# Cavitation, everyone's favorite



## RAWillimann (May 16, 2018)

I have lowered the engine as far as it goes with my Vance jackplate. Unfortunately the DF20A on the back of my LT25 cannot go any lower as the clamp handles will not allow it. 

The boat will get on plane if I tilt the engine down slightly. It will plane fine until I try WOT, Then it cavitates. 

So my options are as I see them:

- thinner piece of starboard on the jackplate, hopefully with a thinner piece I can still manage to clamp the engine down...it will be tight if I can manage it.

- spacers instead of starboard, I forgot where I saw them but that seems like a good option as well.

- hydrofoil - this intrigues me the most as they advertise the elimination of cavitation. I can get one from work pretty cheap and use one with a drill-less installation. 

Anybody have any suggestions Besides those? I'm open to ideas


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## Smackdaddy53 (Dec 31, 2012)

It could be the wrong prop.


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## FlyBy (Jul 12, 2013)

This is the position I had to run mine in to keep it from cavitating. Doesn't look right to me.







I tried a 4-blade prop and it didn't help.


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## RAWillimann (May 16, 2018)

FlyBy said:


> This is the position I had to run mine in to keep it from cavitating. Doesn't look right to me.
> View attachment 29100
> I tried a 4-blade prop and it didn't help.



That's exactly what mine looks like, don't like it. And it still will blow out before I get to WOT.


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## NativeBone (Aug 16, 2017)

Sounds like the prop may not be cupped as needed? I do not believe the 4b prop will help only the opposite. Others on here know much more....


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## Smackdaddy53 (Dec 31, 2012)

You need a prop guy to look at it and give him some numbers before you get there. Top speed, when it cavitates, rpm etc.


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## Vertigo (Jun 3, 2012)

If you have to tilt the motor as much as shown in the photo, you're probably beyond the point where a different prop will help. I'd do whatever you have to do to get the motor a few inches lower. Your transom height is 16" and it looks like your jack plate has raised the motor another few inches. Coupled with a 15" shaft motor, your cav plate is probably an inch or two higher than it should be. With the motor trimmed with the thrust axis parallel to the keel, the cav plate should be about an inch (give or take an inch) above the line of the keel for best performance.


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## RAWillimann (May 16, 2018)

Vertigo said:


> If you have to tilt the motor as much as shown in the photo, you're probably beyond the point where a different prop will help. I'd do whatever you have to do to get the motor a few inches lower. Your transom height is 16" and it looks like your jack plate has raised the motor another few inches. Coupled with a 15" shaft motor, your cav plate is probably an inch or two higher than it should be. With the motor trimmed with the thrust axis parallel to the keel, the cav plate should be about an inch (give or take an inch) above the line of the keel for best performance.


With that said, would adding a hydrofoil help? I'm not disregarding what you've said, I'm well aware it is higher than it needs to be but the problem is I cannot lower it any further. I'm wondering if that hydrofoil will keep enough water on the blades to combat the problem.


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## Guest (May 19, 2018)

Maybe, cheap enough to try.


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## RAWillimann (May 16, 2018)

Boatbrains said:


> Maybe, cheap enough to try.


That's what I was thinking


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## Vertigo (Jun 3, 2012)

A compression plate (hydrofoil) might help, a different prop might help, but you'd be throwing money at your problem and probably not fix it. It's hard to imagine that a boat would be built such that with the proper motor mounted properly it wouldn't run right. Is the jack plate necessary? Maybe it's the reason the motor is too high.


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## Guest (May 19, 2018)

Vertigo said:


> A compression plate (hydrofoil) might help, a different prop might help, but you'd be throwing money at your problem and probably not fix it. It's hard to imagine that a boat would be built such that with the proper motor mounted properly it wouldn't run right. Is the jack plate necessary? Maybe it's the reason the motor is too high.


Another thought here, don’t just buy any hydrofoil type. A compression plate is what you need. The dol fins and the like will create lift and do not normally keep water around the prop. Like vertigo said, it the jack plate necessary? Your cavitating now, motor appears to be a couple inches too high. Jack plate appears to be raised only a couple inches. Remove jack plate, mount motor on transom. Problem solved?


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## Guest (May 19, 2018)

What prop is on this thing? Prop in picture looks tiny. Also, post some picture of it from behind so we can get a better look at elevation.


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## RAWillimann (May 16, 2018)

Boatbrains said:


> Another thought here, don’t just buy any hydrofoil type. A compression plate is what you need. The dol fins and the like will create lift and do not normally keep water around the prop. Like vertigo said, it the jack plate necessary? Your cavitating now, motor appears to be a couple inches too high. Jack plate appears to be raised only a couple inches. Remove jack plate, mount motor on transom. Problem solved?


Yes...the problem is the jackplate is on there good. Removing it would mean removing half the gel coat off the transom as I can see what I believe to be 5200 on the jackplate. 

Throwing parts and money at a problem isnt the best solution, but if I can avoid destroying that brand new transom and gel coat I'm all for it. I'll look into compression plates and at the same time prepare for the worst. Ain't nothing easy.


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## Guest (May 20, 2018)

There are a couple products out there that claim to break 5200’s bond. Debond and I believe the other is unhesive. I have never used them, usually by the time I get the boat it’s saw and grinder time anyway lol. If you try them let us know how they work please.


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## Smackdaddy53 (Dec 31, 2012)

Boatbrains said:


> There are a couple products out there that claim to break 5200’s bond. Debond and I believe the other is unhesive. I have never used them, usually by the time I get the boat it’s saw and grinder time anyway lol. If you try them let us know how they work please.


Heat will break that plate loose if you hitch the trailer to your truck and use ratchet straps to put some constant pressure on the plate it will slowly lift off as you heat the jackplate. If that doesn’t work you could try a hot knife used to cut styrofoam to get between the plate and transom. If you think the jackplate is the issue or compounding the issue then it’s not doing you any hood leaving it on there. There’s nothing worse than an inefficient boat and motor combination.
A hydrofoil is a big trim tab that mounts to your motor, a compression plate covers more forward area and the sides curve down to compress water around the prop and water pickups and also acts as a hydrofoil.


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## Vertigo (Jun 3, 2012)

Don't waste money on Debond or the other products that claim to release 5200. Been there, done that. 5200 can be cut off with a razor knife and sharp chisel, and then sanded. Otherwise dynamite and a flamethrower. Why people use it in semi-permanent applications is beyond me. There are plenty of good sealants out there that can be removed. Loctite makes a fast cure marine sealant that works fine and can actually be removed if the need arises.


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## Guest (May 20, 2018)

Thanks for clarifaication on the debond Vertigo. Like I said, I’ve never used them. The dynamite will work for sure!


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## DuckNut (Apr 3, 2009)

Maybe I am seeing something that doesn't exist.

It looks like your plate is above the transom and the motor can not be mounted at the proper height. If this is the case then remove the plate and mount the motor as originally designed.


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## Smackdaddy53 (Dec 31, 2012)

DuckNut said:


> Maybe I am seeing something that doesn't exist.
> 
> It looks like your plate is above the transom and the motor can not be mounted at the proper height. If this is the case then remove the plate and mount the motor as originally designed.


That’s why we are trying to help him figure out how to remove the jackplate.


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## Brett (Jul 16, 2008)

A jack plate without a cupped prop is an exercise in frustration.
Based on the picture of the setup
You are attempting to run a factory prop with the tips above the water line.
Not designed for it.
Check for an aluminum 3 blade cupped prop to replace the current one.
I used a Turning Point prop on my tunnel skiff in order to obtain maximum mounting height.
The top two inches of water coming out from under the hull is highly aerated.
A basic factory prop is intended to be mounted with the tips below that level.


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## DuckNut (Apr 3, 2009)

Smackdaddy53 said:


> That’s why we are trying to help him figure out how to remove the jackplate.


I needed to read further: I quit reading after about the 5th post on cav plate.

A "Roundwound" guitar string can be used as a saw to get the plate off without using sharp tools. Then you can clean the hull.


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## Smackdaddy53 (Dec 31, 2012)

DuckNut said:


> I needed to read further: I quit reading after about the 5th post on cav plate.
> 
> A "Roundwound" guitar string can be used as a saw to get the plate off without using sharp tools. Then you can clean the hull.


I’ve never used one but guys have pretty good luck with the hot knife for cutting foam blocks. It’s basically a guitar string that heats up.


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## RAWillimann (May 16, 2018)

Thank you all for the input. I too despise the concept of throwing 5200 at everything on a boat, makes my job hell and when it finds its way into my private life that's even more annoying.

So last night I went outside, leveled the boat and trailer, removed the top two bolts on the jackplate rider and started measuring. I just barely managed to squeeze the clamp bracket bolts into to the jackplate while dropping it down so it is about 1.5" higher than the transom. It took drilling a couple of new holes in the jackplate and in order to remove the motor you will have to remove the jackplate riser....but it's lowered.

If it stops raining anytime soon I'll get it out and see how it goes, I will keep you all updated, thank you for the motivating input. There is nothing like the talk of dynamite and hot knives to motivate me to try and make this thing work.


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## RAWillimann (May 16, 2018)

Ha! Got it. 

https://instagram.com/p/BjE_AkqHH5s/

Final measurement for cavitation plate to keel is 1.75"
And that seems to be the sweet spot with 5" of setback. 
She jumps right up on plane. Thank you to all again for the advice, I'm sure it won't be the last time I ask for it.

Now it is on to electrical and then a front deck.


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## No Bait / Lures Only (Apr 10, 2011)

DuckNut said:


> I needed to read further: I quit reading after about the 5th post on cav plate.
> 
> A "Roundwound" guitar string can be used as a saw to get the plate off without using sharp tools. Then you can clean the hull.


Excellent


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