I had a trailer sailor for 10 years and kept an Optima AGM on board. Outgassing is negligible, and the battery won't freeze so you can leave it on the boat all year. More expensive than a deep cycle lead acid.
Three recommendations:
1) Estimate your daily power usage and charging capacity (e.g. a reasonable estimate of output from a solar panel or how often you'll have shore power). This will help you decide how much battery capacity you need. Remember that you shouldn't discharge a deep-cycle battery below 50%, so an 85 amp-hour battery is really good for (at most) ~42 Ah, or about 500 watt-hours. And 50% is the absolute minimum - the deeper you discharge, the sooner your battery will wear out.
2) Decide whether you want a 'flooded' battery or an AGM. AGMs can take a charge more quickly, don't offgas when charging (at least not when charged properly), can't leak acid if broken, etc. But (of course) they're more expensive.
3) Read whatever you can find of what @Maine Sail has to say about batteries (e.g.:
https://forums.sailboatowners.com/i...ecommendation-for-a-c30t.163933/#post-1131570). Spend a little time on his featured-contributor forum. When I bought my batts, I got Lifeline AGMs, based on his comments. More recently, he's had great things to say about Firefly. Apparently they can be cycled safely below 50%, so you might not need as big a battery. They're expensive, but if you can buy a smaller battery, the cost might be about the same (and lighter, too).
1) Estimate your daily power usage and charging capacity (e.g. a reasonable estimate of output from a solar panel or how often you'll have shore power). This will help you decide how much battery capacity you need. Remember that you shouldn't discharge a deep-cycle battery below 50%, so an 85 amp-hour battery is really good for (at most) ~42 Ah, or about 500 watt-hours. And 50% is the absolute minimum - the deeper you discharge, the sooner your battery will wear out.
2) Decide whether you want a 'flooded' battery or an AGM. AGMs can take a charge more quickly, don't offgas when charging (at least not when charged properly), can't leak acid if broken, etc. But (of course) they're more expensive.
3) Read whatever you can find of what @Maine Sail has to say about batteries (e.g.:
https://forums.sailboatowners.com/i...ecommendation-for-a-c30t.163933/#post-1131570). Spend a little time on his featured-contributor forum. When I bought my batts, I got Lifeline AGMs, based on his comments. More recently, he's had great things to say about Firefly. Apparently they can be cycled safely below 50%, so you might not need as big a battery. They're expensive, but if you can buy a smaller battery, the cost might be about the same (and lighter, too).
While a group 24 may fit the bill for a few more dollars you can get a group 27 with around 20% more capacity. Get a battery labeled for deep discharge and stay away from dual purpose batteries and starting batteries (mostly used in power boats). I like lead/acid wet cell batteries as they are less expensive and less finicky than some of the other types plus if it fails you can get off the boat and purchase a replacement almost anywhere.
