How many hours per day should the engine be run to keep the batteries charged?

Running the engine for two hours per day is generally sufficient for keeping the batteries charged on a sailing vessel. This duration allows the alternator to effectively replenish the charge in the batteries, especially if they are of a standard size and not heavily depleted.

The alternator produces electricity as the engine runs, and the amount of time required to charge the batteries can depend on factors like the size of the battery bank, the state of discharge, and the load on the electrical system. In most cases, two hours strikes a balance, giving enough time to replace energy consumed while not consuming excessive fuel or putting undue stress on the engine.

Longer durations, such as three or four hours, may not be necessary unless the battery bank is particularly large or has been heavily drained. Conversely, running the engine for only one hour might not be adequate to collect enough energy back into the batteries if they have been used considerably during the day. Thus, the choice of two hours is a practical recommendation for maintaining a healthy charge in the battery system.