[C320-list] BAttery Hook up question

[Crosby Roper]

This is pretty basic ( and a bit embarassing). I am replacing the batteries on my boat and have connected them, but the battery # 2 is not providing any power. I blithely disconnected the old batteries without paying close attention to the set up on the second battery because everything was tightly bundled together and I thought you'd have to be a moron to get it wrong....
 
My question is this. On the forward battery the are two red cables  that are bundled with a 3rd black cable that is longer. I have connected the two red cables to the positive post and the black cable to the negatrive post (along with two other black cables.) No power from the 2nd battery this way. Is the black cable bundled with the two reds, actually meant to connect to the positive post on the forward battery ( like the negative to positive in series configuration in a flash light??? Am I risking dong serious dam,age here and should I just call someone to sort it out rather that trying  switching things around?

Any advice would ber helpful.
 
Crosby Roper
Tethys
Hull 1054