Re: THRUST POWER

[Subscuba@aol.com]

Steve,

I looked into the horsepower rating and found, according to Minnkota, that it
is largely  marketing bull----. What I was told was that they took a certain
size boat with an X horsepower gasoline outboard and measured how long it
would take for a set length of distance. Then they took one of their trolling
motors and said that if the trolling motor took the same time as the X
horsepower outboard then it was rated as that horsepower. This is a load of
crap! I wish marketeers and Lawyers would stay out of the technical arena. 

The  Minnkota motors now usually use an electrical strain gage to measure the
actual thrust in pounds which may be better or at least it is more repeatable.
The motor I am using from Minnkota is their 70 pound thrust. The motor ratings
are 24 volts at 45 amperes, that's 1080 watts. Bear in mind that when they run
the tests the motor is powered by a voltage of 13.8 times two (27.6 volts) to
help marketing. One horsepower is 745.7 watts. Thus my motor has an input
power of 1.448 horsepower baed on the ratings. The motor efficiency according
to Lake Motors who make this motor for Minnkota is over 90%. The motor then
has a shaft horsepower rating of 1.3 horsepower. Then you have to consider the
actual propellor efficiency and that's where I draw the line. I've been told
the three bladed propellor is more efficient but the Minnkota motors have a
two bladed weedless prop. What I decided to do was to get the biggest single
motor I could about two years ago and put it in my Sub. Based on that
information I don't know if a clear relationship exists between horsepower and
thrust. My motor on a static basis produces 70 pounds of thrust for 1.3
horsepower. I do run the motor at the higher voltage.

When you look at the human powered Sub literature you can get an equation from
some guy named Pool that relates, on a conservative basis, speed versus Sub
size and horsepower. I used his equation and found my predicted Sub's speed at
a little over 5 knots which is sufficient for me. I hope he's right.

Good Luck,

Ken Martindale