# Rule of thumb JackPlate ht.



## Redbelly (Jan 23, 2016)

What is the set back and relative ht. of the cav plate above the bottom you can set the motor?

I'm wondering if you can get the same running ht. as with a standard tunnel you see in aluminum hulls.


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## Guest (Jul 31, 2018)

As high as you can go without cavitation or cooling water loss.


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## Brett (Jul 16, 2008)

The answer above is the correct answer.
As a starting point, measure the distance from the bottom edge of the transom
to the leading edge of the water inlet on the lower unit.
The distance measured in inches divided by 8 is the height in inches
you can have the cav plate above the bottom of the hull to start with.
Measure 14 inches divide by 8 = 1.75 inches.
That is the starting point before you begin water testing.


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## Redbelly (Jan 23, 2016)

Ahhh...thanks for the lesson!


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## Smackdaddy53 (Dec 31, 2012)

If you have any interest in adding a compression plate I recommend doing it before setting up to adjust height because you will generally be able to run much higher without losing water pressure and prop bite on the water.


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## Redbelly (Jan 23, 2016)

Smackdaddy53 said:


> If you have any interest in adding a compression plate I recommend doing it before setting up to adjust height because you will generally be able to run much higher without losing water pressure and prop bite on the water.


Are you talking about something like a Bob's cav plate?


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## Smackdaddy53 (Dec 31, 2012)

Redbelly said:


> Are you talking about something like a Bob's cav plate?


Bob’s, Tran, Shaw Wing etc. are compression plates. Hydrofoils that don’t have curved down sides that hold water are just a trim tab mounted on the motor. 
You also want the correct prop for shallow water running and higher mounting positions because generally speaking a prop with very little or no cup will cavitate and lose grip at higher settings.


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## Capnredfish (Feb 1, 2012)

I have to ask. Compression plates. Do they work? I ask because in theory they should not. They operate above water line in an area with water that is mostly slung by the prop and hull and is very disturbed. Not really giving the prop anything to screw against. Those that run them, have you ran numbers after getting one or does it just sound a lot different Giving one the impression of performance. I’m not doubting any of you. I am asking.


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## Smackdaddy53 (Dec 31, 2012)

Capnredfish said:


> I have to ask. Compression plates. Do they work? I ask because in theory they should not. They operate above water line in an area with water that is mostly slung by the prop and hull and is very disturbed. Not really giving the prop anything to screw against. Those that run them, have you ran numbers after getting one or does it just sound a lot different Giving one the impression of performance. I’m not doubting any of you. I am asking.


Yes they improve performance a lot, no BS just real world experience with them on many different hulls. I don’t understand your “theory”.


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## Capnredfish (Feb 1, 2012)

My theory is a prop works by screwing its way through the water. When it is half way out of the water or in very aerated water leaves it with less screwing power. That's why a prop slips less when fully submerged in clean water. Cupping increases bite where prop is in clean water. My thoughts are the top of prop is out of good water. What is the compression plate doing other than acting as a splash guard? Maybe the water that gets slung stays within the splash guard and prevents prop from drawing in air? Maybe that's why it works? To technical?


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## Smackdaddy53 (Dec 31, 2012)

Capnredfish said:


> My theory is a prop works by screwing its way through the water. When it is half way out of the water or in very aerated water leaves it with less screwing power. That's why a prop slips less when fully submerged in clean water. Cupping increases bite where prop is in clean water. My thoughts are the top of prop is out of good water. What is the compression plate doing other than acting as a splash guard? Maybe the water that gets slung stays within the splash guard and prevents prop from drawing in air? Maybe that's why it works? To technical?


Your thoughts are correct and that’s why it’s called a compression plate. Speed, water pressure, maneuverability, holeshot and prop grip are all increased with a compression plate. They just work and to me it really doesn’t matter how. I know when myself and many others bolted one on their boats performance increased. The only people that I know that had no change in performance were being stubborn and not running the correct prop, jackplate setback and motor height. There may be setups that don’t work with a compression plate but I have never seen one and I’ve worked on setting up/rigging everything from 12’ aluminum hulls with a 9.9 to 26’ cat hulls with a 250.


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## sjrobin (Jul 13, 2015)

I just removed the Shaw wing from my F60 and will note the differences. I have engine data now that I did not have the first 375 hours on the engine. Using the same props three and four blade I was able to run about 1 1/2" higher on the micro jack setting, standard weight load, non-tunnel. Now that I know the RPM I will test without the plate and then back to the plate to more accurately measure the difference in micro jack setting. One notable difference with the plate is less to almost no need for trim tabs if the load is balanced.


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## Capnredfish (Feb 1, 2012)

I don't doubt you guys one bit. I suppose it is a bandaid for a prop in a poor situation. I don't run one. I have motor mounted all the way up on a bobs plate. Only loose pressure when jacked 6inches at full speed. 5 inches is ok. I can go slower all the way up not loosing pressure. That's why I also mounted motor on highest setting. Compression plate would probably allow full throttle on plane jacked all the way up.


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## State fish rob (Jan 25, 2017)

Capnredfish said:


> I don't doubt you guys one bit. I suppose it is a bandaid for a prop in a poor situation. I don't run one. I have motor mounted all the way up on a bobs plate. Only loose pressure when jacked 6inches at full speed. 5 inches is ok. I can go slower all the way up not loosing pressure. That's why I also mounted motor on highest setting. Compression play would probably allow full throttle on plane jacked all the way up.


Ive got the bob ‘s plate on a 22’ tunnel. Still blows out at full height wo throttle (6”) ,unless trimmed way down


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## Guest (Aug 1, 2018)

A lot has to do with tunnel design in tunnel hulls! Some do better than others at directing the water where it needs to go. Some boats improve with a plate, some do not.


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## Smackdaddy53 (Dec 31, 2012)

Boatbrains said:


> A lot has to do with tunnel design in tunnel hulls! Some do better than others at directing the water where it needs to go. Some boats improve with a plate, some do not.


Exactly! Throwing just any plate on a motor is not always going to be a cure-all but if set up correctly it will definitely help most. I don’t think it’s a band-aid for a motor mounted too high any more than trim tabs are a band-aid for poor weight distrubution. I see my plate and most I’ve mounted abd boats I’ve run with them is an integral part of a system to make a boat/motor perform at it’s peak.


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## permitchaser (Aug 26, 2013)

I dont have a tunnel. When i jack my motor up as high as it will go, 6", the prop doesn't push near as much as when its down. I think i can run about 20


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## Guest (Aug 2, 2018)

permitchaser said:


> I dont have a tunnel. When i jack my motor up as high as it will go, 6", the prop doesn't push near as much as when its down. I think i can run about 20


That’s typical of all my old rigs, I ran 4blades with cup but they still lose a little bite and top speed jacked all the way up. But they would scream at about 3” lift.


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## permitchaser (Aug 26, 2013)

Boatbrains said:


> That’s typical of all my old rigs, I ran 4blades with cup but they still lose a little bite and top speed jacked all the way up. But they would scream at about 3” lift.


Yep, i think mine will run faster if jacked up a little. But 40 mph in that boat is scary. I usually run about 27 mph


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## State fish rob (Jan 25, 2017)

I m w smackdaddy, gives you more options than not


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## Derek Radtke (Feb 24, 2018)

This may be a dumb question but I’m gonna ask it any. I’m relatively new to operating a boat with both a jack plate and trim tabs. My question is as I raise the jack plate do I need to be trimming the bow of the boat down to help maintain water pressure?


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## Guest (Aug 2, 2018)

Trim is relative to what you want the boat to do at the time. You might need to trim down if losing water pressure but if that’s the case I’d go with a low water pickup


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