Speed calculation formula
#1
Gold Member
![](https://www.offshoreonly.com/forums/images/icons/gold_member_star.gif)
Thread Starter
Join Date: May 2002
Location: DFW
Posts: 1,127
Likes: 0
Received 0 Likes
on
0 Posts
![Default](/forums/images/icons/icon1.gif)
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.
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.
#2
Registered
![Default](/forums/images/icons/icon1.gif)
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
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
Last edited by Biggus; 07-11-2006 at 08:48 PM.
#4
![Default](/forums/images/icons/icon1.gif)
Originally Posted by GOODT
that formula doesnt allow for 3 or 4 blade or hull wieght or dispacement but im still impressed
#5
VIP Member
![](https://www.offshoreonly.com/forums/images/icons/prop.gif)
![Default](/forums/images/icons/icon1.gif)
#7
Registered
#8
Registered
![Default](/forums/images/icons/icon1.gif)
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
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