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From: The Marchands (no email)
Date: Mon Feb 03 2003 - 07:50:13 EST
Bob:
I had a 35' sailboat in Galveston Bay with a 16,000 BTU A/C. This unit would
barely keep up on a hot summer's day, but would be loafing in the evening. A
lot depends on shading and head liner insulation. You can probably get by
with 12,000 BTU on your 28' boat.
Boat air conditioners roughly draw 1 amp at 115 v for each thousand BTU/hr
of capacity. Whether you can run your a/c from an inverter depends on how
long and when. To try to run a 12,000 btu/hr a/c for 24 hours on a hot
Florida summers day on a battery/inverter system is practically infeasible.
To run it for 8-10 hours overnight might be possible.
Here are some numbers for the latter situation: 10 hrs x 12 amps x 115 v x
0.5 service factor (the time the unit cycles on and off) = 6900 watt hours
AC. 6900 watt hours would equal 6900 div 12 volts div 0.9 efficiency = 640
amp hours DC. To supply 640 amphours DC, you are going to need a battery
bank of at least 1280 amphours so you don't discharge it below 50% for
decent life. This is about a dozen group 27 12v batteries at about 40 lbs
each or about 500 lbs of batteries. A 2000 watt inverter would handle this
load adequately.
To recharge 640 amp hours would require more than 4 hours of charging with a
high output, large case alternator charging at 150+ amps (see Balmar's web
site at www.balmar.net).
So, it may be feasible, but it doesn't seem too practical. If all you want
to do is cool the forward cabin at night, you might install a separate 6,000
BTU/hr A/C for only that cabin. The foregoing numbers should drop in half.
That might be getting into the realm of practicality.
David
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