18’ crest w/40hp Prop size and engine height help

[pita482]

Let me apologize in advance if these questions have been asked before! Or if I should be in the prop forum?
this is my first pontoon and I’ve been running vee bottoms for the last 30yrs,

Just bought a 1991 18’ crest pontoon and it had a 9.9 on her, I replaced the 9.9 with a 40hp force that
I rebuilt, the 9.9 was clamped on so I had to drill mounting holes, was wondering the correct engine height (see enclosed picture), was wondering if I’m close,
My main question is prop size, I don’t have an rpm gauge installed yet, and not had her in the water yet, wondering if someone on here has the similar set up, 18’ with a 40hp, I put a 10x11 pitch on and hoping the 40hp will turn her.

 

[HotTommy]

The customary starting height for the motor is with the ventilation / cavitation plate even with the bottom of the engine pod (i.e., the large structure that supports the transom and motor). This prop selector website should get you in the ballpark.

 

[pita482]

Thanks for prop selector! By looking at my pictures am I to low in the water?

 

[ahicks]

I would say you are at least a couple inches low, but unless you intend this boat to plane out (not very likely), I'm not real sure what the advantage to raising it might be - unless shallow water might be an issue?

 

[HotTommy]

Other than striking bottom sooner, the main drawback of having the motor lower is the extra drag the motor leg has in the water. Given your boat and motor combo, I'd be surprised if it made more than a 1 MPH difference in top speed.

 

[pita482]

Thanks everyone! Kinda thought I was a couple inches low! Now to figure prop out! Thx again

 

[ahicks]

It's going to depend on what you are looking for from this boat. Is a 1 or 2 mph increase in speed at wide open throttle interesting to you, or is this mostly about just putting around at a fairly low throttle setting? If putting around, the prop you have is probably fine.

This combo is not going to be capable of 20mph at best.

 

[clemsonfor]

I have a 24ft Crest II pontoon with a 1998 60hp Evinrude on it. Your motor is lower than mine, but like said it probably won't make much difference. My top speed with full tank of fuel (about 20gallons) and just me on board is like 21mph. My prop if I remember right is 11P 14" diameter. My RPMs are in the acceptable range at WOT.

But comparisons between our setups kind of are like apples to oranges. Different length boats, motor brands, HP size and prop size.