Dear Boat Doctor,
I own a Quickstep 24 sailboat called Hogwash that, when I bought it, had a group 27 battery, which seemed to be overkill for the electrical needs and it was also heavy at 63 pounds.

I replaced it with a group 24 battery at 53 pounds and I'm wondering if I can decrease the battery size again. I would like to take even more weight from the back of the boat.

The boat has a depthsounder, VHF radio and a single cabin light that gets used occasionally. My motor is a Honda 8-horsepower without electric start.
Can I get away with a smaller battery?
John Dyson
Brooklyn, NY

Dear John,
The amount of battery capacity you need is dictated by the electrical equipment aboard the boat as well as how often you are running the lights, radio, etc. 
The gear you list would typically draw about 1.5 amps, and I assume that you also have navigation lights, which would draw a couple more amps. It is safe to assume that you have about a 2-amp average load per hour in a full day.

The group 27 battery that came with your boat likely had a capacity of 90 amp-hours, while the group 24 has a capacity of about 80 amp-hours. This means the group 27 could run your boat for about 35 to 40 hours before needing to be charged, while the 24 would run for a little less time.
Your only other option in a standard marine battery is a U-1 size. This battery is smaller at only 24 pounds but has a significantly smaller capacity of only 23 amp-hours. This battery would only run your boat for about 10 to 12 hours before needing to be charged. So you will need to decide if the lower weight is worth the limited capacity.

The largest consumer of power on your boat is lighting. You could reduce your power consumption significantly by changing to LED lighting. Of course, LED interior and navigation lighting is not inexpensive. However, LED lights do last longer, which means they need to be replaced less often, which is also something to consider.

You can also trickle charge your battery and extend its run time by installing alternative energy sources such as solar panels or a wind generator. In the case of your boat, where weight and space is an issue, I would go in the direction of the solar panels. Some of the new flexible solar panels are showing a lot of promise and are ideal for small boats as they can easily be positioned for maximum efficiency, and when they're not needed they simply can be rolled up and stored away.