cavitation plate on tunnel hull flat bottom

Have a question about your motor? All motor talk and questions go here.
Post Reply
alwayslookin
Posts: 8
Joined: 24 May 2016, 11:22
Location: third coast

cavitation plate on tunnel hull flat bottom

Post by alwayslookin » 28 Jul 2016, 13:56

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

Xtremeboats
User avatar
stinkfoot
Posts: 399
Joined: 05 Feb 2016, 03:42
Location: Langley, BC

cavitation plate on tunnel hull flat bottom

Post by stinkfoot » 28 Jul 2016, 16:51

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

User avatar
RiverBottomOutdoors
Posts: 1729
Joined: 20 Aug 2010, 14:46
Location: Halifax, Virginia
Contact:

cavitation plate on tunnel hull flat bottom

Post by RiverBottomOutdoors » 28 Jul 2016, 17:05

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.

alwayslookin
Posts: 8
Joined: 24 May 2016, 11:22
Location: third coast

cavitation plate on tunnel hull flat bottom

Post by alwayslookin » 28 Jul 2016, 17:26

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

User avatar
wmk0002
Posts: 659
Joined: 17 Apr 2014, 15:39
Location: Cullman, Alabama

cavitation plate on tunnel hull flat bottom

Post by wmk0002 » 28 Jul 2016, 17:53

stinkfoot wrote: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.
2016 Alumacraft 1648 NCS - 1992 Johnson 9.9 HP Tiller
http://www.tinboats.net/forum/viewtopic ... 21&t=40764

User avatar
AllOutdoors
Posts: 209
Joined: 13 Oct 2011, 23:54
Location: Tennessee

Re: cavitation plate on tunnel hull flat bottom

Post by AllOutdoors » 28 Jul 2016, 23:30

stinkfoot wrote: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.
On a shallow river in a RiverPro!

jsmoody
Posts: 37
Joined: 25 Sep 2014, 14:19

cavitation plate on tunnel hull flat bottom

Post by jsmoody » 29 Jul 2016, 00:02

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.

User avatar
richg99
Posts: 4945
Joined: 27 Nov 2010, 09:36
Location: Houston, TX & Crossville, TN

cavitation plate on tunnel hull flat bottom

Post by richg99 » 29 Jul 2016, 14:48


User avatar
stinkfoot
Posts: 399
Joined: 05 Feb 2016, 03:42
Location: Langley, BC

cavitation plate on tunnel hull flat bottom

Post by stinkfoot » 29 Jul 2016, 21:37

wmk0002 wrote:
stinkfoot wrote: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.
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.

jsmoody
Posts: 37
Joined: 25 Sep 2014, 14:19

cavitation plate on tunnel hull flat bottom

Post by jsmoody » 30 Jul 2016, 02:11

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.

alwayslookin
Posts: 8
Joined: 24 May 2016, 11:22
Location: third coast

cavitation plate on tunnel hull flat bottom

Post by alwayslookin » 01 Aug 2016, 17:48

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.

Post Reply