Larry<br> Your absolutly correct about the speed
for you hull. I was only picking 8 knots as an
example. For your boat we should use a number around 5.8
knots or say 6 knots. The calculations are based on
Gerr's propeller handbook and you are also correct the
hull configuration is a major factor. That is where
some educated guess work is first used since the
"SLIP" of the prop is a factor of you hull shape and the
screws RPM for a given speed. But in your case it we can
determine the slip. We know that a 12 pitch prop will try
to move throught the water at a specific speed
depending on it speed of rotation the difference between
the threoretical speed and the actual speed is the
slip. If you could measure the shaft speed at a given
known speed say 6 knots we could then calculate the
SLIP. If you don't have a tachomoter then you can get a
good estimate by measuring the motor armature voltage.
we know from the manufacturer that at 36 volts the
motor should be turning at 2100 RPM's and the
relationship is linear at 24 volts the speed will be about
1400 RPM's This is not exact and it will vary with
load but it is within 5 %. If WE know the slip then we
can calculate the ideal gear ratio to get the motor
to max speed at your most effecient speed.
