Like Dennis said, if you have room for two full size batteries, then you have room for two 6-volt batteries.
As for Optima batteries, I'm not a fan. Neither is the manager of the Autozone at home, who wishes Autozone would just quit carrying them because of all the problems and returns they have with them.
The Red Top, which are strictly starting batteries, cannot be used out here on the road. You can use either the Yellow Top (marketed as truck batteries), or the Blue Top (which are marketed as marine deep cycle batteries). Neither are deep cycle batteries, and are instead hybrid batteries, both still deigned primarily for starting engines. Optima's Battery Selector will direct you towards a marine Blue Top, based on the starting requirements of your boat's motor, not on how many hours you'll be offshore or your amp hour requirements. That should tell you something.
The biggest, beefiest Yellow Top is the D27F (Deep Cycle, Group 27, and weighs 53 pounds and will give you 66 amp hours. The biggest, beefiest Blue Top weighs is the D31M (Deep Cycle, Group 31, Marine) and 60 pounds and will give you 75 amp hours.
A couple of D31M Blue Tops are gonna cost you around $450. That's just a snotload of money for, like, no amps.
Two Trojan J305-AE batteries will run you less than $400 (mine were $160 each), and will give you 305 amp hours. That's four times the amps, in a true deep cycle battery, for less money than the Optimas.
If it's the 6-volts that is giving you pause, as it does a lot of folks, stop it. Once you connect the positive terminal from one of the batteries to the negative terminal of the other, you instantly have a 12-volt battery and all will be right with the world again.
As for Optima batteries, I'm not a fan. Neither is the manager of the Autozone at home, who wishes Autozone would just quit carrying them because of all the problems and returns they have with them.
The Red Top, which are strictly starting batteries, cannot be used out here on the road. You can use either the Yellow Top (marketed as truck batteries), or the Blue Top (which are marketed as marine deep cycle batteries). Neither are deep cycle batteries, and are instead hybrid batteries, both still deigned primarily for starting engines. Optima's Battery Selector will direct you towards a marine Blue Top, based on the starting requirements of your boat's motor, not on how many hours you'll be offshore or your amp hour requirements. That should tell you something.
The biggest, beefiest Yellow Top is the D27F (Deep Cycle, Group 27, and weighs 53 pounds and will give you 66 amp hours. The biggest, beefiest Blue Top weighs is the D31M (Deep Cycle, Group 31, Marine) and 60 pounds and will give you 75 amp hours.
A couple of D31M Blue Tops are gonna cost you around $450. That's just a snotload of money for, like, no amps.
Two Trojan J305-AE batteries will run you less than $400 (mine were $160 each), and will give you 305 amp hours. That's four times the amps, in a true deep cycle battery, for less money than the Optimas.
If it's the 6-volts that is giving you pause, as it does a lot of folks, stop it. Once you connect the positive terminal from one of the batteries to the negative terminal of the other, you instantly have a 12-volt battery and all will be right with the world again.
