What is the minimum and maxium rpm I need for twin motors in a 36" tug running near 3" props with korts?I already learned that more turns mean more torque,but you must need a certain rpm to go from low to full speed and have the grunt for pushing or pulling things.I've seen on other forums that the little Springer boats using a stock motor can push loads that seem impossible for their little 8"x18" size.

Grunt is not a easy number of pounds of push or speed when heavily loaded or lightly loaded.

How deep & wide is the waterline ? A couple of pictures with the ruler along side would help.

My old Artasania 40" tug with 1 large, 6" x 2" Pittman servo motor on 12 v, with a 3 : 1 timing belt reduction, driving a Octura plastic 2 blade 2.8" x 5.0" prop can pull a Aluminium fishing boat in no wind.

Rich

And another way of saying that is it's a matter of how much water you can move with the motor/prop. Moving lots of water slowly means a slow start and lots of pre-thinking of stopping. Being able to move lots of water faster means a faster start and still worrying about stopping the @#$ thing. If you can juggle the size of the prop, it's pitch, the amount of torque the motor develops and at what speed (rpm), then the size of the battery(s) needed to supply that power, you've got it made. If that wasn't hard enough, you then get to consider the shape of the hull, it's displacement, and I don't know what else into the equation. (Having fun yet?) There's no 'one' single answer, just depends on all the variables. There's only three ways of finding that 'best' solution that I'm aware of. That's by doing the experimentation, copying someone's results of experimenting, or by doing all the "brain sweat" needed for that equation thingy. So, what'cha gonna do? (I have no problem copying someone else's solution! But I'm lazy.) Wish I could give you -the- answer, but...
- 'Doc

And completely off topic... would copying someone else's solution then make you a 'bandit'??

I have a Darby with twin Marx Hectoperm motors with 4 to 1 pile gear reduction. I bought them from Dynamic models way back when. It will push/pull....

My "Rule of thumb" for gear reduction is for every inch of prop use one rpm reduction. 4 inch prop, 4 to 1 reduction. It seams to work well without generating alot of heat.