"tony thomas" wrote in message
news:kFyfe.67380$c24.3956@attbi_s72...
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.

--

I understand, but it still seems to me that the prop slippage is the real
unknown. If you change props then your top RPM will change too, right? If
you put more or less weight on the boat, or clean the hull, then things will
change too. With handheld GPS so cheap it seems strange to me to use an iffy
formulas to calculate speed.


--
Peter Aitken