Hmm. So you think a fin would be bad, because it makes the plane to hard to turn? I suppose that makes sense. Maybe making the lower hull sharper than scale on the bottom was a bad idea. I wanted to make sure it didn't wobble all over the place, but maybe it's too stable and just wants to go straight.
There are so many forces at work in this case, it's hard to wrap my brain around which ones are beneficial and which are causing the problem. I think adding a fixed fin would make it that much harder to turn when you wanted to, and if the wind kicks up or shifts the plane will be less likely to weather-vane into that wind. Meaning, it's more likely for the plane to remain sideways to the wind a lift a wing tip, burying a float.
One of the problems you might be having is that buoyancy, like lift, doesn't scale. If you halve the dimensions of something, you cut the volume by an eighth but the area by only a quarter. This means that for your scale model (what's this plane, about 1:22 scale?) to float the same as the full scale your total weight needs to be 1/10648th of the full scale, or about 3.32 pounds (53oz.). (This is assuming I did my math correctly. 22x22x22 should be 10,648. Max takeoff weight:35,420lbs.) And, your wing area is only 1/484th less than full scale. (22x22=484)
Where was I going with this...
Right. Buoyancy doesn't scale equally with lift and weight. Because your plane is so light, I'm guessing less than 32oz, your wingtip floats are nowhere near the water. If that is the only thing affecting water handling, longer struts for your floats would make up for that.
You mention sharpening the lower hull, does that mean you deepened the V of the hull for better tracking? If so this could be adding to your difficulties. By deepening that V you put more buoyancy deeper, lifting the whole hull higher. This means less of the edges (chines) of the hull are in the water to stabilize it. Essentially, it's like turning a roller-skate into a roller-blade. If you have or add enough weight to pull those edges back down into the water the deeper V should give you straighter tracking without affecting the stability, but if you're too light it's like a sea-saw with the keel as the pivot-point.
Lots of words, mostly to say, you could either lengthen the struts, or add weight, or flatten the V in the hull. Any one of those by itself should improve things. I wouldn't do more than one at once. Try one (whichever is easiest), analyze, try another if you don't like the results.
*Edit* I see now your AUW on the plans is 26.8oz If that were full scale (multiply by 22^3) it'd be about 3,000 pounds lighter than the Catalina's empty weight and 14,000 pounds lighter than max weight. No wonder it looks like it's sitting so high in the water.
Retractable floats starts around post 353. For those to lazy to read through it the video is below.
