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
Tailpiece
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.”
Showing posts with label wire rigging. Show all posts
Showing posts with label wire rigging. Show all posts
November 19, 2009
February 3, 2009
Tuning the rig
ONE OF THE most frequent questions I hear around the marina is “How tight should my stays and shrouds be?” Tuning the rig seems to be a source of deep mystery for many sailboat owners, but there’s nothing mysterious about it at all, really.
What you need to do first is lay hands on a wire-tension gauge. The Loos company makes gauges that work well on boats, with an accuracy of about 5 percent. They’re available from all reputable marine stores and cost between $60 and $90, depending on where you buy them. You may find a used one for far less on eBay, but in many cases you’ll need two gauges, one for wire of 3/16th inch to 9/32 inch diameter, and another for wire sized between 3/32nd inch and 5/32nd inch.
Now, armed with your bought, begged, borrowed, or stolen gauge, you can get to work on your mast’s standing rigging. The first thing to check is whether or not the rigging is strong enough.
Shrouds: All the shrouds on one side of your boat should be capable of lifting a weight equal to 1.2 times your boat’s displacement when she’s fully loaded with all liquids, gear, provisions, and crew. Ocean cruisers should make that figure 1.4 times. In the case of double lower shrouds, use only one lower shroud for this calculation. The other lower should be the same size as the first because the presumption is that the load is carried by only one lower shroud at a time.
Headstays: Make them the same size as the heaviest shroud, or one size bigger.
Backstay: Make it the size of the headstays, or one size smaller.
So now, knowing the displacement of your boat and the number of shrouds on one side, you can check the correct wire size from the following table. It’s for 1 x 19 stainless steel wire, type 302/304. (Type 316, more resistant to corrosion, is about 15 percent weaker.)
Diameter in inches -- Breaking strength in pounds
1/8 -- 2,100
5/32 --3,300
3/16 -- 4,700
7/32 -- 6,300
1/4 -- 8,200
9/32 -- 10,300
The correct tension
The old rule of thumb was that under normal working sail in a moderate breeze, the lee shrouds should feel slightly slack when you wiggle them, but they shouldn’t look particularly slack to the casual observer.
We can do better than that with a tension gauge. Follow these steps:
1. Tighten each upper shroud, and the backstay, to 10 percent of your boat’s displacement. Not 10 percent of the wire’s breaking strength. Ten percent of the displacement. In doing this, you’ll notice that the forestay becomes tighter than the backstay. This is because of the differing angles of attachment to the mast. But don’t worry, it’s a good thing.
2. Now you can tension the forward lower stays, or the babystay, to give the mast a slight but noticeable forward bow at the spreaders.
3. Tighten the aft lower shrouds until, as you sight up the mainsail track, the mast is straight up-and-down again.
4. Go sailing in moderate winds and let the rig stretch into its new position.
5. After a few hours of stretching, and while still sailing in a moderate breeze, get the mast straight from side to side by tightening or loosening the appropriate shrouds. To do this without losing the correct tension you’ve just obtained, you must loosen a turnbuckle on one side a few turns before you tighten up the turnbuckle on the other side by exactly the same number of turns. Never, ever, try to take the slack out of the leeward shrouds by tightening the windward shrouds, and then going about and repeating the action. The turnbuckles are so powerful that you’ll tend to push the mast down through the bottom of the boat.
6. In most cases, adjust the lower shrouds first. They’ll get the mast centered and straight up as far as the spreaders.
7. Then work on the cap shrouds to get the top half of the mast into column.
There we are. Your rig is tuned. Wasn’t so mysterious, was it?
Today’s Thought
He that will use all winds, must shift his sail. —John Fletcher, Faithful Shepherdess.
Tailpiece
At our local city council meeting the other day the chairman of the Works Committee was asked to give figures for how many people are employed by the City, broken down by sex.
“Not too many,” he replied, “liquor is more of a problem for us.”
What you need to do first is lay hands on a wire-tension gauge. The Loos company makes gauges that work well on boats, with an accuracy of about 5 percent. They’re available from all reputable marine stores and cost between $60 and $90, depending on where you buy them. You may find a used one for far less on eBay, but in many cases you’ll need two gauges, one for wire of 3/16th inch to 9/32 inch diameter, and another for wire sized between 3/32nd inch and 5/32nd inch.
Now, armed with your bought, begged, borrowed, or stolen gauge, you can get to work on your mast’s standing rigging. The first thing to check is whether or not the rigging is strong enough.
Shrouds: All the shrouds on one side of your boat should be capable of lifting a weight equal to 1.2 times your boat’s displacement when she’s fully loaded with all liquids, gear, provisions, and crew. Ocean cruisers should make that figure 1.4 times. In the case of double lower shrouds, use only one lower shroud for this calculation. The other lower should be the same size as the first because the presumption is that the load is carried by only one lower shroud at a time.
Headstays: Make them the same size as the heaviest shroud, or one size bigger.
Backstay: Make it the size of the headstays, or one size smaller.
So now, knowing the displacement of your boat and the number of shrouds on one side, you can check the correct wire size from the following table. It’s for 1 x 19 stainless steel wire, type 302/304. (Type 316, more resistant to corrosion, is about 15 percent weaker.)
Diameter in inches -- Breaking strength in pounds
1/8 -- 2,100
5/32 --3,300
3/16 -- 4,700
7/32 -- 6,300
1/4 -- 8,200
9/32 -- 10,300
The correct tension
The old rule of thumb was that under normal working sail in a moderate breeze, the lee shrouds should feel slightly slack when you wiggle them, but they shouldn’t look particularly slack to the casual observer.
We can do better than that with a tension gauge. Follow these steps:
1. Tighten each upper shroud, and the backstay, to 10 percent of your boat’s displacement. Not 10 percent of the wire’s breaking strength. Ten percent of the displacement. In doing this, you’ll notice that the forestay becomes tighter than the backstay. This is because of the differing angles of attachment to the mast. But don’t worry, it’s a good thing.
2. Now you can tension the forward lower stays, or the babystay, to give the mast a slight but noticeable forward bow at the spreaders.
3. Tighten the aft lower shrouds until, as you sight up the mainsail track, the mast is straight up-and-down again.
4. Go sailing in moderate winds and let the rig stretch into its new position.
5. After a few hours of stretching, and while still sailing in a moderate breeze, get the mast straight from side to side by tightening or loosening the appropriate shrouds. To do this without losing the correct tension you’ve just obtained, you must loosen a turnbuckle on one side a few turns before you tighten up the turnbuckle on the other side by exactly the same number of turns. Never, ever, try to take the slack out of the leeward shrouds by tightening the windward shrouds, and then going about and repeating the action. The turnbuckles are so powerful that you’ll tend to push the mast down through the bottom of the boat.
6. In most cases, adjust the lower shrouds first. They’ll get the mast centered and straight up as far as the spreaders.
7. Then work on the cap shrouds to get the top half of the mast into column.
There we are. Your rig is tuned. Wasn’t so mysterious, was it?
Today’s Thought
He that will use all winds, must shift his sail. —John Fletcher, Faithful Shepherdess.
Tailpiece
At our local city council meeting the other day the chairman of the Works Committee was asked to give figures for how many people are employed by the City, broken down by sex.
“Not too many,” he replied, “liquor is more of a problem for us.”
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