Prop Efficency

[Halo]

Dear All
Does anyone have a "rule of thumb" figure for the efficency of a well matched 3 blade prop working at cruising speed on a modern displacement yacht?.
By efficency I mean to convert shaft output in kW to the effective output in kW
or effective drive thrust (N) x speed in ms-1
Thanks
Martin

 

[pampas]

Only way I know is to run for say 1 hour at constant speed (engine revs) and count the revs turned. then calculate the distance from the prop pitch for the same number of revs, from that you can work out the efficiency. (from memory many years ago) but with a small craft one will not have the means of telling you the actual revs turned, but that is how its done.

 

[Marsupial]

Dear All
Does anyone have a "rule of thumb" figure for the efficency of a well matched 3 blade prop working at cruising speed on a modern displacement yacht?.
By efficency I mean to convert shaft output in kW to the effective output in kW
or effective drive thrust (N) x speed in ms-1
Thanks
Martin

50% on a good day

 

[theoldsalt]

I have always thought that "prop slip" (the ratio of distance travelled by the boat and that calculated using prop pitch and engine revs) is a lot less than 50% in calm water at constant speed.

However the slip does depend on the hull/prop condition and speed of the boat. The slip will be high as the boat accelerates and also when close to hull speed.

So on average maybe 50% may be a fair assumption.

 

[theoldsalt]

Out of curiosity I have just done a calculation for my boat.

Boat speed 7.3 knots, prop pitch 12.5", engine 2500 rpm, gearbox reduction 1.79. That works out at 50.8% efficient.

That is close to hull speed though, so the efficency should be better at lower speeds. This would be reflected in better fuel economy.