Is there a formula

[Bubba1235]

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.

 

[tashasdaddy]

Re: Is there a formula

i've used one that was online, it was not accurate. i was suppose to be doing 90mph.

 

[Bob_VT]

Re: Is there a formula

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.

This is a link to the post and look at JB's responses http://forums.iboats.com/showthread.php?t=4747&highlight=theoretical+speed

That formula has no regard to the type of prop. Many of the prop calculators can be close too.

 

[stubblejumper]

Re: Is there a formula

http://hhscott.com/evinrude/docs/BSLAC.htm

 

[MikDee]

Re: Is there a formula

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

Simple, Accurate