Oh, that. It's a cut and past insert showing the enlarged surface finish of the existing chainplate. It's not part of the discussion at all. It's all going to get cleaned up to a mirror finish.
Chainplate refurbishment
- Thread starter Roger Long
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It's thick. These boats are tanks. I have a 3/4" thick plug (no core) where I drilled for new cockpit scupper through hulls. It isn't a secondary bond in the hull flange either. Even if the flange fractured in the bend, you would have to lift up the whole aluminum toe rail that's bolted and screwed into the hull both through the deck and through the hull sides. It's "T" shaped with the fastenings through the guard rail portion going into the fiberglass below the point where it could fracture at your "stress riser". Those chainplates aren't going anywhere.
I've got about 11 square inches of surface bearing on the structure, about 950 psi when the wire breaks. A half dozen half inch bolts in 3/8" fiberglass would give me a tenth of that, 9500 psi on the fiberglass edges of the holes. Sure, the friction of well torqued bolts helps but you've ultimately got to design something like that to hang together on the bolts alone.
Failure modes
You should consider all failure modes
The bolts could break (tension failure) bolt area times number of bolts
The bolts could hold and the backing plate could tear out the fiber glass. In that case it is shear failure of the smallest sheared area or the perimeter of the backing plate times the deck thickness.
The shroud wire to chainplate fitting could tear out. pin diameter times the plate thickness times the distance parallel to the load from pin hole to edge of plate (shear failure) or the width of the plate at the pin depth times the plate thickness (tension failure)
Shroud wire to chainplate fitting pin failure (shear failure) 2 times the pin diameter area
As long as the backing plate shear failure is guaranteed to be last (never happens in other words) you should be fine. I’d design a gradual failure so you have some time to unload the rig before it comes down but that has been meeting with some poo-pooing lately.
You should consider all failure modes
The bolts could break (tension failure) bolt area times number of bolts
The bolts could hold and the backing plate could tear out the fiber glass. In that case it is shear failure of the smallest sheared area or the perimeter of the backing plate times the deck thickness.
The shroud wire to chainplate fitting could tear out. pin diameter times the plate thickness times the distance parallel to the load from pin hole to edge of plate (shear failure) or the width of the plate at the pin depth times the plate thickness (tension failure)
Shroud wire to chainplate fitting pin failure (shear failure) 2 times the pin diameter area
As long as the backing plate shear failure is guaranteed to be last (never happens in other words) you should be fine. I’d design a gradual failure so you have some time to unload the rig before it comes down but that has been meeting with some poo-pooing lately.
More poo-pooing:
Rigging stresses are limited by vessel stability. It's therefore possible to design a rig that simply won't fail from wind force. Unless, of course, the strength of some part degrades due to corrosion or fatigue. Once you are allowing that to happen through poor maintenance and inspection, all your careful calculations to make sure that something non critical breaks first go out the window. It may be a part other than your planned weak link that degrades and goes first.
A simple rig is is like a chain. How do you propose designing the links for this gradual failure mode? You could design a stretchable part with an indicator that tells you to reduce heel or luff up when a critical stress is reached. Maybe a strain gauge on a shroud with a read out in the cockpit and possibly an alarm.
My boat already has that, a big needle called the mast backed up by the water surface and the cockpit coamings. When the mast gets to be greater than 45 degrees to the horizon, fairly easy to measure mentally, and the water is pouring over the cockpit coamings, it's time to do something. The screaming of the crew and guests works well as an alarm system and isn't subject to the electrical problems of wiring up a strain gauge in a salt water environment. The rig should be no where near a critical stress point in these conditions and the strain on it isn't going to increase any further with wind velocity since righting moment will be dropping off.
The stress that breaks an intact rig in which all parts are up to their designed strength will be something like a wave induced roll over. We're talking about massive forces here that few rigs can be designed to withstand. You aren't going to take any stress reducing actions in that situation. The most vital part of heavy weather sailing is avoiding a roll over.
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