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M-384 Mast Vibration

cfnava

Carlos F. Navarrete
I have a Morgan 384 (sail number 39). I would like to know if anyone else with a 383 or 384 has experienced a mast vibration while tied to a dock. Whenever the wind is on the beam (at the dock) I have an intermittent low frequency (4-8 hz) vibration in the mast. The amplitude is not great, but still large enough to rattle the internal cables/halyards. I have randomly adjusted the tension in the stays in an attempt to reduce the vibration but this doesn't seem to have any effect. Does anyone have anything to offer about this problem?
 
i experience the same thing with my M38....i think its those shivering timbers the pirates used to talk about....anyway i dont see it as a problem with the rigging or tuning but perhaps someone else can help us both with a better answer.....

 
<div>It may not be your mast but your toping lift. Next time you notice the "humm" reach up and grab the toping lift. If it is the toping lift you will feel the vibration. Loosening the tension on the main sheet and / or changing from rope to wire for the toping lift may solve the problem...but then again it may not... Harmonics, sometimes are just un-solveable.Good luck.

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Bud, I have experienced the same on my Morgan 384
(sail # 032), both vibration and some mast pumping
at dock in certain wind conditions. Some more tuning
seemed to cure it. Also, a mast covering inside when
re-decorated seemed to silence some halyard mast
slapping noise noticed in certain wind conditions.

 
I have the problem as you discribed. In addition to tuning the lowers, I believe that the problem is a result of the prebend that I have in my mast. Every year I try to tune this problem away but, have not had much success.
 
In response to the radar cable clang inside I have tried a simple solution. Foam Pipe insulation. Slit open and wrap the cable tape closed and push up adding about 12 feet total. It really quieted it down. Some folks say you can just make a group of the foam sections and just stuff them up the mast to deaden the sound.
 
The latest issue (May 98)of Mainsheet magazine (from Catalina) has a definitive article on this subject.
In summary: Any cylinder in a airstream will generate vortices. The frequency depends on the airspeed, diameter, etc. When the vortices occur at a frequency near natural frequency(s) of the mast, the vibration will be noticable.
The writer notes that the spiral staircase like structure which can be seen on modern smokestacks serves to disrupt the formation of these effects.
There is no cure. Altering the compression load on the mast column by adjusting shroud and stay tension has a minor effect on the natural frequencies of the mast. For long term storage, winding a halyard around the mast in a spiral will disrupt vortex formation.
 
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