# Rig vibration during sail and at dock?



## g0twind (Oct 5, 2010)

While out sailing in a slight breeze (10 knots or so) I noticed a vibration on the boat. The mainsail did not have it battens in and the leach was blowing around a bit. Could this be causing the boat to vibrate? 

Then, while sitting down below at the dock the next day, i noticed a slightly different vibration. I went up on deck and could feel the mast vibrating when the wind blew. Is this normal? 

This is the first year we have spent much time just hanging out at the dock, which is why I probably never noticed the vibration before....


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## peterconway (Sep 23, 2010)

what do sailboats and guitars have in common? they both have tight strings (shrouds), necks (mast) and acoustic chambers(hulls). add a little wind to activate the strings and music is made.


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## MJBrown (Apr 1, 2009)

Don't forget the topping lift. Too tight and it will sing.


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## Faster (Sep 13, 2005)

Quite normal... some boats make more noise than others, some rigs will rattle quite a bit in a crosswind.

We chartered a C&C 38 one time, had a good sail and then moored for the night. We spent a half hour trying to find the pump we could hear running below only to figure out in the end it was rig vibration.


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## Boasun (Feb 10, 2007)

In port you will with a elipical mast with spreaders have various wind eddies along that mast and it will set up a vibration. Some harmonic and some not.
Move your halyards away from the mast, one way or another to stop them banging against the mast. It is very very annoying to have to listen to those halyards banging because some people can't, won't or ignorant of the practice of 'Good Neighbor Policies'.


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## g0twind (Oct 5, 2010)

The topping lift is pretty tight now that I think about it. I will loosen it a bit while at the dock. 

I need to get some battens into the main to make sure the vibration while sailing goes away.

I always tie my halyards away from the mast, so not to bother all the "other" boaters....


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## Barquito (Dec 5, 2007)

I was also suprised by the noise passing motor boats make.


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## peterconway (Sep 23, 2010)

i like to weave an old bungee cord thru the rigging while docked to muffle the vibs and clangs


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## Frogwatch (Jan 22, 2011)

You'll know when you've been aboard for a long time when somebody points out the noise you no longer notice.


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## WDS123 (Apr 2, 2011)

Rig tension ?

Rudder bearing ?


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## RichH (Jul 10, 2000)

What you most probably have is 'mast pumping' - an induced harmonic vibration that is caused by the eddy currents of wind crossing over the mast. This is the exact same dynamic vibratory response that makes a violin string 'vibrate'. 
RIG TENSION is critical and changing your rig tension 'up' or 'down' will change the 'natural frequency' that your mast is 'pumping'. If you have lower aft and forward stays in addition to the typical 'cap' shrouds, increase the forward lowers and ease off the aft lowers ... to induce a 3/4" forward BOW into the mast. This 3/4" BOW will very significantly change the 'stiffness' of the mast and the harmonic natural frequency at which the mast becomes 'excited'. Do websearch for "prebend + mast" or check out the many SPAR manufacturers to see exactly what is needed for YOUR mast .... OR if the vibration is becoming 'severe' then add a 'check stay' to the front of the mast to get that desired 'forward bow' shape -3/4" for a single spreader rig and 1/2" per set of spreaders on a multiple spreader rig. 

Rx: change the rig TENSION and add some 'prebend' in the mast. 

;-)


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## puddinlegs (Jul 5, 2006)

Adding prebend opens up a whole range of issues, most predominately, you'll be looking at recutting your main as you've effectively flattened the sail before you leave the dock. If the main is cut with prebend in mind, your fine. General rig tension I'll buy. Shrouds, etc... are the guitar strings you allude to. It deserves a look, but in my experience, prebend is about many things, just not about harmonic vibration. Mast pumping is about fore/aft mid mast movement that is wave or seaway induced on some rigs depending on the mast section, and as you say,this is controlled with check and/or a baby stay. We use check stays for this when it's rough, but our mast doesn't vibrate in it's current configuration at all whether the checks are on or not.


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## RichH (Jul 10, 2000)

puddinlegs said:


> Adding prebend opens up a whole range of issues, most predominately, you'll be looking at recutting your main as you've effectively flattened the sail before you leave the dock. If the main is cut with prebend in mind, your fine. General rig tension I'll buy. Shrouds, etc... are the guitar strings you allude to. It deserves a look, but in my experience, prebend is about many things, just not about harmonic vibration.
> 
> Mast pumping is about fore/aft mid mast movement that is wave or seaway induced on some rigs depending on the mast section, and as you say,this is controlled with check and/or a baby stay.


Thats entirely INCORRECT. 
A 'stock' plain vanilla mainsail will 95% of the time be cut EXPECTING the correct amount of 'prebend' in the mast ... and that small amount of 'prebend' will typically decrease the amount of maximum draft by about 2-3 inches (as measured at the 'point of maximum draft'. So, in reality, without setting the proper prebend will result in a slightly powered-up sail with a slightly 'hooked-up' leech.

Setting PROPER prebend increases the relative mast stiffness by increasing the "I (structural moment of inertia) to the third power" a vitally important characteristic of 'stiffness' / vibrations. The previous posting was about INDUCED HARMONIC VIBRATIONS (caused by the mast shedding 'votices' - Karman Vortices ... not from wave or sea action - BIG difference.

More applicable info on 'mast vibrations' (although the below is Seldén mast specific the can be applicable in a 'general sense': Seldén Mast AB
Seldén Mast AB .... but see page 24 for correctly setting up 'pre-bend' on a (Seldén) mast

So the previous recommendation/posting stands, as the OP probably has a thin section mast that is vibrating because its being 'induced' by the wind to oscillate as its 'natural harmonic frequency' and the typical remedy is to change the rig tension AND to add 'pre-bend' to artificaily change that natural frequency.

;-)


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## puddinlegs (Jul 5, 2006)

RichH said:


> Thats entirely INCORRECT.
> A 'stock' plain vanilla mainsail will 95% of the time be cut EXPECTING the correct amount of 'prebend' in the mast ... and that small amount of 'prebend' will typically decrease the amount of maximum draft by about 2-3 inches (as measured at the 'point of maximum draft'. So, in reality, without setting the proper prebend will result in a slightly powered-up sail with a slightly 'hooked-up' leech.
> 
> Setting PROPER prebend increases the relative mast stiffness by increasing the "I (structural moment of inertia) to the third power" a vitally important characteristic of 'stiffness' / vibrations. The previous posting was about INDUCED HARMONIC VIBRATIONS (caused by the mast shedding 'votices' - Karman Vortices ... not from wave or sea action - BIG difference.
> ...


Depends on the spar, spar section, and the boat. A friend just went through this process on his boat. Our rig is of similar design (Bellanger).. Neither his boat or ours has any prebend, though both rigs are raked aft. In the process of raking their mast aft, the old spartite induced prebend. The sail wasn't designed for it. Long story short, old spar tite was removed, mast straightened, and raked. My mistake is that I'm thinking about racing sails that are taylored more than the vanilla mains that you're describing, and your description of mast harmonic vibration I'm absolutely fine with. It's the description of that vibration as 'mast pumping' that was problematic.


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## g0twind (Oct 5, 2010)

Confirmed last night that the vibration during sailing is the centerboard. When I pull it up just a little bit the vibration goes away.


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## lancelot9898 (Dec 30, 2008)

A few years ago I went to the boat after an ice storm to see if everything was alright and I observed that the headstay was vibrating severely in the 20 plus knots of wind. Never seen that before after even living aboard during the winters over the years. All sails are removed over the winter months and just the foil remains on the headstay, but apparently a natural frequency was reached. I now use a rolling hitch to tie a line to the headstay as part of the winter storage to dampen the vibration. There has also been times when sailing in higher winds that the yankee starts to vibrate the headsay. Adjustment to the jib cars seem to lessen that tendency plus making sure that the headstay has proper tension also helps.


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## puddinlegs (Jul 5, 2006)

A naked head foil rattling in a breeze is pretty normal. You'll notice most race boats have something attached to the headfoil, whether it's a topping lift wrapped around it, or a pendant, etc... to keep the live aboard neighbors from going nuts!


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## cruisingdream (Feb 7, 2007)

I miss my old swing keel , the cable to raise it would sing nicely to let you know you were moving quickly. You could trim the sails for extra speed just by listening to the pitch of the cable humming.


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## Sublime (Sep 11, 2010)

cruisingdream said:


> I miss my old swing keel , the cable to raise it would sing nicely to let you know you were moving quickly. You could trim the sails for extra speed just by listening to the pitch of the cable humming.


Had a hobie cat that did that.

ZZzzZZZZzzzt....zzzzzZZZZZZZZzzzzt...ZZZZZZZZztzzzz :laugher

I had a vibration at dock. The tension in the stays were too loose.
As far as halyard slaps, I hook them to a bracket on the life line stanchions to keep them away from the mast. I don't like to replace them so it's to keep them from unnecessarily rubbing against things.


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## T37Chef (Oct 9, 2006)

Frogwatch said:


> You'll know when you've been aboard for a long time when somebody points out the noise you no longer notice.


 :laugher:laugher:laugher:laugher


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## MedSailor (Mar 30, 2008)

Take note if the "vibration" at the dock is happening only when the wind is exactly abeam. If this is the case it is caused by vortex shedding (as alluded to by Bosun). My mast does this in a bad way if we're side on to the wind. Usually we're at anchor or at a slip with winds that previail from bow or stern. Occasionally when we're at a guest dock (just about never) we will get the harmonic pumping. It's really profound on my boat and my triatic stay causes the mizzen to whiplash!

So how do you fix it? You can orient your boat so that you're not side on to the wind, or you can hoist or install something alongside the mast to disrupt the vortex shedding. The easiest way to do this is to hoist a fender in the rigging. (don't forget to tie a downhaul line to the fender otherwise you'll have a new perminant fixture in your rigging!) Unlike more solid objects it doesn't make noise if it bumps the mast a little and you can move it up and down until you find the spot that disrupts the harmoics. The vortex shedding will likely happen all along the mast but you only need to disrupt it in just the right spot to stop the mast from developing just the right harmonic to "pump".

I pass this information along to you in the hopes that you don't do what I did, which was re-tune my rig 20 times (and invent a few new words) before I found out what was really going on. Unlike RichH's experience, no amount of rig tuning helped my situation. Besides, my rig was tuned for sailing, not to allieviate pumping, before I messed with it. Now it's back to being tuned for it's original purpose.

Once you find the right spot with your fender, if it's convenient to do so, consider installing a radar reflector, mast steps, deck light, or whatever piece of mast gear you might want so that you'll always have a perminant piece of gear "hoisted" to disrupt vertices in the future. I prefered this option over a check-stay or baby stay, which would be one more force direction on my already overly complicatedly (sic) rigged ketch.

Good luck!

Vortex Shedding:Vortex shedding - Wikipedia, the free encyclopedia









Medsailor

PS It's really fun when your neighbors finally get the courage to ask, "Is that a FENDER in your rigging?":laugher


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## Boasun (Feb 10, 2007)

Try taking a large line and wrap it around your mast in a spirial manner. so it looks like a giant screw. Have done this when I was working on a research vessel when they had to lower a transducer on a long pipe leg over the side and attach rigidly... at first it was vibrating madly. but after we wrap a line in a spirial manner around it. The vibration was reduced a great deal. 
As that cereal commercial goes; Try it, you might like it.


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## TimofBlindSquirrel (May 21, 2009)

*Vibration*

Often this will be caused by the mast pumping in the top third. My C&C 36 did this. You should only notice this in the slip or in very light winds.

The easiest way to correct this is to get a small line with no stretch, connect it to the main halyward, and raise up to within 5 feet of the top of the mast. You will have to play with the height to get the correct setting. The other end of the line needs to be connected to a cleat at the rear of the boat. By changing the halyard tension, you can change the harmonic damper of the rig and end the vibration.


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## MedSailor (Mar 30, 2008)

Boasun said:


> Try taking a large line and wrap it around your mast in a spirial manner. so it looks like a giant screw. Have done this when I was working on a research vessel when they had to lower a transducer on a long pipe leg over the side and attach rigidly... at first it was vibrating madly. but after we wrap a line in a spirial manner around it. The vibration was reduced a great deal.
> As that cereal commercial goes; Try it, you might like it.


I've done this also. Worked great. Not as good of a conversation starter at the hoisted fender though.

MedSailor


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