Cleat Backing Plate

[ausnp84]

Hey guys,

Did a search and couldn’t find anything on this. I’m fitting a midships cleat on a 40ft /9.5t catamaran and wondering what thickness G10 to use as a backing plate? 10mm looks pretty solid and I could fit a piece 100mm x 200mm to really keep it solid... does this sound about right?

N

[TheOffice]

sounds right. If hte area is cored you need to overdrill, fill with thickened epoxy and then re-drill for the bolt.

[thinwater]

As a rule of thumb, it should be about 65% of the fastener diameter and the radius should be about 5 times the fastener diameter (based on testing of cored hulls). These are minimums and depend on the lay up, what is nearby, and how well the plate is supported by the hull (no gaps--bonding is best).

[ausnp84]

Ok, so for an M6 bolt the G10 should be 4mm thick (5mm preferable) and the backing plate should be 30mm in diameter?

I’m ok with 5mm for thickness but the diameter seems light. I was thinking more a long strip that’s butted up against the underside of the deck....

N

[thinwater]

Originally Posted by ausnp84

Ok, so for an M6 bolt the G10 should be 4mm thick (5mm preferable) and the backing plate should be 30mm in diameter?

I’m ok with 5mm for thickness but the diameter seems light. I was thinking more a long strip that’s butted up against the underside of the deck....

N

Those are minimums. The thickness is based on the strength of the bolt. The radius has more to do with the lay-up in the area. Testing was based on 2 layers of 17-ounce biax and balsa core. If it is lighter than that, increase the size, or better yet, bond the plate to the hull with thickened epoxy (taper edges and ALWAYS round the corners ~ 1-inch).

[ausnp84]

So the next question... I’ve just bought a pair of straight through davita that were on an open 40. They bolt to the back of the transom, then continue through the transom and brace on the other side.

What sort of backing plate to use? Each davit is 75mm diameter and they’ll carry 75kg each. Transom is circa 8-10mm thick.

N