Fairleads > Parrels
January/February 2006
A group of square-rig sailors was gathered 'round, talking about interesting problems they'd encountered while under sail. Given the complexity of the rigs they dealt with, and the shortage of crews truly qualified to deal with that complexity, the stories tended towards the scary/hilarious. It's amazing how much trouble one can get into when confronted with high winds, steep seas, huge sails, and several miles of running and standing rigging.
One skipper was describing what happened when, in an extremely lumpy sea, the main yard parrel on his boat let go. This parrel, on pre-19th-century-style ships, is essentially a massive, very formally-constructed lashing. It holds the yard to its mast, yet allows the yard to pivot and tilt, as well as to be raised and lowered. If a parrel breaks, the yard is still restrained by the tie or halyard that provides overhead support in the middle, as well as the lifts that suspend the yard, the robands and head outhaul lashings along the top, and the sheets at the lower corners. But even with all these attachments intact, failure of the parrel means that the yard, which in this case weighed around half a ton, can and will slide a good ways laterally, as well as swing fore and aft, bashing the mast and tearing up assorted gear, both its own and that of adjacent sails. That's why, in this instance, an experienced hand went very carefully aloft to constrain the yard. All eyes were on him as he struggled to lasso and lash it back into place.
"And of course," said the skipper telling the story, "it's especially bad when something like that happens." And another, greener skipper looked puzzled and asked, "Why's that? Because the crewmember is in danger? Because the sail can carry away? Because the yard can break, or the parrel get caught in the –"
"No, no," chorused several old salts, and then the first skipper explained, as the others nodded agreement, "It's because that's when the generator catches fire."
Now the young skipper was really puzzled, because how could a parted parrel have anything to do with a flaming generator?
The old hands weren't in a mood to explain, and the conversation drifted on to other topics. But later on the youngster cornered the storyteller, insisting on an answer. "Well", came the reply, " let's start with redundancy. I recently had my house rewired, and got into a bit of a set-to with the electricians about backups and failsafes. Working by the codes, the redundancies are about three deep, which is why it is very unlikely that a properly-wired house will either burn down or electrocute its occupants. But these guys treated the failsafes as if they were some sort of optional cushions, layers of protection that they could choose to bypass some of, in the interests of getting the job done quicker or easier."
"Well that's terrible and stupid of course," said the young skipper, "but I don't see how that has anything to do with the –"
"Just wait, there's more to it. See, even houses that don't have all those safety layers are pretty safe; it takes extraordinary circumstances to cause an accident. The codes are written for circumstances that hardly ever happen, but they apply for every house, because nobody can know when or where those circumstances might arise. Skipping some of the backups is going to terrify anyone who has actually dealt with the possible consequences, but it's sort of understandable if you've grown up in houses made quite safe by those very same codes – you know, it's like what's the big deal? Wiring never hurts anybody, right?"
"Okay, so you're saying that there weren't enough redundancies in the rig, and that's why the parrel failed."
"Not exactly. It's true that it takes layers of protection to avoid mishaps in the long run. In the case of the lower yard parrel, there was first a redundancy in scantling. That is, the thing is – or should be – way stronger than the loads require. It should never break. In addition the yard is part of a system comprising several other yards, as well as sails without yards, any several of which we could get along without.
"And finally, the actions of the crew – regular inspections, cycling of materials, ongoing awareness – form another layer of redundancy.
"The lower yard is perhaps the most important piece in its system, since it's the biggest, and because all the other yards are sheeted to it, directly or indirectly. You can still work around it, but without that parrel it can do a lot of damage, which is why we sent someone up there to secure it. So you could say that the parrel's redundancies failed, but hey, that can happen. What we're talking about here, though, is a larger system, with deeper opportunities for chaos. Let's look at the generator now."
"At last, the generator."
"This was one of two, both good machines, both properly maintained. Nothing the matter with them, lots of warnings and failsafes. But when they were installed, there was a flaw in how the venting was laid out. Nothing really bad about that, just one less layer between you and catastrophe. But as it turned out, it was precisely the kind of flaw that could be exploited by violent vessel motion, with water pouring into places that it isn't meant to pour, and merrily shorting things out.
"Meanwhile that yard is thrashing around, some poor sod is aloft, trying to secure it without getting crushed, the wind is still screaming, the boat is lurching to and fro, and everyone's attention is either on handling the ship or watching the drama aloft. That's why the only person who sees the smoke coming out of the deck, way aft, is the person aloft. He shouts, but of course no one can hear a word. He waves and points, but everyone assumes that his gestures have something to do with the yard. They're scratching their heads and shouting at each other and at him in an attempt to confer, and all the while that smoke is getting thicker."
"This is just an example, isn't it?", says the young skipper.
"Exactly. It could just as easily have been someone falling down a hatch, or a line about to get wrapped in the prop, or any of a hundred other things.
"The point to remember is that catastrophes don't pick on one weak spot at a time; they're after all of them, always. So when one thing fails, even though you might have your hands really full dealing with the consequences, it might be a good idea to look around for whatever else might be failing. And when you find that, you'd better start looking around for another."
" That would also mean," said the young skipper," that when the smoke has cleared, and you've cleaned up whatever godawful mess has resulted, the repairs should involve putting in layers of redundancy where they were lacking. Otherwise you're just setting yourself up for the next catastrophe."
"Ahhh," says the old hand.
Finis
Brion Toss