Oh, one more thing regarding prop choice. You could do far worse than to simply size the prop to the max you can swing under the hull. Figure on having a blade/hull clearance of 1.5" or 10% of prop diameter, whichever is greater. Let pitch be somewhere between 2/3 and equal to the diameter. Maximum prop diameter will let you operate at lower RPM, and both lower RPM and diameter positively affect the prop's efficiency. Higher pitch can contribute to cavitation at low speed through the water. The lower the boat speed, the lower the optimum pitch. Once prop choice is hammered out you can then select a reduction ratio that will properly load the motor, and if you use belt drive this can be adjusted by changing pulleys. You should have a target max prop RPM and if the motor is powerful enough to achieve this, you want max power setting used to give you the optimum motor RPM, too. This can be found easily in the V/RPM constant of most BLDC/PMAC motors and at 48V for most of this type motor this will be probably between 2400 and 3200RPM. You MUST monitor current during initial sea trials, and motor case temperature. Many motors have internal temp sensors that you can connect to most controllers but you also need to KNOW the motor temp at the highest power level that you will be using.
With electric, it is all about the batteries, on the one end, and all about the prop, at the other end. Everything in the middle is adjusted to make the two opposite ends play nicely together.
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