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Re: Electric Sail Boat Motor
Sorry for bumping this old thread, but it seemed a shame to start a new one on the same topic!
I'm looking at installing an electric motor on a 32ft 9,500lb yacht which currently has a (horrible) 18hp diesel. I've read through all the pages on this thread, and there are clearly lots of electric powered yacht owners for whom it works really well. It's also been said several times that 'the maths might not add up, but electric motors work in the real world'. I'm currently in that situation. Let me explain!! (and sorry in advance for a big lump of numbers).
Most of my sailing is coastal cruising. This would involve perhaps 1/2 hour of motoring per day (15min in/out of harbour) at 3-4ish knots. Sails up ASAP, I hate motoring! I reckon the boat would need a 10kW motor, and it would probably use 5kW to reach these speeds. Electric Yacht produce a motor which would do the job. Clearly, there is no problem at all in getting this range (and a lot more) from a sensibly sized battery bank. The problem comes in recharging the batteries. I don't really like being a sardine in a marina (who would choose one over a secluded anchorage or a quaint harbour?), I'm on a swinging mooring, and enjoy being self sufficient. Shore power every night is therefore just not a guarantee when out cruising. Using the motor at 5kW for 1/2 hour every day means, then, that I will need to generate, on average, around 100 watts to recharge the batteries (5000 watts divided by 48 half hours in a day = 104). I prefer not to work in amps, it gets confusing! At 15kt, the average wind generator produces 50 watts. The problem is... most harbours or anchorages don't have 15kt of wind, or even close. The wind generator will only really work out on the water, 8 hours per day... in other words average 17 watts. 6 wind generators is clearly not an option! Solar panels... at best, 8 hours of sun per day, and from memory solar panels normally put out about half their rated capacity. To average 100 watts, then, I would need 600 watts worth of solar panels. Not an option either! The same goes for towed generators, 100 watts output but only for eight hours per day.
If I have a realistic 1 wind generator (50w for 8hr), 1 towed generator (100w for 8hr) and 200 watts of solar panels (100w for 8hr), I would average 83 watts output in a normal day. This doesn't even make up for the power use by the motor, let alone normal domestic consumption (and no, I don't have a TV or anything silly!). Basically, with a fairly impressive array of generating stuff, the battery charge would have dropped to risky levels after a few days, especially when relying on it for propulsion - and I would have to find shore power.
Will any electric yacht owners either back up or dispute my maths? It would be great if it could work, but suspect that electric motors are best either for marina-hopping, or for people that spend days at sea and therefore have plenty of time to recharge batteries - not the average sailor who goes coastal cruising and often finds themselves without shore power. But I would love to be proved wrong! Maybe regeneration with the motor when sailing fills the gap? Maybe I underestimate wind/solar? Overestimate the power needed to push the yacht? Look forward to your replies!!