Question

With ref to rudder state

Why a breached hollow rudder adds to fuel cost

Why excessive pintle clearance should not be tolerated

Why fitted bolts are used

Answer.

A hollow rudder provides a certain amount of buoyancy to the ship

However when a hollow rudder is breached it will fill up with water and the buoyancy is lost

As a result of this the aft draught of the ship will increase and the propeller will move through a greater depth requiring greater amount of thrust to move the ship at its original speed

Also a breached hollow rudder will slightly increase the drag because of the disturbance caused to the stream line motion by eddy current, hence ships resistance increases

The hollow breached rudder also increases load on rudder carrier and more load on steering gerar

This in turn results in increased load on the steering motor and hence increase on A\E load and fuel consumption

More over the troque required to turn the rudder will increase due to added weight of water in the rudder and the loss of buoyancy causes the movement of rudder sluggish

This will adversely affect in maintaining a steady course of the vessel which in turn result in more distance being travelled

Since the cost of fuel is directly related to the distance travelled by the ship

Therefore the cost of fuel increases

Refer to fig

The rudder is supported by pintle which fit into gudgeon

The upper part of each pintle is tapered which fits into similar taper in the rudder gudgeon

The pintle is pulled hard against the taper by means of large nuts with some suitable locking devices

A brass liner if fitted into the lower part of the pintle

If the bearing surface is worn the pintle clearance increases the side wise movement will start and can result in bending moment in the stack

Any shock load can further increase this B.M. and misalign the stock

It is essential that C.G. of rudder stock and C.L. of pintle are all in the same line otherwise it will not turn

The top and bottom forging of the rudder are extended to form upper and lower gudgeon

The upper forging is opened into a palm, forming a part of horizontal coupling

This palm is stepped to provide a shoulder which reduces the possibility of shearing of the bolts

It also assist in proper positioning of the rudder stock W.R.T. rudder

Fitted bolts are provided because these are of exact diameter as the hole, thus prevent any relative movement of the stock W.R.T. rudder

They help in proper centring of the rudder and stock and stock and bearing

The bending moment and stresses set up due to heavy seas is taken by fitted bolts.