Help me Understand?

[Bluewater Bowfishing]

So, a prop will lose thrust after 3000 rpms, if my motor turns 6000, whish its propably right there since it is souped up, a 2-1 reduction drive will run a larger prop right at the "boil out" speed. At least this is how I understand it b/c a outboard "boils out" on takeoff if you throttle it too hard I assume a airboat would do the same, it don't mattter how fast it spins if it don't push air, right? I have a 23 Vangaurd from the go cart racing cucuit puts out 34 hp I am looking at a 2-1 reduction and a 60 inch 3 blade composite prop. What kind of performance will I have on a 11.5 by 5.5 alum hull of .100 gauge right at 225 lbs? I will want 2 people sometimes and a small amount of gear, maybe a few fish. Should I sell my engine and buy a larger Rotax engine? Maybe like a 40 horse? I don't want but about 25 mph with one person and gear, but I would need it to at least plane off with me in deep water. Thanks for helping me, I am going to put a 45* angle on my sides all the way up to get a good slide out of it.

[chuckitt]

Hey Bluewater, sorry I missed your post. If your engine puts out about 34 HP, It should be fine. The hull would work
better if the bottom was 6 ft wide. The sides don't have to be 45 degrees, 15 to 20 degrees would be fine.
Will that engine stay together at 6000 RPM? If so, I would use a 2 to 1 drive and pitch the prop to hold the engine
down to about 5000 RPM. Than would give you about 2500 on the prop. It should work good and want be as loud when
your turning the prop slower. On our RD2 Aluminum Mini, We made it 11 ft 8 inches long and 7 ft wide at the top, the
bottom is about 6 ft wide. It only sets about 1.5 inch in the water with one person and get's on plane easy in deep
water. It ran 35 MPH with one person. I hope this info helps. Let us know as you build your boat. Chuck

[Spyda-Ryda]

I agree with chuckitt, that set up sounds like it should work well for your application and expectations. Purchasing a rotax, gear box, prop, and accessories can get to be quite pricey. Although, if you do decide to go that route, I would suggest a 503 Rotax DCDI. In my experience this engine is very reliable and you can usually find them online fairly easy and for a resonable price. As far as a reduction goes, a B-Box is all you'd need. (2.58:1)