Mast shaking in high winds

[Obi]

Hi,
my yacht is on its berth and the mast is shaking and flexing back and forth below the spreaders during the high winds in the UK at the moment; F7/8/9/10. It was flexing about 1/2 an inch.

It wasnt too really noticable, until the wind changed direction, and when the wind turned to the north it was blowing across the topping lift that comes down to a cleat on the mast. The topping lift was taught, and vibrating/oscillating in the wind, I moved the topping lift line and this reduced the vibration on the topping lift, this had the result in stopping the mast shaking so much, but it was still doing it a little bit - noticably.

There are a 4 of halyards & pole uphauls from near the mast head, attached to a granny rail infront of the mast, these are also shaking in the wind but are not excessively taught - running from the shackle attached to the granny rail, upto the block on the mast and then down to a rolling hitch to hold them fairly tight.

I had a look at my nieghbours, but couldnt see any other masts behaving like this, but it was more noticable close up.

The yacht is a 1979 Oyster 39 ketch, which I have taken ownership of this year. I havent noticed this before.

The rigging is 2 years old.

Is this normal, or anything to worry about?

Thanks
Obi.

 

[Sans Bateau]

If the rigging was replaced 2 years ago and has not been adjusted since, then there is probably your answer, the rigging needs adjusting, tightening up. Nothing too much to worry about but get your rigger to look at it before you find yourself in heavy weather conditions. A badly adjusted rig is more like to fail than one that is tensioned properly. It will also imped the sailing capability of the boat.

 

[michael_w]

What you are suffering from is vortex shedding This is easily cured by wrapping one of your halyards round and round the mast below the lower spreaders. Works like magic! Has the added benefit of stoping your halyards slatting too.

 

[saxonpirate]

Yeah... michaels right, it's vortex shedding. We had a thread on this problem a week or so back. Follow the link below

http://www.ybw.com/forums/showthread.php?t=220082

Another good link here on mast pumping

http://www.c34.org/faq-pages/faq-mast-pumping.html

 

[saltwater_gypsy]

I can't turn up the reference right away but Selden have an extensive discussion of this problem in their data sheets. The halyard wrap seems a good idea.

 

[Twister_Ken]

Can't help thinking that the uppers and stays are going to make it a bit of problem to spiral wrap a halyard on the mast.

 

[contessaman]

Can't help thinking that the uppers and stays are going to make it a bit of problem to spiral wrap a halyard on the mast.

I agree. I hadn't noticed this thread and posted a new one as my mast has been shaking around a lot in these winds. I have 2 sets of spreaders so at best a halyard wrap will only help the lower 1/3 rd of my mast.

I have shinned up the mast and put a rolling hitch (with a line led aft on the deck and hove in tight) opposite my babystay. Solves the problem over the winter but I'm not doing that everytime I need to use the boat in the season...

It seems 'checkstays' are the only real answer.

 

[pugwash60]

If it's being caused by the topping lift try hanging the boom from the backstay and loosening the topping lift off (or use the boom crutch if you have one), I had an irritatingly noisy topping lift which used to vibrate badly in a strong wind at anchor,( ie wind ahead,) so probably to do with these vortices which I've not heard of before but which sound like they make sense.

 

[roger]

Vortex shedding

In anything but a gentle breeze, vortices are shed alternately from each side of mast, sheet halyard etc. Each vortex gives a push to the mast as it leaves - thus shaking it continually. When the shake frequency coincides with the natural mast frequancy the shaking builds up - Think Tacoma Narrows bridge.
Remedies are :-
Destroy the vortices - by winding rope round (Its like the odd things at the top of tall chimneys
Add damping
Change the natural frequency - by tightening or slackening halyards etc.
It is often difficult to work out what is doing the shaking. My boat whole shakes because of vortex shedding from the furled jib. For that I tighten up the jib sheets.
When the sourc is a tight topping lift then slacken it off a bit.
Only damping will cure the problem at all wind speeds tension adjustment just changes the wind speed at which it happens.

 

[michael_w]

Ken, you don't need to do the whole mast. Usually the area below the lowest spreaders will do. Just like the partial spiral spoilers on chimneys.

It seems that this problem is mainly confined to masthead rigs. None of my fractional boats have suffered. Maybe its somthing to do with the way a fractional mast is pre-bent ans set up with very tight cap shrouds.