Cyril,
I have a 34" Fireboat, and have spent ages experimenting with the running gear, if this helps, or you get a better combo, let me know. I have tried 30mm and 35mm "x" props, ok performance, then tried 40mm brass 3 blade (11.5 amp draw), 45mm plastic 3 blade (less pitch than the brass prop, 8.5 amp draw), 50mm plastic 3 blade (11.7 amp draw.)
The amps where measured full throttle, 1 minute, holding boat, so probably not accurate, but a constant test. 2 x 8.4v (wired as 16.8v) 3300 nimh, graupner 12v 700bb turbo
The brass 40mm gives the best speed, as in the video, I am now struggling with the bow "digging in". With a good charger, this makes such a difference, I get approx 15 mins running almost flat out, with a couple of brief stops, which I thimnk is ok on this boat. I have just got 9.6v packs, at 4600, so I think I can maintain the speed, longer run time, and use the lower performance 45mm prop, that has the least amp draw. Hopefully will be able to test again at the wekend. The second half is the speed half!!
http://uk.youtube.com/watch?v=so5Ge9CNEV4Hope it helps,
Paul