The stock prop is a plastic version of an x440/3. I've used an x445 and it runs very well. I haven't tried an x645 because we are trying to limit the props to plastic props. The speed secret to this boat is prop choice and prop tweaking. Sine prop mods are a black art that few have perfected, using plastic props helps to level the racing field. If your friend is not interested in organized racing metal props rule - but run too long and you can overheat the motor, ESC and cells. Racing is easier because the race time is limited. Sport running can place bigger demands due to longer run time......
We have found that running the strut deeper than stock - around 1" down - helps a lot as speeds increase. It should be almost level toom, depending on the prop. Like Brian said, block-sand the sponsons bottoms flat and true, and leave them rough from 400-grit paper. Don't polish them.
Some have had problems with the water jacket leaking. Get an aluminum jacket from Offshoreelectrics, bullet proof.
All the power in the boat comes from the cells - top end cells give more speed, although cheap cells are not too far behind. I'd use 4000-5000mAh packs just to maximize run time, but get a minimum of 3300 mAh.
I build my air dams from pool noodle foam or vinyl tape and fuel tubing. Cut a length of large tubing just long enough to fit across the tunnel and tape it down. That will work for most applications.
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