101

1 INTRODUCTION

Safenavigationthroughtheseaandespeciallywithin

the narrow canals is extremely significant for the

various types of vessels with different lengths and

tonnages. Passing vessels encounter many different

situations and positions with each other.

Accordingly,therulesarecollectedinCOLREGsand

suggested to navigators.There

 often exist collisions

between the vessels at the head‐on situation. The

rules of the head‐on situation is expressed in the

COLREGs, Rule 14.However, COLREGs do not

directly or indirectly express the obligations of the

vessels at the head‐on situation based on their

propeller types. In case

of a collision, liability

apportionmentisnotconductedconsideringtypeof

the propellers. COLREGs declare the head‐on

situationasallvesselsshoulddirectlyalterthecourse

to starboard. This is quoted in the COLGREGs as

follows:

Rule14

Head‐onsituation

(a) When two power‐driven vessels are meeting on



reciprocalornearlyreciprocalcoursessoastoinvolverisk

ofcollisioneachshallalterhercoursetostarboardsothat

eachshallpassontheportsideoftheother.

(b)Suchasituationshallbedeemedtoexistwhenavessel

sees the other ahead or nearly ahead

and by night she

would see the mast head lights of the other in a line or

nearly in a line and or both sidelights and by day she

observesthecorrespondingaspectoftheothervessel.

(c) When a vessel is in any doubt as to whether such a

situation

existssheshallassumethatitdoesexistandact

accordingly.

In case of a collision, after a detailed analysis,

collision liability is apportioned cyclical all to the

mastersofthevesselsortheirskillsofwhetherornot

they apply the COLREGs wholly and correctly.In

The Obligations of Single-Propeller Vessels at the

Head-On Situation

B.Sahin

KaradenizTechnicalUniversity,Trabzon,Turkey

ABSTRACT:Manoeuvringcharacteristicsofthevesselsatthehead‐onsituationareexaminedinthisstudy.The

meetingsbetweenthepower‐drivenvesselsareconsideredbasedontheirpropellers.Thesevesselscaneither

haveasinglepropellerordoublepropellers.Avesselwithasingleright

‐handedpropelleraltershercourseto

port side easier than the starboard side. There exists an unnoticed gap, therefore the authors discuss the

International Regulations for Preventing  Collisions at Sea, 1972 (COLREGs), Rule 14, considering the vessel

orientationbased onitspropellerwalk.Afterpresentingallpossiblecasesandtheir

probableconsequences,

this paper offers authorities to embed the information of propeller characteristics into the Automatic

IdentificationSystems(AIS)inordertopreventmisunderstandingsduringtheVHFcommunications,probable

collisionrisksanddiscussionsonliabilityissuesincaseofmarineaccidents.



http://www.transnav.eu

the International Journal

on Marine Navigation

and Safety of Sea Transportation

Volume 10

Number 1

March 2016

DOI:10.12716/1001.10.01.11

102

fact, the above‐mentioned rule might construct the

infrastructureofunsafepassing forthevesselsatthe

head‐on situation if they did not maneuver to

starboardtooearly.Besidesallmastersarerequired

toknowtheirvesselsperfectly,agoodseamanshipis

expected for the vessels to keep clear

 of each other

beforefiveorsixmiles.

Thiscaseismostlyseenatthenarrowstraitssuch

as Istanbul Strait, shallow waters and the narrow

canalsincludenavigationalimpediments.Inpractice,

the vessels do not maneuver considering  the other

vessel’s propeller characteristics. In this study, two

scenariosaresimulated

basedonthepreviousmarine

accidents. Finally, as a suggestion, Automatic

Information System (AIS) must include the

informationoftypeandnumberofapropellersofthe

vessels.

The rest of this paper is organized as follows:

Section2formallydefinestheconceptofapropeller

work and maneuvering characteristics of

 a vessel.

Section 3 provides the previous marine accidents

based on the given unnoticed problem. Section 4

presentscasestudybasedonallprobablescenarios.

Conclusion and directions for authorities are

expressedinSection5.

2 THECONCEPTOFAPROPELLERWORKAND

MANEUVERINGCHARACTERISTICSOFA

VESSEL

The methods

applied to define the maneuvering

characteristics of the vessels are not new concepts.

International Maritime Organization (IMO)

Resolution MSC.137 (76) has previously defined it

and provided its calculations. In the Figure 1 a

diagram of maneuvering characteristics is provided

based on the practical experiences of the captains. 

This diagram is available

 for all vessels based on

their design, tonnage, and type of propeller and so

on. According to these diagrams, maneuvering to

starboard side and maneuvering to port side are

different. Maneuvering diameter is calculated by

multiplying the vessel length and coefficient of the

maneuveringtostarboardorport.

The vessels

moving ahead or astern are always

undersidewiselatenteffectswhichcausedeviations

from the motion directions because of propeller

design and its characteristics. These forces are

propeller discharge current effect and transverse

thrust (propeller walk). However, propeller

dischargecurrentisnegligibleforthevesselsmoving

ahead.Whenthevesselsmoving

aheadthereexistsa

strong inertia. The transverse thrust strongly comes

outwhentheshipisintheshallowwaterwithalow

speed.

Inthisstudy,followingfiguresdepicttheanalysis

ofinteractionofallvectorswiththedifferentrudder

anglesandalleffectstothevesselaredetermined

as

integrated vectors.Then, horizontal and vertical

componentsofresultantforceareshown.TheFigures

2a,3aand4ashowtheeffectsofforces(vectors) to

theshipinwhich“T”representsthemotionfromaft

to forward, “f” symbolizes rudder force and “P

s” is

for sidewise effect. Vectors are assigned relatively

consideringtheaheadmotionofthevessels.

TACTICALDIAMETER

TRANSFER

FINALDIAMETER

ADVANCE

TACTICALDIAMETER

FINALDIAMETER

TRANSFER



TurningCircleDiagram/Angle=35deg.

ShipSpeedPort.=6knots

ShipSpeedStarboard=6knots

MeanDraught=10m.

Displacement=30000t.

Figure1.Manoeuvringcharacteristicsofthevessel

Itisfoundthatsidewisecomponentoffinalfforce

isincreasedbytheparallel effectofP

stoN.Thus,the

force that alters the bow of the ship to port side is

increased(Figure2a).



Figure2a.Vectoranalysisincaseofrudderhardtoport.

Ifruddercommandisgivenashardtoport,stern

ofthevesselmovesmoretostarboardandsimilarly

aheadofthevesselmovesmoretoport(Figure2b).



Figure2b.Resultantforceforthevesselalteringitscourse

toport

SecondlyasshownintheFigure3a,finalfvector

tendstoaltership’safttostarboardthus,ship’sbow

tends to move to port side even the rudder is

midship.Here,transversethrustcausesP

Sasaforce

topullafttostarboard.

103



Figure3a.Vectoranalysisincaseofruddermidship

Duringthesmallaheadmotionofthevesselwith

itsruddermidship,becauseofthetransversethrust,

the stern of the vessel is pulled to starboard and

therebyasternofthevesselare dragged to the port

(Figure3b).



Figure3b.Resultantforceforthevesselmovingahead

Thirdly,asseenintheFigure4a,whentherudder

commandisgivenashardtostarboard,P

SaffectsN

in negative direction. For manoeuvring to the

starboard,thesidewisecomponentoffinalfvectoris

lessthanthecaseofrudderismidship.



Figure4a. Vector analysis in case of rudder hard to

starboard.

Ifruddercommandisgivenashardtostarboard,

because of the negative  directed transverse effect,

stern of the vessel moves less to the port and

similarlyaheadofthevesselmoveslesstostarboard

(Figure4b).



Figure4b.Resultantforceforthevesselalteringitscourse

tostarboard.

However, if the submergence of the rudder is

adequateenoughandtheinertiaofthevesselistoo

high, transverse effect less considerably affect the

vessel.

3 PREVIOUSMARINEACCIDENTS

In case of Catherine Desgagnés, the significance of

transversthrustisreportedas“Thetransversethrust

createdbyaright‐handed

propellertendstoswinga

vesselʹs head to starboard when the propeller is

turning astern and to port when the propeller is

turningahead.Theeffectofthetransversethrustisat

itsmaximumwhenthevesselhaslittlewayon and

decreases as the vesselʹs speed increases“.

Besides,

caseofAmarantosidentifiesnegligenceofthiseffect

asacontributingfactortotheincidentandpointsout

thattransversethrustunpredictablyaffectsthevessel

largely in shallow water.  Kapitan Serykh shows

effect of transverse thrust that might cause

grounding of vessel.In addition, in case of

Enterprise, although master

 and pilot are aware of

transverse thrust, while manoeuvring off the berth,

unexpected large transverse thrust created by

propeller caused grounding on rocks. Case of

Marjorie Jackson accident, she has double propeller

butnotcounterrotating,duetothisfactshehaslarge

transverse thrust that requires constant steering

adjustments

forstraightlinenavigationandshehas

trouble to fix this gap, and accordingly collision

occurs.IncaseofPrideofCherbourgandBriarthorn,

lessonsarecirculatedwhichcontain being aware of

theeffectsoftransversethrusttobelearntfromthis

incident to its master and deck officers. In case of

SichemMelbourne,duetotransversethrustofright

handed propeller, she crashes shore and mooring

dolphin. In grounding of Coaster Whilst, it is

reported that “the Master should have been aware

that under the prevailing conditions and with the

effectoftransversethrustthatitwouldtendtoswing

her stern and bow.” On the other hand, in

investigation report of Mv Katika, it is noted

 that

effect of transverse thrust could beneficially be

utilized, if its effect is well‐known. However, in

many accidents, it can be observed that transverse

thrust involves marine accidents directly or

indirectly.

4 CASESTUDY

In case of head on situation, two right‐handed

propeller vessels maneuvering to the port

side are

simulatedintheFigure5.AsitisseenintheFigure

5,bythehelpofthepositivetransversethrust,both

vessels keep clear while passing safely starboard to

starboard.Thisisthemostpropermaneuverforthe

right‐handedsinglepropellervesselsincaseofhead‐

 situation to avoid collision. At this point,

COLREGs,declare“…involveriskofcollisioneachshall

alterhercoursetostarboardsothateachshallpassonthe

portsideoftheother.”Instead,asasuggestion:

Whentwopower‐drivenvesselsaremeetingonreciprocal

ornearlyreciprocalcoursesso

astoinvolveriskofcollision

eachshallalterhercourseconsideringtheirmaneuvering

and propeller characteristics so that each shall pass

safelyeachother.

104

Figure5. Bothvessels are alteringtheir coursestoport in

caseofhead‐onsituation.

However, if both vessels turn rudder hard to

starboardsideinthesamesituation,theymightnot

maneuver properly because of the negative

transversethrust.AssimulatedintheFigure 6,itis

foundthatbothvesselscannotmaneuverproperlyto

keep clear each other. Accordingly, risk of collision

or dangerous

 contact may occur in this situation.

Similarmarineaccidentsrelatedtotransversethrust

aregivenintheprevioussection.



Figure6. Collision between the vessels which they are

altering their courses to starboard in case of head‐on

situation.

5 CONCLUSIONS

This study provides a comprehensive overview

about orientations and obligations of right‐handed

singlepropellervesselsatthehead‐onsituation.The

difference between the maneuvering characteristics

of starboard and port should be considered as a

disadvantageforalteringthecourse.Itisexpected

to tolerate and to fill

 this gap by an experienced

seamanship with a perfect marine and navigation

knowledge.Moreover,COLREGsdonotprovideany

active,positiveandrationalprecautionthatbringsa

certainsolutionforthecollisionriskcausedfromthis

well‐known difference. In this study, maritime

authorities are offered adding a feature to

AIS that

showsthemaneuveringcharacteristicsofthevessels

inordertoinformtheusersforallcases.Thus,after

getting the information of the propeller and

maneuveringcharacteristicsofeachother,thevessels

will have the opportunity to discuss the probable

future positions of the vessels during the VHF

communications.

Finally,allvesselswillalwaysbe

ready to take an action for all scenarios including

emergencycases.

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