Connie; Your motorcycle battery sounds like a good lightweight plan with only one drawback. A good MC battery is 80-100 bucks. My Honda 1100 Shadow's is around $90. A SMALL car battery is maybe $40 and only weighs 25lbs (a guess). The amp hours will also be a lot longer for the car battery. Another post mentioned not needing lights on a 15. True, for a true sailboat, but if you have an engine mounted, lights are required for night sailing and motoring. Have you ever sailed to Manhasset Bay and stayed at the Riviera Marina. I was raised on a boat there in the '60's. We had a 1929 50' Elco Flattop. It was a good life. Sandy