Help avoid "egging out" through-bolt holes in fiberglass; hatch hold-open install

[ajgraz]

Help avoid "egging out" through-bolt holes in fiberglass; hatch hold-open install

17' CC has an anchor locker in the bow with a very large triangular-shaped hatch lid that is basically almost the entire foredeck. This thing is definitely heavy enough to maim or kill if it fell on you in rough seas. So I'm thinking of installing a heavy-duty stainless spring-type hatch hold-open, something like this one:
http://www.iboats.com/Hatch-Spring-...763847--session_id.389320049--view_id.1183035

Mounting the spring to the ~1" thick hatch lid looks simple enough with oval-head through-bolts, finish washers and sealant. The other end would have to mount on a vertical wall of the anchor compartment, four pan-head #10 through-bolts fairly closely spaced. But that wall is just a layer of fiberglass, I dunno maybe 3/16" thick, gel coat on one side, the unfinished backside is easily accessible inside another hatched compartment.

Given the direction of forces, I'm afraid the through bolts will "egg out" the fiberglass in that wall. Washers or even a backing plate won't prevent that movement, I think.

So, how do I prevent this metal-on-fiberglass from egging out? Lay-up a small square of 1/2" plywood on the unfinished side before through-bolting? Just oversize the mount holes and run the through-bolts through rubber panel grommets? Creative use of T-nuts? Other ideas?

(For that matter, anyone got comments on those spring-type hatch hold-opens? Never used one before.)

 

[ajgraz]

Surprised at lack of responses...

I am going to try using rubber grommets in over-sized holes in the panel. I did that very same method of repair to fix egged-out mount holes for the 5/16" studs on the wrap-around grab rail at the top of the console, and that (so far) supports my massive bulk and gives an ever-so-slight nice "shock absorber" character to the grabbed rail.

If that fails, I can always go back and lay up some plywood backing instead.

 

[GA_Boater]

Missed seeing your question until now.

The nice thing about the hatch springs is the only time any pressure is exerted is when the spring is straight and supporting the weight. A heavy hatch might need a pair of hatch springs to spread the weight. That would minimize the chances of egging the mounting holes.

Using a plywood backer plate larger around than the spring plate and epoxied to the side wall will also eliminate flex. Bulletproof is to use a metal sleeve liner in the screw holes to make a more solid thru-hole so the screw threads aren't working directly against the wood.