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Richard Kollmann Richard Kollmann is offline
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First recorded activity by BoatBanter: Feb 2007
Posts: 35
Default Ping Richard Kohlmann


Skip, I did estimate that the total load on your boats DC electrical
grid daily could exceed 200 amp-hrs. This estimate was based on you
removing the on board generator and providing AC power from an
inverter. The amp-hr estimate was based on experience with other boats
in the 46 foot range having similar energy consuming equipment, with
true refrigerator and separate freezer operated at low temperatures.
The boat was intended to be used for extended cruising in tropical
waters. In addition to refrigeration daily power consumption there is
typically an additional 50 amp-hours per day when at anchor. During
the concept design phase you should be able to project the size of the
DC power grid and size the alternator and sources of alternative
energy to replace daily the amp-hours used. It Sounds like you do have
an adequate power grid although I have no information on equipment
sizes.

If you have read the refrigeration study done by Cruising World some
20 years ago comparing manufactures equipment you will see my box Btu
heat loads are 50% higher than the others, that's because I use real
world conditions projecting daily product through put and tropical
conditions.
Here are the guidelines I use to project daily Btu and the amp-hours
needed for refrigeration on any sail boat:
1.All calculations are based on worst case tropical waters of 86
degrees and ambient air temperature of 90 degrees at midnight.
2. Refrigerator box insulated with 3 inches of moisture free
polyurethane and freezer to have at least four inches of polyurethane.
3. Each cu ft of refrigerator will require 600 Btu per day of heat
removal while each cu ft of freezer will require 1200 Btu removal per
day.
4. For each person above two onboard add an additional 1000 Btu
daily.
5. Refrigerator temperature maintained at 33 degrees bottom to 40
degrees at top, Freezer temperature 6 degrees bottom 22 degrees at
top.
For more detail on this see the slide show on my web site.

The amperage to remove this amount of heat will depend on type
refrigeration unit used and the efficiency of the refrigeration's
condensing unit.

Seawater temperature against the hull of a sailboat has a major affect
on cabin and refrigerated box temperature. I developed a correction
factor to my tropical daily power consumption. When the cruising area
water temperature is cooler than 86 degrees the daily amperage in my
estimate can be reduced 2% for each degree of cooler seawater.

Projected power consumption
I will list a chart of typical onboard electrical equipment you might
have on board so you can get an idea of what each piece draws, then by
adding the hours each item is used per day you can see how many amps
are needed daily to replace what is consumed.

First Skip's refrigeration. Six inches of polystyrene is equal to
four inches of polyurethane that OK.
Refrigerator 7 cu ft in tropical waters 4200 Btu per day, 3.5 cu ft
freezer 4200 Btu per day Total daily Btu heat removal required 8,400.
Estimated daily power consumed in tropics by Frigoboat Capri 50 with
SSC controller 120 to 140 amp hours depending on average speed of
compressor. Correction factor to refrigeration consumed in ocean off
New York where water is at least 15 degrees cooler today 84 to 98 amp-
hrs per day.

All other electrical equipment requiring current from DC power grid is
listed in watts per hour. Total all watts and divide by 12 to get DC
amps. AC 110 volt items to be operated by inverter increase by 10 % do
to inverter efficiency. The inverter manufacture may advertise 5% but
that is based on minimum current draw.

One Lap top computer 55 watts
Portable vacuum 506 watts
Coffee Maker 60 watts
Compact Microwave 935 watts
Anchor Light 12 watts
Steaming light 12 watts
Running lights 40 watts
Bilge Pump 50 watts
Fresh water pump 80 watts
Ham or SSB receive mode 30 watts Transmitting 300 watts
Weather Fax 30 Watts
Dept and speed log 12 watts
Radar 50 Watts
Windless 1000 watts
Spreader lights 90 watts
Auto pilot 50 watts
Florescent lights 10 watts
Cabin lights, radio, Water maker, or TV unknown watts