Stephen:

In this biz, that's as simple as it gets...

We have worked for many of the small trolling motor and propulsion equipment
companies, so here's some more insight into trolling motors.

1. Trolling motor drives are typically less than 1 hp. One of the most
powerful is Minn Kota's e-Drive at 2 hp (see www.minnkotamotors.com and look
for the motor spec chart).

2. Thrust figures are indeed for maximum thrust at low speed (as one would
expect from their principal market - recreational fishing). These products
(including the propellers) are not typically designed to push boats at
higher speeds.

(As an aside, we designed a custom prop for one of the mid-90's AC
syndicates to use in side-by-side testing. They wanted to match drag figures
as closely as possible, so they were looking to add some 30-35 lbs thrust to
one hull. The existing trolling motor props lost all of their thrust as boat
speed increased, so we did a prop with much higher pitch to get a better
speed-rpm-pitch match where they needed to be. Who knows, there might be a
nice market for replacement props with higher pitch for small boats.)

3. Unlike gas or diesel engines, these electric motors have their highest
torque (and power) output at lowest RPM. This further complicates the
ability to run at higher speeds, since an increase in revs (associated with
getting to higher speed) doesn't buy you more available power.

4. If you need to get over 2 hp into the 5-10 hp range - and you've got some
deep pockets - you can consider the Briggs & Stratton gas outboard or a new
elec outboard from eCycle (www.ecyclemarine.com). There are other elec
outboards on the market, as well.

Regards,

Don

"Stephen Baker" wrote in message
...
Don McPh says:

snip technicalities

Don, The OP asked for a "simple formula".......

;-)