Max HP vs Max RPM

[LTCMontana]

I've been reading as much as possible about props and our boats.

A little background before the question: I have a 24 SSL with twin 25" tubes. It has a 115 Honda.

The engine has a max HP of 115 at 5250 RPM. The Redline is 6000. The engine probably doesn't drop much HP after 5250 but I'm sure it is some. Because this is direct drive, more RPMs = more prop speed = more MPH (notwithstanding slip at WOT). However, the max HP is only at 5250; where you would think the 'sweet spot' is for the engine.

So, should I work a prop to run at WOT at 5250 or 6000?

What say you experts?

 

[ericscher]

Generally speaking you want a prop that lets you get all the way to redline, or at least pretty close; when the logs are clean and the water calm.

That way, as the season progresses and you begin to lose RPM (as a function of additional resistance limiting top speed), you'll still be able to reach your engine's max power range.

And don't worry about hitting an exact RPM. Just get yourself in the vicinity.

 

[cwag911]

+1 on what Eric said.

 

[BulldogsCadillac]

Also have an idea of how many/how much weight you will have on the boat on MOST days and prop for that, I think, Eric is smart at this stuff, he'll know! Haha

 

[LTCMontana]

Thanks. I understand the weight/drag issue very well. I will make the goal to get max rpm, without over rev, and call it good.

Interestingly, my dealer (30 years) is starting me out with a 15.5 X 11. He just outfitted a 24 SSL with the same engine with elipticals and ran a 15.5 X 13 and nailed it (27 MPH at 4K ASL).

I am at 4000 ASL so altitude certainly plays into the equation.

Trying to use my knowledge of cars (shift points at HP vs TQ vs speed) and apply it to boats which may not be the best idea.