Best approach to attach plywood base to v-shaped hull?

I'm looking to add a new base to mount a pair of Group 31 batteries near the bow of my fiberglass boat. I have a good location selected right at the centerline of the boat along the bottom hull. Naturally this location has a subtle V shape to it (measured about 5/8" difference from centerline to ends).

The factory applied gelcoat to the fiberglass, so my plan is to rough up the thin gelcoat layer, then wipe with acetone. Then apply 3M 5200 generously, and lay a piece of 3/4" marine plywood on top of it. I will have the bottom side of the marine plywood bottom routered to roughly match the curvature of the hull for maximum contact area. Then, we will screw a piece of 1" plywood into the first, and paint the whole thing with gelcoat for water protection. Then I'll mount the battery boxes to that and wire them up.

Any feedback on this approach or alternate suggestions?

 

5200 will delaminate from raw wood if it gets wet. I think Sika has a better product for the application. Otherwise, sealing the wood with epoxy will work too.

 

Thank you. Do you recommend Sika 291?

 

291 will work,as will 221,but either of them need to have a sealed surface on the wood.Sika have a dedicated wood primer,but if you have epoxy or polyester those will work too.Take care to seal all the edges of the plywood or water will creep in there and eventually rot it.You could just apply a couple of plies of glass over the whole thing to be extra sure.

 

Have you considered not using wood at all? It sounds like you're just installing a nailer to attach your battery box to. Would you get just as good performance from directly glassing in four steel tiedown brackets for less work than glassing in a nailer platform to attach your battery boxes to? I'd suggest just glassing in the boxes, but you might want to remove them easily.

Normally, I'm all for wood, but it sounds more complicated: glue down a shaped piece of plywood to attach another piece of plywood to, seal them both, glass them in, then mount your boxes. I don't know what kind of pounding those batteries are expected to get riding near the bow of your boat, but they aren't hanging upside down.

-Will

 

Your approach is solid, but consider using thickened epoxy instead of 3M 5200 for a stronger, more rigid bond. Sand slightly beyond the gelcoat for better adhesion, and epoxy-coat the plywood before applying gelcoat for better waterproofing. Adding fiberglass tabbing around the plywood edges will further strengthen the mount. Pre-drill and epoxy-seal any screw holes to prevent water ingress. This will ensure a more durable and long-lasting installation.