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-   -   Vortex shedding-certainly real (https://www.sailnet.com/forums/general-discussion-sailing-related/329556-vortex-shedding-certainly-real.html)

alanr77 05-09-2019 12:39 PM

Vortex shedding-certainly real
 
Had an interesting phenomenon happen last night during a storm that was blowing force 6 and gusting force 7 according to my anemometer. The mast was vibrating something fierce and the “pumping” felt like it was going to shake my boat apart. I’ve never had this happen to this degree so it caught me unprepared and I was definitely baffled for a minute. I first went and double checked all halyards to ensure they were secure, eventually even checked all my static rigging tension. Everything was fine but the severe vibration persisted. I’m on land at the moment and the wind was being funneled around a building and it was constantly shifting from about 30-95 degrees off the starboard beam. Eventually I ended up reading an article from Selden discussing a phenomenon called vortex shedding. I’ve never had to deal with that for some strange reason though it’s apparently common. So mid storm I took the spinnaker halyard and wrapped it around the mast from about the spreaders down to the deck, looked like a candy cane....the vibration stopped. Crazy stuff and I think I learned something new here. Anyone else dealt with this and why only now has this happened? I’ve been in much stronger winds and did not have this issue. Perhaps being on land combined with the buildings and wind angle? Only other thing that’s different is I have no sails mounted. Strange.

MarkofSeaLife 05-09-2019 12:58 PM

Re: Vortex shedding-certainly real
 
Wow, how bizzare. and not harmonic resonance.

https://upload.wikimedia.org/wikiped...-animation.gif

alanr77 05-09-2019 01:08 PM

Quote:

Originally Posted by MarkofSeaLife (Post 2051600940)
Wow, how bizzare. and not harmonic resonance.

https://upload.wikimedia.org/wikiped...-animation.gif

Bizarre is definitely what I was thinking. The vibration was definitely athwartship, which makes sense based on the wind direction. It was violent! I was afraid it would knock the boat off it’s stands. My sea swing was rotating 4-5”..That a simple candy cane like wrap of a halyard corrected the issue blows me away (bad choice of words?) lol.

overbored 05-09-2019 01:56 PM

Re: Vortex shedding-certainly real
 
it happens because when on land the mast is held rigid to the ground and the vortex is allowed to form, if the boat is in the water the mast is allowed to sway in the wind and not held rigid to the ground . the halyard changed the shape of the mast and made the cross section vary along the length which acts as a vortex breaker happens to antenna towers also.

alanr77 05-09-2019 04:05 PM

Quote:

Originally Posted by overbored (Post 2051600952)
it happens because when on land the mast is held rigid to the ground and the vortex is allowed to form, if the boat is in the water the mast is allowed to sway in the wind and not held rigid to the ground . the halyard changed the shape of the mast and made the cross section vary along the length which acts as a vortex breaker happens to antenna towers also.

That explains a lot. I went out this morning after an uncomfortable night and found the boat had shifted 1” on its cradle. That’s how bad it was. The wrapped halyard corrected it so I should be ok. I hate doing refits on land. Boats not in its natural environment and all kinds of weird things happen. I think the boats safe as it’s sitting on it’s large keel (T34) and the cradle just keeps it from tipping over. Time for a nap, up all damn night dealing with this and I think I now know what it was, why the halyard corrected it and the boats secure.....god I can’t get back in the water soon enough...🤨

mbianka 05-09-2019 05:59 PM

Re: Vortex shedding-certainly real
 
Quote:

Originally Posted by alanr77 (Post 2051600962)
That explains a lot. I went out this morning after an uncomfortable night and found the boat had shifted 1Ē on its cradle. Thatís how bad it was. The wrapped halyard corrected it so I should be ok. I hate doing refits on land. Boats not in its natural environment and all kinds of weird things happen. I think the boats safe as itís sitting on itís large keel (T34) and the cradle just keeps it from tipping over. Time for a nap, up all damn night dealing with this and I think I now know what it was, why the halyard corrected it and the boats secure.....god I canít get back in the water soon enough...🤨

Things like this are why I always drop the mast for winter storage. Left it up once and could feel the vibration all through the boat in even a moderate blow.

fallard 05-10-2019 03:22 AM

Re: Vortex shedding-certainly real
 
About 30 years ago we sailed our 18í catboat to Block Island for a rendezvous. The weatherman got it wrong, however, and that night the wind came out of the NW at 50 mph (Force 8). It was pandemonium in the Salt Pond, with lots of boats dragging anchor and fouling each other. As the wind arose, the standing rigging started buzzing and eventually the mast would periodically go into resonance, driven by vortex shedding. At that point the whole boat vibrated. Didnít get to sleep that night, but at least our 7 lb danforth held our 2500 lb boat and no one came down on us. Being in shallow water shielded us from the keel boats.

Vortex shedding is a well-known phenomena: just not appreciated in those circumstances.

BTW, weíve stored our 35í sloop with mast up for 22 years without incident. The mast vibration in higher winds is minimal, and nowhere near what we experienced 30 years ago on our catboat. I will say that I have never visited the boat during a noríeaster, however.

alanr77 05-10-2019 09:05 AM

Quote:

Originally Posted by fallard (Post 2051601056)
About 30 years ago we sailed our 18’ catboat to Block Island for a rendezvous. The weatherman got it wrong, however, and that night the wind came out of the NW at 50 mph (Force 8). It was pandemonium in the Salt Pond, with lots of boats dragging anchor and fouling each other. As the wind arose, the standing rigging started buzzing and eventually the mast would periodically go into resonance, driven by vortex shedding. At that point the whole boat vibrated. Didn’t get to sleep that night, but at least our 7 lb danforth held our 2500 lb boat and no one came down on us. Being in shallow water shielded us from the keel boats.

Vortex shedding is a well-known phenomena: just not appreciated in those circumstances.

BTW, we’ve stored our 35’ sloop with mast up for 22 years without incident. The mast vibration in higher winds is minimal, and nowhere near what we experienced 30 years ago on our catboat. I will say that I have never visited the boat during a nor’easter, however.

I think the wind angle and surrounding “funnels” had a lot to do with it. There’s a 35’+ IP next to me with a giant tarp wrap around it and a large building in front of me. A large hill is behind me. I didn’t notice the violence effecting any boats around me but it’s possible. It’s one of those things I guess you don’t research unless it’s happening to you. I’ve had mast resonance happen and sometimes even a slight vibration in other boats. This was the first time it was so violent as to move a 12 ton boat in its cradle. I’m not kidding when I say my sea swing and various other gimbaled items were swinging 4-5” athwartship and this went on for hours. Then again, I’m not usually out of the water for a couple months so the previous reply regarding the boat not being able to move in the water may be the answer.

SanderO 05-10-2019 09:33 AM

Re: Vortex shedding-certainly real
 
Obviously the forces on the hull and right from wind are transferred to and buffered by the water it floats in. In a cradle this cannot take place and obviously some sort of vibrations and harmonics get the mast pumping energy into the hull. Better to store in water if the rig is left in place.

JimsCAL 05-10-2019 09:37 AM

Re: Vortex shedding-certainly real
 
I had a Pearson 26 many years ago, and that mast was very prone to pumping. It was due to the single lower shrouds and no baby stay. Basically when the frequency of the vortex is close to the natural frequency of the mast, you get a resonance. With only single lowers, the middle of the mast can move fore and aft pretty easily (lower natural frequency). With double lowers and/or a babystay, the mast is stiffer fore and aft and usually pushes the natural frequency well above the vortex frequency.


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