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Old 16-11-2017, 11:45   #46
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Re: Hurricane wind effect on rigging on the hard

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I think the infamous Tacoma Narrows Bridge collapse should easily demonstrate just how sympathetic vibration and unexpected movements can lead to a structural failure.
We had a brand new replacement suspension walkway installed here across a narrow canyon. Builder forgot to install the side stays (or more likely, decided they were unnecessary, what with the massive steel cables supporting it). Four cables were maybe 1 or 1.5 inches thick. About a month or two after installation we had 50 kt winds aligned with the gorge. There was almost nothing left of the bridge or its cables. (Builder claimed the river level had risen and wiped it out but that would have been an insane amount of water, and forces would have been nothing compared to the destructive power of wind.)
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Old 16-11-2017, 13:24   #47
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Re: Hurricane wind effect on rigging on the hard

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The pits that I have seen are mostly only for the keel, not so much for the hull, so there isn't much volume to generate flotation. It has been a pretty successful scheme as far as I have heard.

Jim
Every hurricane that I've been near has arrived with a prediction of a large tidal surge. None have ever actually been as large as predicted. It looks like the boats stored in a hole would need a few feet of flooding to start partially floating. That would be really unlikely at Vuda without a really large tidal surge.
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Old 16-11-2017, 13:39   #48
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Re: Hurricane wind effect on rigging on the hard

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Every hurricane that I've been near has arrived with a prediction of a large tidal surge. None have ever actually been as large as predicted. It looks like the boats stored in a hole would need a few feet of flooding to start partially floating. That would be really unlikely at Vuda without a really large tidal surge.
No surge, Consider yourself exsteamly lucky.
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Old 16-11-2017, 13:53   #49
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Re: Hurricane wind effect on rigging on the hard

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No surge, Consider yourself exsteamly lucky.
Not no surge, just never as much as the weather service predicts.
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Old 16-11-2017, 14:18   #50
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Re: Hurricane wind effect on rigging on the hard

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Not no surge, just never as much as the weather service predicts.
I don't think they can. It's a scientific wild ass guess. I think they can but it depends on the underwater and coastal terrain. A 7ft. can become a 22ft. when it piles up. 20 miles one way or the other can make a difference.
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Old 16-11-2017, 15:29   #51
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Re: Hurricane wind effect on rigging on the hard

I know this is different from masts and rigging, but I was in Joplin Missouri during the 2011 tornado, 22nd may, when I walked into the zone, I noticed every street lamp had been broken and the base, and each one was laying in the same direction, it looked weird. They said that wind reached 250 mph, a tubular steel street lamp, dosen't offer a lot of resistance to the wind, but it broke every one of them.

I am staying on my boat just now, its out of the water, I had to get the mast taken down, as in the wind, the mast was shaking the boat around in the cradle, it felt like being back out at sea, since I took it down, the boat dosen't move.

If you work out the area of that mast, its a lot of area,
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Old 16-11-2017, 17:42   #52
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Hurricane wind effect on rigging on the hard

The street lamps are designed to break so that when hit with a car, they donít kill the occupants, and they have impact sensors that cuts power to them.
I know because as a kid we used to wack them with a piece of 2x4 and they would turn off for quite awhile.

The harmonic vibration you guys are alluding to with the bridges is called aeroelasticity, or flutter in the aircraft world. It is a destructive force and I guess maybe that sailboat rigging is susceptible, but many different kinds of boats with completely different rigging would each have different frequencies where they flutter, and itís just not really all that likely for them all to suffer from flutter around similar wind velocities.

Aircraft wise, you first do a GVT or ground vibration test where you shake the aircraft with what amount to huge speaker drivers looking for resonance frequencies.
If it passes that then you get to do a flutter Dive which is done to Vd, velocity Dive which is 1.2 times VNE or velocity never exceed.

The resonance frequencies even among identical boats would be different unless every rigging wire were tensioned exactly the same, and even then the ďsoftnessĒof the hulls are likely different.
In other words, flutter or aeroelasticity certainly could tear rigging and mast apart, but the likelihood for all boats in one area to have the same or very close to the same flutter frequencies is pretty remote.
Be almost as bad as saying they were all hit by lightning
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Old 16-11-2017, 18:57   #53
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Re: Hurricane wind effect on rigging on the hard

Think about it - every tall, circular chimney that doesn't have protection from vortex-shedding will fail in strong winds. Some will fail early, some later, but none are resistant to harmonics, given the right wind speed. Wind speed is never constant, all speeds are encountered at some point, hence all will fail at some point.
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Old 16-11-2017, 18:58   #54
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Re: Hurricane wind effect on rigging on the hard

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The street lamps are designed to break so that when hit with a car, they donít kill the occupants, and they have impact sensors that cuts power to them.
I know because as a kid we used to wack them with a piece of 2x4 and they would turn off for quite awhile.

The harmonic vibration you guys are alluding to with the bridges is called aeroelasticity, or flutter in the aircraft world. It is a destructive force and I guess maybe that sailboat rigging is susceptible, but many different kinds of boats with completely different rigging would each have different frequencies where they flutter, and itís just not really all that likely for them all to suffer from flutter around similar wind velocities.

Aircraft wise, you first do a GVT or ground vibration test where you shake the aircraft with what amount to huge speaker drivers looking for resonance frequencies.
If it passes that then you get to do a flutter Dive which is done to Vd, velocity Dive which is 1.2 times VNE or velocity never exceed.

The resonance frequencies even among identical boats would be different unless every rigging wire were tensioned exactly the same, and even then the ďsoftnessĒof the hulls are likely different.
In other words, flutter or aeroelasticity certainly could tear rigging and mast apart, but the likelihood for all boats in one area to have the same or very close to the same flutter frequencies is pretty remote.
Be almost as bad as saying they were all hit by lightning
I am confident that I am unable to offer the equations and physics principals behind the loss of masts of the sailboats (especially catamarans with higher masts) but I can say with certainty that the wind alone picked up our boat and threw it a few yards as well as several cats around ours (see photo). Our boat was on Virgin Gorda and tied down. Sure, the brush deck provides a wing and tunnel for uplift but the boat ways 26,000 lbs. I believe many of the mast structural failures we saw photos of were at the step or coachroof while shrouds largely stayed intact (our chainplate is bent 90 degrees on these and other photos). We saw several photos of such masts on catamarans that weíre on the hard at Nanny Cay and still on the stands. Seems as though the mast profile and height may have been sufficient for some of the masts to fracture and may have contributed to domino effects occurring on neighboring boats.

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Old 18-11-2017, 07:19   #55
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Re: Hurricane wind effect on rigging on the hard

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...Aircraft wise, you first do a GVT or ground vibration test where you shake the aircraft with what amount to huge speaker drivers looking for resonance frequencies. If it passes that then you get to do a flutter Dive which is done to Vd, velocity Dive which is 1.2 times VNE or velocity never exceed...
Thinking about your analogy with aircraft, they are far more rigorously designed and tested than any yacht mast:
1. What happens when the velocity dive test is a failure? I'm guessing the wings vibrate excessively and the dive is cancelled - back to drawing board?
2. What happens if the dive velocity of 1.2 VNE is exceeded, let's say an out of control dive? I'm guessing the wings vibrate excessively due to ever larger wingtip vortex shedding? Then eventually the wings fall off - right?
Now take those wings and try to prevent them ripping off (or fluttering) by securing them with wire rigging to the aircraft: instead of falling off, the next failure mode may be the wings simply crushing inwards through the sides of the aircraft, or collapsing in compression?
I don't suppose many aircraft are subjected to such forces far beyond their design parameters but as engineers we should be able to visualise those limiting factors. No need to be struck by flying objects - failure occurs eventually, when design parameters are exceeded. In a mono (ignoring waves) failure mode is lying flat on the water and hoping downflooding does not occur through some open hatch, as has happened (rarely) with a sudden downdraft within a sheltered anchorage. With a multihull in winds over 200 mph, the failure mode is of course capsize or pitchpole. If we prevent them flipping then the next failure mode must kick in - not hard to predict: the mast is crushed by downward forces imparted by the rigging, combined with vibration induced out-of-column alignment.

200 mph and still upright is beyond any realistic mast design parameters.
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Old 18-11-2017, 11:03   #56
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Re: Hurricane wind effect on rigging on the hard

Old saying in boat and car racing circles alike: If you didn't break anything, you weren't pushing hard enough. Or, it was built too heavy, and heavy is slow.

You could make the case that especially for cats, perhaps the designer SHOULD make a rig that fails in high winds. So the rig will come down, instead of creating enough force to capsize the boat.

Good engineering, really good engineering, always considers the system as a whole and actually encourages a system to "fail soft" as best it can. That was the radical philosophy that Mercedes(?) introduced to the auto market in the 60's with "crumple zones" where the vehicle would be ruined, as it collapsed from impacts, in order to keep that force away from crumpling the occupants.

Sacrificial mast, anyone?
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Old 18-11-2017, 18:48   #57
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Hurricane wind effect on rigging on the hard

I have no doubt that in high winds a Cat can fly, there have been at least a few cases of that, there is a lot of area between those hulls, weight isnít the issue, itís weight per surface area. A 747 can weigh almost a million lbs, but flies very well.
I just canít believe that a bare mast will fail in 200 mph winds, yet be fine with sails up in 40 mph winds, the forces on sails due to surface area has to be many times bare poles at 200.

Think about this, a mast is many times stronger wind resistance wise than any building or tree, yet some buildings and trees survived?

My boat has three stays port and starboard, each oneís breaking strength is I think over 20,000 lbs, and are decently equally loaded.
Just 200mph of wind on a thin mast is over 60,000 lbs? No way, the boat would have been blown away long before those forces could be reached on the mast, the surface area of the hull is many times as much, you would have millions of lbs of force on the hull prior to getting to 60,000 on a bare mast.
Problem is of course that the boats were dismasted, how to explain that?
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Old 19-11-2017, 00:56   #58
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Re: Hurricane wind effect on rigging on the hard

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My boat has three stays port and starboard, each oneís breaking strength is I think over 20,000 lbs, and are decently equally loaded.
Just 200mph of wind on a thin mast is over 60,000 lbs? No way, the boat would have been blown away long before those forces could be reached on the mast, the surface area of the hull is many times as much, you would have millions of lbs of force on the hull prior to getting to 60,000 on a bare mast.
Problem is of course that the boats were dismasted, how to explain that?
Buildings or trees are not in compression, a mast is. Once it gets out of column either from a steady force of 200 mph or vibrates out of column the compression wins and it can collapse. A boat on the hard will not heel to dissipate loading.
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Old 19-11-2017, 10:32   #59
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Re: Hurricane wind effect on rigging on the hard

Circular or near-circular is the worst case scenario for vortex shedding - no cylindrical parts exposed anywhere on an aircraft. Aircraft tend to have air pressure from a single direction, so they are streamlined to cope; masts don't and aren't. Imagine the vibration from shrouds in 200 mph - mast is even more prone to vibrate (and hardly likely to be validated to have no harmonic vibration at 200 mph while remaining vertical).
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Old 19-11-2017, 11:08   #60
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Re: Hurricane wind effect on rigging on the hard

a64-
There is inevitably Something Really Stupid Obvious that accounts for baffling problems like "Why should it fail at 200mph when it can take much bigger loads?"
Like the tale you've no doubt heard, that when the Brits found out that the US was testing jet aircraft windshields for bird strikes, by firing chickens out of an air cannon directly at the windshield.
And the Brits complained that the glass was always breaking.
Apparently no one had pointed out, you need to let the handy frozen chickens defrost before firing them. Ooops.

It could be something as simple as hurricane debris hitting the shrouds, causing a severe and basically invisible twist on the mast perhaps? Hurricane "impact resistance" is, after all, normally tested in the US by literally firing a 2x4 at various objects, and finding lareger and heavier objects embedded in walls and trees after the storm is normal, so we know there IS heavy flying debris to be dealt with.
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