# Homemade jackplate mounting



## Batt34786 (Apr 7, 2011)

I am going to fabricate a jackplate for my old Highsider, copying what Caucasian Sensation did (he has some good pics of his on his Bragging Spot thread). I just want to make sure I get my motor mounted at the correct/best height above the transom.

Right now, the plate directly above the prop to which I've mounted my hydrofoil (I always thought this was the cavitation plate, but I think Brett said it had a real name) is about 2 1/2" below the bottom of the hull. I'm more concerned with motoring skinny than getting peak performance from my 8 hp Merc, but also want to make sure I don't raise it too high. I'll have about 4" of setback to the mounting plate.

Any suggestions on how high above the transom I should set the top of the plate?


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## HaMm3r (Dec 11, 2006)

I have a highsider with a Merc 8 and hydrofoil installed on a TSG manual jackplate. I spent a lot of time adjusting it to get it as skinny as possible without breaking loose. I'll take some measurements for you when I get home, but every boat is going to be a little different. For example, I have fuel center and battery forward, which really changes things on such a small hull.


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## Flyline (Mar 3, 2008)

I just finished built my homemade slim profile jackplate for my 56lbs 8hp tohatsu with a Gheenoe highsider. I'm trying to keep light and run skinny as possible.

My JP built 12" long and 10" wide, .250 1/4" thick aluminum frame with starboard. I'm going to mount mine starting 3" above the transom to full 7" above the transom. It's 2.75" setback and I like slim profile jackplate cuz it's save the weight and draft.




























Also I made a compression plate for my 8hp tohatsu



















Hope this helps.....


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## Brett (Jul 16, 2008)

Nice fabrication work, Blake. Looks good.



> I always thought this was the cavitation plate, but I think Brett said it had a real name


Wasn't me! Just repeating what I learned years ago from an outboard mechanic.  

There is a difference between cavitation and ventilation.
The causes and cures are both different.



> Ventilation and Cavitation
> 
> Ventilation occurs when surface air or exhaust gases are drawn into the propeller blades. The load on the propeller is reduced by the mixing of air or exhaust into the water stream causing over revving.
> 
> ...


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## Batt34786 (Apr 7, 2011)

Man - this site is awesome.

Whitesnooky - I like your rig. Where do you get the Starboard? I don't want to spend much on this build and don't want to have to buy an entire sheet to get an 8" x 10" piece. I also don't have the tools to countersink the bolt holes like you did - kind of hoping to get a best guess on the height and may go 1/2" lower just to be sure I don't cavitate/ventilate (my apologies to Brett's mechanic).

My gas tank is in the back of the boat, but the hyrdofoil seems to do a good job of keeping the nose down when running.

Thanks again to all you guys - any measurements you can provide would be greatly appreciated.


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## Flyline (Mar 3, 2008)

Thanks for the comments.

I got the startboard at Teak Isle MFG.

Teak Isle MFG
401 Capitol Court
Ocoee, Fl 34761

I got 30" long x 12" wide by 1/2 thick black stardboard for $29.95

Hope to see your ride gets done and go fishing sometime!


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## Batt34786 (Apr 7, 2011)

Whitesnooky - small world - I live right on the edge of Windermere/Gotha, and have done a ton of development in Ocoee, so I'll swing by the supply shop and see about snagging a piece of Starboard. Looks like you put one piece on the inside face of the outer angles and once piece on the outside face - is there any possibility of getting flex in the gap between the 2 pieces that could let the motor get loose? I'd rather not bolt the motor to the plate so I can keep it in my garage.


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## Flyline (Mar 3, 2008)

I dont know if it will flex but probably not because it's so stiff and strong.

I have always bolt my motor down to the plate so it wont fall off. If your motor get hit by a submerged objects or turning like crazy or trailering......more chance the motor will pop off the transom or jackplate. My motors stay in the garage but I always bolt the motor on when I put the motor on the plate or transom.


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## HaMm3r (Dec 11, 2006)

Ok, took a measurement for you. With the motor trimmed all the way down, the bottom of the cavitation plate is 2.5" above the bottom of the hull.

Just so you know how I got that measurement, I took a yardstick and laid it flat against the bottom of the hull so that it stuck out past the lower unit, then measured up to the cavitation plate.

Good luck!


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## Batt34786 (Apr 7, 2011)

Thanks, HaMm3r - I guess having the cav/vent plate 2.5" above the bottom of the hull works since the boat probably draws 4" of water or so, and the back end squats a bit when under power? No issues with cavitation/ventilation at idle speeds?


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## HaMm3r (Dec 11, 2006)

It doesn't squat under power unless I have the motor tilted up. Certainly no problems at idle speeds, but I will say that when taking tight corners near WOT, it'll slip sometimes. I'd rather have that than draft more though.


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## Batt34786 (Apr 7, 2011)

Thanks again, HaMm3r. I'll mount it this weekend and see what happens!


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