Boat shuddering during storm.

[Locquatious]

It wasn't a shudder as such, but we noticed that the boat seemed to shake each roll while on a mooring. We dove to see if we were hitting bottom, but it was deep enough.
The next sail, a chainplate broke. We suspect that the plate was broken before the sail but had not yet pulled out from the deck slot. Or the last threads of the plate crack had loosened the rigging such that the mast would flex away from the broken plate and then snap upright against the tight rigging on the other side.

[taxwizz]

Maybe the boat is just expressing its nervousness and dislike of your abilities as skipper?

Maybe your boat just doesn't like you.

[FionaJC]

Originally Posted by Alan Mighty

Possibly mast pumping. That's mast vibration due to shedding of a von Kármán vortex street (named after Theodore von Kármán) along the mast.

What a fantastically fascinating post and brilliant answers! Ive often wondered why tall chimneys had that but never bothered to research it.

"von Kármán vortex street" is definitely a phase I am going to embed in my knowledge cells. Looking forward to seeing one when I am in company or if asked at the next quiz. Now there's a quiz question

[Ken Z]

Probably your mast, do you have spreaders? Is your mast mounted on your cabin top or thru the deck? I rode out three hurricanes with a thru deck mastwith spreaders and didnt experience much shutter , had another boat with the mast stepped on cabin top and had a lot of shutter.

[Flagman101]

My C&C 30MK does the same thing. I use the spinnaker pole topping lift to stop it.
I attache it to the baby stay deck tackle and winch a little pressure on the toping lift. This puts tension on the middle of the mast and prevents it from pumping and vibrating.
Next time maybe try and figure out what causes that vibration.
It's usually pretty easy to find.

[Smokeys Kitchen]

Reading the Wikipedia article was interesting - except for the formulas!

I read the example of the vortex occurring around the Juan Fernandez Islands off the Chilean coast. Anyone have any ideas how this would affect sailing around them and if this is a common occurrence near an island chain?

[CShawn]

Originally Posted by Steady Hand

Alan,

Thanks for adding the info about Vortex Shedding.
I enjoyed learning about that, especially after seeing the effects in photos of clouds and windstream tests show on Wikipedia article.

The spiral strakes on towers and chimneys was something I had wondered about for years, and now I know their reason for being added to such structures.

Each time I come here to CF I learn something, and sometimes it is an answer to a thing that I had wondered about for years.

Thanks to you and everyone else that contributes and shares knowledge here.

[Flagman101]

Originally Posted by Smokeys Kitchen

Reading the Wikipedia article was interesting - except for the formulas!

I read the example of the vortex occurring around the Juan Fernandez Islands off the Chilean coast. Anyone have any ideas how this would affect sailing around them and if this is a common occurrence near an island chain?

Any tall mountain will affect wind.
I sailed from St-Pierre Martinique to Fort de France. The wind forecast was for NE winds 20kn. I was observing 10 kn westerly winds. As soon as I cleared the shadow of the volcano the NE winds kicked right back in and were as per forecast.
If at all possible I would avoid sailing in the and shadow of a high mountain. Unless you have to sail there.
The Capo Verde islands are notorious for there wind shadow. All racers know to avoid it.

Also keep in mind that any large land mass will also affect wind.
Thermals effects and friction can completely change the wind direction and speed. And this as the day goes by you can experience very dramatic land effects on the wind.

[Cadence]

Originally Posted by nzmal

My wife and I just survived a few tense hours at anchor while a tropical storm blew through. We had gusts to 60 knots but held on with a little dragging.
One thing I hadn't experienced before we're occasional instances where the entire boat went through a *shudder* phase that lasted perhaps 15 to 20 seconds.
Can anyone shed light on what may produce this? I'm speculating either rigging resonations or chain vibration? Certainly it was a harrowing Night!

I feel you're correct about it being The rigging singing.

[Sailormantx]

Originally Posted by Alan Mighty

See the Wikipedia page for vortex shedding for both a description of the phenomenon and an image of a chimney with what I call a helical fence and they call a helical strake.

https://en.wikipedia.org/wiki/Vortex_shedding

Thanks for the link! That explains a lot that I have figured out on my own; I didn't know there was a name for that. Nomad is junk rigged, hence there are no spreaders in the rig and the entire mast column is only supported at the ends. The mainmast in particular would get pumping in certain conditions and I had tried tying lines around it only to have it start again when the wind changed. I discovered quite by accident that wrapping my staysail halyards in a spiral around the mast stopped it from pumping. This is now a permanent storage location for the halyards when I am not sailing

[dannc]

Originally Posted by Sailormantx

Thanks for the link!
..
Nomad is junk rigged, hence there are no spreaders in the rig and the entire mast column is only supported at the ends. The mainmast in particular would get pumping in certain conditions and I had tried tying lines around it only to have it start again when the wind changed. I discovered quite by accident that wrapping my staysail halyards in a spiral around the mast stopped it from pumping. This is now a permanent storage location for the halyards when I am not sailing

And Thank You!

We are interested in Junk Rigs, and when I was reading this thread, I started to wonder if vortex shedding affected junk rigs, and if so, would the spiral halyard technique prevent the pumping.

Note: I always thought the "fins" on exhaust stacks were structural and had no idea it was wind related.

Thanks,
Dan

[CaptTom]

My vocabulary expanded a lot today, thanks!!

And I always thought those helical strakes on chimneys were something to do with structural strength and rigidity.

Who says cruising isn't educational?!

[thomm225]

You can make it as complicated as you want or as simple.

Crank in a bit of mast prebend and your problem will disappear. (mast pumping problem)

You can't put prebend in your Chimney!

Plus with the prebend, you should get better upwind performance especially in heavier winds (think flatter mainsail) but it may not be noticeable unless you are racing

[Cadence]

Some years age auto antennas had a spiral around them to counter that at higher speeds.

[Mike Banks]

I think it is most likely the rigging vibrating due to resonance--caused by the turbulence of the wind.

However--there is another possibility, one that is more concerning--and that is a dragging anchor on a gravel sea bed. If you are using an all-chain rode, add a kellet, also known as an angel--which alters the angle at which the rode pulls on the anchor and can, in good ground, effectively double the holding power of an anchor. It needs to be shackled on to the chain at about one third of the total scope, taken as measured from the anchor. I used a kettle weight which I bought at a sale for ten dollars. Previous to that, a length of railway iron track with a loop welded to one end. It needs to be about half of the weight of the anchor to be super-effective, but any weight at all will be helpful. Not only does it make for better holding--it makes for a soft and gentle anchoring.

I have on two occasions drifted during a blow. Both times it was a dragging anchor. Now I take anchoring extremely seriously. A kellet will give you the most holding for the total mass of all anchor tackle carried aboard.


Still--I think it sounds more like the rigging to me. It depends on the violence and period of the vibration. Resonant vibration can be quite severe, but it is more or less regular in its pulsation.