Or better yet, a general rule of thumb for boat length, width, weight vs. horse power to get a ball park speed estimate? I know boat design makes a big difference but I just need rough guestimates.
Bubba..... As a matter of fact there is a fairly accurate formula that was used here. I believe JB himself introduced it and it actually is fairly accurate providing you have the ability to do the math.
Here's a simple Formula I got from an old boating handbook that comes amazingly close:
WOT RPM- divided by gear ratio X prop pitch -1 = speed -15-20% slip = Top Speed
I/O
Ex: 4500rpm divided by 1 1/2 (gear ratio) = 3000rpm X 19 pitch -1 (18) =
54mph -15% = 45.9 actual speed
Outboard
6000rpm Div. by 2 (gear ratio) = 3000rpm X 19 P (18) = 54mph -15% = 45.9