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