Help with new engine ventilation/cavitation issue

absentx

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Hello!

Just spent the last few weeks getting our 1988 30 foot pontoon boat (23 inch pontoons) outfitted with a new transom so that we can have a brand new engine installed. The transom is rated for a 150hp engine and is 19 inches deep, designed for a regular long shaft engine.

Well, our dealership has the new engine on (65 horse etec pontoon series) but they are saying they can't get much past 1/4 throttle before it either cavitates or ventilates. I am thinking it ventilates because they are describing the issue as the transom is basically directing all the water away from the engine and gearcase. Almost seems like the transom is plowing water away from where it needs to be.

Modifying the transom seems a bit extreme to me. Multiple people are agreeing that the setup/height/location of the engine and transom are sound.

Other variable that isn't realistic right now is the boat is missing four to six hundred pounds of base equipment. Just a battery, steering console and fuel tank on the boat right now. While I understand getting the extra equipment on their could have some influence, I feel if we take delivery of the boat with new engine now, we are going to be fighting this for ages.

Pictures attached. Any suggestions? They are currently using an 11 pitch prop.
 

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HotTommy

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My instinct says it is cavitation from the propeller trying to turn faster than that motor can propel that big boat. I suggest you try the Propeller Forum here on iboats and be prepared to provide the following Information:
  • Number of blades
  • Diameter
  • Pitch
  • Maximum engine RPM
  • Gear ratio
Using those data and a good guess at the slip percentage, someone there will be able to tell you how fast that propeller should be traveling through the water. If the propeller is trying to go 25 MPH and the motor is only capable of pushing that big boat at 10 MPH, there will be a much higher slippage and that is a recipe for cavitation.
 

absentx

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Good advice. Our old 1988 Johnson 50 horse could push the boat about 14.5 mph in optimal conditions, and it was low on compression. I would hope this brand new 65 horse could at least get us up to the 17 or 18 range - but top end is really not all that critical. We re-powered for reliability and low speed control. Obviously though with the money we are laying out for a new engine, we need it to be able to work at its complete throttle range!

Most people I have asked so far believe it to be prop related even with this grand description of the transom plowing water away.
 

jbcurt00

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absentx, please dont start multiple topics about the same subject. I deleted the new duplicate topic in Props.

If you think you started a topic in the wrong forum or would get more (better?) help in a different forum, ask that the topic be moved. Any Mod can do it.

Want this moved to props?
 

ahicks

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With the engine trimmed the way it is, that's not helping a thing. With the angle the pics are taken at it's difficult to see how the engine lines up on the bottom of the transom. If the cavitation plate is clearly below it, I doubt the engine pod could be messing with the water flow enough to cause too much trouble.

Yet we know it's ventilating...

I'd be tempted to fabricate some shims to place between the floor and the engine pod to lower the pod and engine to see if that helps. For testing, shims could be 2x4's drilled to allow the use of the existing holes with longer bolts.

Purely out of curiosity, it appears there are no cross members supporting the floor? Instead, an extra layer of plywood? Not saying it won't work, only that I've never seen anything like that....
 

Sea Rider

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Sound means that OB sits on transom while AV plate is even, flushed with lower keel/transom ? If so, a bad way to start boating. That setting means nothing, if OB sits and rides at 90? while deck weight is evenly distributed, OB sits too high, less water ideal height for prop to grip as is should, losts of lost prop thrust.

4 Trim Angle Effects.JPG

Check on flat calm water cond at which lower leg water flow is passing by. There are only 2 ways : Above small upper plate or Under it. If flow passes slight above AV plate much worse ventilation/aeration issues..

Happy Boating
 

absentx

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There are certainly cross members. It's just like any other pontoon.

What you are seeing on the rear view is just two rear decks we made that hold the gas tank and the battery. There are no cross member supports for those two pieces. There is a small platform on the back of each pontoon to help support those pieces.

We spent an entire weekend removing about eight inches of custom aluminum the past owner had put on to lower the old transom. Many thought it was done that way to accommodate a short shaft engine although we have never seen a short shaft on the boat. The original engine was constantly swamped with water, so at the suggestion of many people, including pontoon fabricators, we removed all the old transom material and lowering brackets and connected the new transom directly to the cross members.

I have attached another picture that might help see things better.

Are you saying it's current natural position is not ideal? Does it need to sit more perpendicular to the water?
 

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Sea Rider

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Check Prop Help ? Post....try to match that...sitting any OB on transom doesn't mean a thing, need to check on a wot spin if OB/transom height is Tip-Top or not, not on a "dry installation"..

Happy Boating
 

absentx

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Okay so you are saying load her up with fuel, batteries, all that pontoon furniture, a couple people and get out there and test and see what really happens right?

Checked that post - good info thanks.

Just spoke to the dealership further. They said about 5 to 7 mph before ventilation occurred. Prop is 14x11.
 

ahicks

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Geez, re: crossmembers, that makes much more sense!

It's likely not the biggest key to your issue, but the engine is "tucked under" a bit in the pictures. Cavitation plate/prop shaft should be just about parallel with the bottom of the pontoons. Running it the way it is lifts the rear and pushes the front of the boat lower in the water. Not very productive in most cases. Minor point anyway...

Regarding the bigger issue, I'm thinking the engine is high because the way I see the pics, it looks like the bottom of the transom is only maybe 2" below the water line on the 'toons (yes, realize this is a WAG/wild a$$ guess). If the transom is a 19" and your engine has a 20" shaft, that would place the cavitation plate maybe 3" below the water line on the toons. So, that in mind, it's not hard to imagine air getting to the prop when you consider this is all happening on one end of a 30' boat. Any front to rear rocking motion (due to wave action maybe?) is going to affect what we're talking about as well.

I'm thinking the cavitation plate, especially one doing what you describe, should be no higher than maybe 2" above a line drawn across the bottom of the tubes.

Another thought. As the engine is lowered in the water, the water becomes denser, making the prop less likely to cavitate.

Last, I am not a 'toon rigging pro by any shape of the imagination. These are just my opinions. There are people that know a lot more about it than I do. I'm approaching the issue as I would if it were my boat.

Good Lord Sea Rider. Share some of your wisdom. How do you propose our OP take the boat "on a WOT spin" if the engine ventilates at 1/4 throttle?
 

HotTommy

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Absentx,
You didn't provide the information I suggested so I looked it up myself. Here's what the prop slip calculator says about your situation. If the prop starts to cavitate (not ventilate) at around 6 MPH (and I guess that 1/4 throttle is about 2500 RPM), the the prop is experiencing 57% slippage. That's about 3-4 times the typical 15% noted as normal by some folks. So, instead of the propeller screwing its way through the water it is having to suck slow moving water from ahead of it to push out the back. The low pressure area created by the suction causes the water to transform from a liquid to a gas (i.e., boil) so the propeller is no longer working against water. As the gaseous water vapor provides much less resistance to the turning of the propeller, it starts spinning faster and you have .... cavitation.

The prop slip calculator says that to get your slip back to 15-20% you need a prop with a pitch of 6. Of course that means your top speed would be only about 14 MPH. Frankly, that sounds about right to me for a 30' pontoon boat being pushed by a 65 HP engine.
 

absentx

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Sorry HotTommy, I put that information in the thread when I posted it here but then it got deleted and the old one got moved here and I forgot to update.

Here it is now to verify with your info:

Engine RPM: 5000-5500
Gear ratio: 2.36:1
Prop is 14x11 and three blades

Yep with the 1988 50 hp we topped out about 12 or 13. Was hoping we could maybe get 16 to 17 with this engine, but we aren't expecting any miracles, we just want the reliability of a new engine, the ability to troll for fish all day long and then the ability to quickly (relatively speaking) get off the lake when bad weather moves in.

I wish I knew what the prop on our old engine was...I am not with the boat or old engine right now.
 

HotTommy

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Those number are a little better. They indicate about 45% slip if 2500 RPM yields 6 MPH and that is still more than double the desired amount. They also indicate you might see 19 MPH at WOT if you can get the slip down to 20%. That still sounds optimistic to me given my experience with a 24' pontoon boat and motors of 90, 115 and 150 HP. .... I still favor cavitation over ventilation as the cause of the problem. But if it is being caused by disturbed water coming off the logs, perhaps there is a way to add plates to the sides of the engine pod to effectively make the sides of the pod go deeper into the water and shield the prop from the wash from the logs. As a test you could clamp a 12"x18" metal plate to each side of the pod so it hangs down about 6" below the pod and see if it has any effect.
 

absentx

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Yeah I think the strategy when I get back to the boat will have to be starting with a lower pitch prop. Even if we drop it to a ten, I just want to see if that gives us an improvement.

If it ends up being just an absolute mess of bad water back there and prop pitch isn't doing anything, we'll look to transom modifications...and I have absolutely what I would need to clamp some sheets to the transom for a test. I like that idea.

One other thing the dealership mentioned is that the transom was not taking in any water at all...so that tells me the wakes of each pontoon running together wasn't getting high enough to enter the front of the transom. Now thats fine, but doesn't it also suggest we are then getting some added flotation in the center rear? Seems to me like we would be having the exact same problem if we added a center log style transom to the boat!

Attached a picture of the transom before install just for reference on the shape of that etc.

Also, if I needed to go down to some really low pitch like six or seven, where would I even get that seems a bit hard to track down on initial google searct? Our dealer has stated that the 11 pitch is about as low as you can go. Not sure if they mean they don't suggest going lower or if finding props to work with our engine in that pitch range are hard to find.
 

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HotTommy

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The lowest pitch prop I've found is a 9". .... It sounds like someone needs to be watching the motor while the boat is underway. If you can't easily see it by looking over the back, then perhaps you can ride in a second boat to see what is happening back there.
 

absentx

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Yeah we can stand right next to the engine on one of the back decks and watch. We'll take some videos too, but it will be a few weeks.
 

Sea Rider

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This post should clarify prop aeration/ventilation issues. Check modified pic...

Pontoon.JPG

You need to match water flow shape exiting under middle box for it to skim right under protruding lip and consequently under upper water def plate. If OB sits at 90? and having aeration/ventilating prop issues, OB definitely sits way high on transom. Need to lower OB more till water flow passes right under blue line, right now flow is passing around or even under red line, if so, to much prop aeration best prop thrust is cancelled. But there's a bummer, OB already sits at the lowest position.

This example says it all...
This is a 60 HP elongated lower leg done locally, moron OB dealer did well telling boater to sit OB at 90? for best prop thrust while pushing heavy loads...

OB Badly Height Seated.JPG

but didn't know that water must pass way over for OB to work pushing as expected. This combo has huge prop aeration issues, when forward or reverse. I know, will say that with added weight hull will sit much deeper, you're right, what if boater needs to go with bare empty boat say 100 miles off coast to fish, can you imagine the excessive prop aeration that OB will experience at open sea along excessive fuel consumption till the boat is filled with fish to lower hull along lower leg.

Aeration Issues.JPG

The only possible way to check if OB sits at the Tip-Top transom height is pulling head out transom if possible or have other boater follow toon at speed to visually check what's going on at back transom, after that water test will know much much transom needs to be chopped down, install a jack plate or whatever needs to be done to lower OB on transom till Tip-Top OB height is finally dialed..

Happy Boating
 
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absentx

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Thanks for the very informative post Sea Rider, We'll finally be up with the boat this weekend to do some diagnosing and testing. We'll use all your reference points from the images to see where the engine sits. No issues for us to watch the engine during testing and see exactly where the water is in relation to the engine parts.

Dealer really insists that OB height on transom is solid but that transom is strangely deflecting tons of water away from gear case area and too low. They want to cut some holes or something to allow some pass through.

We are going to pick the boat up Friday, put all the gear and furniture back on and do a real world test. Thinking with furniture, full gas, batteries, anchor and two or three guys we'll be adding 1200 or more pounds to it.

We will have an eleven pitch and a nine pitch to test with.

Also wondering if a hydrofoil would be good to have just to see if it makes any difference if the prop and weight don't first work?

I understand the height has to be right but to lower that engine we have to start looking at setback plates etc. So on this first weekend of testing, we want to rule out all other fixes first.

CMC has one where we could drop the engine 3.5 inches but it sets it back 7 I believe...looks like a pretty easy install though.
 
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