RCU Forums - View Single Post - Help sizing props to motor, voltage, amps, ect.
Old 01-19-2021, 03:42 PM
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Default Help sizing props to motor, voltage, amps, ect.

Howdy. I'm in the early stages of attempting to build an electric kayak, so this isn't really a question about RC stuff, but I will be using RC components and I want to go a lot faster than what a trolling motor will push me to, so I figured you guys might be able to help me get a baseline on the setup.

I've yet to build the hull, but it will be a planing vee hull with an OAL of 8' and a beam of 3'. I'm not sure what it's going to weigh, but the bare hull should be under 30 lbs. Call it an even 30 for the sake of argument. I weigh 180 lbs. and have yet to determine how much battery I'm going to want or need, but I think the all up weight will be around 250- 275 lbs.

Due to the limited number of props out there for something of this scale, my choices are limited, but have settled on 100mm props with a PD ratio of 1.6. I can't post a link yet, but if you go to aliexpress and search for "100mm pitch 1.6 prop" you'll see what I bought. I got a counter rotating pair and I won't lie... they're much stiffer and much higher quality than I thought they'd be. The blade form has all the right elements- reverse rake and sweep at the entry of the blades leading edge, decent radial cupping to keep the water on the blade, and a generous amount of cup on the trailing edge. Chucking a shaft into a drill and spinning the prop at 2000ish RPM in the water makes a considerable amount of thrust and I feel that two of these will be enough prop for what I want to do.

Where I'm stuck is which motors to use. Before I build the hull, I'd like to get the propulsion sorted out so there will be more experimenting, but I'd like to be able to spin these things up to 10,000 RPM. Purely guessing based on no experience whatsoever, I tend to think that a direct drive 360KV 56104 motor at 100v and 95A would get me close but before I drop the money, I wanted to run that by you guys. If they don't hold up to 10K RPM, I'll need to look for metal alternatives and will cross that road when I get to it.

I did do some searching before signing up to post and didn't see anything that would help me, but if I missed it and this sort of thing has been covered, just tell me to search and I'll go dig deeper. I appreciate it and if anyone wants me to, I'll make a build thread as I get closer to beginning work.