Sustainable Max RPM?

Matt Gurnsey · Posted: Tue Dec 16, 2008 2:01 pm Post subject:

Try to prop the engine to get close to the 6000 rpm mark. Getting lower in the range can lug the engine a a bit, and if you wind up heavy for some reason you might wind up closer to the low end of the range.

The smaller prop to get to 6000 will also give a better hole shot as it will spin up faster. Of course proping can be a bit science and a bit luck.

If you did wind up at 5500-5600, and happy with how the boat runs, I would not lose sleep over it.

marco422 · Posted: Thu Dec 25, 2008 5:16 pm Post subject:

I'm no expert but it would seem that the most logical approach would be to load your boat as lightly as you expected to use it and then find a prop that would give you 6000 rpm. Then whenever you load it more heavily your WOT rpm will be lower, keeping you below the maximum allowable (making sure to keep your WOT rpm above 5000 rpm to obviate lugging).

Tortuga · Posted: Thu Dec 25, 2008 5:36 pm Post subject:

I agree. Here are my most recent numbers with the 13.5x15 aluminum prop:

4500 RPM -- 24 mph*
5000 RPM -- 25-26 mph**
5500 RPM -- 26 mph**
5700 RPM -- 29 mph**
5800 RPM -- 30 mph**
WOT -- 5800 RPM

* Surprisingly comfortable;
** In general this was not comfortable, but with the bow down there was little slamming.

Later, when the water flattened out more:

4500 RPM -- 26 mph
4700 RPM -- 27 mph
4900 RPM -- 28 mph
5000 RPM -- 28-30 mph
5100 RPM -- 29-31 mph
5500 RPM -- 31-33 mph
WOT -- 5800 RPM

This seems to me to be a comfortable place. I'll see when I run with a heavier load. I may need to drop down to a 13" pitch for very heavy loads.

Matt