This is what I put my old K&B.40 in. It is almost ready to test. I usually try to use plain bearing motors for water, but this motor is expendable. I had another one in a control line slow combat that went quite well until the ground turned the plane to styrofoam beads.

Many years ago, I had a Fox Hawk .60 engine on one of those Dumas Airboats like yours. I had to be careful as it had more than enough power to go airborne, but it didn't have a elevator or ailerons on it to control it with when airborne of course (also it would be quite tail heavy too). I remember the kit saying for up to .60 engines, so that was what I used. It went plenty fast at half throttle.

I figured I could use the .40 and go bigger if I had to. I tried it in the spring on some snow, and it didn't idle low enough to sit still, so I think it will be enough power. It revved up pretty good to go on the snow, although I didn't actually do it. I replaced the throttle cable a while ago. I should go out and try it somewhere.

I overpower my airboats heavily as I can always throttle back. My little hydrofoil is less than 4.5 lbs, and with a Jett .35 on it its Way overpowered. 50mph is fun, 60mph is more fun.

Aspeed - what are the dimensions of that boat? Is that a BSB?

I had one of those once, it's a Dumas Big Swamp Buggy. I powered mine with a Super Tigre .51, I still have the engine.

3150 grams or 6.9 lb. 17 1/2" x 34" including the rudders. Yes it is the BSB. I can see Lake Erie from my house, and there is nowhere to play with this or my floatplanes.with beach access. Got a Supertiger .51 on the Seamaster.

A friend gave me a BSB in need of refurbishment. I was gonna put a K&B Sportster .65 on it with a 13x6 prop. Mine is a good bit heavier than 7lbs though.