Formulae etc.

[edgewing]

Hi Again Folks,

I was wondering if anybody knew where I can get hold of various boating formulae. eg. How to calculate speed based on prop pitch, revs etc. I know there is software out there, thanks to DaveR, but I would still like the basic maths in behind it all.

Thanks

[Raceman]

If you want the basic common sense formula it's:

RPM divided by gear ratio X pitch X 60 (minutes in an hour), divided by 12 (inches in a foot), divided by 5280 (feet in a mile) X estimated factor for slip. Many people use .90 as the slip factor.

Example, using 6000 RPM's, 1.87 gear ratio, 26" pitch:

6000 divided by l.87 = 3208.56 (this is actual prop RPM)
3208.56 X 26" = 83422.56 (inches per minute at 100%)
83422.56 X 60 = 5005353.60 (inches per hour at 100%)
5005353.60 div. by 12 = 417112.80 (feet per hour at 100%)
417112.8 div. by 5280 = 79. (miles per hour at 100%)
79 X .90 = 71.10 (miles per hour with 10% slip)

There are several ways to combine the values into factors, but then you've got to remember the factors. If you think about and understand the above formula, you can always work it out without memorizing any numbers.

Here's a factor:

RPM/gear ratio X pitch X .000947 X slip.








[Edited by Raceman on 06-27-2001 at 06:25 PM]