For me, it works very well as my props have about 8% to 10% slip and it's
verified by GPS. But then again I'm running a hi-performance boat over
100mph. The props I have are Mercury stainless steel lab cleavers. Most
average consumer props have alot 15% slip for stainless steel and near 20%
for aluminum. The calculation provides a good starting point when you want
to change pitch, RPM, mph, etc


"frosty" wrote in message
...
Barry asked:
Hello,
This has probably been flogged to death but I am going to ask anyway.

What is the formula for calculating boat speed?

From memory the propeller pitch, engine rpm a reduction for slippage
and a couple of other things are needed.

I did have the formula on the computer but a recent crash of the
system caused it to be "misplaced"

Many thanks..

Woodchuck responded:
http://www.rbbi.com/folders/prop/propcalc.htm

Good link, Woodchuck, thanks. Nice JavaScript app. A caveat, though:
You answer is only going to be as accurate as the "Propeller Slip" figure.

Their suggestion that it's "10 to 20% slower at top speed" has an error
margin of 50%(!) Of course, they go on to say, "This ratio is found by
dividing the actual boat speed by the theoretical boat speed..." Great!
So, if you start with the actual speed, you can calculate the percent
slip,
by which you can accurately determine the actual speed. *chuckle*

--
frosty (Circular Reasoning: see 'Circular Reasoning.')