Join Date: Nov 2008
Location: Deale, MD
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Here is a plate with a latch.
OK, first I found source for a commercial plate: ABI. It is 5/16" thick SS with no rated working load. Workable, similar to Sailingdog's plate, and I found some sailors with experience. One, with a 39" 12,000# Privilege cat is using one on a circumnavigation and is happy with it. He noted that it has distorted a bit, as the cat sails at anchor sometimes. I am sure he has used it hard.
Though I have worked with chain hooks, I have always wonder if they could come off, however seldom. A few sailors noted that yes, it has happened to them. Two noted that their hooks had released when their bridle touched bottom, which is not uncommon for a cat at slack tide. Perhaps a rock or the mud lifted it off - we do not know. Thus, I came up with this design, in red.
It is 3" x 4" x 3/8" plate, galvanized after fabrication, A-460 steel, as I recall. The calculated working load is 3200# (one leg attached, gate open) and I pulled it to 5500# (one leg attached, gate open) with no permanent distortion (I have a test rig, used to destroy rock climbing gear - my other passion). With both legs attached and the gate closed, the working load is ~ 10,000 pounds, but it does not matter because no other link in the system is so strong. I have since replaced the gate shown in the photograph with a thicker piece of 316 stainless, for better feel, durability, and strength in the gate, should there be some odd bind.
Some have commented on the longer slot and strength; yes, the arm is longer but the angle is less and the calculations show only minor differences. I did pull test it. With the gate in place it is MUCH stronger, just as a carabiner is 3x stronger closed with gate (thay are generally rated closed gate, but sometimes marked with open gate ratings as well). I sized it for my 8000# cat, so if you have something heavier, adjust accordingly. Also the carbon steel plate I used is tough but not exceptionally strong; there are better materials.
The hole at the bottom is for attaching fiber rode (rolling hitch) should I let out more line and run out of chain. If you have all chain, delete it.
Thoughts? My blog has another photo and additional discussion.
Perhaps it is mostly a catamaran thing. If I like it well enough - I have only used it twice - I may fabricated in 304 or 316 SS in the Fall. I suspect I will find 5/16" is then enough for my boat, and as strong as the other components in the system.
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Last edited by pdqaltair; 03-16-2009 at 07:03 PM.