I am running a Windy with an old .25 (like 15 years old, but not used much) and it flies. I started with a new .15, but it just did not get up to any 'fun' speed. With a .25 it zips along the water at an excellent speed and you see signs of speed instability, but nothing thats difficult to handle. My buddy who built one with me has a .32 on his and it is ballistic! Flat stick down the straight and it will become unstable and flip, but full power on sweeping turns is great! This thing really flies and is awesome fun.

This is a time consuming but fun boat to build, but there are a few mods I would make right of the bat if I was to do it again:
1. Modify the top for a 4oz tank right from the start
2. Build engine mount a little higher than plans if you are using a larger engine. Swinging a 9inch prop really brings out the power of a .25
3. Extra ply on tail fin to make it a bit thicker/sturdier (with bigger engine)
4. Make radio box cover/opening a little bigger. you need damn small hands to fix stuff in there otherwise (i.e. antenna mount)
5. Do not use standard rudder hinge system. It works great in the beginning, but will eventually fall apart and become a real hassle. Use aircraft hinges right from the start.

Go build it and have some serious fun!

Johan