boat shakes in slip.

[brad minda]

I know this may sound strange but if any wind is blowing the boat will shake. This shaking sometimes is so bad that the screen on my open lap top will shake. I have tried the see if anything is loose and can't find anything. Any ideas, I think it may be my back stay as it is the longest wire vibrating or shaking in the wind. Any ideas on how to stop it would be great. One note the standing rigging is new, probably has stretched and needs to be tightened.

 

[vhoisington]

What is causing the shake? If the shrouds and stays need to benightedness try that first. Are the dock lines too tight? Is the dock shaking as well or just the boat.

Try these and let us know the cause.

 

[Joe]

Do you use those rubber snubbers on your dock lines?

 

[All U Get]

Let's see, California and shaking. Now that's funny. However I can feel the wind vibrate through the rigging if we're at a dock. Do you feel it when you anchor out? Maybe your boat wants to move a little more.

All U Get

 

[Anonymous 121765]

Brad,

While docked, when the wind is blowing around 15 kts, I get what I refer to as," My mast pump."
I keep my rigging tuned like a piano & as wind hits my mast from the beam, I occasionally feel this.
It's a vertical pump, pump, action. When I first encountered this, I was below & concerned. After abit, it became apparent that my mast was flexing abit above the spreaders. Not much, but enough to feel it throughout the boat. It now tells me my rigging tension is good.

Hope this is what you have pal.

CR

 

[JohnVTX]

Ditto on the mast pumping. I got it allot on my C25. Not some much on my 30TR

 

[Anonymous 121765]

Brad,

Yea, John agrees with me on the pumping.

What I should've added, was that if a steady wind has gusts, this is when I feel my mast pump.

CR

 

[The Garbone]

In out last marina our boat neighbor had a in-mast furling main. If the wind was over 10 out of the ENE the sucker would howl like a train whistle.

 

[JK_Boston_Catalina310]

I was going to say mast pumping as well. Check out the Catalina 34 IA tech wiki. They have some info on mast pumping. Not much you can do I believe.

 

[Johnb]

It is most likely caused by a phenomenon called vortex shedding.

https://en.wikipedia.org/wiki/Vortex_shedding
"In fluid dynamics, vortex shedding is an oscillating flow that takes place when a fluid such as air or water flows past a bluff (as opposed to streamlined) body at certain velocities, depending on the size and shape of the body. In this flow vortices are created at the back of the body and detach periodically from either side of the body."

One way to mitigate vortex shedding is to alter the shape of the "bluff body" (your mast) by wrapping something (your halyards, lazy jacks or whatever) in a helix around it. This is why car antennas sometimes have that helical addition along their length, or telephone cables are twisted between poles - it stops the whole length from shedding vortices in unison.

Changing the tension of the rig may only change the "natural frequency" and therefore the wind speed at which the phenomenon peaks.

 

[justsomeguy]

More write-ups in these forums under "von Karman effect".

 

[Jongleur]

Yes to the vortex shedding. I solve it by wrapping
all halyards, twisted together around the mast. No
more pumping. Not a good idea to overly tighten
rigging. Won't help in vortex shedding anyway.
Just changes the frequency.

 

[jssailem]

Do you feel it when you anchor out? Maybe your boat wants to move a little more.

Laughing out loud.

 

[Bob4203]

"Vortex Shedding".....................love it!