the thing about "cranking batteries" is discharge rate. I have three 1100 amp interstate batteries on my boat, I have a 90 hp yamaha engine, the discharge rate of my "deep cycle" batteries is within the "draw rate" of my starter on the engine, therefore I can start my motor with my deep cycle batteries. big motors take more discharge rate to spin the starters (takes more energy out of the battery at a faster rate) so a cranking battery provides less amperage ussually at a higher rate of discharge (so when you turn the key, you get a heck of alot of amperage for a short time) a deep cycle is designed to give you steady 12v power with lower amperage load for long periods of time. a cranking battery is designed to give you big bursts of power for short periods of time. hope this helps, rule of thumb, if you have the resources get a cranking battery for the engines, and a deep cycle for the electronics/trolling motor/bilge/ and whatnot.