Motor Position on 20' Pontoon

[carhopper]

Good afternoon boat experts. I purchased a 20' pontoon this summer and the performance is less than stellar. It has a retrofitted 90hp Yamaha 2-stroke and I get about 21mph, but at WOT I run through a gallon a mile. I know that one issue is my prop is worse for wear and I'm going to replace that next season. But I also think the motor wasn't properly positioned. Please see the attached picture and tell me what you think. The motor mount has 4 holes and it is in the lowest position. From what I've read, the AV plate should only be about 1" below the water line, but mine is about 6" as shown in the picture. Does the 1" standard apply to Pontoon boats?

 

[steelespike]

Re: Motor Position on 20' Pontoon

It appears your using about 20 gallons per hour. A 90 hp will use about 9 gph whether pushing a houseboat or a little runabout.Your prop won't affect gph,only the speed which changes mpg.21 is a little slow but typical speeds run 23- 27.
Pontoon setup varies widely I would raise the motor to a point of best performance with the least ventilation.If the
prop is bad I would replace it first but get us the prop size and the rpm at 21 mph.Is 21 by gps?
If your really using a gallon a mile you have a motor issue. Perhaps a leaky fuel pump diaphram.
Perhaps just a plain old fuel leak.

 

[carhopper]

Re: Motor Position on 20' Pontoon

It appears your using about 20 gallons per hour. A 90 hp will use about 9 gph whether pushing a houseboat or a little runabout.Your prop won't affect gph,only the speed which changes mpg.21 is a little slow but typical speeds run 23- 27.
Pontoon setup varies widely I would raise the motor to a point of best performance with the least ventilation.If the
prop is bad I would replace it first but get us the prop size and the rpm at 21 mph.Is 21 by gps?
If your really using a gallon a mile you have a motor issue. Perhaps a leaky fuel pump diaphram.
Perhaps just a plain old fuel leak.

I probably exaggerated a little on the GPM, it's more like .7, and that's at WOT which is currently 4,800. I believe my engine is rated at up to 5,400. BTW, when I reduce my RPMs to 3,500 my gas mileage over doubles and I still get 16mph. I clocked my time by GPS. My prop is a 13-5/8x13k. I was thinking about going to a 14x11 to increase RPM a little, at the same time raising the engine a few inches. Like I said, the prop is chewed up a little and needs replacing.

 

[steelespike]

Re: Motor Position on 20' Pontoon

If I'm figuring right at .7 your still is about 14 GPH. Any marine motor will use about 10% of the hp rating in fuel at wot rpm.
90 hp = 9 gph.
Typical prop size for a 90 on a 20 to 25 ft pontoon is a 13"
producing speeds 23 to 27 mph.at about 5200.There are those with an 11" but it seems to be the exception.
As you reduce throttle it will use less fuel naturally and increase mileage as you get closer to the ideal speed for the boat.
Maybe you could get us some pics of the prop. If its really poor it could affect setup and speed.But wot GPH would remain about the same.
Are the tubes clean?

 

[limitout]

Re: Motor Position on 20' Pontoon

I think you just never had the correct prop on it and tuning the right prop with the right pitch will increase your speed and help with the fuel usage. if you not getting "fully" up to full plane then you have a lot more drag that's burning fuel

 

[steelespike]

Re: Motor Position on 20' Pontoon

A pontoon really doesn't plane.I have a feeling the test that does 27 may be a tritoon.
carhopper,if you look up performance reports for Yamaha it may give you an better idea where you are at.