Why should uppers be thinner than lowers?

[cmm]

Originally Posted by Brion Toss

Hi again
I prefer to put a heavy-duty thimble in, just as I would for wire. The radius of the clevis pin would be a bit minimal, and there would also be the possibility of chafe, unthimbled.
As for the shrouds, how about 5/16" for the lowers?
Fair leads,
Brion Toss

I apologize for entering this thread from the side, but my eye caught Brion's recommendation to keep the lowers 5/16 instead of 3/8.

In this game it is very easy for novices like me to think, "the thicker the better".

The only counter advice to oversizing my rigging wires I have heard, came from a fellow yacht owner who feels it is better that wire is the weakest link - if it was not, something more threatening to yacht integrity may suffer damage (e.g. chainplate may rip out or a bulkhead may be damaged) in the event of an abnormal stress.

Brion, is your advice to Matt to reduce his lowers form 3/8 to 5/16, at all related to the same consideration - i.e. if the wire is too strong something else may break, or is there a different reason?

Re-framing it as a more general question, heavily oversized rigging wire will resist breakage more; but are there any negative, structural consequences (setting aside things like wire and terminal costs, windage and weight)?

Thanks
Martin

[Matthew Sebring]

As a partial response (based only on my limited understanding on the subject - and this is removed from the obvious such as weight and windage). In order to get the slack and stretch out of a wire you need to tension it to a specific point which usually ends up being a percentage of it's ultimate strength. To get that same performance out of a larger wire you need to tension it somewhere in the same percentage range. However, since the breaking strength of that wire is so much larger the corresponding tension you need is much higher delivering much greater loads to the boat and rig.
Take an overly simplified and extreme example: Let's say we've got a columbia 22 with some 3/16th wires. We've decided we're going to take this boat around the world and simply don't believe the 3/16 wires are strength enough so we beef it all up with 1/2" Now you can easily visualize that tension needed simply to take the slack out of the shouds and the sag out of the stays would be enough to damn near punch that tiny deck supported mast right through the cabin top.

[cmm]

Thank you for the input - I didn't know that 1x19 wire had to be pre-streched (like pre-stretched nylon rope) before or during initial use.

However, one can imagine that a Valiant 40 could handle the tightening of its lowers, whether they are 5/16" or 3/8" in diameter. So are there any other structural issues that may come into play?

Regards
Martin

[Brion Toss]

Weight, windage, and cost are far from trivial issues; any one of them might make for a compelling reason to go with smaller wire size. And the loads needed to tune oversize wire, as pointed out above, are not trivial either. But I don't get the feeling that we are talking about the same thing when we say "structural difference." If I understand you correctly, you think that objections to 3/8" lowers are minor compared to some imagined benefit(s). It is at times like these that I become aware of how evangelistic I can be on this subject. Really, I know I take it all too seriously. But what we are talking about here is a series of relationships in an astonishing, elegant machine. Nothing in a well-designed rig is arbitrary. It isn't a matter of having the wire break before something else, it is a matter of everything being in harmony. And there is no benefit in having it out of harmony. Yes, bigger wire is stronger, but what does that get you, if the loads don't require it, except more frequent reefing (more weight, more windage), more compression on the mast, more tension on the hull, an unsatisfactory tune, and thus worse performance, and more expense? Once you have met the loads, with whatever the appropriate safety factor is included, going bigger is not better.
I recommend going heavier where it is needed. If you want to hoist large heavy objects over people's heads, a safety factor of at least 10 to 1 makes sense, but safety is heavy, and a sailing vessel can only afford so much before the weight results in danger.
Think of this question as a window into your rig. Look at the whole thing, and discover the elegance that is available to you there.
Fair leads,
Brion Toss

[cmm]

I wish there were more evangelistic operators in the yachting industry - I'd love my engine installer to become evangelistic about using stainless fasteners in my bilge.
Thanks, I fully take your point, Brion, and re-reading your earlier post "the lowers should be sharing less than half the total load", I interpret that to mean a pair of fore and aft lowers working together share less than 50% of the total load on one side of the boat, and so each lower carries less than 25% of the load (and therefore need not be as strong as the 3/8" upper shroud).

Although it hasn't been said in the thread, I presume Matt's Valiant has Upper and Intermediate shrouds, and that the "total load" referred to by Brion is the sum of the Upper, Intermediate and Lower loads.

Martin

[Brion Toss]

Hi again,
And oops, I mispoke. The lowers share about 55% of the total transverse load in this rig, so each of them takes less than half of that load. Because the load moves around some, due to things like reefing and dynamic motion, it is prudent to make them a bit stronger than 22.5% of the total, but each wire will still bear significantly less than the upper, so can be smaller.
In our example, if the uppers are 3/8" at 17500lbs, and that is something like 45% of the load (the upper's share), then 55% would be on the order of 21000lbs. Half of this is in the neighborhood of the strength of 5/16".
Is that clearer? Thank you for bearing with me.
Fair leads,
Brion Toss