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Theo, good suggestion.? There have been some lengthy discussions about this topic in several forums, but the bottom line is this: You want to know what size motor to use for your specific application.? There's not really any good way to standardize that with a formula as there are so many factors such as prop diameter and pitch, gear reduction, water line length, beam, hull shape, etc, all factors which are part of the complex relationship between power and speed and which should be considered when choosing your motor.? In my case, I eliminated my transmission and installed a reduction gear, and changed my prop diameter and pitch.? All of these factors are different for each conversion and all affect each other.? There's some math for sure, but there's also a lot of educated guesses and experimentation necessary.? What I did was look for conversions of boats the same size as mine and try to learn from their successes and failures.? If you can, get them to share some data with you.? Ultimately, the most useful formula will be an empirical formula for how much power in kilowatts?it takes to get their?boat to a certain speed.? If you can develop that curve for their?conversion, and if your boat is a close?analog to theirs, you will then be able to select an appropriate combination of motor, nominal battery voltage, prop diameter and pitch, and gear reduction needed to move your boat efficiently and without burning up your motor or overdesigning (and overspending).? I've considered putting together a website where we can all submit our basic?info and data to?generate a power?curve.? Alas, the older I get, the more it's about time.? Cheers and welcome aboard!??

Darin

On Mon, Jul 13, 2020 at 8:54 PM Theo B <theo.brillhart@...> wrote:

Hi all - first time poster. I think I have some insights to the calculator question, but maybe not a totally straight forward answer.

What's missing in traditional calculators like VicProp is the torque that the electric motor brings to the party. The electric motor can supply almost constant torque across its RPM range and near full torque from the time you turn it on. So there's a mathematical relationship that predicts this power discrepancy and uncovers the false assumption that electric hp can be treated like ICE hp.

Mathematically, horsepower equals torque multiplied by rpm. Or, torque of one newton-meter, twisting a shaft at an angular speed of one radian per second, requires a power of one watt. So power is torque times angular speed (with no extra constants or conversions if you work everything in radians and SI units). So for the same angular velocity (RPM) the electric motor has all of its torque available starting at zero RPM, an ICE does not, and will not until the RPM gets sufficiently high. Thus the exaggerated discrepancies seen at slower speeds.

Given enough time and brain cells one could likely develop a compensation factor for typical ICE hp, but it would have to be non-linear due to the non-linear torque of the ICE. I've seen a 5:3 hp ratio tossed around in the electric car communities, but this does nothing to compensate for exaggerations at slower speeds. Might get you in the ballpark though.