thanks all for help, its not so much a new build, but a redevelopment really. I have almost finished a big refurb of a 42" Huntsman 28. On the plane it looks great, I am trying to optimse the speed/run time. The current set up is as listed at the start of the thread. After having a long conversation with a rc good battery supplier, I will opt for running 2 x 9.6v (as 19.2v) 4600 mah packs. He has calculated that my run time, excluding the variables such as "is the drive line perfect", and "is the prop the most efficient", that I would actually achieve a better run time with 2 x 8.4v packs, as the motor will not be sucking as much current, however if I can resist the urge to run at max power, and settle for a bit less, then the run time will increase,as I will not be using max volts, 17 to 25 mins hopefully, which would be very acceptable.
So before I commit to the batteries, I want to experiment with the part that I do not understand, the propshaft, and try to minimise the drain, whilst retaining acceptable performance. Up to now, I have been swopping the props around, giving the boat a quick run, and if it looks faster keeping that prop!. Now its time to see what amps the prop is drawing, as the fastest may not be the best, if you know what I mean!
I suppose I am more interested in the difference between the readings with different props, not so much the actual reading, unless it was a lot different from that stated by graupner, at 12.5v max efficiency .
Malc, batteries are fully charged, 2 x 3300mah 8.4v nimh as 16.8v, 10 to 14mins run, mostly at flat out, but I think I could do better. Tried 3 x 6v (as 18v) lead acids, died after 4 mins!