1 Battery or 2??

I have 2 perfectly good batteries in my boat, fully charged. But neither one by themselves can start the boat. When I run both batteries, the boat will turn over fine, but on one or the other...barely any juice. Any suggestions???

If neither battery by itself will start the boat but both together will then it is evident you dont have enough in one battry or the other for engine start. Have you checked the batteries with a meter to find out what kind of amperage each has? How do you know the batteries are perfectly good? I suspect as soon as you install a new battery the poblem will go away. Good Luck.

This maybe a stupid question, but are you sure of the batteries voltage, i have seen people use 6 volt batteries connected in series to make 12 volt, apparently the 6 volt have better capacity. Also you can buy or borrow a load tester and test your batteries seperately.

Had both batteries checked before putting boat in water. One was perfect, the other needed charged and was fully charged when put in the boat. Both tested at 12.4 ????}

What does "one was perfect" mean? Since this is not quantifiable it is not significant in trou blshooting the issue.

What is the actual reserve of the battery and the voltage. If you are reading 12.4 volts you probably dont have batteries that are fully charged to the rated manufacturing spec. You need to put the battery(s) on a tester that will tell you current reserve or cold cranking amperage and volgtage. For a fully charged 12V battery you should have around 12.6 volts or more. Compare amperage with the spec listed on the battery.

Are you testing the batteries independantly of one another? If they are connected in parallel by a switch then they will both read the same(in other words measuring the batteries with the switch in BOTH). It is somewhat unusual for the batteries to read exactly the same voltage.

By the way what is the age and type of batteries.