Ask SAIL: Generating Energy While Underway

I just read Nigel Calder’s article on the costs of generating energy (“The Cost of Energy,” Dec. 2012) and found it fascinating. I understand there is a high cost for generating energy at anchor by running an engine, but is there an additional cost to generating energy while underway? If my engine is running the alternator to recharge the batteries, as well as moving the boat, surely it must add to the load and increase my fuel consumption. Is the increase significant, miniscule or somewhere in between?
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Jay Adamsson of Kingston, Ontario asks:

I just read Nigel Calder’s article on the costs of generating energy (“The Cost of Energy,” Dec. 2012) and found it fascinating. I understand there is a high cost for generating energy at anchor by running an engine, but is there an additional cost to generating energy while underway? If my engine is running the alternator to recharge the batteries, as well as moving the boat, surely it must add to the load and increase my fuel consumption. Is the increase significant, miniscule or somewhere in between?

Nigel Calder replies: 

In almost all cases, due to the manner in which a propeller loads an engine, engines are never operating at peak efficiency underway. However, as a general rule, the higher the ancillary loads from things like alternators, refrigerators, etc., the more efficient they become. Fuel consumption rises, because the engine is doing more work, but it rises proportionately less than the increase in work.

NigelCalder_4

To put it in terms of a power bill, instead of paying around $0.20 per kilowatt-hour (kWh) of energy used, you will pay $0.15 per kWh. If usage goes from 10 kWh to 20 kWh, the bill may rise from $2.00 to $3.00, but the unit cost will decline. Running an alternator underway, we are not only getting housepower at a much cheaper rate than when charging at anchor, we also get our propulsion power at a cheaper, more fuel-efficient rate, because the engine is operating more efficiently. The bottom line is that any time you are running your engine for propulsion purposes at less than, say, two-thirds of its full speed, there is a significant benefit to loading it up with other jobs.

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