How do you figure the thrust you need for the trolling motor and the length.

For the shaft length, you need to measure the distance from where you are wanting to mount the motor to the waterline. Add 18" and round up to the next foot.

So, if you you measure 36 inches from your bow to the water line then you would add 18" and round up to the next whole foot.

36 + 18 = 54" round up to the next whole foot measure and that would be 60" shaft.

As far as thrust, I dont know a formula but, I have a 18' bay style boat and have a 65# thrust 24v and have wished I had more power on a couple of occasions but, it's adequate 99% of the time.

Go to www.minnkotamotors.com and in the top right hand corner
you'll see a tap labeled 'select motor'. Click on it and it will step you throught the selection process.
Okie Bob

I did the math and then asked around. Everyone said get the longest shaft and most power you can. Well I have a Lund Pro Angler 16 and I bought a 55#, 12v 54" MK Autopilot, and a 55#, 12v Vector 3x. They are just fine based on calculations. Fishing in wind where your bow mount comes out of the water is a booger, but I just use my transom mounted Vector and back troll in big waves, works mucho better.

I think I should point out ... 99% of the time my shaft is long enough, I do have the collar up quite a bit. When it gets windy like that I am usually on my way back to the boat ramp. Unless I am catching crappie, then I back troll and get wet.