I fitted a 10 watt fully flexible solar panel to my last boat with a 110 ah battery. Result - fully charged battery at all times with no whirling blades or noise, brilliant. I was so impressed I have just bought the same panel for my new boat. Go to http://www.ampair.com They had a deal on at the boat show, 130ukp.
For the average weekend sailor a small panel is just the job to keep the batteries topped up even over the winter. I have 2 topped up by a 10w panel and 2 without, after 3 months there is nearly 2 volts difference in them. Worth every penny and the best investment I've made in years.
I heartily endorse both of the other replies. I fitted an 18 watt solar panel to my boat 10 years ago originally to keep one 80 amp/hr battery charged up. I eventually fitted a second 105 amp/hr battery in paralell and now have 2 fully charged batteries at all times. I originally bought a regulator for £45 but was told it was not necessary in these northern latitudes so returned it. The batteries dont seem to have suffered at all
Stuart Jones