"Stephen Baker" wrote :
Don McPh says:

snip technicalities

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

;-)

Actually, I asked if a simple formula existed. I had a sneeking suspicion
that the answer would be no. I often recall the words of a colleague I was
working on a bank's data model with almost 20 years ago: "For every
complex, intricate & detailed problem there is a solution that is simple,
concise, to the point, and wrong."

I am pleased with the technicalities in the responses to my question as I am
always keen to learn. And I have now learned that thrust can only be
compared to HP if distance & time are considered.

I haven't quite reached full understanding of the responses and will think
on them more. In the meantime, I'd be grateful if someone who did have a
good understanding would be kind enough to do the calculations for the
particular case I have in mind.

The boat is a Monarch 17 foot trailer sailer. In cruising trim it displaces
2500lb on a waterline of 15'6"(L) x 5.4"(B) x 1'0"(D). On flat water 2HP is
adequate, 5HP would be normal, but if I ever expected to find myself trying
to motor off a lee shore into 30 knots I think I'd want 8hp. Lets use 5hp
for this exercise which equates to 3250 watts. Unfortunately makers of
electric trolling motors rarely specify wattage, preferring to specify lbs
of thrust. Trolling speed is 3-5 knots which is also the speed that the
Monarch would motor at so we're lucky there.

My gut feel is that we'd be looking at an electric trolling motor designed
for a 25-30' inboard launch. This is larger than the Monarch but the
trolling motor is not expected to push the vessel off a lee shore - the main
motor would be called in to play for that. I have no science to back up
this guess and do not know what thrust the manufacturers would recommend for
this sized launch but it feels about right.

So, If I've understood the technicalities, the variables necessary to
calculate the thrust required a

5 Hp at 4 knots. = ? lbs thrust.

Paul Squire.