Help with prop size!!!

[MercPower]

So I took some measurements and found I probably have a completely wrong sized prop on my boat.

First off. The boat is a 19' tri-hull with a 140 hp I/O Mercruiser. Last time I took the boat out with a full load (7 grown adults) it had a bit of trouble leveling on a plane.

The current prop is a 14.25 x 21. I'm no expert but I think this pitch might be a little too high for this boat. The rpm gauge is screwed up, but I'm estimating WOT around 4000 revs.

The prop is all messed up and needs to be replaced.. question is, with what sized prop?

 

[steelespike]

Re: Help with prop size!!!

While it appears you need a little less prop.Without a tach no way to know how much less or if the prop selected is right.It's critical that the motor be able to operate within its wot rpm range.
A 19" pitch will help your hole shot with a load but you need the tach to be sure it doesn't over rev with a light load.

 

[MercPower]

Re: Help with prop size!!!

First obstacle, get the rpm gauge working PROPERLY. It works sometimes, but I'm not very trusting with it. WOT says 5,000 rpm but the engine isn't screaming 5k. I know what an engine sounds like at 5k, that's why I'm guessing somewhere in the 4,000 range. Gut instinct was telling me 19 pitch but next question is diameter.

How does diameter effect speed and is it possible to change the diameter of the prop?

 

[reddogg]

Re: Help with prop size!!!

A change of 1" diameter up or down has little effect as far as I know.

Red