Larry Rick of Vancouver, WA, asks:

Considering the fact that you have two freezers, a refrigerator and numerous other electronics, what is your average amperage load and what effect would this have on fuel burn? It's great following your progress.

Dan Streech responds:

Your suspicion is correct. We do indeed have a higher than average 12VDC amperage load on the ATW boat. The Sub Zero freezer, Sub Zero refrigerator, household chest freezer and the microwave are 120VAC and operate from the inverter output circuit. This means that ultimately, they consume 12VDC if the generator is not running. The inverter loads must also be added together with the regular 12VDC loads such as the electronics, interior lights, fresh water pump, running lights, auto pilot, Naiad cooling pump, and blowers.

Many of the loads are intermittent or cyclical, so one tends to see a somewhat steady demand for 12VDC power and the resultant alternator output amperage. On the ATW boat, we are seeing average alternator output of about 100 amps.

Jim Leishman was a little surprised by the high steady load on the alternator and after he flew back from Hawaii, he and I discussed it. As with nearly all of the equipment on the ATW boat, we are using the standard engine, alternators and inverter. The main alternator is the sturdy Leece-Neville (the same alternator that was previously called Prestolite and before that Motorola) which has a continuous output rating of 130 amps. We have used this same alternator for a number of years and have found it to be a wonderfully dependable unit.

After discussing it, Jim and I concluded that while the loading on the alternator is rather high, it is still acceptable and we decided to leave it as is. Actually, it is an interesting study to see if the alternator will survive the trip and once again we will gain some new knowledge from the ATW. We do have a complete spare alternator on board as well as two spare regulators if needed.

The washer/dryer is 120VAC and can only operate from the "non inverter" circuit. This means that the generator must be run when doing laundry. When the generator is running, its AC output "passes through" the inverter directly to the AC loads and also operates the inverter as a battery charger. Therefore, the main engine alternator output is pushed down to nearly nothing and it gets a rest until the generator is turned off.

As you might have read elsewhere, we have decided to install air conditioning when NORDHAVN reaches Yap. The air conditioning unit could (very carefully) be operated from inverted power, but considering that the loads are already rather heavy, we will probably run the AC generator continuously to operate the air conditioning. This will relieve the engine alternator of its heavy load.

The fuel burn from the alternator load is rather simple arithmetic as follows: 100 amps at 14.2VDC = 1420 watts (1.42KW). 1 KW = 1.341 horsepower, therefore 1.42KW X 1.341 = 1.90 horsepower. An alternator is about 50% efficient, so we will double that figure to 3.80 HP (1.90 X 2). We need to add another 15% for belt loss and the cooling fan so this brings our total to 4.38 horsepower (3.80 X 1.15).

While not perfectly accurate for all engines and load conditions, we generally say that 1 gallon per hour of diesel fuel consumption will produce 20 horsepower. So, 4.38 HP/20 = .219 GPH or 5.25 gallons of diesel per day to feed the parasitic load of the alternator. Interesting to think that the passage to Hawaii required nearly 85 gallons of fuel to feed the electrical requirements.


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