Mounting screws for battery tray

Willhound

Looking for suggested size of mounting screws for a battery tray on my 2000 FV270. 
Looking to add a second house battery that I'm thinking to locate up near the aft cabin wall, slightly to Port to offset the original two batteries that are mounted on the starboard side about 3/4 of the way up towards the aft cabin wall from the transom. Want to use a decent length screw with 3M 4200 to assist.
I assume there should be some decent meat to screw into at that location?

Lake_Bum

I'd remove one of the existing screws, and just match it up to that. 

Willhound

Didn't want to because the existing screws are sealed with, I assume 4200 or similar, and moving over to a different location, so I was wondering if the underlaying material was a different thickness. But another look late yesterday leads me to believe that the little "shelf" area against the aft cabin wall looks like 3/4" ply with fibreglass mat over it, so will likely just use 1" screws and 4200.

Cableguy Greg

I would only use 3/4" screws. You can also bed them, and cover the holes with 3M4000. Since it is in the engine room you will be fine with that.

Willhound

#8 or 10 okay Greg?

jme097

Really needed to seal the holes in the engine room?

Willhound

More to keep things from vibrating loose or any chance if any water getting in between layers. I've never screwed anything to any boat without a dab of some kind of sealer or adhesive. Cheap insurance. 

Cableguy Greg

I would go with #10's