Rick..
Here are a few things to consider: 1) It's not generally a good idea to regularly draw your batteries down bellow 50% as it will greatly reduce the effective cycle life. 2) You should do an energy audit so you know what your actual use is and where it's going. The engine actually draws very little from the battery in terms of real amp hours. I can start my 44hp Westerbeke, with glow plugs, on less than .5% of my battery banks capacity! 3) Wet cell batteries need to be maintained on a regular basis and should be toped up with distilled water when low. When the level gets close to the lead plates you've waited to long and should consider upping your maintenance interval. 4) Make sure you don't have a stand by draw. By this I mean the only thing that should be live when you leave your boat is the bilge pump. A bilge cycle timer and a stop watch, timed to see how long it takes to drain the bilge after it automatically kicks on, will show you exactly how many amps you are using when combined with the count of the bilge pump cycle counter. 5) If you run your engine for short periods of time you may never even be getting close to a full re-charge. Getting back to 100% can take 7-12 hours or more of straight engine run time. Getting to 85 or 90% does not take nearly as long. If you only ever get back to 90% your energy budget needs to start at 90% capacity not 100% when calculating draw and battery state for cycle life. 6) The size of your bank will depend entirely on your draw and without knowing that with some level of accuracy it's impossible to say if two 27's will work in your application. They certainly should, but, there can;t be a great deal of certainty, based on what you've written. It would be good to know how you use them? Are they always combined as one bank or do you use them separately? Is there a third battery and these are the "house bank". If it is like you say and all you have is two group 27's then you run a big risk by combining them and having NO back up. If you don't combine, and have one as an emergency/start battery, then your dealing with about 90-100+/- available amp hours. A 50% discharge give you about 45-50 total amp hours of use before you need to fully recharge. If you regularly re-charge to only 85 or 90% you now have between 38 - 45 amp hours to consume before you are shortening it's effective cycle life.. As a general rule you always want a back up battery that should be at full charge & ready in case you drain the house bank... The more detail you can give the better. P.S. The Peterson 34 is a cool boat! (see a filled out profile is good) Have you seen Tim A.'s Peterson 34 web site? http://members.acadia.net/catamount/Greyhawk/
