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Broad Reachin'
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I'm looking at a used boat that's set up with a single lower shroud on each side that is then connected to the same chainplate as the upper shroud. What are the pros and cons of this type of rig? My current boat has the more common arrangement of one upper shroud and two lower shrouds per side connected to their own indiviudual chainplates.
 

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Wandering Aimlessly
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My Hunter 26 had that arrangement (also no backstay). I had no problems with it.
 

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The Pearson 26 I used to own is rigged that way. Main problem is "pumping" of the mast in strong winds. This is due to the lack of fore-and-aft support that multuiple lowers and/or a baby stay provides. Annoying but not a problem as long as the mast is strong enough and stays "in column".
 

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It's not too uncommon, especially nowadays, even on some no-nonsense cruising designs.

Depending on how the rig is "sprung" and how solid the chainplates are, it might not be any less robust than having double lower shrouds. Usually, you'd like to see a bit of pre-bend tuned into the rig, with aft-swept spreaders, and the chainplates positioned aft of the mast base (rather than even with it.) Sometimes this would include a babystay too.

On some earlier designs, this was not always the case. Often, the single lower shroud and upper shroud were tied into a single chainplate even with the mast, with perpendicular spreaders. That is an arrangment that would give me some pause for serious off-shore work.

In the mid-late 90's (I think), Practical Sailor published the results of some research they did on why quite a few folks were experiencing mast failures while sailing under headsail alone. It had become an issue because, with the advent of roller furling, more sailors were opting for a lazy sail under headsail alone. Practical Sailor found that one almost universal common denominator among the boats that lost their rigs while sailing under headsail alone, was that they had single lower shrouds.

A few years back, the late Robert Gainer and I had a discussion about this subject here on SailNet. I seem to recall that Robert was somewhat skeptical that the single lower shroud could be the problem with the dismastings, so you might want to dig up that old thread and review his thoughts.
 

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The loss of a lower shroud on a rig with single lowers will often result in loss of the mast when under sail in med to heavy air. The mast will fold like a jackknife and typically break at the spreaders. The loss of an upper shroud on such a rig will usually not result in mast loss since there is less pressure (i.e. sail area) up high. Of course, a lot depends on the rig and the strength of the mast itself.
 

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It is common on smaller boats to have only single lowers.

On larger, more racy boats, it is also common to have only single lowers, since forward and aft lowers would be too limiting of one's ability to bend the mast and thus re-shape the mainsail. The disadvantage is that the mast can tend to "pump," potentially requiring the use of check stays or running backstays to control.
 

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The loss of a lower shroud on a rig with single lowers will often result in loss of the mast when under sail in med to heavy air. The mast will fold like a jackknife and typically break at the spreaders. The loss of an upper shroud on such a rig will usually not result in mast loss since there is less pressure (i.e. sail area) up high. Of course, a lot depends on the rig and the strength of the mast itself.
I have to disagree with most of this (unless there are typos in there).

Loss of an upper shroud on the windward side of the rig will certainly result in mast failure at the spreaders, with the now-unsupported upper panel folding like a cheap straw.

The loss of a loaded lower shroud may cause a mast to fail, especially if the crew is not quick enough to tack and unload that side, but the mast can tolerate significant side bend before failing under those conditions. I'd think too that deck stepped vs keel stepped could make a difference as well.

Single in-line lowers are common, as mentioned before. Not a reason to reject the boat unless, as JRP alluded to, you are planning some serious offshore passagemaking.
 

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Old as Dirt!
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While an older design, the Irwin 28 MkIV, is a prettry sturdy boat. Moreover, the yacht only displaces 7800 lbs. Accordingly, if the standing rigging is good, it is more than sufficient for the boat. Given the age of the boat, and the relatively minor cost, unless the rig has been renewed, it might be wise to very carefully examine and perhaps replace the chain-plates and replace the wire. With that you should have no difficulties, at least with the rig.

FWIW...

s/v HyLyte
 

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To clarify my earlier post (that Faster took exception to) - it was an attempt to point out that a lower shroud can be as important (in some rigs) as upper when the rig is under load - for example in more flexible fractional rigs where the uppers attach to the mast at points lower than the masthead. Sorry to confuse.
 
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