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Improving a cruising vessels charging by right-sizing the alternator

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What is the issue?
Most all inboard sailboat engines have a standard alternator size range from 35 to 55 amps and use it to top up batteries. However the modern cruising life is electrically hungry and we quickly add batteries without considering the alternators capability to charge an expansion.

Why address this?
Battery capacity and charging enhancements should go hand in hand otherwise engine charging will become long, consume unnecessary fuel, adds engine time and increase the length noise has to be endured in the cabin.

How to address this?
Fit an upgraded alternator to your diesel engines that is sized to the extended battery capacity.

We started off with a 55amps alternator and upgraded to an 80 amps made the world of difference to us. You can upgrade to more amps ranging from anything from 75 to upwards of 200 amps but we lived with just three batteries, start plus two domestic and did not require excessive charging.

The charging sweet spot rule is the alternator should be one quarter as large as the vessels battery capacity. Investing in anything larger is pointless as it will not charge efficiently. Just be careful to use the hot rating when sizing an alternator. We found manufacturers listed the cold rating but in operation the hot rating is the one that will be applicable and it can be 20-30% less than the vaulted cold value. In our case we had three 100 amp hour batteries. This totalled to 300 divided by four to provide the alternator sweet spot output of 75 amps that drove our 80 amps choice as it was the only trade in available where we were.

The larger alternator required a larger 1/2" belt. The general rule is once you go beyond 75 amps you will want to move up from the standard 3/8" to the 1/2" belt and once you exceed 150 amps a dual belt is required to deliver the additional pull. However when you go very large that dramatically increased wide side load on the fan belt may cause rapid bearings wear. Before you dramatically increase the size of your alternator you should consult the engine manufacturer for approval.

A "soft start" 90 second turn on delay to minimise belt stress on a smart regulator can dramatically help the belts. We traded our 55 amp alternator in for a reconditioned 80 amps but many people hold onto the old alternator. If you size it right you can keep the old alternator as spare.

There is no free lunch however in the world of physics and big alternators take significant power from the engine. For every 25 amps of 12-volt alternator output will consume about one horsepower. In our case our 80 amps removed the best part of 4 horsepower from driving the vessel through the water. This we did not miss as we were well ahead of the 1 x HP to foot of boat ratio. If you feel your boat is underpowered however you may need to consider this reduction in engine power before scaling up as it may prove critical in tight situations.

With thanks to:
Michael Harpur, Yacht Obsession.

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