Boat Shudders With High Wind Gusts

[GordMay]

Greetings and welcome aboard the CF, Salty Skipper, and MVNightWatch.

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 be drawn 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.

[carstenb]

Originally Posted by Sid at SailAway

My boat, a Lagoon 410 will get these vibrations also. It never happens at anchor, only at the dock. The vibrations will shake the whole boat. I narrowed down what was happening to the fact that under certain wind directions and velocities the mast will create a vortice on the downwind side. I solved the problem by hauling up a fender about 3/4 the way up the mast using the spinnaker halyard. The fender disrupts the formation of the vortice and thus the vibration. It doesn't happen all that often, so hauling the fender up hasn't been a hassle.
Next time you see a tall smoke stack or other similar object take a look. It will have some sort of spiral bands going up the stack to break up the vortices. Try the fender idea and see if it works for you...Sid

This is correct. If the fender doesn't solve the problem immediately, experiment with it at various heights. My guess is about 2/3 the way up will probably be about right.

It frequently sounds like at train going by quite closely

[minaret]

Originally Posted by MarkJ

It may be Harmonic Resonance.

This little trick of science has bugged architects and engineers for thousands of years. In fact it totally pissed off the romans when they were building bridges because when long lines of troops marched over them the harmonic Resonance could cause the bridge to fall down!

All engineers will see a film in their first year at university showing a bridge in the USA turning into a fluttering ribbon in quite low winds, maybe 40 knots, till it ripped itself apart.

Another new bridge that had it is the ANZAC bridge in Sydney over a portion of Sydney Harbour. It started to resonate along its stay wires when there was NO wind! It happened when a very light drizzle would fall.

But their solution could be your solution. They tied short ropes from part way up the stays to the road way. Problem solved.
So experiemnt with a bit of rope from your shrouds to a toe rail at 90 degrees and see what happens. It doesnt need to be strained, just enough to change the resonance.


Mark

http://m.youtube.com/watch?v=uWoiMMLIvco

http://m.youtube.com/watch?v=3mclp9QmCGs

Resonan bridge is designed to harmonically resonate at very common low frequencies.



The Tacoma Narrows bridge is likely the one you mentioned. Around here it was known as " Galloping Gurdy".



Nikola Tesla invented a harmonic resonance device which when properly tuned and left running long enough could destroy large structures like buildings and bridges. The NY fire department shut him down. Mythbusters did an episode on it. At first they couldn't even get close to his results, but when they realized just how fine Teslas tuning device was they began to make it work, and quickly realized this is very dangerous stuff.

[MarkJ]

Yes Tacoma bridge is the one.
Science is fascinating.

[Zednotzee]

I tried the halyard thing- it was worthless. Maybe it would work for a round mast, but it had no effect on mine. It may also be different with a fender attached, couldn'y say.

Sent from my SGH-I257M using Cruisers Sailing Forum mobile app

[Carl Richardson]

My experience with this problem:

1) First ease all your running rigging attached to mast and boom ( halyards, topping lift and spinnaker pole lift) and listen…….

2) Now harden (trim/tighten) up this rigging, listen…..

3) If the sound is different/ disappears make adjustments to running rigging
- try putting halyards out on pulpit / pad eyes, attach your jib halyard to the bow roller and harden up.

If the sound is the same/ doesn't go away completely, call a rigger to discuss potential for creating stress fractures in spars, shackles, tangs etc- noise is energy and if its not doing something positive it just may be doing something negative to your boat.

I had this problem and rigged the jib halyard to the bow anchor roller, hardened up the jib halyard and the vibration disappeared.

Some sailors have attached a 4 ft. single braid > 3/8 spectra line with snapshackle to their bow and attach their Jib halyard to it- we call this a "fractionator" and it is used on racing and cruising boats to move the tip of the mast forward when flying the spinnaker - don't forget to ease your backstay.

Good luck- CRR