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.

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.”


Oded Kishony said...

When I tune my bandsaw I pluck the blade to a certain note. I wonder if you could pluck the stays to adjust them? Oded

Anonymous said...

Oded, once you had the rig tuned the normal way, your method would probably work. You'd probably have to hire a recording artist to register the harmonic frequency, and then a musician to replicate it, though. And lord knows what different temperatures would do to the tuning. The aluminum mast would expand with heat in much the same way as the stainless steel stays, but the overall length of the boat changes with temperature, so in hot weather the tension in the stays would be greater than in cold. Frankly, I think using a Loos gauge would be cheaper and less bother.
--John Vigor

Oded Kishony said...

Hi John,

You wouldn't need the absolute pitch just have to be sure that the pitch matched. I'll have to try it soon and report.
> using a Loos gauge would be cheaper and less bother. <

That a meager recommendation ;-)