Hurricane wind effect on rigging on the hard

[quandary]

For what its worth, I'm reminded of the articles by Donald Jordan (google them) who invented the series drogue. He was an aeronautical engineer who designed jets which dealt with air speeds of thousands of miles an hour. His view was that the aerodynamics of sail boats in severe storms at sea was pretty irrelevant compared to the forces on the boat generated by waves. Storm survival depends primarily on preventing the boat from being thrown off a breaking wave because of excess speed and by keeping it orientated so it could not never be broadside to a breaking wave....hence the drogue design which achieves that.
It would seem to me that the storm damage to rigging might be caused more by shock impacts from falling and flying objects than from direct wind pressure in most cases? Rigidly secured boats on the hard would be most likely to suffer such damage?

[Paul L]

Originally Posted by Seas the Day

I'm moving from Sydney to Vanuatu over the next year where they get a cyclone (NH Hurricane) every year or so. My preparation choices are 1) to leave the boat on a 'cyclone rated mooring' in my deep harbour or hard stand in town. If I go the hard stand way then I would remove the mast and standing rigging and strap everything down hopefully away from other boats that leave their masts on.

Have you looked at the shoreline along the mooring field in Port Vila? It is covered with wrecked commercial and recreational boats. Probably half a dozen there.

Havannah Harbour had a large mega-yacht washed up too.

[Paul L]

Originally Posted by Jim Cate

Seems that taking halyards out to the sides to anchors buried in the ground would give great support to the mast on a "pitted" boat.

Lots of places that don't have mast cranes available, and there might well be a lot of folks needing that service just before a cyclone strikes if there WAS one available! But agreed that pitting the keel has worked pretty well in Vunda Point, Fiji, where they've done it for years. Safer than cradles, "cause ya can't fall out of a pit!

Jim

I know it has not been an issue in Vuda, but I'd be concerned about floating out of a pit in some places.

[jimbunyard]

To illustrate why getting real answers to the original question is an engineering nightmare;


With all other things being equal, does 1 hr at 200mph shorten the life span of the shroud the same as 2 hrs at 100? Or 4 hrs at 50?

Does snap rolling at anchor (or at sea, for that matter) put more load on a shroud than sustained, constant wind against a rigidly held mast on land?


Best to inspect, measure and keep your fingers crossed...or you could just replace it all. (though that wouldn't be my first choice)

[NevisDog]

Originally Posted by a64pilot

...However I think 100 mph of wind is something like .17 PSI?
If you have a 50’ mast that is .5’ wide, that is 25 sq ft?
25 Sg ft is 3,600 sq in?
If 100 mph is actually .17psi then force on mast is like 600 lbs?
... Unlikely to take a mast down with that kind of force, however...

I'll run with your figures - seem closer than any others.
Now double that wind speed to 200 mph: 600 lbs goes up to 2,400, which is over one tonne of force on that mast (plus the forces on all those wires). One tonne of force creating vortices around that cylinder will mean the force is around double for a second, say two tonnes, then half that the next second as the vortex is shed - that's enough to bring down any rigidly held cylinder in my book.

Forces at sea are entirely different as the boat heels and releases almost all of the energy, at least for monos; multis just accelerate before the wind, even with a drogue, but 200 mph must get pd close to failure!?

[Tbonem17]

I would agree with those who have said that most rigging failures were due to flying debris. The same probably applies to hull damage as well whether moored or on the hard.
Your boat is doomed if it's down wind of any building or mooring failure.

[Jim Cate]

Originally Posted by Paul L

I know it has not been an issue in Vuda, but I'd be concerned about floating out of a pit in some places.

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

[zippy]

Originally Posted by zippy

Here is a simplified calculation of the force generated by the wind on an object.

F=A x P x Cd

F= force generated
A= area exposed to the wind
P= pressure generated in psi = .00256 x V^2
V= wind velocity in mph
Cd= drag coefficient with depends on the shape of the object

Assume a 60' by 1' mast area for an A= 60 ft^2
Assume wind speed V = 150mph

P = .00256 x 150^2
=57.6 psi

The drag coefficient for a tall cylinder Cd=1.2, Cd = 2 for a flat plate. I will use 1.4 for a rounded rectangular mast.

F = 60 x 57.6 x 1.4
=4,838 lbs of force on the mast

The force would be half that if we used the narrow profile of the mast using half the width.

Increasing the wind speed to 200mph almost doubles the pressure to 102 psi, making the force increase to 8568 lbs of force on the wide cross section of our mast.

Note: The numbers are unchanged but the wind pressure unit should be psf not psi.

P= 57.6 pounds per square foot


You can now remain on your motorcycle [emoji5]

[a64pilot]

Originally Posted by NevisDog

I'll run with your figures - seem closer than any others.

Now double that wind speed to 200 mph: 600 lbs goes up to 2,400, which is over one tonne of force on that mast (plus the forces on all those wires). One tonne of force creating vortices around that cylinder will mean the force is around double for a second, say two tonnes, then half that the next second as the vortex is shed - that's enough to bring down any rigidly held cylinder in my book.



Forces at sea are entirely different as the boat heels and releases almost all of the energy, at least for monos; multis just accelerate before the wind, even with a drogue, but 200 mph must get pd close to failure!?

Lord I am no real Sailor, much less a rigger, so I don’t speak as an expert. But mast should never fail without rigging going first, if it does, then it’s not properly designed, or was subjected to forces outside of the design, like a piece of roofing tin wrapped around the mast or being hit by flying debris. Mast is in compression, rigging in tension.
I look at it like an externally braced wing, the strut takes all the load.
Anyway 2400 lbs is nothing to the rigging. I have no idea as to the strength of my rigging, but I am sure that any one piece will withstand more than 2400 lbs, and there should never be an instance of only one taking all the load. Possibly forestay.

Just a quick look, but one half inch SS wire ought to break at around 20,000 lbs of force? That is new of course and zero corrosion etc.

[admiralslater]

I talked to a chap in Grenada last week who was in charge of getting new masts on three cats .theboats had not flipped. My best interpretation of the French that was spoken was that the rigs had let go by being forced out of column.not by failure of the shroud.suggests violent shaking or vibration. Not sur if they where in the water or not

[NevisDog]

Originally Posted by a64pilot

...Just a quick look, but one half inch SS wire ought to break at around 20,000 lbs of force? That is new of course and zero corrosion etc.

Think of it as a one tonne jackhammer on the side of your mast, slamming the mast once every second at its weakest point. Folks are still thinking static loadings - think vibration at 200 mph. Masts are not designed for this - they are supposed to heel. They fail when they hit the water too, not because rigging breaks... think jackhammer.

[Cadence]

Originally Posted by zippy

Note: The numbers are unchanged but the wind pressure unit should be psf not psi.

P= 57.6 pounds per square foot


You can now remain on your motorcycle [emoji5]

Thank you. What would that be 57.6/144 or .4 psi?

[NevisDog]

Originally Posted by Cadence

Thank you. What would that be 57.6/144 or .4 psi?

This ties in exactly with a64: .1 psi at around 70-75 mph (34 m/s) so 4 times the pressure at double the windspeed (150 mph) = .4 psi. All in agreement now.

[Cadence]

Originally Posted by NevisDog

This ties in exactly with a64: .1 psi at around 70-75 mph (34 m/s) so 4 times the pressure at double the windspeed (150 mph) = .4 psi. All in agreement now.

I seriously doubt wind on the mast would be a structural problem on the hill. Unless the standing rigging is not up to snuff. I had posted, my bad, a harmonic from slack shrouds. I should have said singing shrouds causing a sympathetic oscillation in the mast. Possible even if unlikely.

[hellosailor]

I think the infamous Tacoma Narrows Bridge collapse should easily demonstrate just how sympathetic vibration and unexpected movements can lead to a structural failure. Granted that's not a mast and rigging--but it shows what kind of surprises can happen.

Interestingly, NYC's Whitestone Bridge suffered from similar "fluttering" of the roadway when it was first opened. The bridge was actually shut down for something like another year, and the clunky heavy extra side struts were retrofitted to make sure it didn't fall down (or terrorize motorists) in the same way. A couple of budget crises ago, NYC actually had planned to add more subtle reinforcing under the bridge and remove all the crap from the sides, but that was shelved.

As with those bridges, I think the folks who designed the rigging on most sailboats simply never considered the case of "What happens at 150 mph after a couple of hours?" with or without flying debris impacting anything. That's just outside the normal and usual design considerations.