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## A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding

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Re: FW: Barrels was DR thread from Nov-Dec '04
From: Bill Noyce
Date: 2005 Feb 2, 08:44 -0500

```All the formulas that are based on (old/new) (possibly raised
to some power), or the log equivalent (log old - log new)
(possibly times some factor) are insensitive to the units
used -- tonnes or grams, knots or meters/day, etc -- as long
as old and new are measured in the same units.

The formula for rpm->speed has the magic numbers 60
(presumably minutes per hour) and 6080 (presumably
feet per nautical mile), revealing that pitch is
measured in feet per revolution.

As a programmer, I would add parentheses to this
formula so the reader doesn't get confused:

rpm = 6080/((pitch/(1 + % slip)) x 60) x speed in Kts

and might even reorder it (without changing the meaning) to
rpm = 6080*speed in Kts * (1+%slip)/(pitch*60)

A mathematician might notice the RPM formulas are related by
the approximation (1+%slip)/(1-%slip) = 1, which is valid
for reasonable (small) values of slip, to the precision we
need.
-- Bill

```
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