ORIGINAL: Phoenixangel
With the kind of drag and weight air boats have you may need to go up a size or two to get your 40+ mph.
The size and weight of these .90 class engines is not much different then the .60-75 class. In fact the G90 uses the small crankcase. The O.S. weighs less then other brands .75's.
For the PRICE and POWER you will have a hard time doing better then a [link=http://www3.towerhobbies.com/cgi-bin/wti0001p?&I=LXFV79&P=0]G.90[/link] from Super Tigre.
It will turn a 13x9 at 12000ish on the ground once fully broken in, running 10% nitro.
If you are wanting more RPM it will cost you more. An O.S. 95 AX will rev higher turning even bigger props. I have seen the AX turn a 14x8 at 13,200 on the ground. That is impressive.
http://www3.towerhobbies.com/cgi-bin...?&I=LXXMU4&P=0
Either one of these engines with the right prop are going to get you real close if not over your speed goal.
Another option would be Thunder Tiger PRO in the 90 range. I am not real sure but I am guessing it is somewhere in the middle between these two.
I didnt mention - the boat in question is about 6lb10oz with the K&B on it and wet rudder.
I'm not real keen to OS engines, namely because of the price. I read up a little bit on earlwb's Fox Eagle IV .60 but he only had RPM figures for a 11x7 prop of which is too small for my airboat application but was getting over 14,000rpm from it.
I'm not dead set on any given RPM target as I'm more concerned with staying with a 13" prop or less. I make my own fuel with castor oil so my engine choice would need to tolerate the castor lube.