20 foot pontoon prop size question

[Gizmostowers]

I have a 1990 sun toon 20 i just purchased it has a 90hp mercury force. I can not read the serial number. It has a 14x11p prop on it but it goes really slow at full speed and dranks gas. Any ideas on why and if prop is right size?

 

[Scott Danforth]

First, Welcome Aboard

I have a 1990 sun toon 20 i just purchased it has a 90hp mercury force. I can not read the serial number. It has a 14x11p prop on it but it goes really slow at full speed and dranks gas. Any ideas on why and if prop is right size?

most pontoons go really really slow, and drink gas. in fact, most run the same speed with a 30hp as they do a 90hp motor (they just used more fuel with the bigger motor) that is the downfall of a displacement hull

however BEFORE you ever assume its a prop issue, have you done the basics?

have you done a compression test?
is the motor's anti-cavitation plate just below the surface of the water, or is it well below that?
how do you have your motor trimmed?

Now, assuming the motor is not too deep, you have a decent angle of trim, what is the speed you are getting in RPM, and what is your GPS speed?

 

[steelespike]

A 90 hp will use about 12 or 13 gallons per hour at wide open throttle.
Your setup would probably go about 21 - 27 mph.
If it runs at about 5000 rpm slowing to about 4000 will save some fuel
yet still be closer to the best speed.
Be careful trouble shooting the ignition as it is sensitive to careless handling.
Check that the throttle is opening all the way.

 

[Sea Rider]

Was water trial done while combo was lightly loaded, solo boating to say trial was really slow ? check speed with a GPS. You need to tach that OB to know what's the max achieved wot rpm at full speed, then can move to other prop to better that poor fast displacement speed. You plan boating with few boaters or is a family oriented leisure water craft ?

If OB is that old as toon, a 25 year oldie, probably prop hub is already shot and not revving as it should

Happy Boating

 

[Gizmostowers]

We have done a tune up checked everything like fuel filter,pump,carburetors,spark plugs,coils etc... everything looks good. My rpm tack is broken.

We are just doing this as family outing for fun.

 

[Gizmostowers]

This motor was rebuilt last year

 

[steelespike]

A lightly loaded wot rpm and gps speed will tell a lot.

 

[ron nh]

We definitely need to know RPM's @ wide open throttle to help if it's a prop issue. Speed would also help. by the way, a 30hp on 20'er will do about 14mph, and a 90hp should hit about 27mph as mentioned, if set up is correct.