max-props

[daveperce]

Fitted in my light displacement 34' yacht is a 16hp Yanmar 2GM20 which has a max of 3400 rpm. At a more bearable 2900 rpm the boat plods through the water at 4 - 4.2knts in slack water. I also have a max-prop fitted and my question is this.. If I alter the pitch of the prop will that have a beneficial/negative effect on the speed of the boat? At the moment the pitch is set at 16deg RH. If I can increase the speed this way do I increase or decrease the pitch angle?

 

[chippie]

Increasing the pitch is much the same as moving to a higher gear.

The boat speed should theoretically be more for any given engine revs. The problem is whether the engine has enough power to deliver the increased speed theoretically available. I guess trial and error would determine the answer. Try a google search for a propellor manufacturer that can work it out beforehand .Maybe Max Props themselves.

Good luck.

 

[pandroid]

Darglow Enginering (Max-prop) seem to usually set the prop pitch to coincide your ideal cruising speed with max-tourqe from the engine. This is less than max-revs, and from your manufacturers engine data, this appears to be about 3100 revs. Ideal cruising speed should be reached about this engine speed, and little extra speed (say a knot) will be reached between this setting and max revs.

Boats vary, but assuming your boat is average shape, hull speed would be about 7 knots, and it therefore seems that you should be getting around 6 knots at 3100 revs. If you are not (and it seems like you arent), you need to pitch up the prop. One other simple test is that if you can reach max-revs before the throttle is wide open (WOT), the prop is pitched to low.

These figures are all rough and Darglow can calculate the correct pitch if you give them all the data. If its a popular boat, they may already have the setting. Give them a ring.

One thing, if you do pitch up the prop, expect some loss of acceleration (and some more initial cavitation) from a standing start.