Jack/Transom Plate Questions

OK,

I have to move my motor up on the rear transom, I've done a bunch of searching on both this site and the internet.

I have pretty much figured out how to make the transom/jack plate, but what I'm reading there are come posts about moving the motor further away from the transom so here is my technical question: ](*,)

Background:
* The way I'm planning on making the jack plate it moves the motor about 2 -2 1/2" inches more away from the existing boats transom, and the plate will be adjustable. I can only get about an 1" of up travel with shims on the transom with having nothing to clamp the motor to, so I opted to use some aluminium angle slotted to allow for plenty of adjustment, milled them with slots and will allow me to dial in the height of the motor vs. the transom dept/height.

* The boat this is going on is a 12' Sears Gamefisher, with a 9.9 hp Johnson motor. The boat is stamped as rated for a 9.9 motor, and my over-sized rear end and rest of my body weighs 190 lbs. I don't store the trolling motor battery in the rear of the boat only the fuel container. Everything else is stored when fishing or other, in front of the middle seat to help with weight distribution.

* I don't have anything fancy on the boat like decks or such, basically a stock boat, normally it's just myself and my son fishing, and he weighs in at about 160 lbs soaking wet, I make him my ballast depending on how the boat is running.

* This is my first 9.9, I have been running a Merc 1979 7.5 hp, and it never got on a plane, but that didn't matter to me, as we are normally fishing on the Arizona Rim Lakes, which caps horsepower to 9.9, so basically the old motor just got me from point A to B. The lengths of both motors are the same, however, I have never really paid attention to transom heights vs. shaft lengths/anti-cavitation plates. Now with the new motor, I want to set it up correctly.


The question:
* What are any and all that you can or have experienced ill effects of moving the motor farther away from the transom?

Thanks =D> :mrgreen:
Rob

i love jackplates....but in my opinion, NO NO NO...you dont need to try to put a jackplate on that boat...

first, not all jonboat transoms can handle the additional leverage of a jackplate...9.9, yeah not that much hp but its still weight that is magnified by leverage...(i had a jackplate on a little 14 footer several years ago, and hit a log, and the transom buckled out exactly where the jackplate was mounted...my 14 footoer now, ive overbuilt the transom to handle it..

second, thats a small boat...putting your motor another 3 inches back WILL make a difference in trim..

how high do you need to go? you wont see any performance gains by raising the motor 1 inch on a 9.9...or, if you do, it will be very little....

i suggest just putting a piece of 1x2 or something similar on the top of the transom, then clamp her back down...(and put 2 bolts)

What your asking about is called "Set Back" setting your motor back can allow you the raise it a little higher.

OK,

Let me clarify a few thing about the boat and the old motor that was on the boat, sorry I did not do this in the first post:

Additional Clarification
* The transom on the boat has be totally beefed up by me, 3/4" plywood across the compete inside of the boat and laminated to the Aluminum, and the exterior of the transom was through bolted to the inside of the transom, it's pretty much bullet proof.
* My old 1979 Merc 7.5 weighed just at 73 lbs, I personally weighed it at work, on our shipping scale.
* My new 1986 Johnson 9.9 weighs just at 74 lbs, again weighed it at work on the scale.

Basically there is no difference between the weight of the two motor, the real issue is the height of the motor on the transom. I can only put about an 1" block on top of the transom and I will run out of clamping room on the inside of the transom.

I can however build and extension to the transom to raise the transom, but until I know how the motor reacts to the boat, I won't know exactly how high to build the extension to the transom. I have unlimited resources to fabricate just about anything, we race Ultra4 off-road buggies, so anything is possible.


Question/Conclusion?
So is the conclusion that the additional leverage of the jack plate too much for the transom or too much for the boat to be able to plane/trim out at speed?

Thanks again
Rob

how about the obvious?? how high do you need to go, and what do you expect to gain??

how tall is your transom now?? most short shaft transoms are 15-16 inches tall... a short shaft engine is 15-16 inches.....

it seems that youre set on building a jackplate....why not just go ahead and build it rather than ask...then you can report on the gains of adding a jackplate to a 12 foot jon with a 9.9...

I take it that you have a long shaft motor if I understand you and that is why you need to raise it a little. I had the same problem with a 12 foot semi-V. I made a jack plate similar to what you have described and had no problems with the setback, the plate worked perfect, now this was with a heavy 4 stroke motor and although I never had any problems with my transom due to river strikes it did bow some from the road travel. What I did was run a ratchet strap from around the jack plate to the wench post at the front of the trailer to relieve some strain.

Thanks everyone for the replies.

I'm going for the Jack Plate, I rarely travel to the lakes with the motor attached to the boat.

I will report back with final results.

Thanks
Rob

Just experiment with different heights, start with cavitations plate on same plane as hull, then move up as desired until you get ventilation or lose coolant intake. In theory the higher the motor, the less resistance until you lose your bite in the water. Good luck!