Influence of wake on efficency and hull induced vibration

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.

propeller slip and its effects

(a)        Define propeller slip                                                                (2)

(b)        State, with reasons, FOUR conditions which will affect the propeller slip.    (8)

Since the propeller works in a fluid medium, the helical path it follows will advance less than in a solid medium in the same time.   The speed in a fluid medium, measured relative to a fixed position, is called the ship speed (V) whilst the theoretical speed (V_T) in a solid  medium would simply be pitch multiplied by revs per sec.

            Usually V_T is greater than V and one form of the slip, called the apparent slip (S_a) calculated from:         

                                                S_a        =          V_T  -  V

                                                                            V_T

            If the speed of the ship were measured relative to the surrounding water it would be called the speed of advance (V_a) then this would give a true slip (S) calculated from

                       

                                                S          =          V_T  -  V_a

                                                                              V_T

It can be seen from (a), that in both cases, if pitch and revs were constant, then theoretical speed would be constant and hence:

                                    if ship speed or speed of advance reduced, slip would increase.

            conversely,     if ship speed or speed of advance increased slip would reduce

            The following conditions could therefore affect slip (FOUR required).

●          Apparent slip is affected by currents since ship speed is measured relative to land therefore in an opposing current, speed would reduce and slip increases, whilst in a following current speed increases and slip reduces (and in fact can become negative)

●          If the hull is fouled it would give increased resistance, hence speed reduces for a given power and revs,  therefore slip increases.

●          A propeller is designed to work at optimum efficiency at service revs and speed, therefore a change of power and revs will give a speed which is greater or less than the optimum and therefore less efficiency.  Lower efficiency means less speed for a given revs and therefore more slip

●          Damaged/roughened  propeller blades will mean the propeller is working less efficiently and less speed results hence more slip.  Also, damage is generally non-uniform which can lead to severe vibration. May need to reduce speed to a region where efficiency is less, hence more slip.

●          The water can only support a limited suction from the low pressure side of the propeller before cavitation occurs.  If the propeller is cavitating, it will produce less thrust, hence less speed and more slip