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Oops.

Upon further consideration, it appears that the new 210A alternator I got may not be appropriate for the boat. They suggest that you need to run 2 belts to drive the unit, which I may not be able to do.  (I’m not certain if I really need 2 belts, though - my batteries will ‘only’ take 120A, a load that’s more comparable to the smaller alternators that use 1 belt.   I’m not using anywhere near the 210A rating of the device.)

I’d also have to find/fabricate a bracket to mount the unit, do additional wiring, and so forth.  It seems time-consuming.  For now, I think we’ll go with the original 75A unit.     This means that we’re only charging at 60% of our potential rate using a bigger alternator.   Anyway, we’ll have to wait and see how much engine time we’ll be requiring.. if consumption is lower than expected, and wind speed is good (which it ought to be in the trades), we should be OK.

I’ve been doing the math on our electrical supply.  It’s all guesswork, but hopefully it’s “worst-case” and our actual electrical usage is less than projected.

Here’s a fairly generous budget for electrical demands on a daily basis:

Electrical usage

(for reference, that’s 2.35 kilowatt-hours in home terms - about the draw of a single 100W incandescent light bulb running 24hrs)

All the power that is used must also be replaced/regenerated. Due to inefficiencies, 20% more power has to be put back into batteries than is taken out.   We’re looking at potentially 235AH of generation required per day, based on the numbers above.

That 235Ah has to be generated by a combination of solar, wind, and lastly the alternator on the vessel’s diesel.

Solar generates a predictable amount of power, in most conditions.   It’s said on average to assume you’ll get 5 hours of rated capacity a day.    This comes out to 600W-hrs, or 50Ah/day.   Nice, but we’re still looking at some 185Ah remaining.

Wind power is variable.   The following chart shows current versus wind speed :

(the curve is because wind power is related to the square of wind speed)

hmm, 185/24hrs = 7.7a.    It looks that a 16 knot wind (average) is roughly our “break even” wind speed.   On windy days, we’ll have tons of power.

All that remains after wind and sun must be generated via the alternator.   If our consumption is in line with estimates, there will be days where we have to run the engine to recharge.

Our current alternator should generate nearly 75A.  But, we’re in the process of fitting/upgrading to an alternator that should generate 120A.  Effective charging ampacity is very important, because in the extreme conditions that there’s no wind or sun, all 235Ah has to be generated via the alternator.     The difference between our old alternator and new would mean running the engine 2 hours instead of 3 to completely top off the batteries.