Speed calculation formula
 

Prop pitch divided by gear ratio,divided by 12",divided by 5,280',times 60 minutes=Factor constant.
Factor constant times engine RPM=Gross speed (no slip)
Known speed divided by Gross calculated speed=% of efficiency.Subtract % of efficiency from 100 to find % of slip.
Example:35"prop pitch w/1.50 gears @ 4600 RPM=101.64 MPH,gross.If net is 94 MPH the efficiency is .9248% and the slip is 7.5 % +/-
The factor constant for the above example is .022096 as found using the above formula.Multiply the factor by any engine RPM to find gross speed,for the example combination.

Re: Speed calculation formula
 

The formula that I've used for estimating speed (theoretical speed) is as follows:

RPM X prop pitch x gear ratio x .00094697 = theoretical speed

Example, 5600 rpm x 26 pitch x.667 (decimal equivalent of 1.5:1) x .00094697 = 91.9 mph. Actual speed, 81.8 = 11% slip.

I'm not a math wiz, how accurate is this formula?

Kurt

Re: Speed calculation formula
 

that formula doesnt allow for 3 or 4 blade or hull wieght or dispacement but im still impressed

Re: Speed calculation formula
 

why would it need to? thats where the slip factor comes into play.

Re: Speed calculation formula
 

just use this courtesy of BAM..:)

http://www.go-fast.com/boat_speed_predictions.htm

Re: Speed calculation formula
 

Ttt

Re: Speed calculation formula
 

Try this one...
http://continuouswave.com/cgi-bin/crouchcalc.pl

Re: Speed calculation formula
 

I'm somewhat familiar with Crouch's formula for calculating speed but does anyone have Crouch's formula for determining the hull efficiency factor?

The hull factor of 180 in the above link is for a Boston Whaler. I think that a stepped bottom v hull would be about 220

Good stuff, thanks for the link!

Kurt

Re: Speed calculation formula
 

Mine worked out to about 215 by filling in the other known fields. Velocity 30 w/pad.