1. If you are wanting to know what a different prop will do for your speed.
Most know that a 2" pitch change will be about 300 rpms.
2. If you want to know how accurate your speedo is. I have found that most
aluminum props are about 85% efficient. Performance SS is about 90%. A
High performance prop like a Merc Lab, Mazco, etc.. are about 92%.

These are just examples. I use the formula all the time. When someone says
they have a 150hp engine that is turning 5500 rpms w/ a 22 pitch prop and
running 80 mph you know they are lying. 61.27 is theoritical and w/ a good
prop at 90% they would be at 55 mph.
Now a lot of speedometers will show 80 mph. Without a GPS the formula is
the only way to really know.

--
Tony
my boats and cars at http://t.thomas.home.mchsi.com

-
"Peter Aitken" wrote in message
om...
"Woodchuck" wrote in message
...
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

No one has asked the obvious question - why would anyone want to calculate
boat speed this way?

--
Peter Aitken