Got my jack plate installed!

[Destroyer]

Originally Posted by Gfunk

BTW, without the jackplate the transom was solid, but with all the added leverage, I was able to move it a bit. So I'd rather be safe than sorry.

Well, even the Empire State building moves in the wind. Doesn't mean it's not solid. Point is, you have added extra leverage which can put additional pressure on the transom. Even the most solid transom will flex under the right conditions. As Archemedes once said, "Give me a lever long enough, and a point to anchor it and I'll move the world". And of course what he meant was that as you increase the length of of a lever you also increase the force it can exert against whatever it's anchored to, providing the fulcrum point stays the same. So in your case, by increasing the length of the engine, you've also increased the amount of force your engine can exert against your transom. That doesn't mean that you have a bad transom that needs to be replaced.

[jasoncooperpcola]

I second what Destroyer said. Did you know there is a quarter inch gap between the actual transom core and the surface your motor bolts hit? its packed full of putty or it is supposed to be. If you are really worried about it add in some backing plates.

[Gfunk]

Thanks for the words of encouragement. I thought the same thing, probably just me being paranoid. It really was pretty solid before the jack plate. The bolt holes appear to be solid , no putty or anything. Given its age, this has been my number one worry though. Glad to have educated opinions weigh in on it.
Going to run it in the spring then monkey with the adjustment. Still have a 21" prop on, so I know I'm looking at a new one, I was just waiting till the plate was on to buy it. I was getting about 4800 rpms and about 38 mph before the plate, so pretty sure this will help break the 40 mph barrier pretty easy.

[RidgeRunner]

I wouldn't advise running the motor WOT for very long if it will only achieve 4800 rpm. That is a little low and potentially harmful to the internals on a 2-stroke. (pre-detonation)
I would try a 17p prop if going Mirage or maybe a 19p if smaller in diameter. Your MPH and MPG should increase.
My friend Lawrence installed a 30" Yamaha OX66 on a 25" transom height Parker Cuddy Cabin. That was 5 years ago and it had major movement in the form of transom flex from the very beginning. I think you will be OK for a season or two based on your description.