Question.

(a) Explain the reasons for the adoption of the following features in propellers:

(i) Six or more blades;

(ii) Highly skewed blades;

(iii) Variable angle to the blades.

(b) Explain with the aid of a sketch how the propeller curve varies from the chosen design curve under the following conditions:

(i) Hull fouling;

(ii) Ballast condition with a clean hull.

Answer.

(a) (i) The propeller diameter is maximised and revolutions minimised for maximum propeller efficiency.

The number of blades is also kept to a minimum since this also tends towards better efficiency.

However, there may be a dynamic balance problem with large thrust and torque variations in the more problematical low frequency modes.

This may lead to serious aft end vibration in the structure and shafting.

Increasing the number of blades to 6 or more will give better static and dynamic balance.

Thrust and torque variations will be reduced and hence vibration will be reduced. It is also possible to slightly increase the blade area through more blade 'overlap'.

Thus, a smaller diameter propeller may be fitted, increasing tip clearance to the hull. Cavitation may also be reduced, which would otherwise also add to the vibration problem, (ii) Angle of attack will change as wake varies and this can cause vibration due to the varying thrust and torque components of the propeller force generated.

A propeller blade with no skew will have high thrust and torque variations as it passes through the variable wake around the propeller disc with possible vibration problems. Highly skewed propeller blades arc designed so that each blade has areas in different wake regions simultaneously so that large variations of thrust and torque do not occur as it rotates. This is referred to as a 'softer interaction'.

(iii) Does this mean CPP or varied pitch angle of a blade from root to tip?

If CPP: answer using 'advantages' stated in propeller handout.

If fixed pitch propeller:

The blade needs to vary in pitch angle from root to tip to follow the different flow encountered for a particular forward pitch distance the propeller would advance if it were assumed to work in a solid medium. It is also possible to design a varied pitch angle to suit variations, in the wake that occur at lines of constant radius.

(b) The propeller curve is a plot of power versus revs.

(i) If the hull is fouled, the propeller curve will be much steeper than the chosen M.C.R. curve,

(ii) In the ballast condition, with a clean hull, the curve will be much flatter than the chosen M.C.R. curve.