Chain Plate Sizing

[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

[Storm]

Originally Posted by 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

Hi Brion,
Thanks for your input. The S&S drawing is dated 1968 (S&S 34 design #1959). The drawing calls out for the forestay and backstay to be 5/16 inch BSF wire with a breaking strength of 9,640 lbs. The fore and aft lowers are called out 1/4 inch BSF wire with a breaking strength of 5,000 lbs. The drawings do not say if 1x19 or 7x, and I am curious as to what the "BSF" means. I plan to use 316 ss 1x19 wire.

[Brion Toss]

Hi again,
I do not yet know what BSF means for wire (for threads, it means British Standard Fine), but I do know that the wire in use today is significantly stronger than what S&S listed. See here: http://www.sailingservices.com/stand...d_1x19wire.htm. And here: http://www.riggingonly.com/wire.htm#WIRE. 9/32" for your uppers. Smaller for the lowers and backstay.
Fair leads,
Brion Toss