Boat shuddering during storm.

[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!

[nzmal]

Sorry should qualify
39 ft fibreglass sloop.( Cavalier 39)
1974 vintage. 100ft of 5/8 chain with 40ft of rode.
Rigging new this year.
3/4 integral keel.

[robwilk37]

likely a combination of both, and the flexi nature of fiberglass. i used to race on 40' ULDB that would constantly quake every time we pounded or fell off a wave. disconcerting at first but got used to it after a bit...

[Alan Mighty]

Originally Posted by nzmal

Can anyone shed light on what may produce this? I'm speculating either rigging resonations or chain vibration? Certainly it was a harrowing Night!

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

When you're sailing, your mainsail prevents a vortex street forming.

If you look around at tall chimneys and such like narrow towers, you'll see various ways to defeat mast pumping. One is to attach a helical fence around the chimney.

Wrapping a hawser in a spiral around the mast is an easy way to make a helical fence.

Or you can hoist a small fender (longer than the diameter of the mast) using the main halyard and a tag line, to about 2/3 mast height.

[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

[Jim Cate]

Agreed, Alan... very likely mast pumping. On the Yankee 30 that I owned years ago (single spreader, single lowers, baby stay and a fat mast) when strong winds hit at the right angle the mast would pump vigorously. You could see the center section moving maybe a half diameter at a couple of hertz. Would shake the whole boat for a few seconds (which seemed much longer!) until the wind angle changed and it would quit.

Never tried the helix wrap idea... seemed like it would take a pretty big diameter line to do much to the air flow and I had nothing that met that description and couldn't see how to conveniently wrap it above the spreaders... so sloth won out!

Jim

[Alan Mighty]

I assume that mast pumping take place when the frequency of vortex shedding is close to the resonant frequency of the mast. Something on that line.

I had a length of a nasty orange polypropylene hawser, about 2" or 5 cm, in diameter that I found floating one day (assumed lost from a trawler or the like) and removed from the sea.

And when in an anchorage sheltering from 35 knot SE on the QLD coast, when some gusts of >35 knots pumped the mast, I wove it through the mast steps and around the mast as a helical fence. Seemed to do the trick in terms of stopping the mast pumping, but I cannot prove that the mast would have pumped without the hawser. The mast steps certainly did not stop the pumping.

[four winds]

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!

The shudder caused by mast pumping or "panting", as some say, is certainly an unnerving type of vibration. Like something bad is about to happen.

On a single spreader masthead rig sloop one thing to consider is having the forward lower shrouds just I bit tighter than aft lowers. Makes it harder for the oscillation to set in.

[savoir]

It might be a resonating hull caused by vibration in the forestay.

It is also unlikely that the mast of a 1974 production boat would be so sensitive as to vibrate in this fashion. By all means check for loose chocks at the mast collar which would exaggerate the problem.

[Sailmonkey]

Originally Posted by savoir

It might be a resonating hull caused by vibration in the forestay.

It is also unlikely that the mast of a 1974 production boat would be so sensitive as to vibrate in this fashion. By all means check for loose chocks at the mast collar which would exaggerate the problem.

I have a 1976 boat with a tree stump of a mast. I can promise you that it is quite capable of pumping in the type of winds the OP has described. It’s annoying, and I have found nothing that works on all wind strengths and angles. I’ve even played around with tension on the lowers. When it’s stopped from pumping in a 30 knot blow at 0 deg bearing it will pump at 25 knots at 90 degrees.

[CaptTom]

For the record, power boats can shudder, too. You don't need a mast.

It's especially noticeable on larger ships. I once had a stateroom near the stern on a very rough crossing (small cruise ship) and was "rocked" to sleep feeling the bow plunge into the waves, then feeling the shudder run all the way down the keel. I found it relaxing. I've felt it on smaller boats, but thankfully not as pronounced.

[Steadman Uhlich]

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.

[malbert73]

I have an adjustable baby stay on a track that I can adjust for cockpit. That seems to stop pumping and I can adjust if frequency changes with wind speed change.
One could do same with staysail halyard or spin pole topping lift halyard. If you have these...

Even if normal to have pumping I would want to stop the pumping given effect of fatigue and snatch loads on rigging.

[thomm225]

You might try putting in some mast prebend.

Mine used to pump quite a bit in 30 knots or so at anchor but has pretty much stopped since I put in some prebend.

If I remember correctly, I have 440 lbs on all shrouds and stays except the forward lowers which have 540 lbs or so

The prebend looks to be maybe one and one half inches or so but that's with an eyeball guestimate. I used to measure it when I raced but not on this cruising boat.....

[S/V Alchemy]

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.

When you're sailing, your mainsail prevents a vortex street forming.

If you look around at tall chimneys and such like narrow towers, you'll see various ways to defeat mast pumping. One is to attach a helical fence around the chimney.

Wrapping a hawser in a spiral around the mast is an easy way to make a helical fence.

Or you can hoist a small fender (longer than the diameter of the mast) using the main halyard and a tag line, to about 2/3 mast height.

Well, you learn something every day. I was going to go with "resonant harmonics".