Backing plate

I would like to add mid ship,cleats
The boat is an 8000lb cat with cored decks
The cleats are 8in 4bolt.
I would use 1/4in aluminum for the backing plate.
Is there a rule of thumb for size of the plates??
Thanks. Nick

 

I think that ideally you want to try to use the largest (re surface area) backing plate that is possible. 1/4" thick plate should be a good start.
Is the core material foam or balsa?
Ideally you should also fit some compression pillars (eg made from resin mixed with a suitable filler) in the core in way of where the bolts will be located.

 

Your boat weighs a little less than mine. I used 8" cleats as well. When I installed new decks I added hardwood backing plates but also added top plates, right under the cleat on top of the deck. Why? I've seen quite a few boats with deck damage from the cleat digging into plywood or cracking fiberglass laminate from the side loads placed on the cleat in storms. The top plate adds additional bearing area right under the cleat. A little more work but I had the table saw and router set up for the under deck cleats anyway so what the heck. Make certain your core is in good shape and not saturated with water. One last thing. When you're making up those backing plates don't just make a 90 degree cut, round off the corners. You could use a bench grinder for this. This will eliminate a hard spot and spread the load more effectively.

 

The standard is to use a plate that is at least 5/8 as thick as the bolt diameter, and to have about 5/8 bolt diameter edge clearance around the bolt holes. This is smaller than what most would guess at, but this services the needs of the bolts with standard washers, and that is really what backing plates are intended to do. If you have concerns about the laminate in between, that is a different matter. You may be able to get away with using bigger top and bottom plates, but this isn't what they were intended to do. These should have their own bolt pattern.

Another common practice is to drill significantly oversized holes and fill with high strength epoxy, then redrill for the bolts. These are called compression annuli. Don't be shy. Don't drill through the bottom skin, use a Fostner bit following a hole saw and save the top skins for patching the holes. Use the standard backing plate described above with these. A third option is to remove the coring and inner skin and tab an adequate patch of glass in there to support the cleat or other hard point. I think this is considered the best method, but rather awkward to do where most cleats want to go. I have frequently reinforced the top skin with a slightly oversized patch of glass in an attempt to prevent skin deformation and water retention around the bolt holes. I did this to every seat leg on a 100 pax tourboat after having to replacing the plywood deck after 5 years of service. The deck got hosed down about 30 times a week, so any sucked-down mounting was perpetually wet.

1. Make a set of backing plates as per above.
2. Use these to send a pilot hole from below to above for at least one hole in each cleat.
3. Template the topsides for the holes.
4. Figure out what size annulus you can get to go on the outboard holes, the bolt hole doesn't have to be centered perfectly, you can cheat it a bit.
5. Figure out what size annulus you can use on the inner holes. These will have an appearance impact.
6. Holesaw the skin and drill out the core leaving the inner skin in place. One option here is to just put a slightly oversized hole in the top skin, then put an allen wrench in a drill, cut the end off to an appropriate distance, and hog out the annulus with the bent allen wrench so as to undercut the top skin. This is okay if you want to add one cleat, but I wouldn't want to put ten on this way. This works decently on polyurethane foam, but less well on anything you make boats out of.
7. Bog the holes with epoxy, put the top skins back on, and glass in the patches to the top skin.
8. Drill the holes and testfit everything, then prep and paint.