New Outboard Motor on Old Pontoon Boat

[oldtortuga]

Newbie question.
Put a new 60HP 4 Stroke EFI Mercury on my old (1973 28 foot Funtoon) pontoon this summer. It replaced a 60HP 2 Stroke Evinrude.
Had a marine tech mount the motor and controls for me.
Motor runs great, boat is all aluminum deck and in good condition.
I am satisfied but have one issue with water splash.
My question/problem.
At 1/2 throttle and higher with the motor trimmed correctly I get water splashback on the back deck on the boat.
I can stop the splashback by trimming out the motor but,, then the prop starts cavitating occasionally.
The boat is just a flat deck front to back with a wrap around rail and one operator chair at the console. It is great for fishing and river cruising. (no tubing, ski etc.)
The motor mount bracket is presently set to the lowest notch.
We lowered it to this setting to get the prop lower in the water to address cavitation problems.
Any suggestions/info from this group would be much appreciated.

 

[SpinnerBait_Nut]

Re: New Outboard Motor on Old Pontoon Boat

I have a '82 Funtoon.
Love the boat.
No rot to worry about.
I had a 50 Merc on mine to start with and same thing was going on with it.
I just got use to it and went on.
In 2000, because of the small lake I used it on and they had idle speed only inforced, I went to a new 9.9 Merc and ofcourse, no problem with it.
Does your's have the hard fold down top?
Mine does and I want to go bimini on mine.
Also has the little booth thing going on back by the control station.

 

[tashasdaddy]

Re: New Outboard Motor on Old Pontoon Boat

sound like you have too much in the water, move up one hole, at a time.

 

[SpinnerBait_Nut]

Re: New Outboard Motor on Old Pontoon Boat

Has your's got the slanted piece of metal attached to the adjusting transom?
If it does, that is where it's coming from.
Get to much of it in the water and it will backwash.
Your's got the big toons under it?

 

[Ride The Lightning]

Re: New Outboard Motor on Old Pontoon Boat

I think your outboard motor does not have the right height, so try to move the motor up.

Lars

 

[oldtortuga]

Re: New Outboard Motor on Old Pontoon Boat

Thanks for the quick replies.
The pontoons are 23 inch diameter, the fold down hard top was gone when I bought the boat. I bought a 10 foot bimini top from Pontoonstuff and it installed without any problems. We love it for the simplicity of use (less than a minute open or retract for one person)
Can you tell me more about the "slanting piece on the bottom of the adjusting transom" ? Can I tweak this to deflect the water ?
I don't know how raising the motor mount bracket is going to work.
The bracket was left in its existing setting when we installed the new motor.
Thats when the occasional cavitation issue first arose. So we set the motor down a notch (the last one available). Now, no cavitation but mucho agua on the rear deck at 1/2 WOT and higher.
Dumb question 1?
How is the water catching me and climbing my boat when I am going forward 9 or 10 MPH ?
Dumb question 2?
How much splash should the engine be taking ?
At 1/2 WOT and higher the splash is just below the power unit.

 

[External Combustion]

Re: New Outboard Motor on Old Pontoon Boat

The water is "climbing" up your rear deck is because your hull is transitioning from a displacement craft to a semi planing one at those speeds. Until your pontoons get up on step (where the resistance of the water supports the weight of the hull, much like a stone skipping across the water) the bow wave created by you progress will get higher, but not longer. The flip side of that is the trough of that wave is what supports the rear of your boat. The greater the horsepower transmitted to the water, the deeper the trough is and the more the rear of your hull drops into the water. As the rear of your hull drops closer to the water, the wetter you get.

Add some additional horsepower to push the hull up the slope of the wave and your hull will get up on step and suddenly become much drier; also the horsepower requirment to go faster will suddenly decrease. Go out and watch other peoples boats as they accelerate from standing still to past planing speeds. Notice how the bow of their boats point to the sky for a few seconds after a level start? It is simply a function of the length of the hull. There are some designs of hull that will never be able to plane on the water. With those, additional horsepower is converted into larger waves, but not greater speed.

At displacment speeds it takes very little horsepower to push a hull. The rule of thumb is one horsepower per marine ton (2240 pounds) to power the hull to roughly the square root of its waterline length in knots. So you 28 footer would only require one horspower per ton to go six miles per hour.

It is nice to have more power than this though to take care of such things as pushing against the wind, freeing anchors, towing others etc. Also remember that the figures above are for horsepower transmitted to the water, not engine rated power. Discount engine rated power by 50% and you will have a good feel for what you need.