engine rpm v propshaft rpm?

[Ric]

On a typical sailing boat with shaft drive, does the prop turn at engine rpm? Or is there a reduction?

And when the prop freewheels while under sail, what would be a typical rpm? Presumably for a given boat speed (whether sail or power), the prop rpm will be less under sail than the rpm under power for the same boat speed, and presumably this ratio is closely linked to the propeller efficiency. I know on aeroplanes that prop efficiency is around 85% - is this a realistic figure for water-screws?

<hr width=100% size=1>

 

[chippie]

Most marine engines have a reduction gearbox with forward and reverse, I have no idea how fast a prop turns while under way. If you put a chalkmark on your flywheel and also one on the shaft flange and then turn the motor over until the shaft has turned one complete rev, you should be able to calculate the approx ratio by counting the relative number of turns and part turns of the engine. I seem to recall that prop efficiency in water loses about 30%.

<hr width=100% size=1><P ID="edit"><FONT SIZE=-1>Edited by chippie on 24/12/2003 22:04 (server time).</FONT></P>

 

[oldharry]

On most smaller engines the gearbox will give 1:1 meaning the prop is not geared down, but very commonly there is a degree of reduction usually 2 or 3 : 1, so that a larger and therefore more efficient prop can be installed.

Propellors for displacement boats start losing effcieincy at around 1750 rpm, and are liable to cavitate at speeds in excess of 2000, causing massive loss of thrust, which is why larger installations are geared down to allow the prop to turn in the range 800 - 1500 rpm as a rule.

A prop for a planing hull is designed very differently, and is extremely inefficient at low shaft speeds.

I do not know if there is any rule of thumb about the speed of rotation of a freewheeling prop, but in optimum conditons of drive displacement props are rarely developing much more than around 50-60% efficiency - much depends on the prop size, design, shaft speed, input hp etc, and some can drop as low as 35 - 40%.

Speed of rotation freewheeling at a given water speed will depend on the hull design - is there a free flow of water into the prop - which is nearly always masked by a keel or skeg of some sort. Again size, number of blades, type and condition of the shaft bearings, and gearbox drag will all affect the shaft speed.

However it will generally rotate fast enough at normal sailing speeds to drive a low speed low power generator - a number of which have been used succesfully as additional power sources for long distance cruisers. But it is most certainly quite a small fraction of the powered shaft speed - at most a few hundred rpm. Most (but not all) marine props are extremely inefficient in reverse, so that water power conversion is quite low as the prop is not designed to be driven efficiently in this way.

<hr width=100% size=1>

 

[philmarks]

Usually a reduction gear. I have a prop rev counter fitted it appears to be a simple magnet on a steel disc forming part of my aquadrive. Pick up via a small coil readout on a rev counter in the cockpit. When engine rev counter reads 2000rpm, the prop rev counter reads 800 rpm ergo reduction ratio of 2.5 to 1.

<hr width=100% size=1>Rgds
Phil