effect of close clearance tunnels on gear ratio and wheel selection

[Dan V]

I´m looking for advice concerning the effect of close clearance tunnels on gear ratio and wheel selection. I was very dissatified with the performance of a boat I designed with a tunnel. I found research of the late Donald Blount and redesigned the tunnel as per his reccomendations. The boat is 32´ and about 14,000 lbs. Cummins 450C w/ 1.48 gear. Upon relaunch w/ a 22 x 26 four blade (medium cup), the top speed was 32 knots but top rpm was only 2350. Calculations show a slip of only 5%, so the tunnel w/ its .375" tip clearance was doing its job perfectly, but that restriction had to be hard on the engine. So thinking hydrodynamic friction, I started whittling away at pitch, then diameter and back to pitch again. I´m finally down to 20-1/2" x 22" and just am getting 2600rpm at wot. Performance sucks: top speed is 24k, 20k cruise at 2200 and I´m burning 15% more fuel than before. I think I have to go back to filling the tunnel w/ propeller to get the system to work as designed and will need to change gear ratios to protect the engine. How do I determine if I need a 1.79:1 gear, 2:1 or something different? Or is there something else I have overlooked?

Thanks,
Dan Varner

[baeckmo]

First: State rated engine power @ rpm, then main dimensions of tunnel, including propeller position.

[Yellowjacket]

Originally Posted by Dan V

I found research of the late Donald Blount and redesigned the tunnel as per his reccomendations.

I had a meeting with DLB a few months ago. He was on time for the meeting, so I wouldn't have characterized him as late.....

We all gotta go sometime, but I think your description of him as deceased is premature....

[daiquiri]

Gentlemen, I propose to keep this discussion in one place and not spread on several identical threads. This one is evolving well, so why not using just that one: effect of close clearance tunnels on gear ratio and wheel selection.

Cheers