They aren't through bolted, just screwed into the deck.
The proper way to screw things into the deck is to overdrill
the hole and ream out the core,
so that a leak won't wreck the boat.
It's even more important with screws, since if there's a leak it will never drip inside the boat, so one would never know that the core is getting rotten.
July 2005 --
These things are odd, and are
specific to the C&C Landfall 38 (I think.)
They tie the deck down to the aft bulkhead.
The old one is on the right. They're right
next to the genoa track. After I removed
the self tending jib setup and put a new headsail on, the track is being used for the first time in 20 years, and the deck tang started to leak.
The plate is too small to really cover a lot
of sealant, so I had them remade with a big plate,
to hide a lot of caulk and
solve the problem for a long time.
After the holes were filled with West System Epoxy and colloidal
silica, the area was covered in wax paper and the mounting screw were pushed
down into the epoxy. It's really important to dip the screws in oil (or
mold release) before pushing them into the epoxy.
Now the holes are perfectly threaded to match the screws.
The slot and old holes were filled with epoxy putty as well, to seal the core. Then the slot was recut.
Here's what it looks like once it's caulked well, screwed into the deck and bolted to the bulkhead underneath.
I really don't know why the boat was built with this thing,
but I do believe it's structural so it's there forever.
Fixing the Rot in the bulkhead was another project...