mast vibration

[jim]

A question from a new Catalina owner ( '85 30' w/tall rig ). When the wind picks up and the boat is moored at a dock and not able to swing to the breeze a vibration sets up in the mast. The central section of the mast will gradually built up a fore and aft motion of approximatily 2 inches for a duration of 30 to 60 seconds before subsiding. Wind speed determines the frequency of the episode.  The boat was well maintained by the previous owner and the rigging appears to be sound. Is this something that can be tuned out? Any comments or advice would be appriciated.

 

[Bob Camarena]

Normal

This is a normal effect with C-30's. As far as I know, the pumping doesn't cause any damage. It's just irritating. There was an article in the Mainsheet (I think this is where I saw it) a while ago that explained this "pumping" as a resonant effect related to the vortex created by the wind blowing past the mast at certain angles and velocities. There were a number of seemingly conflicting suggestions such as tight backstay, loose backstay, etc., but the only one that made some sense had to do with wrapping your halyards (or another line if you have internal halyards) around the mast. This disturbs the vortex and supposedly eliminates the pumping. Perhaps you've seen car radio and cell phone antennae with a wire spiral wrapped around them. This uses the same principle to eliminate wind-generated noise. I tried it one night at anchor and it seemed to work somewhat, but didn't eliminate the problem (probably since you can't wrap the mast above the spreaders).

 

[LaDonna Bubak - Planet Catalina]

Try snugging aft lowers

Howdy!Try snugging up your aft lower a touch. This should help a little but nothing will stop it completely unless you want to install a baby stay. A baby stay is located about 3/4 of the way from the base of the mast to the spreaders and tacks about 12-18" forward of the mast. Tartans & J-boats (along with a number of others, I'm sure) use baby stays to eliminate mast pumping. LaDonna

 

[Frank Corpus Christi, Tx]

Vibration

Both Bob and LaDonna are correct. Changing some things may make a difference and it is normal. Unfortunately I am an engineer and find these kinds of problems interesting. Just some generalities are explained below.What happens as stated before by the others is that Vortex Shedding (Why flags flap) causes a reaction in the mast. If the frequency of the shedding matches the natural frequency of the mast a substantial vibrational displacement can ocur. In your case it sounds like 2". As wind speed goes up the shedding frequency goes up.The two things that affect the natural frequency of is the mass and stiffness of the mast. The stiffness can be varied by the tension in the stays and shrouds or changing the mast cross section shape (not feasible). The mass is pretty well fixed also.If you loosen the supports the natural frequency of the mast gets lower. So if the mast was pumping at 20 knots it may start at 15 but be done by 20. If you tighten the supports (more stiff) the natuaral frequency will increase. So the mast may start pumping at 25 and be done by 30.It will be very difficult to change the frequency since it is a squared relationship. If you used 4 times the tension you would increase the natural frequency by 2. If you increase the Tension by 25% you would increase the frequency sqrt(1.25).By now everyones eyes have rolled back in thier heads with the detail but I hope this helps. Good luck.Frank

 

[Don]

As the others have said it is normal-Living aboard for 5 yrs I got used to it. I would tighten the backstay as tight as possible. This would normally stop it. I used the vibration as a message that the wind was strng enough to either check the anchor or the dock lines.