May 13, 2011

Taming the mast

THERE HAVE BEEN TIMES when the behavior of my boat’s mast has puzzled and even alarmed me. While lying quietly in a sheltered marina slip, with a gentle breeze from abeam, my mast has started to shake back and forth, as if the shrouds and stays had all completely abdicated their dedicated duty.

It took me quite a while to learn that any mast may vibrate in winds of moderate speeds (5 to 14 miles an hour), and the vibrations may become severe when the natural frequency of the mast coincides with the frequency of vibration.

Alternate sideways movement occurs when wind eddies shed from one side then the other. In theory, it’s possible for the mast to vibrate back and forth in any direction perpendicular to the wind direction. Almost always, however, the movement is fore-and-aft, with the wind coming from abeam, or nearly so.

Here are ways to cure, or at least lessen, vibration:

1. First tighten the stays; then tighten the shrouds if necessary.

2. Add a wire or rod inner forestay.

3. For mast with luff-grooves, hoist a stiff narrow strip of heavy sailcloth in the groove to separate wind eddies. The strip needs to be at least 4 inches wide.

4. Turn the boat so the wind is striking the bare mast less from the side, and more from fore or aft.

5. For temporary relief on smaller boats, lead a non-stretch line from a strong point forward, such as a Samson post or an anchor cleat, clove-hitch it securely around the mast as high as you can reach, take it to a winch aft, and tighten as much as you can.

Today’s Thought
The way of the Wind is a strange, wild way.
— Ingram Crockett, The Wind

Boaters’ Rules of Thumb, #198
If you boat has the kind of stern gland that drips water to lubricate the packing, the rule of thumb is to err on the side of too many drips rather than too few. The water flow is needed not only to lubricate the gland, but also to prevent an excessive build-up of heat. About one of two drops a minute is about right when the shaft is not turning. When you’re under power, the rate will naturally be somewhat greater. A flow of water containing oxygen is also needed to protect a stainless-steel propeller shaft from pitting corrosion in the stern gland.

“Tell me, Vicar, do you believe in sex before marriage?”
“Not if it delays the ceremony.”

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


frabjusday said...

1. John, thanks for using the verb "to lie" correctly. So many people would have said "While laying quietly in a sheltered marina..."

2. What meaning of the word "shed" did you have in mind when you said, "wind eddies shed from one side then the other"? I can't find a meaning that makes sense in this context.

Thanks for your great blog.

John Vigor said...

Ah, frajusday, you haven't been reading your quota of bodice-ripper novels lately ... "He stared in awe as she shed her filmy blouse; and he felt his heart pounding as she shed the silk cami that nestled against her soft golden skin. Then in an ecstasy of delight he watched as she shed ..."
To shed is to let fall away, as when you shed your inhibitions, or when your dog sheds his coat in spring. So when the wind is blowing against the mast it will fall away to one side or the other, causing a slight vacuum, which, in turn, forces the mast to move slightly. As the mast moves, the wind ceases to shed on the original side and now starts shedding on the other side, causing the mast to move back. And so it goes, alternately and endlessly shedding eddies from either side and urging the mast into vibrations, sometimes quite large ones. You can see the same effect on an ensign flapping in the breeze from side to side, as the eddies shed, or fall away, first on one side then the other.

Cheers, and happy shedding,

John V.

Anonymous said...

hi all the best to all of you - matt-mays