Don’t always believe prop/speed calculators, they ignore too many factors. That prop is far too small for a 51” boat, it’s efficiency will be very poor with its very small thrust cone and high slippage. A 20% slip estimate is way too optimistic. That prop is suitable for a 30”-36” boat, not a 20# cat over four feet long. The OP should have saved $$$ and purchased a much smaller motor for that tiny prop, that big motor needs a load to run efficiently.
Even a 25% larger 50 mm prop is pretty small. The loaded rpm with that motor and voltage will be around 30,000. Much more realistic performance figures would be:
X450 = 57 mph
X455 = 63 mph
These values assume high quality packs and a well trimmed boat, and are of course estimates. But they are based on plenty of experience with large, high voltage setups. A motor with a lower Kv would have allowed larger, more efficient props to be used, and run on 10S the amp draw would be lower for comparable speeds. Actually, since he has not yet purchased packs he should consider running 8S2P, the more efficient props would give more speed than 1S more voltage. If he has the motor already, that’s what I’d do.
The OP’s 9S setup can work for his speed requirements, but he will need to watch the heat in the motor, ESC and packs if he does much extended running.
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