November 19, 2009

Getting the tension right

OLD WOTSISNAME IS WORRIED about the rigging that holds his mast up. “It’s tight now,” he said, twanging a wire, “but when I go sailing, the leeward shrouds always feel loose.”

“So what do you do?” I asked.

“Well, last time I just tightened up the leeward turnbuckle to take up the slack. And when I went about onto the other tack, I tightened that turnbuckle up, too.”

“And what happened?”

“Nothing. The leeward shroud is still slack. Always.”

I plucked one of OW’s shrouds. It was as tight as a violin string. “You could play Mendelssohn’s violin concerto on this wire,” I said. “But that’s the good news.”

“What’s the bad news?”

“You’re driving your mast down through your deck,” I said. “The rigging is far too tight. A few more turns on the turnbuckles and your cabintop is going to end up on your cabin floor.”

What OW didn’t realize was just how powerful the turnbuckles are. And how he could overstrain the rig by trying to eliminate the slackness in his leeward shroud.

That slackness is always going to be there. The old rule of thumb is that the leeward shroud should feel slack, but not look slack, when you’re beating in a medium-strength wind.

The reason is that stainless-steel wire stretches. There’s construction stretch for a start, which results from the strands settling into place when the first load is applied. That’s a once-only, permanent stretch. And then there’s elastic stretch, which is temporary, allowing the wire to return to its original length each time the load is removed.

Now the elastic stretch may be greater than you imagined. For example, when a 33-foot stainless-steel 1 x 19 wire (of ANY thickness) is loaded to half its breaking strength, it will stretch 2 inches. That’s why the leeward shroud goes slack. It’s fine. It’s meant to.

“So what should I do now?” said OW.

“Loosen it all and start again,” I said. “I’ll lend you my tension gauges.”

The first thing to do in tuning a rig is to find out the displacement of your boat. Then give the upper shrouds and the backstay a tension of approximately 10 percent of the boat’s displacement. A higher tension will automatically be induced in the forestay because it makes a narrower angle to the mast than does the backstay.

Then tighten the forward lower shrouds or babystay until the mast bows forward slightly, but noticeably, at the spreaders.

Tighten the aft lowers to straighten the mast again.

When you’re out sailing in a moderate breeze, check that the mast is straight and adjust the upper or lower shrouds with equal turns of the turnbuckles on both sides, slackening the port side if you tighten the starboard side, and so on.

That’s all there is to it.

Now, if you’re still awake and you’ve been concentrating hard, you may think that 10 percent is quite a lot of tension to preload into the topmast shrouds, and it is; but it’s about right.

The breaking strengths of all the shrouds on one side of the boat should equal a little more than the boat’s displacement, say displacement times 1.4 for offshore cruisers, times 1.2 for inshore cruisers, and times 1.0 for racing boats and daysailers. When you have double lower shrouds, however, (that is, forward lowers and aft lowers) then you should use only one shroud for this calculation, the thinking being that only one lower at a time carries the load.

I don’t know how much extra load OW was placing on his deck, but I can tell you that the normal compression load on a mast step while you’re sailing is as much as 2.5 times the displacement. Now OW’s concrete barge must weigh 15,000 pounds, so his mast is thrusting downward with a force of 37,500 pounds or nearly 17 tons.

Frankly, I’ve never quite understood how thin-walled aluminum tubes can withstand loads as high as that, but they do it all the time. To me, it’s just another of those bits of magic that make sailing so interesting.

Today’s Thought
Technology is so much fun but we can drown in our technology. The fog of information can drive out knowledge.
— Daniel J. Boorstin, Librarian of Congress

Four-year-old Janie had been put to bed for the night when her little brother wandered along and tried to enter her room.
“You can’t come in, Jimmy,” she said, “cos Mom says little boys mustn’t see little girls in their nighties.”
Jimmy went outside, closed the door, and was puzzling about this when the door opened again.
“It’s aw wight Jimmy, you can come in now,” said Janie. “I’se tooked my nightie off.”

1 comment:

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