9.3. Consumption over a 24-hour period when sailing
Our starting point is one 24-hour period under sail (when travelling under power the current consumption is not of importance, because the alternator on the main engine can easily keep up with consumption). We will now determine the battery capacity needed for supplying all consumers during one 24-hour period. In the table that follows the consumers have been divided into continuous (C), long duration (L), and short duration (S) consumers.
Consumers
Consumption
Time / 24- hours
% on
Consumption / 24- hour period
Watt
Amp
Hours
% kWh Ah (12 V)
C Navigation instruments
0.2 0.2 0.1 5 4.2 2.1
24 24
5 5 2 1
C GPS
C VHF standby S transmitting
24 0.2
C Refrigerator, air-cooled heat exchanger L Tricolour navigation light or anchor light
50 25
24
50
50 17 30
8
L Autopilot
5 1
20
30
L Radio
3
3
S Cabin lighting
200
0.6
10
S Other
5
Total consumption per 24-hour period Average consumption per 24-hour period
1.5
128
64
5.3
Minimum battery capacity required, assuming 2 recharges per day (see sect. 8.4.2)
256
It is noticeable that the refrigerator is by far and away the biggest consumer. The refrigerator’s current consumption could be halved by using a more expensive water-cooled heat exchanger instead of an air-cooled heat exchanger and by improving insulation. The total consumption per 24-hour period would then reduce to 103 Ah. Using a gas refrigerator (only useable on motorboats in calm waters) would even reduce current consumption to 78 Ah.
9.4. At anchor or moored without 230V shore power pick-up
Once again, our starting point is one 24-hour period, but this time the following applies for motorboats and sailing boats.
Consumers
Consumption
Time / 24- hours
% on
Consumption / 24- hour period
Watt
Amp
Hours
% kWh Ah (12 V)
C Refrigerator, air-cooled heat exchanger
50 25
4.2
24
50
50 17
L Masthead light
8 3
0.2
L Radio
1
3
S Cabin lighting, ten 20 W lighting points
200
0.6
10
S Other
5
Total consumption per 24-hour period Average consumption per 24-hour period
1.0
85
42
3.5
Minimum battery capacity required, assuming 2 recharges per day (see sect. 8.4.2)
170
9.5. The extra’s
Even the relatively small boats that we are considering here often have (or the crew might wish to have!) some extra safety and comfort on board. A few optional extras are suggested below. For some an inverter is needed. Because today inverter efficiencies are higher than 90%, the losses in the inverter are ignored in the energy consumption calculations.
48
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