Explain
the influence of wake on:
(i) propeller efficiency
(ii) hull induced vibration
(b)(i)
The working position of the propeller is at the aft end and this causes a
different efficiency to that achieved if the propeller were fitted at the bow
(the `open water' efficiency). Since the
propeller is fitted at the aft end, it works in wake water. The wake speed is not constant over the
propeller disc, being greatest near the hull and reducing as the distance from
the hull increases. This can cause
vibration problems. The effect of the
propeller working in the wake is to acquire `wake gain' which results in the
efficiency of the propeller behind the ship being greater than the open water
efficiency.
However, there is also a
propeller-hull interaction called `thrust deduction' due to the low pressure
region on the forward side of the propeller causing a drag on the after end of
the ship. Hull efficiency is a
combination of wake gain and thrust deduction.
(ii) Since
the flow of water into a propeller is not uniform, the propeller blades must
pass through regions where the water velocity is much different from the mean
value. As the wake speed varies, the
angle of attack varies and this gives thrust and torque impulses at blade
frequency (i.e. propeller rev/min x number of blades). These vibration pulses are then passed into
the hull and the problem is compounded if propeller-hull clearances in the
aperture are too small.
Wake variation over the propeller
disc should be reduced to a minimum by careful design of the stern. For single screw ships the waterline at the
aft end should not be made too steep in order to avoid eddying in front of the
propeller. The rudder post should be
shaped and there should be adequate clearance in the aperture. In twin screw ships the propeller tip
clearance should be as large as possible.
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