For a 40', 17,000 lb boat, he would need at least a 5KW generator, but probably a 10-12kw generator would be the correct size.
I sailed from Cape May to Southern Chesapeake with a friend who had a similar size boat, with an electric motor. We had a Honda 5kw generator. We ended up sailing and motor sailing mostly. The generator could not put out enough power to motor at more than 3 knots for a long period. We would just use the motor to gain a knot or 2, and keep the draw at around 2-3kw.
He ended up getting an 8kw generator that directly charged the batteries, which is more efficient, then converting DC->AC->DC.
Agree that it is a sane idea to have such a backup. Particularly if the boat is your home and nest. You will need then an AC to DC charger that will add $$$.Alternately you could construct one from say a small Honda motor and a a rugged off road car alternator plus a DCDC charger. A 3kw motor should drive a 200A (14v) alternator. Say use a 250Amp alternator to drive gross charge into batteries. I am sure one of the subcontinent youtube videos will show you how to rewire an alternator to give higher voltage. ~48 I can see happening without too much strife.It will be interesting to see a 72v alternator built on the body of say a 300W car alternator.. Easy to make rectifier using BIG diodes. Indeed someone could cobble up a synchronous rectifier using 300v 200A MOSFETs. Add some current sense and code running in an Arduino and you have a monster charger system. Water cooled of course. Possibly there is an app note on building a 13.8v dc to 72vdc inverter with 40A output.I have been watching how it is done in "Sailing without a motor": https://www.youtube.com/playlist?list=PLARfxaCYDjDygPQrWVZEqc334JXh0UiTK I see what he does and has done but I feel nervous when he goes near hard things like bridges, rocks and other vessels.
On 3 Jun 2025, at 10:11 am, Scott E Erdman via groups.io <seerdman=syr.edu@groups.io> wrote:Not what the original intention is but a useful backup and maybe needed for some circumstances is a small gas generator- I have a Westinghouse igen 2500 - will help compensate for situations where motor is the sole option (rivers, currents, straight upwind with no option to tack). I keep mine in the starboard locker.
On Jun 2, 2025, at 3:52 PM, David E Marsland via groups.io <david.marsland=gmail.com@groups.io> wrote:
I installed an Electric Yacht Quiet Torque 10 KW motor for my Catalina 27. This would probably not be nearly big enough for your boat. 20 kW sounds like a better bet. It's also a 48 volt system. It's worked out great for local sailing with two 5 kilowatt hour LFP batteries in parallel.
For cruising I'm going to add another 10 kWh of LFP batteries.
The one thing I would caution about going higher than 48 volts DC is that it's very difficult to find electrical components such as switches rated above 48 VDC at high current. Even finding 48 VDC switches is challenging. Many top out at 32 volts.
I charge at the dock and have solar for backup. Theoretically it will regen when going over five knots under sail. Unfortunately I'm going to have to change the propeller to make that work.
All the best,
David
I have researched electric motors for boats a number of years. MY choice for my 32' Buccaneer, which is still being restored, is the Electric Yacht QuierTorque 20.0. It's 48 volts, 400amps current. Maximum power input is up to 20.0 KW, it will accommodate 3/4 to 1.5" shaft size, it weighs 135 lbs. It has two motors in tandem that you can cruise with one motor or both, depending on your speed and current and sea condition. 7.1 knots is my boats hull speed. 6.0 knots can be supported by a 11KW 48 volt gen set. At that speed, it takes 190 amps and 9120 watts. The motor will fit in where the engine would sites on the runners. The cost is about $9K. I think it is the most reliable on the market but that's just the opinion I've developed over the years. Good luck!Dale
On Mon, Jun 2, 2025, 4:25 PM Kevin Pemberton via groups.io <pembertonkevin=gmail.com@groups.io> wrote:
You might want to visit sailing Uma. It is a YouTube channel. They started with a golf cart motor.
On Monday, June 2, 2025, cammcnally10@outlook.com via groups.io <cammcnally10=outlook.com@groups.io> wrote:
Hi everyone,I am restoring a C&C 40' sailboat on a budget, and I am planning on replacing the engine with an electric motor. I know some of the general specifications I would like, including Brushless DC, 48V, and 12 kW. Does anyone have experience or advice on repurposing motors from golf carts, forklifts, or other similar vehicles? I have briefly searched eBay and online motor catalogs.Any insight would be much appreciated.
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