# cavitation plate on tunnel hull flat bottom



## alwayslookin (Jul 28, 2016)

I am working on an older polarkraft 14' flat bottom. The question I have is where should the cav. plate be? Even with the bottom of the boat or above due to the tunnel? When I purchased the boat it had a redneck engineered lift for the motor(40hp nissan)that puts the cav plate about 2-3 inches above bottom of boat. the only reason asking I cannot seem to get the motor to get to full power. It does super on muffs but when getting on the water it seems to bog out about 3/4 throttle. Wondering if cavitation is causing this. Thanks for any input.. Mike from Gtown


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## stinkfoot (Jul 28, 2016)

Level with bottom of boat. The tunnel is for a jet foot to sit in.


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## RiverBottomOutdoors (Jul 28, 2016)

Cavatation makes the motor turn more RPMs since there is less resistance on the prop or impeller for jet. Are your running a prop or jet? Assuming prop since jet doesn't have a cavatation plate.


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## alwayslookin (Jul 28, 2016)

Thanks guys, I guess I have some mods to the motor lift to do.


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## wmk0002 (Jul 28, 2016)

stinkfoot said:


> Level with bottom of boat. The tunnel is for a jet foot to sit in.



They do make different style tunnels, one for a jet foot and one for a prop. So I would think you would want it level with the top of the tunnel since it is there for the purpose of raising your motor.


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## AllOutdoors (Jul 28, 2016)

stinkfoot said:


> Level with bottom of boat. The tunnel is for a jet foot to sit in.


Depends on the tunnel. There are tunnels for jets and for props.


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## jsmoody (Jul 29, 2016)

Check your prop pitch. May be more than your motor can turn. Set your cavitation plate even with the top of the tunnel. Get ready to play around with it a lot. You may want to invest in a manual jack plate. With a 40 you should be able to turn a prop with enough cup in it to jack that motor even higher with a little setback.


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## richg99 (Jul 29, 2016)

From an older G3 adv.

richg99

https://boats.iboats.com/research/2010-g3-boats-1860-cct-dlx/582011.html


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## stinkfoot (Jul 29, 2016)

wmk0002 said:


> stinkfoot said:
> 
> 
> > Level with bottom of boat. The tunnel is for a jet foot to sit in.
> ...


 I understand that but if he is cavitating then he likely doesn't have the latter. I would put the cav plate level with the bottom and see what happens. You can always crank it up incrementally afterwards.


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## jsmoody (Jul 30, 2016)

If its bogging he wont be cavitating. Running a tunnel with the cavitation plate even with the bottom of the boat can cause excessive spray off the foot. Tunnel design has a lot to do with it but you should run it even with the top of the tunnel and then adjust the prop to counter any cavitation.


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## alwayslookin (Aug 1, 2016)

thanks you all I have been out of town. I have been trying to figure this out I am going to try and get some pics. posted of the tunnel and how the motor is sitting currently, as it is it's not even with either the top of tunnel or bottom of boat. I do plan on getting a manual jack plate but I cannot do that for a while yet. Family has to go first. It does make sense to have it even with the tunnel and I do think it may be too low because it does have a bit of spray up sideways from the foot when it is getting up to the speed it will reach before bogging down. Thanks again for all of your input. This is my first tin boat but now have two. Kind of addicting especially when you get crazy good deals on them and willing to put in the elbow grease.


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