Thin intermdiate stay
 

I have a 45ft mast with double spreaders on a 40ft 1980's fibreglass sloop.

The mast has forward and aft lowers, plus a cap shroud. This cap shroud is so-called "discontinuous" - a single 3/8" wire runs from the deck chainplate to the tip of the first spreader. Upwards from the tip of this spreader there are two wires:
1) a 5/16" wire runs to (near) the top of the mast.
2) a 1/4" wire runs diagonally to the base of the upper spreaders (intermediate stay), starting with a turnbuckle at the spreader tip.

So, 3/8 wire into the spreader tip, and 5/16 plus 1/4 wires going out.

I am in the process of replacing my rigging wires using Sta-Lok terminals for all terminations (prev swaged). My question is whether it makes sense for me to upsize the 1/4" intermediate stay to 5/16".

I checked the breaking strengths of the three wires, and the 1/4 plus 5/16 combination comfortably covers the strength of the 3/8 from the deck.

The current rigging sizes were used when the boat was launched. Dockside comments suggest I change to 5/16", but a local rigger said he would just stay with the original, although he could not explain why 1/4 was used.

It is such a short stay that weight saving would not really be significant. You'd think that the 1/4" intermediate stay takes a smaller percentage of the load compared with the 5/16" cap shroud, but I'm not sure.

So, I thought I may pose the question to the rigging experts on this forum - why was 1/4" used originally and is it a good idea to upsize to 5/16" for offshore cruising?

My boat has 8mm uppers and lowers along with 7mm intermediates, so it must not be unusual for the intermediates to be a bit smaller. I don't have the "split" so I can't offer advice, other than when I replace my intermediates, I'm going to up them to 8mm since I can use 3/8" rigging.

Seems to indicate less load on the intermediates, but not clear "how much" less.
Did you make a typo, or are you really going with a big jump from 7mm to 3/8 (which is 9.5mm or 10mm)?

I meant 5/16th. I recently replaced my uppers (survey found a cracked swage) and the rigger suggested using SAE turnbuckles with 5/16 wire (and a special order metric toggle for the chainplate).

Numbers
 

Hello,
Classical load distribution calls for the V2's and D2's being the same size (See the Apprentice, but on most boats, even cutters, more goes to the upper panel, thus the smaller D2. Note that a combination of 5/16" and 9/32" is also an option, and would preserve the existing pin sizes. The question, as always, is "what is the load?" Since this is a sloop, the only lateral load at this height comes from the mains'l. So if there are no signs of accelerated fatigue in the old wire, stay with 1/4".
Fair leads,
Brion Toss

Appreciate your input Brion. Your comment re 9/32 preserving the existing pin sizes is a very good point.
Thanks.
Martin