old question…mounting height for pontoon motors

[johnyrude200]

So after 1 year, I have finally completed my mini tritoon enough and mounted a ’78 35hp Johnson on it (through-hub exhaust). I was able to take it out for a spin earlier today and had some issues.

First, the motor ventilates with anything much above 2/3’s throttle, and has prop slippage. I mounted the motor so the ventilation plate is in line with the bottom of the middle pontoon.

Second, there was a lot of water splashing up into the boat and initially all over the motor (even going up over the top of the cowel!). I will have to fool around with load on the vessel, since with me and 1 passenger, most of the weight is near the back of the boat right now (battery, gas, etc.)

I will end up ‘tweaking’ the configuration with whatever time is left on this season, but wanted to see what people thought of these initial ideas….

1. Lower the motor by 2-3 inches so the ventilation plate sits below the center tube. I noticed even at 2/3rds throttle, that the water right behind the outer tubes and likely the middle one, is basically shooting straight back behind the boat, and is turbulent of course. The vessel follows a traditional hull design in that the middle pontoon is larger than the outer ones, and sits a few inches lower than the outer tubes.

2. Switch to a 4 blade prop.

3. Add a hydrofoil onto the motor to cup water down more.

I can post some pictures tomorrow. I’m going to dial in the binnacle controls and a few other things and yank the boat out of the water.

Thanks for the help!

[garry-in-michigan]

I would raise the transom 4 inches and add a 5 inch extension to the motor.

. . . . . . . . 😉

[johnyrude200]

The motor is already a longshaft motor, and I’m not aware of these motors being in XL longshaft configurations (25").

I’m going to install a 4-blade prop tomorrow along with a hydrofoil and see how this affects the ventilation. If I can solve that problem, I’ll deal with the splashing next. If the 4 blade and hydrofoil don’t fix the ventilation, then I’ll have to lower the motor, but may add some ballast to the nose of the boat to see how that affects things.

I have a power trim/tilt installed (this is a 3rd party, panther trim unit). When I trimmed up, even just a couple of inches, the ventilation/prop slippage became worst.

Looks like I’ll be experimenting a lot with this thing over the next few weeks before it gets too cold out!

[mercuryspecialist]

In quickly reviewing the propeller offerings of some manufacturers, I do not see a 4-blade available, but you likely will need a low pitch, whether being a 3-blade or 4-blade (a low pitch will be needed when the propeller is truly hooked-up). One with very large blade area, and possibly with additional cupping, maybe rolling of the blades as well. Your engine mounting is already too low apparently because of the spray, so I would not lower the mounting height (rather, it may have to be raised a little). You might consider extending the pontoon bottom by adding a plate, kind of like a trim tab, to compensate for the Panther unit’s setback, and trying to prevent the water from striking the area above the gearcase (the likely cause of the spray). I would not add a hydrofoil until I had exhausted All other means, and then only one like a plate rather than a wing in section. You may want (have) to up-size to a larger outboard which can utilize larger diameter propellers, like a 40 or 50 horsepower (and has a larger anti-ventilation plate), making better use of the tritoon design. Mercury has "Bigfoot" models where the next larger gearcase is fitted to smaller horsepower models to accomplish such an issue. Fuel economy likely will be improved by not having to run wide-open so much of the time.

Regards,
Joe

[johnyrude200]

All good info, I’ll fool around with things and report back. The middle pontoon I had trimmed down from a 20 foot to a 17 foot (to accommodate my design), and also had it filled with closed cell foam, so water intrusion in that tube is not a concern. I could take some aluminum plate and extend the tube back to cup the water further back since there is a set back with the panther trim unit.

[johnyrude200]

So I mentioned some stuff about this in my other post, but wanted to report back.

The panther trim screwed up my original designs because it made the ventilation plate be 3" above the bottom of the center pontoon. When I use a yard stick/straight edge off the bottom/keel of the center tube, the prop is only 1/2 way in the water. So DUH, I’m sure we’re going to have ventilation.

The SPLASHING is being caused by water hitting the self-designed transom which acts like a rake/wall/drag in the water.

Observing other pontoon transom designs, they are all made in the shape of wedges/deflectors to prevent this. I can modify mine with some 1/8th aluminum plate to stop the splashing.

Basically the motor is mounted too high, and the splashing is not due to the gearcase/exhaust housing, rather, it’s being deflected onto the stern due to the transom design.

I will have this all dialed in >hopefully< by wednesday and will report back.

[johnyrude200]

I built a deflector and mounted the motor lower, and both issues were solved. HOWEVER, I used a tach and the motor has only hit around 4300RPM, which is 700RPM below the rated specs, and I can only hit about 14mph.

So this is the last week I get to swap out props (it’s been low 50’s to low 40’s for daily highs here in cowhampshire). Current prop is a 11 x 13/12 (can’t remember), but found a NOS 11 x 9 and will try it out this year.

I suspect that previous post(s) were correct. Unfortunately even for an all-aluminum mini-tritoon, a 35hp (really 30hp) motor is underpowered for this vessel. If I hit 5,000 RPM with a smaller prop, I’ll be looking at upgrading to a 50hp over the winter and entering a whole new world of motors with dual carbs…get ready for some questions about how to work on those babies…peanut gallery!

Thanks again as always.

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