Question

(a) Sketch propeller power curves showing EACH of the following:

ii) The effect of hull fouling:

{ii) Operation in clean hull ballast conditions

b) Explain how the fitting of a light propeller may be beneficial.

Answer.

The normal propeller curve is a plot of power versus revs where power is proportional to revs cubed and takes the form of the middle curve in the sketch.

i) When the hull is fouled, more power is required for the same revs and the propeller curve will be of increased gradient compared to the normal curve.

(ii) When the clean hull is in the ballast condition, less power is required for the same revs and the propeller curve will be of reduced gradient compared to the normal curve

(b) A lighter propeller will have a reduced starting torque due to the lower mass moment of inertia, but this is minimal when running since the greater resistance to rotation is from the hydrodynamic effect which depends upon the geometry of the propeller. The lighter propeller will however have a lower bending moment at the stern tube and may be beneficial to bearing life.

c) The fitting of a CP propeller has the following benefits:

Improved and safer manoeuvring:

Quicker speed of response:

Thrusts infinitely variable between full ahead and full astern:

No stop starts, full utilisation of engine power

Stopping times and distances reduced.

Optimum pitch settings for fouled hulls improve efficiency of power and fuel consumption.

Vibration may be reduced by altering pitch and revolutions in contrast to revs control only.

Reduced cylinder liner wear and improved life of both engine and stern tube bearings (constant r.p.m. and no stops and starts for astern operation).

Simplified main machinery if uni-directional prime movers are used particularly control systems and reduction gears

permits use of engine driven auxiliaries.

Reduces air starting capacity requirements

Easier to carry out engine trials.