here is the combo that WILL get you to plane, its tried and tested in my 42" Huntsman 28, currently being used in my Fireboat whilst the Huntsman is being refurbished.
Graupner 700BB Turbo, 12v. This motor will go up to over 20v, get to the max nominal voltage, water cooled
2 x 9.6 4600 nimh's, wired to double your volts, however I dont know if your esc will handle this, mine will
3 blade 40mm brass prop, tried all the other sizes, 2 blade, "x" props etc etc . I dont think I have finally sorted this yet, I think I can go a bit bigger, but this prop on the slightly shorter Fireboat gives longest run time, best speed, its a play off really, I set 15 mins flat out running as a benchmark, and achieved almost 25mins tonight, A good charger is esential!
The rudder was causing a nightmare, and was making the bow submerge on fast turns. Thanks to help of 2 other mayhemers tonight its sorted beautifully. I cut almost all of the rudder off on the prop side of the rudder, and some off the side at the rear of the boat (looking side on) making it more like a racing boat rudder. I have infact cut too much off, but at least I now know the theory. Unfortunately, the play off this time is low speed turns are now the turning circle of the QE2 on the Mersey!, cant have everything, hence I know too much is cut off
To maintain a "scale " plane, the bow was coming too far off the water, moved the batteries more central.
A similar performance, but no more than 15mins run time was with 2 x 8.4v 3300 nimh's wired to double volts, these packs wher too long to have more central.
A video of the boat running pre rudder improvement is in a thread "major problem turning boat" 2nd half is the speed you want, on the 8.4's, now its faster on the 9.6's, and turns beautifully
Hope that helps,
Paul