Mast Vibration

[mark_53]

Hi all, just wondering if anyone else is experiencing main mast vibration when the main sail is down and 15-20mph winds hit.  I get a modal vibration that shudders the boat. Not so bad to do damage but noticeable.   I checked my shrouds and fore and back stays and the all seem tight.  Didn't have a rigger check it yet.  Seems like it has always done this.  I don't notice it when under sail and shrouds and stays seem tight. 

[Stu Jackson]

Mark,

This is from the WAY OLD pre-Forum/Main Message Board days, but there's a lot of valuable information there.  Frankly, I'm amazed I can still remember that some of them exist!  :)

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

This is from this page in the Tech wiki:

FAQs

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

The intro on that page explains where the content came from.

[mark_53]

Thanks Stu, looks like a mini structural dynamics lesson to boot. I'll have to try the trick with the halyards. What a great trove of information you have here.

[fatamorgana]

I have this same problem and it's driving me crazy.  I think I have read just about every post on this forum about this topic, but still getting it.  It happens most when the wind is coming from the side of the boat while we are in the slip. 

I have double checked the rig tensions using a Loos gauge using numbers I found on this site.  The forward lowers are set tighter than the aft lowers.  I even have a bit of bend in the mast (middle going forward). 

Wrapping the halyards certainly helps, but it seems like there should be something else I can do with the mast where I wouldn't need to wrap/unwrap halyards all the time.

The only other thing I've read about is changing the mast rake.  I haven't moved the mast step forward yet as I'd have to shorten my backstay then.  But, I wonder if that would really help, since the mast would still be in the same basic shape, just angled slightly aft.

[Stu Jackson]

>>>>>>>>>>>>>>>>>>>>>>
The only other thing I've read about is changing the mast rake.  I haven't moved the mast step forward yet as I'd have to shorten my backstay then.  But, I wonder if that would really help, since the mast would still be in the same basic shape, just angled slightly aft.

Based on that article I linked to, it would appear that raking it would NOT help at all.  I would surely recommend against doing that kinda surgery to your rigging.

The reality is that sideways wind simply DOES DO THAT because of the reasons in the article.

In the 20 years I have been reading about this subject, that FAQ article is the ONLY one I have seen about that topic.  I understand the idea of "The only other thing I've read about is changing the mast rake..." but wouldn't it help much more to provide either a link or a copy of that article so we can all read it?

[Ed Shankle]

I'm by no means a rigging expert, but the fact that you have some forward bend in the upper part of the mast says to me that the mast isn't properly tensioned. My observation with my own is, more mast pumping when it's been out of column. Once realigned, the pumping goes away, except for when an occasional big gust hits. Of course, I'm on a mooring, so that may impact the frequency.
Regards,
Ed

[Ed Shankle]

Sorry, I should have directed that to Fatamorgana, who noted the forward rake of the mast.
Regards,
Ed

[fatamorgana]

The mast was straight previously, I induced some bend by tensioning the forward shroud more than the aft shroud, and tightening the backstay more than it was.  This helped, but the pumping is still there.

Mast bend discussion: http://c34.org/bbs/index.php/topic,9070.msg65842.html#msg65842
Rig tension guide I used: http://www.c34.org/wiki/images/8/8a/Rigging01.pdf

[pablosgirl]

Ed,

Forward mast bend is not really out of column and will not impact the mast pumping one way or the other.  The pumping has to do with the mast cross section shape and how the air flows over that shape, it's length and where the stays attach all working together to create a harmonic vibration.  It is more pronounced when the broad shape of the mast is facing the wind, like what happens when you have the boat in a slip.  The only way to prevent this is to detach the laminar flow of the wind over as much of the mast as possible.  I have had great success with taking the spin pole toppling lift line and centering it up at the block on the mast and warping each end around the mast as many times as the length of the line will allow, crossing each line in an "X" pattern at the forward and aft section of the mast.  The more the line wraps and hence the more crossing of the line the more you break up the laminar flow, the less pumping you will have.  Just doing the lower portion of the mast is enough to break the harmonic range for our rig setup and stop the pumping.

Also, forward mast bend can be a good thing when you are trying to de-power the mainsail in heavy air.  Mainsails are cut with a forward curve in the luff to provide part of the shape(draft) in the sail. When you hoist that sail on a straight mast this is what provides the belly/draft in the sail.  When you bend the mast forward, the bend of the mast approximates the bend cut into the luff of the sail thus flattening the shape of the sail thus producing less power and heal.  Forward mast bend is usually produced by increasing the back stay tension, but on mast head rig such as ours the forward lowers can be use as well. If you are interested in learning more about sail shape do a Google search on "book on sail shape" and you will have your winter reading list for years to come.

Paul

[Ed Shankle]

Paul,
I agree with your comments on mast bend regarding sail shape, etc. But that is under active conditions. I was referring to an "at rest" state, such as back on the mooring, where presumably the backstay tension would be eased back to tuned levels, allowing the mast to return to an "in column" state.
Regards,
Ed