# Cavitation plate , my experience



## jrasband (Jul 12, 2010)

I concur but I am fat and have a gheenoe with permatrim compression plate.


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## MooreMiller (Sep 10, 2016)

Do cav plates function similarly to a big trim tab bolted to your lower unit?


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## Hunter (Mar 31, 2016)

Yea. This is just a cupped plate made of some kind on composite plastic that attaches to the lower unit.


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## devrep (Feb 22, 2009)

I used an SE300 on a Yamaha 50 2 stroke years ago and it worked well.


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## Smackdaddy53 (Dec 31, 2012)

MooreMiller said:


> Do cav plates function similarly to a big trim tab bolted to your lower unit?


Not just for stern lift but also keeps water around the prop to limit blowing out and cavitation.


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## Battfisher (Jan 18, 2016)

I had a SE200 on my 25 etec, and after install I had significant torque steer - had to really fight to keep the tiller from slamming fully to the right. I tried adjusting the torque tab numerous times with only very minor improvement. I used the sport clip so I wouldn't have to drill holes. It appeared to have been installed square, but my only thought was that it was slightly off and therefore provided more push than the torque tab could compensate for. Has anyone else had this problem? I'd like to reinstall it to help with getting on plane and reducing the flat-bottom slide in turns.


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## Anderson Guide Service (Feb 17, 2017)

Is the engine trimmed out? Sounds like the plate is riding in the water. Try raising your engine to get the plate out of the water when you are running.


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## Battfisher (Jan 18, 2016)

Anderson Guide Service said:


> Is the engine trimmed out? Sounds like the plate is riding in the water. Try raising your engine to get the plate out of the water when you are running.


The SE200 plate is supposed to be out of the water when running???


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## Anderson Guide Service (Feb 17, 2017)

The front half should be. The back half will be in the water until you have it trimmed. Then it will be running pretty much even with the water column. Hope that makes sense.


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## permitchaser (Aug 26, 2013)

My boat had a Bob's cavitation plate on the 150 that came with it. The 150 is gone but I still have the plate. I have wanted to put it on my Suzuki 140 but what if it doesn't work then I have holes in the cavitation plate


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## Battfisher (Jan 18, 2016)

Anderson Guide Service said:


> The front half should be. The back half will be in the water until you have it trimmed. Then it will be running pretty much even with the water column. Hope that makes sense.


I doesn't - but only because of my ignorance of boat engineering. How does a hydrofoil prevent porpoising if it isn't under water to provide stability? And if it is supposed to be out of or skimming the surface of the water, then why would it reduce top end speed slightly? How does it help in turning if it isn't submerged to allow the hydrofoil to prevent the laminar sheer between the hull and water surface? I don't have a jack plate, so dialing in the height of the outboard would be extremely difficult.


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## Smackdaddy53 (Dec 31, 2012)

Battfisher said:


> I doesn't - but only because of my ignorance of boat engineering. How does a hydrofoil prevent porpoising if it isn't under water to provide stability? And if it is supposed to be out of or skimming the surface of the water, then why would it reduce top end speed slightly? How does it help in turning if it isn't submerged to allow the hydrofoil to prevent the laminar sheer between the hull and water surface? I don't have a jack plate, so dialing in the height of the outboard would be extremely difficult.


Because the water column does not exit the stern flat, it rises some then drops back down at different speeds and different water depths. A compression plate catches it and keeps the water around the prop. It's physics. If you have spent enough time running boats you will notice when a boat runs in deeper water it drafts deeper and when it hits a shallow area the whole boat will noticeably begin to ride stern up and bow down because of water pressure pushing against the shallow bottom as the water reaches maximum compression. Run across some deep and shallow areas WITHOUT changing anything and you will actually feel and see what I'm talking about.


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## Capnredfish (Feb 1, 2012)

Smackdaddy53 said:


> Because the water column does not exit the stern flat, it rises some then drops back down at different speeds and different water depths. A compression plate catches it and keeps the water around the prop. It's physics. If you have spent enough time running boats you will notice when a boat runs in deeper water it drafts deeper and when it hits a shallow area the whole boat will noticeably begin to ride stern up and bow down because of water pressure pushing against the shallow bottom as the water reaches maximum compression. Run across some deep and shallow areas WITHOUT changing anything and you will actually feel and see what I'm talking about.


Yup, can feel it in my skiff, that oh shiiit moment when you know the bar is getting shallow. You feel the hull rise.


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## Anderson Guide Service (Feb 17, 2017)

Also that is why when you re in shallow water you can trim as high as you want and the boat will not porpoise.


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## Anderson Guide Service (Feb 17, 2017)

There is drag that occurs not from the compression plate running in the water but from the water being pushed by the prop. It does not allow the water from the prop to go up but instead directs it out and in the direction the engine is trimmed to. The water being pushed from your prop hitting the compression plate causes an upward pressure on the engine and help prevent porpoising. They are really designed to help with your hole shot more than anything. Other effect are secondary.


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## noahvale (May 24, 2016)

Since putting a 4" manual jack plate on my boat, it tends to blow out if I trim up a little too much. The cav plate is about 5 inches under the water, any higher and it blows out. I ordered a SE400. I will report how it does on my 40 Merc.


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## Anderson Guide Service (Feb 17, 2017)

Sounds like a prop issue...


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## noahvale (May 24, 2016)

Anderson Guide Service said:


> Sounds like a prop issue...


It's more of a heavy boat problem. My boat is a '97 16' Aquaforce with a pretty big center console. I'm just trying to get it to run a bit more shallow. It runs great, 33mph at WOT. No trim tabs and I don't really want them. I run a lot more in deep water than I do in flats.


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## Battfisher (Jan 18, 2016)

Smackdaddy53 said:


> Because the water column does not exit the stern flat, it rises some then drops back down at different speeds and different water depths. A compression plate catches it and keeps the water around the prop. It's physics. If you have spent enough time running boats you will notice when a boat runs in deeper water it drafts deeper and when it hits a shallow area the whole boat will noticeably begin to ride stern up and bow down because of water pressure pushing against the shallow bottom as the water reaches maximum compression. Run across some deep and shallow areas WITHOUT changing anything and you will actually feel and see what I'm talking about.


Well said - I get it now.


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## noahvale (May 24, 2016)

I said I would report back on the SE400 and forgot.
I really like it. The boat gets on plane quicker and once on plane it lifts the boat 5 or 6 inches at the motor. The cav plate/hydrofoil is at the surface on plane, so there is only a couple of inches of boat in the water at the stern. Steering is better too. Only issue is that if you trim down too much the front will plow. 
It worked great on my old Merc 40 2 stroke. I'm picking up a Suzuki 50 4 stroke this morning. I will test the motor without it, then with and report back. One thing I really like is that it is a no drill install.
http://www.basspro.com/SE-Sport-400-Hydrofoil/product/1307220921/


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## albuilt51 (Sep 30, 2016)

On my CS12, with an unmodified 9.9 Evinrude, on the factory transom, the cavitation plate was almost 2" under the hull. On the lake, it took what seemed forever for the bow to go up before coming down and on plane. The bow rise reminded me of a jetliner taking off. At top speeds the steering was way too touchy and quick for comfort, feeling like it could flip while cornering if it encountered the right wave. 

So I made a manual jack plate to bring the motor's cavitation plate up to just under the hull (1/4") and added the SE200 that had been gathering dust on a shelf. The jack plate was made from 3"x1-1/2"x1/4" aluminum channel and 3/8" aluminum plate, with nylon backing plates.This resulted in the boat getting up on plane much quicker, much less bow rise, and very solid controllable steering at all speeds, but it felt like I had lost some top end. The SE200 was still under water and not visible even when at full speed.

Next I raised the jack plate another 3/4", which brought the motor's cavitation plate to 1/2" above the hull. Now, out on the lake the boat and motor together act like a match made in Heaven. Quickly up on plane with minimal bow rise, super-controllable steering at all speeds, and what feels like the fastest speeds yet. The top of the SE200 is now above water when on plane, and no blowouts.

I had the same basic experience on my Gheenoe. I'd ever own another boat without a jack plate/power tilt, and a hydrofoil.


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