June 18, 2015

How to cure mast vibration

HAVE YOU EVER experienced one of those times at anchor when a stray puff of wind hits you from abeam, and the mast starts shaking? It’s not a good feeling. I can tell you that from experience. You begin to wonder how on earth the darned mast has managed to keep standing all this time. You begin to wonder how close your mast has been to collapsing, if a little wind from the side can set it dancing like that.

The alarming effect of mast vibration can occur on almost any sailboat, but particularly on those with deck-stepped masts and insufficient fore-and-aft staying. The vibration is caused by wind eddies shedding alternately on either side of the mast, which theoretically oscillates at right angles to the wind.

In practice, nearly all such mast movement occurs when a moderate wind, up to about 15 knots, blows from abeam or thereabouts. When the natural frequency of the mast happens to coincide with the frequency of vibration, the mast can suddenly start shaking quite violently, rattling the whole boat and raising no small amount of alarm among her crew.

You can reduce the possibility of this vibration with an extra stay, such as a wire inner forestay or a removable baby stay. In a pinch, you can use a low-stretch line, made fast to the mast as high as you reach and taken to a bow fitting and hauled taut.

A more certain cure is to hoist in the mast groove a stiff (say 9-oz.) strip of sailcloth at least 4 inches wide. This will break up the regular vortices on the downwind side of the mast. But it’s also pretty certain, of course, that no one will want to go to this trouble.

Another way to improve matters somewhat is to tighten your shrouds and/or stays, thus increasing the downward load on the mast. That will usually reduce the fore-and-aft movement of the mast enough to give you some peace of mind, but I doubt it will help you sleep any better.

Today’s Thought
The wind’s in the east . . . I am always conscious of an uncomfortable sensation when the wind is blowing in the east.
— Dickens, Bleak House

Tailpiece
A man is sitting in a pub having a drink and nibbling peanuts from a bowl on the bar when he hears a voice saying: "You look smart, that's a nice suit.'' He looks around but the bar is empty. Eventually the barman reappears, and the mystified man tells him what happened. "Oh, that would be the peanuts," says the barman. "They're complimentary.''

(Drop by every Monday, Wednesday, Friday for a new Mainly about Boats column.)

3 comments:

Alden Smith said...

Thank goodness I have an inner forestay on my yacht Mariner!

Anonymous said...

On several boats I have owned, mast vibration occurred as a result of a tight boom topping lift vibrating in the breeze, rather than vortices around the mast itself. The simple solution was to reduce tension on the mainsheet, so the topping lift arced back in the breeze rather than vibrating like a guitar string. Try this first before going to the trouble of flying something from the mast while at anchor. That sounds like a rather noisy solution.

Guilherme Castelao said...

Vortex street! The equivalent also happens in the currents passing by islands. There are nice videos on youtube on that subject.