Is there a rule of thumb for how much to increase the recommended engine size when putting a land plane on floats? How much more power is needed in order to compensate for the added drag?

I'm not interested in 3D or breaking the sound barrier.

TIA

Most guys I know don't bother changing the engine...they accept that it will be a bit heavier and a bit slower than the same plane on wheels. The only models I have seen that had troubles due to lack of power were the smaller 25 and 1/2A size floatplanes..

I also think that if you have something like an OS .40 you probably need to up size a tick. I have an Avistar with that engine and she could barely break away from the water. I went to an os .46 and it was like a whole new plane!

My 80" Midwest Citabria flies very well - on wheels - with a Saito 91. On floats I wish I had a significantly larger engine, maybe a 1.20. My floats are pretty much scale, so each one is about the size of a .40- or .60-sized plane fuselage.

I converted a Easyfly2 to a float plane and I could barely get it off the water. I'm going to try a .46 engine.