"I would say that if you run a
"I would say that if you run ancilliaries when motor is switched off and getting home is important to you, use a deep cycle batt (I use cheap car batteries) for the "stuff" and a nice big 13 plate 600cca marine starting battery for the motor, with an isolator switch. Start on 13 plate, run with both in circuit and use lights, fish finder, stereo on just the deep cycle battery.
If you are at sea, then "too big and foolproof" will never let you down....which is an American engineering concept that you should not lose sight of. Saving money at sea is a waste of effort....do it, do it properly and build in a margin for error.
The car battery in my boat won't start my 60HP Evinrude, however the starting battery would probably get me home if I by-passed the ignition cut out and cranked it in gear ;-)
Jump boxes are good!!"