Engine Height

[Brien S]

Thought I had a post about this before, but didn't find it, so here goes.
Dealing with dads Pontoon. 20' with Merc 115. That's how they got it. Current problem is it ventilates easily. I've worked through several other problems with it with your folks help and the search function, but I'm kinda stumped on this.
Anything other than mild throttle off idle, it vents. Can ease it to 3200rpm and cruise there. Yesterday mom and I went to pull it out of the water, and decided to go for a cruise. First time on the water since July and dad passed in August. Temp was pretty cool with the breeze, but it was nice to be out and just reminisce and spend some time with her. He loved to be on the water. I really miss him. Anyway, back to the issue, I decided to see how high I could nurse the throttle, and I got it to 5K trimmed just about all the way in. By nursing, I mean super, super easy throttle increases. Once it was there, all was good. Boat seamed level or bow slightly trimmed up. Wake didn't affect it at all, but if I tried to trim it up a little, it vented.
My first thought was pitch, but the max rpm tells me different.
The water line is way above the Ventilation plate, but that is just the resting water line, above the fins that are above the ventilation plate (sorry I don't know what they're called).
I know I should have looked at the water level while underway, but just spaced, and forgot to get pictures of the boat when we got home. Try and get one today.
I did get a picture of the part number on the prop. Prop is in excellent condition. no pitting or nicks.
Are my assumptions correct that the ventilation plate should be even with the bottom of the toons or should it be a specific measurement from the bottom of the engine pod ? The engine is as low as it can get, and the mounting plate actually hangs slightly lower than the pod.
if memory serves me right, we are pretty close to sea level here, at least under 1000ft.

 

[Brien S]

Doing more research, from what I'm finding is that if the engine is to low it can also cause the prop to ventilate. Is that right ?

 

[gm280]

Yes, a picture or three would certainly help with answers. I personally have no experience with pontoon setups. But you are correct with the cavitation plate being even with the bottom of boats. And even some times a little higher then that depending on circumstances. But with pontoons, I'll let more knowledgeable folks chime in with their experiences.

 

[HotTommy]

The usual starting point is with the engine ventilation plate even with the bottom of the engine pod. Any higher and the pod will block water to the prop, and lower will increase drag from the additional engine leg in the water.

This statement makes me wonder: "The engine is as low as it can get, and the mounting plate actually hangs slightly lower than the pod." I can't visualize this, but I wonder if the mounting plate is blocking the flow of water to the prop.

 

[Brien S]

The usual starting point is with the engine ventilation plate even with the bottom of the engine pod. Any higher and the pod will block water to the prop, and lower will increase drag from the additional engine leg in the water.

This statement makes me wonder: "The engine is as low as it can get, and the mounting plate actually hangs slightly lower than the pod." I can't visualize this, but I wonder if the mounting plate is blocking the flow of water to the prop.

If memory serves me right, the plate is well below the pod. I'm really starting to think the engine is low. Steering is pretty heavy around 3000, but I know I need to pull it apart at the engine and clean and grease everything. I don't feel the water is being blocked as it runs fine at what the tach shows to be 5k, and I'm not 100% sure the tach is super accurate, but I don't know how to check it.

 

[HotTommy]

You may be able to evaluate your tach with a Prop Slip Calculator. Put in your data and see what the slip % is. Less than 10% is good. More than 20% is getting worse. Something way off the scale may indicate one of the inputs (e.g., the RPM) is wrong. ..... I still think there should be nothing hanging below the bottom of your engine pod except the ventilation plate and prop. Anything else could affect the water flow enough to cause the prop to cavitate (not ventilate). Ventilation is when air is pulled from the surface into the prop. Cavitation is when there is insufficent water coming into the prop and the drop in pressure causes bubbles to form. The sensation is the same in both situations, but the cause is different.

 

[Sea Rider]

Doing more research, from what I'm finding is that if the engine is to low it can also cause the prop to ventilate. Is that right ?

Nope if engine sits too low will produce water splashes out or over transom, if sits too high will produce prop aeration/ventilation when combo is on plane.​

​

Happy Boating​

 

[HotTommy]

FYI: Unless your pontoon boat has lifting strakes, it will never be "on plane" and rise up.

 

[Brien S]

I'll try and get out there this week for measurements and pictures.

 

[Sea Rider]

I'll try and get out there this week for measurements and pictures.

With the toon as right now, is it possible to plane out when the trim is set at neutral trim ? aside from a correct motor height install wil also need to go for a correct prop maximization. What's the max wot rpm chieved as loaded and with how many souls on board ?​

Happy Boating​

 

[Brien S]

So I'm assuming that the only way I'm going to solve this issue is with a jack plate ?

 

[racerone]

Try a set of those " whale tail " things.----Might prevent air from being sucked down to the prop.-----It is the nature of a slow moving pontoon boat with a big motor.----I do not believe raising the motor is the answer !

 

[HotTommy]

Your engine looks too deep to ventilate to me. In fact it looks like the long-leg 150 I had on my previous boat. .... My guess is the symptom you attribute to ventilation is actually prop slip (the hub is turning inside the housing). To check for this put a waterproof mark on both and run the engine until the symptom reappears. Then check to see if the marks are sill aligned. If they aren't, your prop is slipping. .... BTW, do you know the pitch and diameter of that prop?

 

[ahicks]

First, sorry to hear of your loss.

I don't believe this engine is too low at all. I do agree it may look that way at first glance, but when you start looking closer, things you don't see at first start coming into focus.

First clue: Look at the top of the transom and how it lines up in relation to the floor of the boat. When was the last time you saw a transom even with or higher than the floor line? 2nd clue: you haven't. They're always way lower.

Look at the engine's gear case and compare it's height to the bottom of the pontoons. This engine is way high - which is exactly how it's acting.

Note also the faint water line left on the engine about 4" above where the water line is now. My bet is that faint water line will be closer to correct than what it is now.

I don't know where this engine pod came from, but my bet is it's a custom to handle the 115hp engine - which is way bigger and heavier than most older 20' toons will have. It likely replaces an engine pod that was too light to handle this engine. Engine size (weight) effect on these pontoons can be confirmed by the waterline visible on the 'toons.

Bottom line, the bottom of that that engine pod needs to be a good 2" lower than it is now. Maybe more. My opinion, FWIW. -Al

 

[HotTommy]

I think ahicks is right. The ususal advice about mounting the engine relative to the engine pod doesn't apply with that unusual setup. But the waterline indicates the ventilation plate is well below the surface, so I still suspect a slipping prop. .... Although, I suppose its possible the wake from the two logs is behaving differently than on most pontoon boats because of the strange engine pod. Someone will need to look over the back of the boat while underway to see what the water is doing.

 

[Scott Danforth]

So I'm assuming that the only way I'm going to solve this issue is with a jack plate ?
View attachment 328773

out of all the boats that could have a jack plate, a pontoon actually needs a jack plate.

 

[ahicks]

I'm not all that familiar with the jack plate install. Would it allow the steering linkage to clear both the jack plate itself, as well as the transom allowing the engine to drop maybe 4" from where it is now??

I do know that USUALLY, you would use a jack plate to RAISE an engine. Not so sure what happens on an install designed/set up to lower one. Can't visualize all the moving parts of an install like that in my head....

 

[Faztbullet]

Where is the transom bracket deflector??

 

[ahicks]

What does a "transom bracket deflector" look like?

 

[HotTommy]

I've never seen one of these in real life, but perhaps this is what he meant.