Pontoon Boat & Deck Boat Forum

Hello everyone,
I just put a bigger motor on my 24' pontoon. I bought a 2001 Yamaha 150 HPDI. (2 Stroke)
The prop is 15 1/4 x 15 ML Stainless.
At WOT I can't get passed 4400 rpm @ 22kts (25mph) measured by GPS. I'm expecting 35kts with a 150hp.
The motor is running smooth as glass, it accelerates like a dragster, throws you back into the seat, gets to 4400 rpm in about 5 seconds, but it won't get above 4400rpm. Redline is 5500 RPM. While at 4400rpm the engine very smooth and not bogging.
The motor has only 300 hours, and ran with no problems on the previous boat.
In the driveway and WOT, the throttle is at the stops under the cowling.
Spark plugs are brand new, previous plugs indicated perfect combustion.

If the 15 pitch prop is too fine, wouldn't I be hitting redline?
If the 15 pitch prop is too course, acceleration would be bad, but it's like a rocket.

If fuel starvation was the problem, wouldn't the engine be running rough at 4400 rpm?

I'm stumped why I can't get more speed/power. Your thoughts?

Honestly your prop sounds about spot on as my Yamaha F150 spins a 14.5x15P and tops out at 6K and upper end of 38ish...

You don't say if this is a pontoon or tritoon or if you have at least 2 lifting strakes? If your a 2 tooner and/or lack lifting strakes then you may never realize the speed you were hoping for...

Also what is your motor height on the transom? There are a series of holes to adjust the motor higher or lower. If mounted too deep you're not going to get speed either. Can you more or less see the big cavitation plate over the top of the prop while underway? It should be roughly even with the bottom of the pontoon when looking at it.

Hello Teecro, thanks for your reply. It's a pontoon, 2 logs. I had to google what a lifting strake was, I don't have them. The motor is mounted in the lowest point, and does sit quite low in the water. I just looked... Cavitation plate is in line with the bottom of the pontoons.

[quote="Ranger"]Hello Teecro, thanks for your reply. It's a pontoon, 2 logs. I had to google what a lifting strake was, I don't have them. The motor is mounted in the lowest point, and does sit quite low in the water. I just looked... Cavitation plate is in line with the bottom of the pontoons.[/quote]

Do you have a means of raising the motor? I'd go up two holes and go water test, you will see right off the bat if it makes you any difference...

its more like the wrong pitch prop

I had a 115 Yamaha 2 stroke engine on a 24' Starcraft twin toon boat, and it wouldn't run but 4900 rpm's with a 15 pitch aluminum prop. Mine would accelerate like a bass boat until hitting a mph wall at about 30 mph.

Your problem is that you're running a displacement hull with your twin toon boat. To get them to improve top end speed, you've got to get those nosecones out of the water. And the only way to do that is have lifting strakes fabricated and welded to the toons.

The good news is that your boat has the capabilities to be a strong performer with strakes. The 150 hp motor like you have is a quite strong motor--properly setup.