my jackplate project

on an unknown quest for speed. boat hit 18mph with me and the wife and 21mph by myself without the jackplate. trying to get below 9% prop slip. motor is an 85' 8hp johnson. boat is a 65' 12' sea nymph. i just need to seal the wood, fill some hole and add the brace across the top of the inside of the transome before a trial run. ill probably trim some of the aluminum too after the final height is set. just figured ide share.---tom

Great work. What size allum angle did you use? 1/4" x 3"?

Look forward to the performance data.

i'd jack it a little more . get the cavitation plate even with the bottom go for the gusto. those little motors will really go . clean that prop up . i'll bet you're less than 9 percent now . weight distribution is very important in a small boat

you cant tell from the angle of the picture but the cavatation plate IS level with the bottom of the boat. this is just the initial setup. i used 3x3x1/8" 6063 aluminum.
i used this websight to figure my prop slip; https://go-fast.com/Prop_Slip_Calculator.htm
2.23 gear ratio, 9" pitch, 5940rpm, actual speed was 21mph.
i dont think theres much more speed in there since even @6200 rpm with 0 slip the speed would be 24mph. im gonna clean up the prop when i figure how. ide like to get more speed with 2 people in the boat. also, i had to trim the motor all the way up to hit 21mph. this caused the boat to do crazy wheelies when taking off untill it planed.

nashua fisherman said:

i had to trim the motor all the way up to hit 21mph. this caused the boat to do crazy wheelies when taking off untill it planed.

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Ok, now I know you are having fun!

What is the recommended wot rpms for that motor? I would have thought that pin drive had lower rpm range.

recomended wot rpm is 5000-6000 rpm. rpm with two people when we ran 18mph was around 5890. hoping to pickup some speed with the trolling motor batery and gear with the jack plate.

nashua fisherman said:

you cant tell from the angle of the picture but the cavatation plate IS level with the bottom of the boat. this is just the initial setup. i used 3x3x1/8" 6063 aluminum.

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I would be a little worried about the 1/8" Aluminum, Is it pretty sturdy?

its plenty sturdy. this is only a 60 pound 8hp motor on a 75 pound boat. they use 1/4" on the 40hp homemade jackplates ive seen.
anyways heres the update;
i took her out for a run by myself(95* out and 70% humidity so dont know if the motor was at its best) with the motor trimmed all the way down, this puts the cavatation plate pefectly parallell with the bottom of the boat and hit 19mph.
trimmed motor to the second pin and hit 21mph consistanly at 5850-5870 rpm. no crazy-*** power wheelie either. without the jack plate i had to trim the motor all the way up to hit 21mph@5940rpm and it wasnt a consistant 21mph either.
tried the third pin and it was crazy-*** power wheelies again, taking forever to plain and when it did it cavitated and the water stream comming out of the pee hole was week.
so... i got my speed without the crazy power whellies and got my prop slip down to 5%. i still need to do a run with two people to be sure it doesnt cavatate and has good water flow.
any comments/suggestions apreciated. thanks

Sounds like you know where to put it.
Other then that that,maybe a prop pitch increase will give you what your looking for but then again,maybe not.
21 mph out of an 8hp sounds real good to me.

anyone think i could go up another inch?

i took the boat out today with my wife to check the perfomance with the jackplate and two people. we actually lost 1 mph and 300rpm!!! the boat was slightly cavatating on the second trim setting and cavated badly on the third setting. i think the moter wants more trim but needs to be lower so it doesnt cavatate so im gonna try lowering it an inch and see what happens. if anybody has any experience with this stuff please chime in. thanks

thanks for the info on your project. I will be makeing or buying a jack plate soon for mine. The new motor is a solid 2-3" below where it should be.

Cole

Thanks for posting your performance results so far. You have done allot with having just a 8hp motor.

The last time I "played" with height changes of this magnitude, was with a 10' semi V and 10hp sporttwin. No gps or tach, so all seat of the pants. I did have cavitation problems with load changes and tight turning. The bottom line was, the fastest straight line speed postion was abondoned, and ended up about half way between stock and the high position.

PS: Your report is tempting to do those tests again (w/gps and tach). I have a bit more ballast than I did back then though.

ya have to stop thinkin in inches and think 1/4 inches when you move a motor , sometimes 1/4 inch makes a difference in cavitation or not . little motors don't have much to work with , so height very important related to preformance . it's not all about speed, but, if you more speed and you don't cavitate you have made it run the best it can good luck ,wear your life jacket and have fun!

I would leave the motor in the current pin/trim position and start moving it up a 1/4 inch at a time (like flatbottom said).. Plenty of guys have motors mounted 1" above dead even with the hull and start playing with trim from there.

Looks like you have about 3" of setback with your homemade jackplate so you should be ok to move the motor up a little from even. Worse case scenario is you drill too many holes in your aluminum angle and you have to buy another piece. Once you dial it in I would beef things up a bit anyway. Maybe by butting/bolting 2 pieces of 1/8 angle together to make it a little stronger. Or just use 1/4" for the final product.. I know you think its strong enough but it would scare me.

the thing is, the boat just seems bow heavy with anyone in the front(middle). when i ran it with my wife last time there was water spray comming from the sides of the boat about where the oar locks are. i wonder if having the motor lower gives it more leverage to lift the front? i remember when i was into remote controlled boats we always ran the trim perfectly parallel to the botom of the boat and adjusted the center of gravity to make the boat run fast. too bad this isnt practical here. is the goal to use the least amount of trim and have the boat run at the optimum angle or is it all just trial and error? im gonna slot the adjustment holes at work on a bridgeport when i get a chance, hopefully this week.

A boat that is only 12 feet in length running on plane at the mid point with 2 people in it is about right.

You could always put your wife on the rear seat beside you and trim it back down one position to get the nose back down a little, and see what happens.

just wondering why we lost a mph and 300 rpm. the cavatation? thought that would make it rev higher. i wonder if throwing the trolling motor battery in the back will help.

slotin them holes is a good idea. makes it a lot easier to adjust it , but its only 1/8" . there is a lot of stress there .

21mph in a 12ft boat is plenty of speed for me , cool project though