Chain plates maintenance

The suspected u-bolt chain plate was unbolted below decks and the fitting tapped upwards. One arm of the u-bolt had snapped off. (Fortunately the remaining side held , bending a little bit. The  mast and entire rig are held up by these deck u-bolts)

Rust stain seeping out on deck was a tell-tale sign that was not noticed before the failure.

The original chain-plate u-bolt had been fitted for over 30 years. It's a good idea to unbolt and inspect these about every 10 years or when rust stains are seen.

A closer look at the damaged indicates that crevice corrosion had taken place, and work-hardening-micro-cracking had occurred allowing salt water in to corrode the stainless steel (in fiberglass deck).

The intact side was also badly corroded.

The investigator 563 class web-forum revealed that this is a common problem in 30 plus year old boats, it has also happened in Compass Careel 18s which have a similar chain-plate system and baots are also reaching this age.

I replaced the chain-plates on both sides with high quality Ronstan 8 mm u-bolts, the same specs as the originals.

The replacement is 8mm rod throughout with small lugs to hold the deck plate in place. The original had 10 mm rod above deck and 8 mm threaded rod below deck, would this size difference contribute to metal fatigue at the deck line?

These fitted nicely into the original through deck holes. Below decks a 12 mm block of timber distributes the load. Clear silicon was used to seal the holes.

The new u-bolt also has a load distribution plate below deck. The original had two washers instead.

The rigging screws shacked above the chain plate were closely inspected. One 6 mm rigging-screw was obviously bent and another had a hairline crack in the toggle, so these were removed.

Hairline crack and rust stain on rigging toggle, a potential failure point.

The replacements are much heavier and twice as strong, 8 mm rigging screws which were fitted to hold the upper shrouds. The two remaining 6 mm rigging screws now hold the lower shrouds to the chain-plates.

8 mm rigging screw (front) is far stronger than the original 6 mm one (back) it replaced. The new chain-plate u-bolt should last another 10-20 years before replacement maintenance is required.

The inner rigging screws should eventually be replaced, the original design is still produced but need to be ordered in.

Will keep an eye on the old rigging for any sign of potential failure. Rust stains are areas to watch for cracks or frayed wires etc. The rigging wires age is not known but it does look younger than the original chain-plates.