ORIGINAL: hooty301
Why doncha get a few different props to experiment with...if that .61FX is still "healthy" (good compression, runs well ) I would try an 11 X 7 APC and a 10 X 8 and 10 X 9 just for the heck of it
The engines only about three months old so shes still healthy, Ive tried a 11/7 APC a12/5 and a 12/6 but get better speed from the 12/4 also I just switched two the coolpower I normaly run Byrun 15% cas/syn mix what fuel do you like? Watch the vid above and tell me if the engine sounds like shes turning where it should with a 12/4. I thought about drilling out the muffler wasent sure if it would hurt anything though, the first thing I did was remove the baffle. Im running a bit under proped becouse of the drag issue with it being on a boat.
You said in your first post "I am trying to break a speed record with a three point hydro airboat". Maybe if you could be more specific about that. Are there specific "contest rules" that you have to follow? Do you have to use a specific hull with a specific engine mount location? If so, then all you can do is add horsepower to get more speed. If not, then there are lots of ways to reduce hull drag (google hydrofoil) which seems to be a huge problem based on your data here . A 12x4 is a "stump pulling" prop for an airplane and not a speed prop. Also, if you can only add horsepower and can't modify the boat to be at least aerodynamicly stable, you will come to the point that every speed run will end with an air-launch and a backflip. At this point aerodynamic stability will be the limiting factor and not horsepower.