torque Formula needed?

[bearwen]

Hello All,
Well I have a small question here goes.
what is the math formula to find the cranck shaft torque to turn a given size prop and pitch?

I want to be able to plug in the prop diameter and the prop pitch then plug in the gear ratio and next the rpm and be able to get the engine torque and hp required to accomplish the numbers I put into the formula. I am looking at a few new boats and well I am considering different power packages and want to be able to figure out what the best set up is to start with of course you will need to adjust but I want to start in a good area. The boats I am looking at are of the high speed go fast type. I realize that tunnel halls and hull design in general play a part but if I can say that B prop with C pitch and D gear ratio spinning at E rpm will require F torque then I will know roughlly what engine/engines I will require to get the boat to a certain speed then I will adjust a little to get reality to where the theoretical formula is.

Thanks all Bearwen.

[stonebreaker]

You can't calculate the torque required to turn the propeller without knowing how much drag the hull creates at a given speed.

http://www.alberg30.org/maintenance/...r/propcalc.xls

[Jango]

Take a look at "Crouch,s" Formulas at:

http://continuouswave.com/cgi-bin/crouchcalc.pl

I have a shallow Vee, very fast (80+mph) runabout and the (218) Hull Factor works for me.

Keep in mind the HP given is "At the Prop" is typically 75% - 90% of Flywheel HP - depending on gear set-up and engine driven accessories. I.Os are usually closer to 75 %.

You can leave the HP blank, fill in the other boxes including desired speed to obtain required HP

[hartley]

See if you can get hold of a copy of ''Propeller Handbook'' by Dave Gerr ,a wealth of information is contained in this book ,and it will steer you in the right direction,it also contains a chapter on Crouch's propeller method
.....cheers Hartley