It Really Works!

[Chrisc]

I appologise if the following is already widely known (I'm always the last to discover such things).....
There is a marina that I occasionally like to use, but it is subject to fresh to strong westerly winds and the berths are positioned north/south and consequently the wind is on the beam, and my boat experiences the most fearful amount of mast pumping as the wind accelerates/decreases through the 14-15knot mark. Apparently all masts will vibrate in a beam wind of a certain strength to a lesser or greater degree, but in my case it was extreme. I have tried endlessly tweaking the rig, but with no success, and after a particularly bad vibrating, sleepless night decided on a little research.
Any long conical structure exposed to the wind will vibrate, this due to the occillation of the wind around the structure and the resultant vortexes thus formed. This is know as the Von Karman effect or the Von Karman vortex, named (if you're interested) after the Hungarian/American physicist and engineer Theodore Von Karman.
Fortunately, he also devised a solution. I had alway assumed that those little spirals around tall metal chimneys were to add strength to the structure. They're not - they are there to disrupt the wind occillation and vortex, thereby eliminating any harmful vibrations that could result in the collapse of the chimney.
My inprovisation was to use the spinnaker halyard and haul a couple of old mooring lines to the masthead and wind the whole mess around the mast a few times and make it fast at the gooseneck.
I had an expectation ranging from nothing to mild, hopefully moderate inprovement. What I got was a complete cure. The mast is absolutely vibration-free. All I need to do is one day make my rope spiral look a little bit more pretty, but just now I'm revelling in my long awaited success. Aint science wonderful!

[Ocean Girl]

Well that is the cat's meow! Von Karman was clever, you too for thinking this up. I have bad mast pumping too, but my other cape dory 30 friends don't, so I am pretty sure I have something out of wack. If I am unable to resolve my pumping I'll try your trick and let ya know how it works.
Cheers,
Erika

[Bloodhound]

Use of a stout rope wrapped in a spiral around a mast to prevent vibrations is certainly new to me, although I've never experienced the problem. Interestingly, the so-called von Karman vortex streets can be shed by all sorts of long narrow objects and as von Karman (an Aeronautical Engineer) correctly diagnosed, the Tacoma Narrows bridge collapsed in 1940 because of them -- the film is quite awe inspiring. So they can have severe consequences if they shed in harmony with the natural frequency of the mast's vibration. The spiral flute or rope disrupts the vortex shedding process. When tying stuff down to a truck, for example, I like to twist the flat strap severl turns to prevent it vibrating whilest driving. Thanks for the suggestion - very useful.

[Jmolan]

Brion Toss told me if your mast vibrates or pumps while at the dock, it is the vortex thing. If it does it while sailing, that is a mast tuning thing and could be serious. Good solution you have there. Love those kind of fixes

[GordMay]

See also ➥ http://www.cruisersforum.com/forums/...mal-27923.html

- Objectionable mast pumping, AT THE DOCK, can often be ameliorated by twisting together, then wrapping the twisted halyards around the mast, and hauling them very tight.
This disrupts the formation of the Von Karman vortices, that cause the harmonic oscillations.
- Another easy way to do this is to run a small fender up a main halyard, with a line attached on other end, so it can held tight.
- You could also move to a dock, oriented at 90 degrees to your present berth, changing the mast foil orientation to the prevailing winds (probably not very practical).
- Changing the tension of the shrouds (rig tuning) will also change the resonant frequency, but may have other unintended deleterious consequences.

[Philsboat]

Another solution might be to fasten the spinnaker pole uphaul(if you have one) to to the bow fitting and crank some pressure on it.

Phil

[Canibul]

In my previous life I was often involved in temporarily mounting sensitive acoustic equipment on the ends of pipes that were mounted to the sides of boats, with the acoustic stuff having to be down below the keel level. These boats were always underway. Strumming of the pipe was a common problem, due to the vortex shedding. Of course for a permanent installation we had other solutions, but for a temporary fix we would tie a length of line to both the top and bottom of the pipe, with enough slack to wrap around the pipe a dozen or so times. Then we would grab the loose line in the middle and wrap it around the pipe until it was tight. We would then tape/ty-wrap/lash the middle of the line to the pipe. This set up counter-rotating spirals, as the line spirals ran clockwise around the pipe from the middle to one end, and counterclockwise from the middle to the other end. This seemed to work consistently better than spiralling it all in the same direction.

I say this was temporary in our case, because the pipe was being drug through the water and eventually the line would chafe, come loose, etc. But for a mast this seems like it would be a good solution. The forces trying to loosen the line in the air are no where near the forces acting on a pipe going several knots through the water.

[Chrisc]

The spiraled mooring line works incredibly well for me and is relatively easy to install as I can just zoom up my mast steps to do the spiral above the spreaders. It would be a bit of a problem without mast steps.
Also, my mooring line is just a copy-cat solution from what I have seen on tall metal chimneys - I don't profess to understand the science behind it at all. That said, if the idea is to prevent the wind oscillation and vortexes, do you need to have a spiral? I am thinking that for ease of application one could perhaps get a mast length of heavy webbing, say 4" wide (?), sew on a few slides and hoist the thing up the mast track. Would this do the same thing?

Chris

[GordMay]

Originally Posted by Chrisc

... if the idea is to prevent the wind oscillation and vortexes, do you need to have a spiral? I am thinking that for ease of application one could perhaps get a mast length of heavy webbing, say 4" wide (?), sew on a few slides and hoist the thing up the mast track. Would this do the same thing?
Chris

I doubt it. Such a “disrupter”, not being continuously attached as you describe, would have little effect on the mast harmonics, and might itself develop a noisy harmonic.

[SkiprJohn]

Wonderful solution to a problem I've never had but may in the future experience.
Thanks to all who input the proper answers.
regards,

[crankysailor]

This is an ancient thread I found by googling .. .. I know.. however, I felt the need to..

TESTIFY!!

I was going mad with this issue on my new (to me) boat). I was about to spend €€ hiring a rigger to help me troubleshoot the issue. The shaking was so strong sometimes, that it even woke me up a few times. I was having visions of the boat being dismasted soon on a passage...

First I did the spiral thing with the twisted main and spinnaker halyard "married" just under the spreaders.. I could sense a bit of a difference I thought but there was still some shaking.. or was I imagining it was less?.. hmm.. then I hauled up a fender just over the spreaders. It looks funny as hell.. and guess what?

IT WORKS!.. HALLELUJAH!!.. TESTIFY! TESTIFY! ...

Some of you I know are still around in this forum so THANKS for this. And to folks who might be running into this issue and going nuts and thinking (as I was) meh.. it isn't that come on.. IT PROBABLY IS!!

WOHOO!!

(can you tell I am so happy?)