Why are Chain Plates Most Often a Straight Strap?

[thinwater]

Is this just tradition, from when boats were wood and any one area could rot?


This topic came up on another form (YWB) regarding JSD plates, and whether other shapes and patterns were more sensible came up. In that case the direction of pull also varies, one more reason why a plain strap is sub-optimal. But none have failed, and that, I'm sure. is why Jordan suggested staying with something proven. But he said that this was the basis of a simple design, not the best or only choice.



For example, Pogo and Lagoon use nearly circular patterns. Certainly, the layup under the plate is calculated to match.




The down sides of thin strap plates:

• The bolts create a tear-here line. This roesn't really matter, since that never happens. But with wooden boats, rot was a concern, so a long plate down a rib was a conservative approach. Fore or aft of the rib there was nothing to bolt to.
• The most common failure is corrosion just below the deck. The length of the strap doesn't even matter, since it fails at or before the first bolt.
• The bolts below the first few are barely stressed. As the load comes on the stainless strap stretches. The first bolt carries half the load, the second 30%, the third 20% and so forth. The only way the lower bolts carry any load is if the plate is way over strength and if the upper bolts are in softened wood and shift (which was very likely, over time, in wooden boats).
• Cheap to make one-off.

Since nearly all boats are composite, and we can lay the hull or bulkhead up any thickness we like, why not a shorter, wider plate, like the Pogo or Lagoon? Just lay up an oblong area that is much stronger. Actually less reinforcement needed than for the long, straight plate. Easy to taper off.

• Less vulnerable to corrosion because there is far greater cross section. We've established that the metal farther down the row is basically wasted.
• Better at handling off-access loads. Matters for JSD plates, but not so much for standing rigging.
• More bolts sharing the load. More bulkhead/hull actually working.

And then there are composite chain plates. Normally built as part of the hull, they always have a distributed load path. Bit I've read of some retrofits. There may be some advantage if the bulkhead is sketchy and somehow a metal replacement is tricky.

https://www.youtube.com/watch?v=8p5E45RnHEE

Of course, anytime the old holes or load path is worn or rotten, changing the shape of the plate to span over it could make sense. Other times they could probably just be moved a few inches.


My chain plates are coming due, so just a discussion starter. I will probably replace them in kind, unless a compelling reason is presented. For example, they are a funny shape and may be difficult to source. It may be as easy to redesign.

[Bowdrie]

Originally Posted by thinwater

The down sides of thin strap plates:[LIST][*]The bolts create a tear-here line. This roesn't really matter, since that never happens. But with wooden boats, rot was a concern, so a long plate down a rib was a conservative approach. Fore or aft of the rib there was nothing to bolt to.[*]The most common failure is corrosion just below the deck. The length of the strap doesn't even matter, since it fails at or before the first bolt.[*]The bolts below the first few are barely stressed. As the load comes on the stainless strap stretches. The first bolt carries half the load, the second 30%, the third 20% and so forth. The only way the lower bolts carry any load is if the plate is way over strength and if the upper bolts are in softened wood and shift (which was very likely, over time, in wooden boats).[*]Cheap to make one-off.

You have come-up with a list that would take several pages to even begin to address.
That said
None of the three main "Scantling Rules" as developed by Lloyds/Nevins/Herreshoff, for the construction of wooden yachts proscribe the fastening of chainplates over a rib/frame, it's universally recognized as bad practice.
Corrosion was not an issue when yachts of even 2nd. rate quality had bronze chainplates.
No, the chainplates don't stretch, the planking of carvel construction in essence produces a boat that might be referred to as a "wicker basket", they bend and the planks "weave", over some time the fasteners end-up sharing the load more than you might think.
Cheaper? But of course, bar stock is one of the original "formations" of usable metal(s), rolled plate stock is a relatively new process and never cheap.
Other posters I'm sure will address other things.

[thinwater]

Originally Posted by Bowdrie

You have come-up with a list that would take several pages to even begin to address.
That said
None of the three main "Scantling Rules" as developed by Lloyds/Nevins/Herreshoff, for the construction of wooden yachts proscribe the fastening of chainplates over a rib/frame, it's universally recognized as bad practice.
Corrosion was not an issue when yachts of even 2nd. rate quality had bronze chainplates.
No, the chainplates don't stretch, the planking of carvel construction in essence produces a boat that might be referred to as a "wicker basket", they bend and the planks "weave", over some time the fasteners end-up sharing the load more than you might think.
Cheaper? But of course, bar stock is one of the original "formations" of usable metal(s), rolled plate stock is a relatively new process and never cheap.
Other posters I'm sure will address other things.

Good point about the ribs. But the whole wooden boat things was just a starter. Let's not wander of into the woods. I'm sure it is different in many ways, but it is also far in the past. That was a point. You did not discuss the positives of broader load distribution, which was the main point point.

The real, and only important question for most of us, is whether strips are still the right answer. Obviously, Pogo and Lagoon don't think so. There are others.

For example, the direction of pull on the tack of a sail is basically in one direction, but you wouldn't use a single strap of webbing to transfer the load. It's an intentional exaggeration to make a point, but I'm pretty sure if you did FEA, you would find a strap is not optimal. Pogo and Lagoon did use FEA.

[Bowdrie]

Originally Posted by thinwater

Good point about the ribs. But the whole wooden boat things was just a starter. Let's not wander of into the woods.

You did not discuss the positives of broader load distribution,

I covered the "starter" points you started with, not heading into the woods.
As to the rest of it, that's why I ended with; "Other posters I'm sure will address other things".
"Discuss the positives of broader load distribution"?
Why? The positives are self-supporting, they need no one to defend them.
But there will always be those who feel the wheel needs to be made rounder.

[s/v Jedi]

They all copied that from Sundeer.

I don’t have good pictures but it has smart reinforcement on the inside, as well as a rubber gasket on the outside for zero leaks.

[TeddyDiver]

A bit aside but I don't really understand why anyone would use metal chainplates instead of composite on composite boats.

[Don C L]

The old Bounty IIs had triangular chainplates bolted through the bulwark. I wish I could find a good shot of those.

[Benz]

When I recently replaced my staysail lead chainplates (they were bronze, but the slot in the deck they went through was susceptible to leaking), I made composite ones. I have a blog post about it:
https://zartmancruising.com/returning-to-normal/
It involves some surgery and some skill in composite work, but my opinion is that composite chainplates are superior to bolted-on ones. If ever I build another boat, I won't even consider bolted-on: composite chainplates with targeted directional fibers will be incorporated into the hull design.

[crosswire]

Cape Dory boats terminated the shrouds at bronze pad eyes on the deck. Presumably, the deck and hull were reinforced under the pad eyes in some manner that was not visible without tearing out some furniture. No evidence of straps connected to bulkheads or separate knees.