Electric Propulsion

I posted about this a few days ago in the midst of my anger about the Atomic 4 motor struggles. I'm strongly considering getting rid of the gas engine that is currently in the boat and replacing it with an electric motor. It's becoming more and more common to have boats swap out their gas and diesels for electric motors. The problem is that it's typically $5k+ to do because of the newness of the technology and markups. I'm in the process of doing research and doing some design and it seems like I may be able to do a conversion for more like $2k conservatively which would pay itself off in a few years.

The plan would revolve around purchasing a 5kw brushless DC motor like the ones you can find at goldenmotors.com. They come straight from China which means cheap. The motors run on 48v DC which means the RPM will be between 2000 and 6000 RPM. That's about double what my current motor churns out and would cause cavatation and lack of power. To resolve this I'm talking to golden motors about building a custom motor that spins from 800-2500rpm. I need to weigh the premium against the cost for a 2:1 reduction drive. I'd rather just go direct drive as it's simpler and eliminates a failure mode in the future.

Once the motor is sorted out I need to figure out batteries. These are the hardest part for me to wrap my head around. I need to balance cost, capacity and reliability. Looking at the 5kw max draw rate I figure I'll need about 100AH of 48v power to get me about an hour of run time. This is quite a bit of battery, and therefore the type of battery matters. At the moment I'm leaning towards using AGM batteries, although the technology is older and they discharge worse and are heavy they are inexpensive. I think that since I'm removing about 500lbs of weight I'm ok in the heavy category. The downside of AGM batteries is they can only be recharged so many times before becoming useless, meaning in a few years I'll need to buy new. Alternately I could use a lithium battery, these are VERY expensive but are also relatively light and can be recharged and discharged very deeply many times. Given my tight budget constraints I'm thinking of going with AGM, I can always upgrade in the future.

One of the best parts of going with an electric motor is that the batteries will be charged in three possible ways. 1) if we get a slip next year I can hook them into shore power keeping them topped up. 2) I can hook them into my already sufficent solar cell to help keep them charged and 3) every time the boat is sailing the propeller can free spin which will charge the batteries!