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Dyneema backstay vibration


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May have already been covered here ...... but I have a Dyneema backstay which howls like a demented mother-in-law whenever wind gets above about 15 knots. Note changes dependent on how much tension is on her (the backstay) 

Anyone have any experience here, or ideas on how to quieten her down? Could just go back to original wire backstay I guess.

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Ha, BP's on fire!  My solution was to tie a piece of about 8mm line on.  It was driving me bonkers and I just grabbed an offcut that was lying around.  3 years later and it's still there.  The 8mm is only about 300mm long and just clove-hitched on.

That was a spare Dyneema main halyard, not a backstay.  But worth a try.  Otherwise, the AC boats run a little flap on their rigging.  Certainly looks a bit more Gucci.

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39 minutes ago, Hapuka said:

May have already been covered here ...... but I have a Dyneema backstay which howls like a demented mother-in-law whenever wind gets above about 15 knots. Note changes dependent on how much tension is on her (the backstay) 

Anyone have any experience here, or ideas on how to quieten her down? Could just go back to original wire backstay I guess.

Von Karman Vortex Street - the same phenonem that causes a Chinese Gybe.   https://en.m.wikipedia.org/wiki/Kármán_vortex_street

You need to break up the air flow with a strake. 

Wrap some 0.5mm dyneema around it for about 1m at the bottom. Or put some balls on it, like a squash ball.

You could also try shortening it by a few inches, reducing the harmonic length. 

image.png.ba86ff24c7ade26c8d1865c8208e0ba2.png

^^ Chimneys with strakes fitted to break up the vortices.

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40 minutes ago, CarpeDiem said:

Von Karman Vortex Street - the same phenonem that causes a Chinese Gybe.   https://en.m.wikipedia.org/wiki/Kármán_vortex_street

You need to break up the air flow with a strake. 

Wrap some 0.5mm dyneema around it for about 1m at the bottom. Or put some balls on it, like a squash ball.

You could also try shortening it by a few inches, reducing the harmonic length. 

image.png.ba86ff24c7ade26c8d1865c8208e0ba2.png

^^ Chimneys with strakes fitted to break up the vortices.

Cheers CarpeDiem thats an interesting read!

Great I'll try spiralling some 0.5mm around the lower metre of backstay. Although wouldnt the top section of it still produce the offending vortex?

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31 minutes ago, Hapuka said:

Cheers CarpeDiem thats an interesting read!

Great I'll try spiralling some 0.5mm around the lower metre of backstay. Although wouldnt the top section of it still produce the offending vortex?

I don't know.

We stopped this humming on our running back stays purely by accident when we tied a bungy in middle to pull the lazy runner out of the way.  The tight bungy stopped it dead by breaking the airflow. 

I noticed that the AC75s seemed to all have their strakes at different heights on their runners so you might have to experiment with it at different heights....

 

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48 minutes ago, Hapuka said:

Cheers CarpeDiem thats an interesting read!

Great I'll try spiralling some 0.5mm around the lower metre of backstay. Although wouldnt the top section of it still produce the offending vortex?

After a bit more lockdown googling, I notice that the strake are very long, almost the full length of the thing being protected. 

https://www.cmrp.com/blog/bending/examples-of-vortex-shredding-mitigated-by-helical-strakes.html

So this idea might not be practical.

I remember racing on a tp52 out of Sydney that had this wire twisted around it all the way up the back stay, I always thought it was part of the ssb antenna until I discovered strakes....  Now that know what they are I can't stop spotting them... 

Eg... 

image.png.3bbc50961686d45e725c3425b3d3f00d.png

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Should work. The harmonic requires the full length of the backstay.  If you effectively shorten it by tying a line to it or wrapping a bit of it as described you break up the harmonic. ( my daughter is a piano tuner). 

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Something to break up the harmonic and/or take energy out. At anchor I use the main halyard as topping lift. I tie a short length of 4mm line between the backstay and the main halyard to stop it humming, about as high up as I can reach. Maybe possible to do something between the backstay and the aft stanchions? Or how about a rubber grommet at the end to absorb some of the energy - or a pad arranged to press on the backstay the like palm muting a guitar string.

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Our dyneema backstay vibrates a loud harmonic through our carbon rig right into the master cabin.

A sharp dig in the ribs fixes it every time. (I get up and let the backstay off completely)

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we have exactly this with a topping lift.  The first night out we thought it was the wind generator on the cruiser parked next to us - a loud, howling vibrato.  We tried all sorts of things including bungy cords and pennants, but it only modified the pitch.

We gave up and the halyard now does the business once the main is down.

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Yes Karman vortices are interesting things and were a key consideration for my father when he was leading a team of supersonic aerodynamicists for the design of Hawk, Tornado and Concorde amongst others. They were a team of scientists working for UK Govt (MOD) but frequently got ‘loaned out’ to help with crucial civilian projects in UK and other friendly allies including civil aircraft (Boeing & Airbus), NASA, the big cross-channel hovercraft fans and various heavy industries eg huge scale airflows through British Steel’s furnaces etc.

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Brilliant thanks team...  cheers for all the ideas and links! Will try them once I can get back on board.

Would be interesting to know how many years a stay could survive such high frequency vibration before it failed.....!

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