I have managed to confuse myself again (not that hard)
I was looking at the graupner speed 600 as they are 1/2 price, these are going in my twin engined Huntsman (approx 34 inches)
it has 540s in it at the moment but these are starting to smoke a bit (probably due to the 12 volt battery I am running them off)
the boat doesn't really run right (another semi built off ebay job) anyway.
then I found the list of details at
http://homepage.ntlworld.com/edward.matthews/ddmbc_files/motors/graupner.htmand have managed to totally confuse myself.
If this is to be believed a speed 600 rated at 7.2 volts has a no load rpm of 18,200
but the speed 600 rated at 8.4 volt has a no load rpm of 15,500 so turns the prop slower, but at a higher Max Efficiency,
these both turn faster than the graupner 700bb turbo that some have said I should use in my mfa spear.
so what is more important on a hull that you wish to have up on the plane High RPM or High Torque?
the spear didn't plane with a very high torque but low rpm motor or am I just making it to complicated?