Ken and all, Sizing a battery for your boat should be determined by expected current draw over a period of time. Granted, if you are day sailing and need your battery for your CD player and nav lights coming in after dark, you can get by with a small gel (AGM) cel (approx 18 to 31 ah). If you are headed out to the San Juans or the Apostles for a 4 to 5 day extended cruise where you are going to be using lots of on board toys and interior lighting, let's go larger, say group 24 to 27. (75 to 100ah) How do expect to charge this battery once you pull her down? Shore power with your regulated charger is ideal, if possible. A solar panel, well, don't expect much unless it is very sizable. Thus, I have used a 31Ah AGM on my M17 with a small 7ah as back-up with fine results. Plus, it doesn't tax my 50 year old back loading it into the boat. Fit the shoe to the foot. Later, Bones