hi all. im building a 40' cutter, 18,000lb disp from a bare yorktown 39 hull. will only have an electric drive for maneuvering (on and off a mooring, in and out of the slip etc.) im thinking 4 hrs at 4kts max capacity. id like to assemble a simple and reliable system from found/re-purposed components and because the boat is wide open i can be very accomadating and build the boat interior around the drive/batteries.
hoping someone with more experience can ballpark the big components of such a system. ive already got a second generation 48vdc motor from a kevit ecar that i hope to use, but maybe its not appropriate? how much battery capacity? source for gearing? prop and shaft advice? the list is long...
im in san diego county if anyone is nearby, im a full service boatwright and carpenter if someone wants to trade services.
thanks in advance
With 9 tons of displacement, you need around 9000 watts or 12 continuous hp, if your motor and controller can put out 9000 watts for 4 hours you will be fine. You will need at least a 48 volt 750 AH battery bank to reach your 4 hour run time at the full 9000 watts.
I dont think you need 9KW to get 4 kts. I`m pretty shore you can hold 4kts with about 2KW in ideal conditions. 9KW is more than the output you get from a 9,9hp outboard motor.
A previus sailboat i had got 5,5kts with the Honda 2.3hp at max. Thats about 1500W..? Though boat was about 4 tonns.
I read other people talking about 30amps @ 48V to get 4kts in their sailboats. Thats less then 1500W.
But i will know more about this when i finnish my own conversion in a 8 tonn motorsailor. I`m 80% finnished, just missing the gearbox.
When it comes to gearbox i found that James Lambden who is a member at this forum have done several innstalls with the Browning 3301 gearbox with thrustbearing. It can do 9,42KW@1750rpm, it should be plenty. I`m trying these days to order one, but its christmas.
Your Kewet motor is about 7KW i guess. It should work .
I will go with 4/1 ratio and get about 750rpm on the prop at max. The bigger prop and slower prop speed is in theory the most efficent. Browning 3301 comes from 1,2/1 - 5/1.
For extended driving I will use a generator and the charger so i dont need to many batteries.