This month, I'd like to discourse on one of those long-term destroyers of rigging, one of those it-doesn't-seem-like-a-big-deal-at-first-but-just-wait-a-while problems that can bring a boat to grief: Point Loading. Any time you have sloppy fit of clevis pin to clevis hole, point loading results; the pin only contacts the rim of the hole at one tiny point, so the pounds-per-square-inch force is immensely greater, resulting in a weakening of material, as well as accelerated fatigue, metal deformation, and chafe. Bad.

   Ordinarily, point loading is very easy to avoid, since it's easy to drill standard holes for standard pins. Ideally the hole is just a teensy bit larger than the pin -- 1/100" to 1/64" larger -- so that the pin won't jam, gall, or corrode in place. The slight extra clearance is slight enough that the pin can deform a bit under load, presenting a full bearing surface to the rim of the hole: no point loading. You'll find more on this in the "Apprentice".

   Inattentive machining and pin selection aside, the most common, maddening source of point loading is the combination of metric and standard pin sizes. There are those who would suggest that the easiest way to deal with this problem would be to stop using the standard measurements and adopt the allegedly more logical metric system. Nonsense. The only thing useful about the metric system is that you can divide things by ten, and of course that is an option that our system of measurement provided for, long before the invention of the meter. Besides, standard pin sizes are so pervasive throughout the world, even in metric countries, and especially in yacht rigging, that it makes more sense to drop the metric sizes.

   I'd be happy to continue this diatribe at a later date. Meanwhile, the topic is what to do about point loading. Just how serious is it? Selden Spars asked that question, and followed it up with tests, and discovered that, with static loads, you can expect about a 5% decrease in strength for every 7% increase in hole size above the ideal. For example, if you put a 1/2" pin in a 5/8" hole, the hole would be about 25% larger than the pin, resulting in a loss of strength of about 18%. Since 1/2" and 5/8" pins can both be specified for 5/16" wire, this is a fairly common, dangerous mismatch: the turnbuckles might be drilled for the smaller size and the chainplates for the larger. But at least the solution is as simple as either bushing the chainplate hole or getting larger turnbuckles. It can be a bit tougher when the disparity is metric-based, as when one hole is 18mm and the other is 5/8". 18mm is just a 64th" or so shy of 3/4", so you could ream the hole out to fit a 3/4" pin. But that means you'd have to ream the other hole a full 1/8' larger, and that would remove too much metal to leave adequate strength.

   And if you just leave the 5/8" pin, it is in a hole that is over 13% oversized, so you've weakened things by almost 10%. This is comparable to cutting two yarns out of a shroud. Now if any sailor saw two broken yarns in a piece of standing rigging, that piece would get fixed in a big hurry. But mismatched pin sizes are a bit less visually compelling, so they often slip in under the radar. In our shop's Bad Example box, we have a clevis pin and linkplate from a Caliber 40. After just four years, the 5/8" pin was significantly chafed and bent (!) by moderate use. The 18mm hole, in a linkplate almost 1/2" thick had also been noticeably deformed (egg-shaped) by the load.

   You can find similar deformations with other metric mismatches, such as putting a 1/2" pin in a 14mm hole, or a 14mm pin in a 5/8" hole. And these holes can be tough to bush, because the disparity, while plenty enough to weaken the system, is too little to allow for a sufficiently thick-walled bushing. The best thing is to avoid mixing the two systems. If there's already a mix on your boat, I recommend converting everything to standard, as spares and replacements are going to be easier to come by.

   As a final note on point loading, I'll re-emphasize that the above-noted weakening is under static conditions; out in the real world, hole/pin disparities will also result in fatigue and stress-corrosion.

Misplaced Babystay

   Moving on, I'd like to present The Case of the Misplaced Babystay. The subject was a single-spreader sloop, with only aft-led lower shrouds. With this configuration, it is common to see a babystay -- a kind of low forestay, enclosing too small a fore triangle to fit a sail into -- providing forward support for the mast at this level, replacing forward-led shrouds. Because it is easy to make this single wire adjustable, a babystay can also be used to induce and control mast bend, so rigs featuring baby stays properly will have supple masts, adjustable backstays, and hulls sufficiently refined as to benefit from these performance tools.

   The problem was that this boat had a weird S-bend, fore-and-aft, that started right down near the deck, and which the very talented, experienced owner couldn't tune out. It's the kind of thing that can drive a rigger nuts. But the answer lay, not in the tune, or the mast, but in how the stay was attached: someone, at some time in the boat's history, had decided that it made sense to attach the upper end of the stay further up the mast, where a forestay would ordinarily attach. This made for a wire with a very steep angle to the mast, as the tack point was much further aft than a forestay's would be. So the mast was getting a lot of extra compression loading, making it more inclined to buckle lower down. In addition the stay was pulling the mast forward at this level, with nothing to oppose the pull, so the mast was unavoidably distorted there. And finally, the aft-led lowers were pulling aft at their level, with nothing to oppose them, and the resulting aft bend was exacerbated by the compression-induced buckling from the misplaced babystay. All this from moving a single wire a few feet on the face of the mast.

   I wish I could say that this was an isolated case, but too many boats are configured by people with no clear idea of how their designs are going to pervert rig integrity. So be sure you understand the flow of forces in your rig before making even innocent-seeming changes. And if some naval artichoke has put an odd-looking design detail into the original plan, go ahead and try to work out whether it really makes sense. The worst that can happen is that you'll learn something about why it was done that way.

Next month: To Engineer is Human

   Fair leads,

      Brion Toss