Chain Plate Sizing

[Storm]

I'm in the process of replacing chain plates. Plan to copy the chain plate size currently on the boat. That brought up a question. I have a mast head sloop rig with forestay, back stay, single upper shrouds and forward and aft lower shrouds. Why would the upper shroud chain plates be the thickest (3/8 inch) and the longest (28 inches and 1-5/8 inch wide) when compared to the back stay which is 1/4 inch thick and only 20 inches long (1-5/8 inch wide). Both are using 3/8 inch diameter wire. Would not the back stay see equal if not more loading than the upper shrouds, and therefore should not the backstay chain plate be at least equal in size to the upper shroud chain plate? Also note the fwd and aft lowers use chain plates sized equal to the backstay chain plate. I should note both chain plates have survived nearly 40 years as designed, and the boat (34 footer) has seen at least 70 knots and 50 foot seas, but still want to be sure it built to standard designed specs.
Regards

[Brion Toss]

Hi,
I'd need to run the numbers, but the odds are good that the chainplates are the right size, and the wires not the right size. The uppers on your boat take about 45% of the total transverse load, so their chainplates should be thickest (along with the jibstay). The lowers share the balance of the load, so would see only a bit more than half of what the uppers bear. Likewise the backstay, which takes less load than the jibstay or uppers.
But really, either you or I or someone should make sure of the load magnitude and distribution before you replace those chainplates.
Fair leads,
Brion Toss

[Storm]

Brion,
Gave the wrong wire size. All wires are actually 5/16 inch (not 3/8 inch as I said in first post).

I did some calcs and figured worst case the upper shroud chain plates would see about 4,000 lbs load. I have not found ways to calculate back stay loading. When is the back stay most loaded? When the boat is on a beat or a run? I would think most loading would occur when running (unless you are a racer and are trying to bend the mast in a beat), especially when flying a spinikar- but that is just my uneducated guess.
Regards

[Brion Toss]

Hi again,
Contrary to what intuition might tell you, backstay loads are lowest, by far, when on a run. Apparent wind is what loads the rig, so the real force comes on a beat. And even then, the backstay takes less load than the jibstay on almost every sailboat.
As for the wire size, 5/16" is the largest that you need for the uppers, and 9/32" might do. Lowers smaller still. What are the pin sizes?
Fair leads,
Brion Toss

[Storm]

The pin size for all the existing chain plates is 1/2 inch. I was able to obtain the original S&S designs specs for the boat's original design and all the wire was specified as 5/16 inch except for the forward and aft lowers that was specd for 1/4 inch. The chain plates were specd at 3/8 inch for back stay and upper shrouds and 1/4 inch for the forward and aft lowers. The dimensions given for the chain plates look to match Skene's.

Looks like I should just up size the back stay chain plate to 3/8 inch thick. I plan to just stay with the 5/16 wire all around. The original S&S design (1968) called for wire to be "BSF". Would you know what that stands for? A search shows no reference to BSF and just curious what it might have met.

[Brion Toss]

Hi again,
Oh dear, the original specs. What is the date on the plans? Even in older versions of Skene's, 5/16" takes a 5/8" pin, unless the wire is a 7x construction; any details available on construction? If the wire is described as 1x19, then either the recommended chainplate has too small a hole, or the wire is oversized for the load. I can tell you from experience that even the justly revered house of S&S was capable of making mistakes.
Please consider at least coming down a size for the backstay; either 9/32" or 1/4" take a 1/2" pin, and the lower will bear significantly less load than the jibstay does, so there is no sense in making both the same size.
Fair leads,
Brion Toss