March 15, 2012

Tension in the rigging

OLD WOTSISNAME, who never believes anything I say, just can't accept the fact that when he's sailing that concrete barge of his, the mast is pushing downward with a force of about 13 tons.
"If that was true, the mast would push the bottom out of the boat," he said.

Well, it IS true, whether OW believes it or not.  Few sailors realize it, but the compression load that the mast step must withstand is generally between 1.5 and 2.5 times the displacement weight of the boat.

That's the word straight from the mouth of an expert. David Potter ran Kemp Masts Ltd., in England, one of the largest spar manufacturers in the world, and in his book, The Care of Alloy Spars and Rigging (Granada Publishing) he offers several other tips that OW probably won't agree with.

Among them is the recommendation that you set up your upper shrouds and backstay with a tension of 10 percent of the boat's displacement.  Most people I've talked to say this is too much, but it's the rule I've always followed and I've never had any trouble on that score. It means that your forestay will automatically be given a higher tension than the backstay because the forestay makes a narrower angle with the mast, but that's exactly what you want, to keep your jib luff nice and straight.

On OW's boat, which probably displaces 15,000 pounds or so, he'd need to pre-load his upper shrouds and backstay with a tension of 1,500 pounds, something he can't bring himself to do, even if he managed to borrow someone's Loos tension gauges.

"At that rate, and under constant tension, day and night, the chainplates would probably pull out," he claims.  "Either that, or the bow would rise up to meet the stern."

Well, there are believers and unbelievers in the world of yachting, and OW is a confirmed atheist when it comes to rigging.  But I think he may have to learn to say some prayers nicely when he next gets caught out in a decent blow.

Today's Thought
Ignorance is preferable to error; and he is less remote from truth who believes nothing than he who believes what is wrong.
— Thomas Jefferson, Writings

The last time they had trouble with the head on the Royal Yacht, Prince Phillip called in a plumber. "Before you begin," said the prince, "I'd like to acquaint you with the cause of the trouble."
"I'm very pleased to meet you, Ma'am," said the plumber, bowing to the Queen.

(Drop by every Monday, Wednesday, Friday for a new Mainly about Boats column.)


Matt Marsh said...

The forces involved in a Bermuda-type rig really are quite dramatic.

If you sketch it out, it becomes pretty clear that O.W. got his three failure modes right: Yes, the mast is trying to punch right through the bottom of the hull. Yes, the shrouds are trying their best to rip the chainplates out. And yes, the stays are trying to pull the bow and stern towards the masthead.

This leads to a few headaches for the designer: The hull has to be stiff enough as a longitudinal girder to handle the stays, and it has to be able to handle enormous point loads at the step and chainplates. (And, of course, the keel root.) So you end up with a lot of complex, highly loaded (therefore expensive) structural components.

If you trace all those forces back through the hull, and think about how much money goes into managing them, a free-standing rig (which is structurally much simpler) starts to look very appealing!

Sun Burst said...

Presumably these calculations (eg 10% of displacement on backstay tension) are for a mast head rig? any suggestions for rule of thumb for a fractional rig?

John Vigor said...

Sun Burst, I can't find any specific reference to fractional rig tuning in my home library. David Potter is not a fan of fractional masts and offers no advice about tuning forces. Brion Toss, the author and professional rigger, makes no mention of fractional rigs in The Rigger's Apprentice, second edition. And he apparently tunes the masthead rig by ear: "Start with the lowers. I like to get them so I can strum a low musical note on them . . ."
My gut feeling is that you should tune a fractional rig in the same way as a masthead rig, with the exception of the backstay, the tension of which is varied to induce mast bend. But I expect Mr. Google will help you out if you search for "tuning fractional rig."


John V.