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Old 17-02-2010, 08:36   #1
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Mast and Standing Rigging Stress

I recently read a comment that recommended unstepping the mast while on the hard as the wind would otherwise stress components that couldn't give as it otherwise would in the water. Following that line of thought: multis sail and sit flatter than monos which roll with the wind. Does that means a multi in the water might stress the mast seat and standing rigging more that a comparable mono which rolls under pressure?
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Old 17-02-2010, 20:55   #2
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Exactly. That's why multi hull rigging is always bigger size. It's a negative for the multi's that most owners don't want to talk about.

The advantage is when they are on the hard... the rigging is already designed to cope with that situation ;-)

cheers,
Nick.
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Old 17-02-2010, 21:25   #3
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Thanks. How about the mast step?
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Old 17-02-2010, 21:57   #4
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Thanks. How about the mast step?
Pls. elaborate, I don't understand the question.

cheers,
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Old 18-02-2010, 06:07   #5
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Thanks. How about the mast step?
The mast step is, or should be, the most robust component of the standing rig.
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Old 18-02-2010, 06:38   #6
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Quote:
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I recently read a comment that recommended unstepping the mast while on the hard as the wind would otherwise stress components that couldn't give as it otherwise would in the water. Following that line of thought: multis sail and sit flatter than monos which roll with the wind. Does that means a multi in the water might stress the mast seat and standing rigging more that a comparable mono which rolls under pressure?
Sounds like nonsense to me. However, I have seen owners loosen shrouds of boats on the hard "to relieve stress" because the hull takes on a different shape deforming slightly on blocks and stands, and mast compression could increase rigging tension beyond safe loads. I would caution those owners not to over-loosen, because I have also see them so loose the mast sways and slams against the limits of travel in such a way that the sudden stress of coming up short against the rigging simulates the force of a gybe (but with greater force because the mast is a longer lever arm). The dynamic force could do damage and cause fatigue -- especially because it would be repeated cyclically over the many months of storage.

The other reason to unstep is to keep the boat from blowing over sideways on narrow stands. Monohulls and folding trimarans should have this concern. Not an issue with cats or open tri's whose amas are supported. (Amas should also be supported by stands or hung from a temporary cross-bar to keep snow/ice loads from overstressing the cross beams. Snow loads could reach thousands of pounds.)
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Old 18-02-2010, 07:17   #7
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s/v Jedi, I can't elaborate too much, as I don't know how or where the mast is stepped in a cat - just that it isn't keel mounted. I'm wondering if the mast base on a multi might be subject to different stresses than a mono, but I suspect not - standing rigging would probably take that stress. Still, I thought I'd ask.

SailFastTri, with respect, I asked a question - I did not put forth a thesis. There might well be nothing to it, but I've seldom heard a question that is by itself 'nonsense.'

While developing the sailing bug over the years, some of my first readings have been about storms at sea. It has not been hard to find references to heavy monos achieving 5 kts-plus under bare poles in such conditions. If a 20,000 pound yacht can move that fast under bare poles, with the same wind velocity on land the same force is being applied, so it isn't unreasonable to wonder where the stress is going. Does the same reasoning apply to flatter sailing multis on water? Again, I suspect it isn't a problem, but it's worth a question.

Thanks.
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Old 18-02-2010, 07:37   #8
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snip

SailFastTri, with respect, I asked a question - I did not put forth a thesis. There might well be nothing to it, but I've seldom heard a question that is by itself 'nonsense.'

snip
Thanks.
I didn't intend to criticize the question so my apologies if that's the way it read.

I did think the the reason cited is unlikely: "stress components that couldn't give as it otherwise would in the water". However, in thinking about it further that's just another way of saying what I wrote above: "... because the hull takes on a different shape deforming slightly on blocks and stands, and mast compression could increase rigging tension beyond safe loads." I do agree that's a legitimate concern... Hull shape does change on the hard when the hull isn't supported evenly as it would be in the water.

To add a fine point -- I doubt wind load by itself would cause rig over-stress unless it was severe enough to blow the boat over, or unless the shrouds were loosened too much allowing the mast to rock and experience cyclical shock loads from hitting ends of travel.
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Old 18-02-2010, 07:47   #9
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Jaywalker,

Our boat was unlucky enough to be in Grenada when hurricane Ivan hit. We were back aboard days after that (Jedi was fine at anchor) and I got to see the devastation in the two boatyards there. Every damaged mono was on it's side, i.e. I didn't see a single one standing up without the mast. I did see some standing up, they weren't all down. So I guess that they fell over before the wind could rip the mast away.

But that doesn't mean much... Ivan was a bad one and may be a cat. 1 or 2 hurricane wouldn't be enough to put them on their sides and some would loose their masts then or if the boats would have been tied down (like they do now) so they can't easily fall over would surely lead to different results.

Most cats that were standing up were destroyed anyway because boats fell on top of it. Many mono's fell over for the same reason: others fell on top of it, they might have been upright otherwise.

What I do know is that I have read the same thing you did and indeed to call that nonsense is tricky because these authors are smart enough people to take notice of what they write and not reject it that easily.

We are now sailing around the high-winds Caribbean for 7 years I think it is and I have never seen a problem with this on any boatyard. We do release some tension in the rigging when on the hard for a longer period (not for a 2 week haul-out).

There are mono's with deck stepped masts. They use a compression post to support the step. For cats this whole area is "cloudy" for me; when I ask cat owners they all assure me that it isn't a problem but I can see on their faces that they never thought about it. Same for their rigging. I see all kinds of strange things like cats with masts that are raked so much that it looks like they are halfway lowering the mast. All that is because they couldn't put the mast where it should be (interior living room more important than the rig) and the rake is needed for correcting the imbalance.

All this doesn't mean I would never buy a cat because I like what I see when looking at cats like the Outremer 50. I wouldn't buy a cat under 45 feet.

ciao!
Nick.
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Old 18-02-2010, 07:57   #10
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I've got to agree with sailfasttri.
If the bare poles cannot take the additional load caused by not being able to heel in the water you had better not raise any sail!
What about all of the dry-sailed racing boats?
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Old 18-02-2010, 11:44   #11
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Thanks. I didn't know hulls took on different shapes under stress - I assumed they were essentially built to be rigid and any flex was something to be designed out (or around), as for instance with cored hulls.

Do builders recommend changing standing rigging tension on the hard or in a blow, or is this something we assume makes sense? Wind force at any given level on an unmoving boat will be constant, and as SailFastTri suggests, by changing tension we simply choose where to have it act - on a small area at the base of the mast or a larger area deforming/changing the shape of the hull.
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Old 18-02-2010, 15:54   #12
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Jaywalker,

For cats this whole area is "cloudy" for me; when I ask cat owners they all assure me that it isn't a problem but I can see on their faces that they never thought about it. Same for their rigging.

Nick.
Owners might not think about it, but designers do. On my boat the mast step sits on the mast beam. The mast beam runs the width of the boat, is a 25mm bulkhead with 1200gsm quadraxial glass, and has huge uniderectional flanges top and bottom, creating in effect a very large I beam, the entire width of the boat.

This beam is heavily glassed to the decks, the bridgedeck floor and the hulls, and is also supported by a perpendicular bulkhead which itself is located by another full width transverse beam, creating a massive box-beam.

It is an extremely strong structure.
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Old 18-02-2010, 19:48   #13
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Hi 44'cruisingcat!

So, that cross beam runs straight through your interior? Wouldn't you hit your head against it often? The cat I described has the mast forward of the cabin hence the wild rake to balance it again. You must know which design I'm talking about. I know the Prout put it at the back hence the tiny main and huge genoa. I don't question the strength, I just don't know enough about cat construction.

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Old 18-02-2010, 19:56   #14
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The mast beam forms the front of the saloon area, the cross bunks fit between it and the front structural beam, which forms part of the box beam.

It does produce a sail plan with the main bigger than the genoa - on my boat the main is 55 sq metres, the genoa 35. Mast rake is around 5 degrees.

There is also provision for a reacher/screecher forward of the genoa, with an area greater than the main.
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Old 18-02-2010, 20:00   #15
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Re-reading my earlier post I think I see that you may have misunderstood. The mast beam is 25mm THICK Duflex, it's not a 25mm deep beam. It is actually the full depth between the deck and the bridgedeck, around 1 metre, and it runs the full depth of the hulls, around 2.1 metres.
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