13 pitch prop okay for 40hp merc tracker on a 17' pontoon boat?

[240sx4u]

Sorry for the gigantic title!

The scenario is this;

I am in the northwoods of WI at my dad/grandparents place. Everytime I come up, I leave it better than when I got here. I was out driving the pontoon boat today and something just isn't right.

It is a sylvan pontoon boat about 17' long or so (guess) and a 40hp tracker mercury oil injected 2 stroke. WOT is like 3800 RPM! The prop is a 11.1x13 (I think I got the numbers straight). 11" diameter 13 pitch.. I know the prop was changed last summer. Is this the reason? Or is the motor in poor tune? I will fix it either way!

I just need to make sure I am on the right track.

Thanks all! - Evan

BTW, there is a third possiblility that I can't fix. Back looks to be sitting about 3" lower than the front when just sitting on the water. Pontoon with a leak comes to mind...

 

[steelespike]

Re: 13 pitch prop okay for 40hp merc tracker on a 17' pontoon boat?

This is an educated guess I'm pretty sure the 13 is too much pitch.
I think a 9 in pitch would be close to what you need however presently
the 9 would only gain about 800 rpm(200 per inch of change) putting you at 4,600.You need to be about 5,500, that would be a very low 5'' pitch.so before buying a prop check a few things.Is the throttle opening all the way,good compression?Varify the tach is accurate.
when was it last tuned.prop in good shape?Any water in the logs?try trimming the motor.Logs clean no excessive growth?

 

[Silvertip]

Re: 13 pitch prop okay for 40hp merc tracker on a 17' pontoon boat?

The issue is not so much the prop as it is the gear ratio in the lower unit. Mercury has whats called a "Big Foot" lower unit that is better suited for pontoon use. It is the same lower unit used on 75/90 HP motors and has a 2.33:1 gear set whereas the standard motor has a 1.83:1 gear set. Big Foot motors use much larger diameter props. Mercury also makes a special pontoon prop for use with the Big Foot motors. That said, I can assure you if the motor you have is NOT a Big Foot, you have way too much prop. But before you change it, make sure the hull is clean and the motor is properly tuned and making full power. If those two things are ok, then you will indeed be looking for a prop that is in the 9P range. I have a similar size boat but with a Suzy DF40 four stroke. It has 2.27:1 gears and spins an 11P prop at 5800 RPM.

 

[240sx4u]

Re: 13 pitch prop okay for 40hp merc tracker on a 17' pontoon boat?

Well, I went out for another spin tonight. Seems to me that the boat is likely overpropped, out of tune AND has water in the pontoons!!

Dad said we have an 11 pitch prop here somewhere that I will reinstall, and I also was having trouble when out. I would open it up, it would run full steam and after a minute RPMS would start dropping. I also noticed the fuel line from tank to motor has a ziptie on it! I am going to replace the hose on the tank, and the primer bulb to see if that cures it.

Thanks for the thoughts and insight, I don't THINK its a bigfoot lower but I may have to turn the motor one rev by hand and do the math to find out.

Evan

 

[steelespike]

Re: 13 pitch prop okay for 40hp merc tracker on a 17' pontoon boat?

You want to be sure it is pumping water/cooling right.
You could CAREFULLY feel the head.Should be too hot to hold your hand on but not so hot it burns you.You could squeeze the primer bulb see if it helps maintain speed.Be sure tank vent is open and clear.
Not a merc pro but I think the big foot is indeed BIG and may be obvious.

 

[240sx4u]

Re: 13 pitch prop okay for 40hp merc tracker on a 17' pontoon boat?

The pee stream is rocking out nicely, so I don't think thats the problemo.

Ill do some more searching on the bigfoot lower now that I am back home, so I can do it much faster than with dialup.

Thanks! - Evan