Nick. This motor is capable of running at 19V or so, according to the quick surf I have had. Putting your batteries in series or getting a 12v pack or 3s 11.1v Lipo would make it go a bit better! You are not getting many revs on the 7.2V pack with this. How fast do you want to go?? Some will advise fitting a brushless/Lipo, but this entails more expense. Do you have other boats, and if so are they all Nimh and brushed??
A change of prop to twin blade 35mm may help. In fact, these props are so cheap it might be interesting to get a sleection…. one each of S30,35, S40 and The correcpondinbg X pitch ones same diameter.
Ashley