115 HP Outboard Motor mounted height on Pontoon???

[bighermHK45]

I am looking for suggestions on how how to mount the 115 HP outboard on my pontoon with 2 logs. I have researched and have not determined a consensus. The prevalent advice is to have the anti-ventilation plate (while underway) to be positioned so it skims the surface of the water. In addition, it was advised to install the motor so the anti-ventilation plate is even with the bottom of the engine pod.

So, looking for some practical experience with this so I have a good starting point. I was going to mount it all the way down on the transom as that was the first advice I got, however with the 115 HP being twice as heavy as the original 40 HP, I have a feeling it will sit lower right of the bat and I should start with it higher first. Just need an idea on where to start.

Thoughts?

 

[ahicks]

Ventilation plate even with the bottom of the engine pod would be my first choice - while realizing that may not be where the engine ends up after "sea trials". If you install it there using the center set of the available holes, moving it up or down a little to tweak that setting not too big a deal. IMHO

 

[1983 ercoa 21']

Remember the load on the boat also effects this setting.

 

[crb478]

You will have to experiment to get the best setting, I would actually set it at least one hole lower than the old motor or in the bottom hole. Take it out run it and see how it performs. Raise it one hole at a time until it starts to cavitatite and then lower it to the hole where it didn't. Do this with your normal load. I have done tis in an afternoon at the boat ramp. I brought along a floor jack the required wrenches and moved the motor around until I got to the best setting.

 

[bighermHK45]

You will have to experiment to get the best setting, I would actually set it at least one hole lower than the old motor or in the bottom hole. Take it out run it and see how it performs. Raise it one hole at a time until it starts to cavitatite and then lower it to the hole where it didn't. Do this with your normal load. I have done tis in an afternoon at the boat ramp. I brought along a floor jack the required wrenches and moved the motor around until I got to the best setting.

I mounted the motor in the highest position on the transom and the ventilation plate is still lower than the bottom of the transom. The characteristics of the 40HP and the new 115HP are totally different. Hopefully, I can get it out on the water in the next couple of weeks to see how it runs.

 

[lmuss53]

If both engines are 20 inch shafts I would mount the 115 right where the 40 was. I pulled the 50 off of my 19 foot two tube 25 inch logs, and put a 90 on. The 90 all the way down on the transom was too high and pulled the prop up out of the water at about 28 to 30 mph. Keep in mind that my boat is rated for a 60 so the 90 was too much motor all the way around.

Rather than start chopping inches off the transom to get the motor down lower, and because of the really high fuel consumption of the 90 I put the 50 back on and sold the 90.

Your transom was designed for a 20 inch motor, and if memory serves me your boat is rated for the 115. Hang it on and go, let it tell you what adjustments need to be made.

 

[bighermHK45]

Okay, finally took the pontoon out with the 115 Merc mounted. I got the 24' pontoon with 2 adults on board to go 24 MPH. At that speed and going in a straight line the prop would blow out. I assume I need to drop the motor height a little more. I also noted when I had the wife steer the boat when I went to the back of boat to observe the motor the speed of the boat increased. I assume the extra weight over the transom contributed to this.

Thoughts???

 

[batman99]

If you lowered the motor all the way down (re: it cannot be lowered anymore) and it still isn't working as planned, you may need to replace its short shaft (20") with a long shaft (25"). Or, replace its long shaft with extra long shaft (if available for your motor). Or, ask loaded passengers to sit in rear position seats - to force the boat's prop lower.

Hope this helps.

 

[bighermHK45]

It is a 25" shaft and I can lower it 3 more holes if necessary. I figured I would drop it down one hole and see how it does.

 

[batman99]

Based on your results of lowering 1 x bolt hole, I'd recommend dropping to the lowest hole. Then do a water test with same amount (and loading) of passengers..

Hope this helps.

 

[bighermHK45]

Based on your results of lowering 1 x bolt hole, I'd recommend dropping to the lowest hole. Then do a water test with same amount (and loading) of passengers..

Hope this helps.

I dropped it 2 holes (only 1 hole of adjustment left before it bottoms out) and took it out and it seems to do fine. I am still trying to determine what pitch prop I need, but I didn't have any ventilation so I think the motor is in a good spot height wise.