8/18/2019 Rudder Torque

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Rudder torque calculationsFormulae for assessing rudder torque's are based upon theexpression Ta ACpV2 Sin q where:-

T rudder torqueC rudder area

Cp !entre of pressure distan!e from !entre line of rudder sto!"V #elo!it$ of ship

q rudder angle measured from mid-ship position

%n pra!ti!e different !onstants obtained empiri!all$ are used withthis expression and ta"e into a!!ount su!h fa!tors as propeller slip

and wa"e speed as appropriate depending upon the relation of therudder and propeller positions& The position of the !entre of

pressure has a signifi!ant effe!t upon rudder torque and hen!e thesie of the steering gear required( the greater the distan!e of the C

of ) from the !entre line of the rudder sto!"* the larger the torquerequired( therefore designers attempt to bring the C of ) as near to

the !entre line as possible& +ith the simple ,barn door, t$pe rudder

on some single s!rew ships* no adustment !an be made* but thesemi-balan!ed and balan!ed-t$pe rudders !an be designed toredu!e the torque required( for instan!e* with the spade t$pe

rudder su!h as fitted to twin s!rew ferries* the position !an beadusted b$ the designer to gi#e optimum position& This lies

between ./ and .2 per !ent abaft the leading edge of the mean

!hord of the rudder& Su!h a rudder would ha#e its C of ) forward ofthe sto!" position at low angles of helm* would balan!e around 0/o 

to 01o and drift aft of the sto!" at higher rudder angles&

POWER The pea" power that a steering gear must de#elop is the produ!t of the maximum torque T3 usuall$ at hard o#er with the ship

tra#elling at full speed* and the maximum speed S3 of rudder

mo#ement i&e& )ower max3 a T x S&

%n graph abo#e is shown a t$pi!al torque !hara!terifor a spade t$pe balan!ed rudder and a ,barn door,

unbalan!ed plate rudder& The astern torque's should

be !al!ulated sin!e this is sometimes higher than thahead torque* this is true for spade rudders&

8/18/2019 Rudder Torque

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The !ombination of maximum power and speed onl$ exists for 2 or. se!onds during ea!h manoeu#re( so !learl$ the a#erage power

required to operate the steering gear is !onsiderabl$ below thepea"& 4e!ause the steering gear must ha#e suffi!ient power to

o#er!ome fri!tion and still ha#e ample reser#e of power* the #aluefor used in the foregoing expression is signifi!antl$ higher than that

used in the expression for rudder torque& +hen !onsidering thediameter of the rudder sto!"* bending and shear stresses must beta"en into a!!ount&