My 42" Huntsman 28 has a graupner 700 bb turbo, 12 v,2 x 8.4 3300 nimh's, wired to double voltage, as the nominal voltage goes up to 19.2 volts. I originally ran a 12v 3300 nimh, going up in voltage was a massive improvement. Luckily I have a water cooled ripmax esc that can handle 19.2 v.
THe prop is like the ebay one, 35mm, "s" blade. I have tried "x" blade, as small as 30mm, as big as 50mm, both 3 and 2 blade, the 35mm and 40mm seem to be the best, the 35mm I think is just bordering on too small, as someone told me it was on the verge of cavitating. Its a heavy old boat, all ply, it flies on this combo, right up on the plane, but I only get 12 to 15 mins.
A second set up was graupner 9.6v speed race bb. Very amp hungry motor, but at 12v battery pack was almost as quick. I am no expert, but in the testing I have done, you have to push the voltage right up on these graupner motors, then they really come into their own. I'm off to Ellesmere show sat to get a battery pack made up, to 16 cells, 2x 8 packs to mount either side of the prop shaft, then I am at the max of the esc and motor! It will prob catch fire! ( then my aerokits Fireboat can come out!)
Hope that helps.
PS....I think Graupner are at tyhe Ellesmere show, perhaps if you can get there they can help?
Paul