development of computer technology,
weobservealsonewchallengestoshipbuilding.This
in turn developednew challengesto ship
hydromechanics. Although basic theory remains the
same, new tools available make
possible to develop
practicalmethodsusedinshipdesign processbased
on rigorous theoretical approach that was never
possibleinthepast.
In the nineteenth century when mechanical
powering replaced sails and oars the need arose to
predictaccuratelythenecessarypower of enginesin
ordertoachievedesiredspeed.
Thiswasproblemof
estimatingforcesactingonmovingshipandproblem
of efficiency of screw propulsion. With lack of
possibility to made prediction on the basis of
theoreticalcalculations,modeltestswereproposedby
William Froude and his method is used until these
days. Model tests found wide application not
only
withregardtopropulsion,butlateron,alsotoother
ship characteristics, as for example motion amongst
thewavesandmanoeuvrability.
Theoretical equations enabling calculation of
forcesactingonthemovingshipandpredictionits
motionswereknown,butpracticalcalculationswere
notpossibleduetoextremely large amount
ofwork
needed.Wideapplicationofcomputertechnologyat
theend of thetwentieth century madeitpossible to
find methods based on Navier‐Stokes equations
applicableto make accurate analysis of flow pattern
around of the body of the moving ship, and to
calculateforcesacting,butmodeltestsremained
still
the most reliable method of simulating the highly
complicatedphenomenaofflowaroundshiphulland
propeller, in particular when the ship is moving
amongstthewaves.
Computer programmes are now widely used for
analysis of flow and for optimisation purposes and
now they consist important part of any
design
procedures of modern ship, where the model tests
stillremainimportanttoolinthedesignprocess.This
356
situationmaychange,however.Bearinginmindthat
the computer capacity is doubling in every three to
fouryearsonemyhopethatinthefuturethecapacity
of available computers will be sufficiently large for
tackling the most sophisticated problems of ship
hydrodynamics. It does not mean, however, that
modeltestswillbetotally abandoned, butthey may
belimitedtotestingfinaldesignsortotestingentirely
newsolutions.
4 NEWCHALLENGESTOSHIP
HYDROMECHANICS
As it often happens,prediction of the future
developmentof technology very oftenappears tobe
wrongandrealityhappenstobequite
differentfrom
theforecast.However,looking carefullyattheworld
politicalandeconomicdevelopmentafterthesecond
world war and in particular at the development of
sensitivity of the society to world warming, air and
seapollutionandsafetyatseaitisnottoodifficultto
predictimportantchallengesto
basicsciencesandin
particular to ship hydromechanics at least for
foreseeablefuture.
It seems that three factors will govern future
challenges in this respect: air and sea pollution and
theconceptof“greenships”,economicconsiderations
and reduction of cost of transport and general
tendencytoenhancesafetyat
sea,inparticularsafety
ofpassengers.Thereisalso,however,thetendencyto
increaseexploitationofsearesources,inparticularin
arcticwatersandtodeveloptransportroutesinarctic
waters,becauseduetoglobalwarmingtheymaybe
accessibleinnearfuture.
4.1 Greenshipconceptandimpactofeconomy
Green ship means a ship with low emission of
noxious gases and wastes but on the other hand
economicallyviable.Thismeanslowconsumptionof
fuel, fuel with low content of poisonous substances,
e.g.sulphur,wasterecoveryinstallation.EEDIvalues
should be respected. From the ship hydrodynamic
pointofview
thisactuallymeansreducedpropulsion
power as a result of smaller resistance and higher
propulsion efficiency. This is important challenge
related to environment protection and also to
economicconsiderations.
Resistance is function of speed and it varies
depending of ship form. Propulsion efficiency
depends on propeller efficiency and wake field in
placewherepropellerworks.Bothdependalsofrom
ship operation, loading conditions, manoeuvres
performedandsomeotherfactors.
The largest gain in overall propulsion efficiency
may be achieved by reduction of speed. Choice of
speedisamatterofeconomiccalculations.Thehigher
speedthehigherturnoverofgoods,but
ontheother
hand the higher cost of transport. With rising fuel
costs rising speed influences the overall cost of
transport negatively. In late twentieth century there
was tendency to increase the speed of transporting
goods and ships were designed for higher speed.
Currently it was discovered that lower speed may
reduceoverallcostofturnovergoods,thereforeships
not necessarily must be built for high speed. Lower
speedmeanslowerpowerneededtopropeltheship,
therefore at lower speed it would be easier to meet
new international regulations related to lower
emissionofnoxiousgasesandwastes.
Optimisationof
shipform,propellerefficiencyand
overallpropulsionefficiencyduringthedesignstage
ofslowsteamingconventionalshipsusuallyresulted
inabout4‐6%powersaving,butincertaincasesgain
maybeashighas15%.ThisisachievedbyusingCFD
computerprogramsforanalysingflowandextensive
model tests.
Development of accurate simulation
models of the propulsion system consisting of
propeller, ship hull and appendages is important
challengetoshiphydrodynamics.
It is well known that the lower loading of the
propeller(thereforeincreaseddiameter) willincrease
propeller efficiency. The problem is, how to
accommodate large diameter propeller at
stern and
find the place wherethe propeller should be
located in order to utilise in the best way mutual
interaction of the propeller and the wake field.
Several proposals are now advanced that must be
investigated,oneofthemisshowinfig.4wherethe
propellerisshiftedfar
towardsthesternwhereitmay
belocatedinthecrestofsternwaveandthepropeller
itself is of large diameter extending lower than the
base line. Overall propulsion efficiency may be
increased even by 15%, as fig. 5 shows, but
manoeuvrability of the ship may be considerably
impaired.
Thereforeproperrudderarrangementmust
be developed or some additional movable fins
installed. Anyway this is new problem or ship
hydrodynamics to be solved. This arrangement
basicallyisproposedfortankersandbulkcarriers.
Figure4.Illustrationofthelargeareapropellerconcept.[3]
With the slower ships other ways of reducing
resistanceandpropulsionpowercametotheattention
of naval architects. Method of air lubrication of the
wetted surface of the hull, idea that is known for a
long time but was never practically applied, is
consideredanewwithsomeprospectsofsuccess.
This
will,however,requirefurtherextensiveresearch.
Twin screw propulsion, where wake field is
modified by suitable arrangement of fins and vanes
and propellers designed to be adjusted to the
modified wake field is another trend that has to be
investigated.Ifappliedtolargecontainervesselsthis
357
solution mayprovide overall gain in propulsion
efficiency up to 12%. Apart from economic gain the
resultofincreasedpropulsionefficiency isreduction
ofemissionoftoxicandnoxiousgases.Withthehigh
cost of fuel as it is now the economic effect is
particularlyimportant(fig.6).
Figure5. Effect of position of the propeller and propeller
diameteronpropulsiveefficiency.Theupperlineshowsthe
effectoflargediameterpropeller.[3]
For tankers and bulk carriers some innovative
arrangements of propulsion systems are now
considered and have to be investigated. Twin screw
propulsionwithhighlyefficientazipodsorazimuting
nozzlesisconsideredaspossiblesolution.(seefig3).
This installation may improve manoeuvrability
considerably,buttheeffectonpropulsionefficiencyis
dubious
and it has to be investigated.In all cases
fighting for higher propulsion efficiency will be
attempted because this may help to achieve EEDI
limits. Method for efficient control of thevelocity
field constitutes important challenge for ship
hydrodynamics
Installationofrigidsailsonslowsteamingshipsis
another possibility.
This was actuallyconsidered
severalyearsagoandthenabandoned,butnowthis
possibility is going to be reconsidered because it
appearsthatitmayberealistic.
Figure6. Arrangement of shrouded propellers and vanes.
[4]
All these proposals constitute important
challenges to ship hydrodynamics and some
international research programmes aimed at
development of new solutions of propulsion
arrangement of ships are already installed. Apart of
this, some large shipping companies installed their
ownhighlyfinancedprogrammeswithsomepositive
resultsachievedalready.AsanexampleMaersk
and
Rolls Royce may be quoted. Several programmes
wereinstalledalsoinChina,thecountrywhereabout
50percentofallsipsarerecentlyunderconstruction.
4.2 Safety
High stress is now put on safety of navigation. In
particular safety of passenger ships, either cruise
vessels or ropax ferries
is considered inadequate.
EstoniadisasterandrecentCostaConcordiadisaster
did show that even ultra large passenger ships may
be endangered. Fortunately in Costa Concordia
casualty there were few fatalities, but that was only
because of the lucky situation that the ship was
driftedtotherockandnotcapsizedor
foundered.
Zeroaccidentrateinpassengershipsisthegoalof
thenewIMOapproachtointactandamagestability
of passenger ships. Ships must not only withstand
adverse weather situations but even in damage
condition it is essential that they must reach safely
nearestportofrefugeorbeing
towedtothatport.If
thatisimpossibletheymustsurvivesufficientlylong
time in order that all passengers and crew could be
safelyevacuated.CostaConcordiaaccidentdidshow
that evacuation was possible only because the ship
was securely sitting on the rock, otherwise the
number of fatalities would possibly
be extremely
high.
Asthesizeofcruiseshipsisincreasingrapidlyand
voyagesarenowarrangedeveninremotepartsofthe
world where distance to the nearest port of refuge
may be as large as thousands miles, achievement of
thisgoalinnotaneasytask. Damaged ship
may be
exposedtoharshenvironmentalconditionsandmust
withstandthosesituation.Behaviourofthedamaged
shipinwavesandunderseverewindconditionposes
serious challenge to ship hydrodynamics. IMO
discussesthisproblemforsometime,butendresultis
notyetpredictable.Thereisofcoursesimplesolution,
putting
many more bulkheads, but this is strongly
opposedbyshipsoperatorsclaimingthatsuchships
willbepracticallyimpossibletooperateeconomically.
Intact stability requirements of all ships are
currently under review and so called second
generation stability criteria are under development.
Thosecriteriaareintendedtotakeintoconsideration
thosehazards,thatarenottakenintoaccountinthe
present criteria. However, development of those
criteriaappearstobenotaneasytaskandapparently
theymightbeinforcemanyyearsfromnowifatall.
It is possible that general approach to intact and
damage stability issues may
be changed entirely.
Instead of prescriptive safety rules risk analysis and
goalorientedrequirementswillbeimplementedasit
is now discussed [5]. Actually risk assessment
approachhasbeenalreadyappliedtosafetyproblems
insomeareas,e.g.infireprotectionchapterofSOLAS
convention.Thisisencouragedbythe
MarineSafety
Committee of IMO and currently international
research programmes were installed with this
ultimateaim.Astheauthordiscussedthisproblemin
anotherpaperpresentedtothisconference,therefore
herefrainsfromdiscussingitfurtheratthisplace.
358
4.3 Off‐shoreandarcticwatersactivity
Off‐shore industry is fast developing industry that
started in the second half of the twentieth century
withexploitationofoilwellslocatedatcomparatively
smallwaterdepths,butlateronexpandingrapidlyall
overtheworld.Now,apartoftraditionaldrillingrigs
andproductionplatformslocatedinlargeoilfieldsat
different depths of water, sometimes as deep as 600
metres,insmalleroilfieldsonlysingularoilwellsare
installed without connecting pipelines and oil is
transportedusingshuttletankers.Shuttletankerisa
specially constructed highly manoeuvrable tanker
taking oil
from the oil rig or FPSO (Floating
Production, Storage and Offloading Unit). Shuttle
tankersarefittedusuallywithsophisticatedsystemof
propulsion devices consisting of azimuthing
propellers and thrusters assuring high
manoeuvrabilityrequiredwhenapproachingtakeoff
tankerordrillingrigorFPSO.Designandoperation
of such ship poses considerable
challenge to ship
hydrodynamics bearing the fact that different
propulsionunitsmaybeemployed,someofthemof
novel type. Optimal construction andmutual
interaction of several propulsion units have to be
investigated and their characteristics estimated.
Automatic systems of operation of multiple
propulsion units, so called dynamic positioning
systems,
usuallyisessential.
Asinnearfutureexploitationofoilfieldsinarctic
watersisplannedwherespecialtypesofvessels,inter
alia, shuttle tankers will be employed, special
environmental conditions in those areas must be
takenintoaccount.Researchprogrammesinthisarea
have been already installed and this is
another
challengetoshiphydrodynamics.
Another type of ships where similar problems
emerged are OSV (Offshore Service Vessels) and
ships servicing wind farms. Wind farms are new
development and in near future thousands of wind
towers may be installed in different parts of the
world. Special type of ship for
installation of wind
towerswasdevelopedthatinsimilarwayasMODU
(MobileOffshoreDrillingUnit)couldbeputonlegs
onthebottomhasbeendeveloped(seefig.2),butfor
servicing wind towers small service ships similar to
OSShavetobeemployed.
In order to achieve good manoeuvring
and
propulsion characteristic of all these ships different
propulsion devices such as shrouded azimuthing
propellers with mechanical gear, Voith‐Schneider
propellers,valvistaarpodswithverticalorhorizontal
axis and others are supposed to be installed. All of
them require thorough theoretical and experimental
investigation,optimisationoftheircharacteristicsand
development
ofdesignproceduresandthatisanother
challengetoshiphydrodynamics.
5 CONCLUSIONS
As it is seen from the above review there are many
new important challenges to ship hydrodynamics in
the twenty first century. The whole development of
sea transport and exploitation of sea resources will
dependonthemeeting
thosechallengesandinorder
to do so, several, preferably international, research
programmeshavetobe installed, possibly involving
scientificinstitutesanduniversitiesfromonesideand
industry and shipping companies fromthe other
side. On the other hand in order to execute such
programs there is urgent need to
train highly
qualified research staff and this is the main task of
universitiesandtheirshipbuildingdepartments.
Poland has fifty years tradition of contributing
effectivelytotheshiphydrodynamicsandanumber
ofscientistofpolishoriginwereactiveinthisfieldin
severalcountries.
RegretfullyinPoland,countrythatwas
formerlya
leading country in shipbuilding, the profession of
ship hydrodynamics almost disappeared. In Polish
universities chairs for ship hydrodynamics
disappeared and specialists in this field are not
trainedpracticallyanymore.
If this situation it will continue, then in Poland
therewillbenopossibilitytodesignshiphull
andits
propulsion effectively and also no research project
couldbeperformedbecauseoflackofspecialistsable
todoso.
Intheopinionofthe author there iscurrently an
urgentneedtore‐installchairofshiphydrodynamics
(or theory of ships), preferably at the Gdansk
TechnicalUniversity,that
practicallyistheonlyplace
whereshipbuildingandoff‐shoretechnologyremains
the subject of teaching and the place where new
cadresofscientistsmaybeeducated.
REFERENCES
[1]TheNavalArchiectApril2013
[2]Germanischer Llloyd (2013): In focus‐ ship efficiency and
economy.Broschure
[3]Streamlining project makes efficiency gains. The Naval
Architect,January2013
[4]Finscanonlygetbetter.ThenavalArchitectJanuary2014
[5]Papanicolau A. et al: Goal based damage stability of
passengerships.SNAMEAnnualMeeting2014