Thursday, December 15, 2011

Heavy Duty Mounting on Cored Panels, Without Through-Bolts

The insides of boats are frequently not a friendly place to mount things to.  Especially if the mount is to be structural, to take weight.  Drew at Sail Delmarva shows us his answer to this problem:
One constant aboard performance cats and even semi-performance cruising cats is cored hulls. If there is anything serious to mount, it's a long and methodical process:
  • Drill through.
  • Route-out the core.
  • Fill with epoxy.
  • Wait.
  • Redrill.
  • Add a backing plate. Yup, perhaps that should have been FRP and epoxied on before you drilled. 
  • Through bolt with washer and sealant.
The previous owner added a winch without following these steps, and I managed to tear it out of the deck. Cored construction can be very strong, but point-loads need to be distributed.

Front of the mount      
What if the mount is on the inside of the hull and you don't want a bolt sticking out the side? A sump pump mount. Some hooks to hang weighty objects. Screws are possible, but the skins are thin and the space between the inner skin, which must fully engage the threads, and the outer skin, which cannot be touched, is less then 1/2-inch, sometimes only 1/4-inch. Since screws come in 1/4-inch increments, this can be a real problem. The answer is a thickened patch on the wall, and to mount real equipment, built-up glass is probably best. Of course, there need to be bolts or enough thickness to get some real screws in it, so glassed in wood is common. But wood rots. There is a simple, faster way, if the surface is close to flat:

Back of the mount

  • Mount the object to pre-laminated FRP about 1/4-inch thick. In this case I tapped-in threads, mounted the hooks with machine screws, and ground off the excess. T-nuts could be used for heavy duty mounts, or simply thicker glass; at 3/8-inch the threads are typically stronger than the bolts.
  • Screw (very short screws only used to apply clamping pressure) and glue (3M 4200 if it's not too heavy and you might remove it, thickened epoxy if it is and you won't remove it).
Piece of cake, easily pre-fabed in the shop, and will last as long as the boat. I'm sure solid glass hulls face the same problem; you still can't just tap a screw into the hull, not unless it is well over 1-inch thick. In some ways, the problem is more ticklish. But glued-on pre-laminated FRP still makes for a tidy solution.

Finished mount. This holds heavy mooring lines and spare rode, perhaps 20-30 pounds on each section. The coils hang well below the  hooks, cow-hitched with slings. The hull is ~ 5/8-inch thick with 3/32-inch triaxial glass skins. Screws alone might hold... for awhile... assuming I don't step on the coil while wading through sails and fenders, which I will.
The hooks are 1/8" x 3/4" aluminum flat bar bent around a double mandrel and ground smooth. I also store my chute in this locker, so I wanted a hook that would hold line but not snag sailcloth. When you can't find what you want, you make it.

I cheat. I get pre-laminated FRP as cast-offs from chemical plant projects. Free is nice. But I've gotten it other places--old signs, cut from old boats. The flat bar were electrician cast-offs. Scrounging 101.

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