Question

(a) Describe how propeller wake occurs.

(b) Explain the influence of wake on:

(i) Propeller efficiency

(ii) Hull induced vibration

Answer.

(a) There are interactions at the aft end between the propeller and the hull affecting the water speed into the propeller disc as follows:

The viscosity of the water causes layers to be dragged along with the hull, thus the water at the propeller will have a forward velocity component

Due to the form of the ship the velocity of water around the hull varies, being less than average at the ends and greater than average at midships. Thus, the relative speed at the propeller is less than the ship speed.

Wave making at the stern of the ship will affect water velocity since particles in the wave tend to take up circular motion giving rise to fore and aft velocity components.

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 clraranc should be as large as possible.