[DELETE] I had my frame fabricated specifically so the motor would not push on rubber alone. Gordo New
[DELETE] Great idea. There is a significant difference in mounting a motor to a cat than to a self-bailer like mine. Gordo New
[DELETE] If they're online you can post links to them here. I'm not the only one who'd like to see them. Gordo New
I've used that exact jackass on several boat configurations. With it mounted dead level on the frame of my big cat -- 30" tubes -- it puts a long shaft motor in the water exactly where it should be.
With the much smaller tubes of your round boat, and the way your transom already is so low, you're going to have a lot more drop than you'll ever really need. All you'll be doing is sinking the back of the raft deeper, and positioning the motor a foot farther back -- making it harder to reach to pull in shallow water, and also increasing the leverage (think structural stress) on the entire frame extension.
I'm not sure what you mean about the motor hitting the back of the boat without the jackass. Can you explain why you think the motor will come near the rubber if it's on the transom alone?
You know I'm a heavy jumbo guy, just like you. I've been there, so I'm not blowing smoke. I'm guessing it's even possible with the jackass in the upper-most position that the prop might still be in the water because of the leverage, your weight at the rear of the boat, and so forth.
But there's a way to test this, sort of. You have the boat up on a trailer in that picture. Get on the back, sit where you'll have to sit to run the motor, and use the jackass to raise and lower it to its several postions. Estimate where your waterline on the boat is going to be (taking into account the sag from your extra weight at the very end of the boat). Then have someone measure where the lower unit is in relation to that point for each of the jackass' four or five positions. As a test, get the measurement at the top of the transom itself, and see if that, in combination with the long shaft motor, isn't plenty of depth for the prop.
Don't mean to beat this to death. But I foresee you sitting on the tube back there, your weight plus that of the motor sinking the rear of the boat way down, you straining to grab the groove on the back of the motor, at a bad angle, trying to yank the motor up, and the prop still too low.
Hang in there.