RE: Engine for a Sea Dancer
(1)Having the torque of a 4-stroke is always nice, but consider that the 4-stroke swings a big prop, and with a pod-mounted engine you can run out of room between the prop and canopy. AND, with all that torque, you MAY need more upthrust in the mount, and PROBABLY will need more right thrust.
(2)Also, seaplanes are more sensitive to weight than wheeldraggers. If you get it too heavy, the nose goes UNDER when you power up to attempt takeoff.
(3) Third, all that weight up high could make the plane less stable.
(4) I flew an OS-91 powered seaplane for 2 years, and as long as I was flying it regularly it was fine. But when I became enamored with another model and let the OS sit for a month, the bearings seized. Upon disassembling the engine I found that the bearings in a 4-stroke are not normally flushed by the fuel/lubricant mixture as in a 2-stroke. If one has an oopsie and the hot 4-stroke gets dunked, the sudden cooling causes it to slurp a dose of water in through the oiler nipple, thus supplying corrosion to the bearings, camshaft, lifters, crank,etc. Once the crankcase is wet, there is not a mechanism in place to flush it clean. A regular dose of corrosion-inhibitor oil is the best guess I have come up with, as well as replacing the factory bearings with stainless ones.
That all being said, I would use the 4-stroke but not larger than a .74.