[rmeador]

My boat, a Gulfstar 37, has developed some sort of rig problem. It's been off and on for a couple years, and I've tried a few things to fix it, but lately it's become much more common. So what happens is I'm just sitting in the slip, no sails raised (though sometimes the jib is on the roller furler), and the boat will start shaking like an earthquake! It seems to be highly dependent on wind speed and even wind direction. I've done a little research online, and I think this might be "mast pumping", but it doesn't quite match the symptoms I've read elsewhere because it doesn't happen when I'm sailing, only when I'm in the slip.

Here are my symptoms:

• A couple years ago this started in the winter when all the sails were off, and I thought I narrowed it down to the forestay being too loose, so I tightened that up and it stopped except during very intense storms.
• The following summer when the jib was on the furler, no problem.
• This past winter with the sail off I had no problem.
• Now this spring, since the shrinkwrap came off, it's been doing it again.
• I thought I spotted the mast itself flexing fore-aft at one point, but it's really hard to pin down what's going on from outside the boat. It can be a violent and loud earthquake inside, but on deck you might not notice the tiny tremor.
• The vibration is almost entirely from the mast forward, you barely notice it when you're in the salon except for the noise coming from the V-berth.
• I re-tuned the rig (by hand, I have no tools, nor specs for what the tension is supposed to be). The mast appears to be perfectly in column, no pre-bend (I don't think my boat is meant to have any, but I have no idea). The vibration went away.
• I put the jib onto the furler, and it came back with a vengeance. It now happens in much lighter winds than it ever has before, and it seems to stop if the wind gets harder. I have a vague idea that it happens most when the wind is coming at an angle from astern (not directly astern).

Any idea what might be going on? Can you suggest possible fixes? Thanks! Please don't hesitate to ask for more info, I'm sure I've overlooked something that seems trivial but might be a vital clue.

 

[Multihullgirl]

Sounds like mast pumping to me. Contact a good rigger and get tuned.

 

[RichH]

Called an 'induced harmonic vibration' and is very common if the mast (rigging) is set up with absence of 'pre-bend' of the mast. Prebend is a slight 'forward bowing' of the mast which mathematically strengthens the mast, and changes the 'natural frequency of oscilations' of the mast. You need to change the mast's 'frequency' by slightly forwards 'bowing' it as indicated below.

A single spreader rig should have 3/4" forward bow; a multi-spreader must should have 1/2 prebend for each spreader set.
FYI - Your mainsail was 'cut' expecting this pre-bend to be present; so, without the mast having pre-bow, your mainsail will have a bit more 'draft' (more power but less speed output) than its OEM design

Go to: http://www.riggingandsails.com/pdf/selden-tuning.pdf , choose your rig type and carefully follow the directions. Also note that there is a section in this manual that includes setting 'proper rig tension' tension WITHOUT use of a tension gage --- a good summary of that 'gage-less' technique is found here: http://www.sailnet.com/forums/gear-maintenance/42542-adjusting-your-rig.html

 

[rmeador]

As far as I know, there are only two riggers who travel to your boat in the Boston area. One of them is expensive, overworked, and have terrible service (came to my boat multiple times to check things, gave me the runaround for 2 months, finally told me he was too busy to work on my boat). Last summer I had the other guy (Ed of Buzzards Bay Yacht Services, highly recommended) replace much of my standing rigging, and he re-tuned the rig at that time. I think the new rigging stretched after a season of use, hence why I needed to re-tune. I tried to follow Giulietta's instructions as best I could without actually dismantling half my boat... I'll give that Selden page a read. Can anyone recommend a decent rigger who services Boston that perhaps I haven't heard about?

 

[Minnewaska]

This is certainly looking like a job for a pro rigger. It's not that expensive and truly worth it, if you don't know what your doing. They can also check all the rigging and give you piece of mind that everything looks in good shape.

Just as a long shot, is there any chance that your mainsheet or boom vang is cranked down tight when this happens? Ours will sing in that case and just letting off some tension stops it.

 

[Delta-T]

I find that if I have my running rigging too tight it will do that. Sorry, I don't know your boat, is it a Mast head rig? Are your halyards external?

 

[MedSailor]

I've had that. I't caused by something called "vortex shedding". On my Formosa it happened whenever the wind was 6-15 knots, the sails down, and the wind was DIRECTLY from the beam.

My understanding is that it can't be fixed by tuning, unless you want to add stays to the section that is pumping (if you watch, it's usually between the uppers and lowers). You can fix the problem by disrupting the specific airflow pattern that causes it. Examples are:

1: Get a different slip that has a different angle to the prevailing winds.
2: Hoist a fender along midway up the area that is pumping. (this is also a great diagnostic tool to prove the concept and conversation starter)
3: Permanently install a radar reflector, antenna, mast steps, or rubber chicken on the mast at the area of pumping to disrupt the airflow.

I spent a lot of time tuning and re-tuning the rig before I figured all this out. Hoist a fender and I bet it'll stop.


MedSailor

 

[Bene505]

Ours does this too sometimes.

Medsailor -- Rubber chicken?

Regards,
Brad

 

[Faster]

Vortex shedding - Wikipedia, the free encyclopedia

.......r

Here's some good news.. this phenomenom is used to measure liquid and gas flow rates in many industries.. so when this starts up you have an audible/physical wind velocity gauge!!

 

[desert rat]

Something as big as a fender shouldn't be necessary. A 3 in. rubber ball cut to the center and taped to the wire (one at a time) 1/3 of the way along the wire should help to locate the oscillation. I could only guess and experiment to figure out how to cure it.

 

[svHyLyte]

My boat, a Gulfstar 37, has developed some sort of rig problem. It's been off and on for a couple years, and I've tried a few things to fix it, but lately it's become much more common. So what happens is I'm just sitting in the slip, no sails raised (though sometimes the jib is on the roller furler), and the boat will start shaking like an earthquake! It seems to be highly dependent on wind speed and even wind direction. I've done a little research online, and I think this might be "mast pumping", but it doesn't quite match the symptoms I've read elsewhere because it doesn't happen when I'm sailing, only when I'm in the slip.

Here are my symptoms:

• A couple years ago this started in the winter when all the sails were off, and I thought I narrowed it down to the forestay being too loose, so I tightened that up and it stopped except during very intense storms.
• The following summer when the jib was on the furler, no problem.
• This past winter with the sail off I had no problem.
• Now this spring, since the shrinkwrap came off, it's been doing it again.
• I thought I spotted the mast itself flexing fore-aft at one point, but it's really hard to pin down what's going on from outside the boat. It can be a violent and loud earthquake inside, but on deck you might not notice the tiny tremor.
• The vibration is almost entirely from the mast forward, you barely notice it when you're in the salon except for the noise coming from the V-berth.
• I re-tuned the rig (by hand, I have no tools, nor specs for what the tension is supposed to be). The mast appears to be perfectly in column, no pre-bend (I don't think my boat is meant to have any, but I have no idea). The vibration went away.
• I put the jib onto the furler, and it came back with a vengeance. It now happens in much lighter winds than it ever has before, and it seems to stop if the wind gets harder. I have a vague idea that it happens most when the wind is coming at an angle from astern (not directly astern).

Any idea what might be going on? Can you suggest possible fixes? Thanks! Please don't hesitate to ask for more info, I'm sure I've overlooked something that seems trivial but might be a vital clue.

Induced vibration in cable stays in wind/rain conditions is a well known phenomenon resulting from wind vorticies spinning off the leeward sides of cables that give rise to isolated harmonic displacements of the cables, hence vibration. The phenomenon is occasionally seen in cable-stay bridge structures. The induced vibrations, if not dealt with, can induce fatigue fractures in the cable stay anchorage fittings leading to failure. There are two solutions. Either to introduce "damping" which eats up the energy giving rise to the vibration of the cable or adjusting the tension in the cables, thereby changing their natural period of oscillation, with greater tension increasing the period/frequency (just as tightening the strings on a stringed instrument increases the frequency-hence pitch-of the strings). On one's yacht there is a practical limit to the tension one can introduce in a cable stay (some hold 70% of the cable's breaking strength) considering the loading introduced in the mast while at rest and more so when the yacht is heavily burdened in use. Often the most simple solution, once the yacht's rig has been properly set up, is adding damping, often by passing a halyard or lift around the stays and drawing it tight. The point(s) of contact of the lines with the says create deflected "fixed" nodal points that the frequency of vibration would have to match before it could resume in the stays while the flexibility of the lines used "eats up" the energy. On can often also effect this by simply connecting the stays together with shock-cord and passing the lines, lifts or whatever under or over the cord(s) and drawing them tight, tensioning the cord between the stays. BTDT...

FWIW...

 

[RichH]

The mast in this case is what is being described as vibrating, not the rigging!

Vortex shedding indeed is the primary cause.

Such induced harmonic vibrations can be easily 'tuned out' by simply changing the natural frequency of the mast by changing its 'stiffness' through 1. 'pre-bending' or 2. changing the mass of the mast -- hanging/attaching an appropriate weight between the 'nodes' of the vibration or 3. Less effective but useful is to spiral-wrap 'a rope' around the mast to interfere with the vortex shedding.

Changing rig tension will work if and only if such tuning changes the 'natural frequency' of the mast but in doing so one can easily exceed the elastic limits of the rigging when under normal sailing conditions. Changing tension only of those wire elements that affect pre-bending in the rigging to affect 'pre-bow' is the time honored and safest way to do this. As stated previously, a sailmaker will expect and will assume that the mast has this pre-bow when designing a new sail.

The easiest method is to change the stiffness of the mast by properly 'pre-bending' or 'pre-bowing' it.

 

[rmeador]

I do have a masthead rig, but the halyards are internal. I tie the main halyard off to one of the aft shrouds when not in use to keep it from slapping the mast and keeping me awake all night. It's fairly tight, and so is my mainsheet, to keep the boom from swinging back and forth. I guess I should try loosening them some before I call a rigger? I think that will just move the oscillation from the standing rig to the boom/halyard, but we'll see. I'll try the shock cord trick too. Thanks for the input, everyone.

 

[MedSailor]

Something as big as a fender shouldn't be necessary. A 3 in. rubber ball cut to the center and taped to the wire (one at a time) 1/3 of the way along the wire should help to locate the oscillation. I could only guess and experiment to figure out how to cure it.

Yeah but a rubber chicken would be so much more fun!

MedSailor

 

[mgmhead]

I do have a masthead rig, but the halyards are internal. I tie the main halyard off to one of the aft shrouds when not in use to keep it from slapping the mast and keeping me awake all night. It's fairly tight, and so is my mainsheet, to keep the boom from swinging back and forth. I guess I should try loosening them some before I call a rigger? I think that will just move the oscillation from the standing rig to the boom/halyard, but we'll see. I'll try the shock cord trick too. Thanks for the input, everyone.

On my boat, when the vibration begins, I find simply relaxing the mainsheet is the solution. Give it a try sometime.

 

[svHyLyte]

The mast in this case is what is being described as vibrating, not the rigging!

Vortex shedding indeed is the primary cause.

Such induced harmonic vibrations can be easily 'tuned out' by simply changing the natural frequency of the mast by changing its 'stiffness' through 1. 'pre-bending' or 2. changing the mass of the mast -- hanging/attaching an appropriate weight between the 'nodes' of the vibration or 3. Less effective but useful is to spiral-wrap 'a rope' around the mast to interfere with the vortex shedding.

Changing rig tension will work if and only if such tuning changes the 'natural frequency' of the mast but in doing so one can easily exceed the elastic limits of the rigging when under normal sailing conditions. Changing tension only of those wire elements that affect pre-bending in the rigging to affect 'pre-bow' is the time honored and safest way to do this. As stated previously, a sailmaker will expect and will assume that the mast has this pre-bow when designing a new sail.

The easiest method is to change the stiffness of the mast by properly 'pre-bending' or 'pre-bowing' it.

Ah... Are your really suggesting that "vortex shedding" off the spar is the problem? Do you really think so? That makes no sense to me, what-so-ever, compared with vibration induced in the rigging and sympathetically transferred to the spar at the connections, which functions as nothing more than a reverberation tube in the event, No?

 

[MedSailor]

Ah... Are your really suggesting that "vortex shedding" off the spar is the problem?

Yup. That's what I'm suggesting too. Why doesn't that make sense? If you read the article I linked to they talk about it happening around thin walled chimneys and destroying them from the induced vibration. A mast looks like a chimney no?

When this was happening on my boat, I couldn't reproduce it by shaking any of the stays and I couldn't stop it by tuning and re-tuning the mast. Hoisting a fender (which presumably blocked the flow of air that caused the vortex shedding) worked great.

MedSailor

 

[svHyLyte]

Yup. That's what I'm suggesting too. Why doesn't that make sense? If you read the article I linked to they talk about it happening around thin walled chimneys and destroying them from the induced vibration. A mast looks like a chimney no?

When this was happening on my boat, I couldn't reproduce it by shaking any of the stays and I couldn't stop it by tuning and re-tuning the mast. Hoisting a fender (which presumably blocked the flow of air that caused the vortex shedding) worked great.

MedSailor

With all due respect, I suggest you look at the polar moment of inertia of the spar on that Gulfstar and, with that, calculate the natural period of vibration of the spar between the supports (including the locations of the spreaders). With that, look at the wind velocity that would be necessary to develop sufficiently strong vorticies to induce vibration in the spar. Use a shape coefficient of .9 and go from there. Of course you can't/couldn't manually duplicate the vibration in the mast by pulling on the rig by hand. One simply can't move the rigging quickly enough by hand to get to the harmonics necessary to achieve resonance in the spar. Moreover, hoisting an object that disturbs the air-flow around the rig would be enough to mitigate formation of vorticies on the wire. N'any case, enough of this discussion already, No?

 

[RichH]

The classic example of an harmonic inducement failure was the infamous Tacoma Narrows bridge whose vortex shedding was the inducement of the 'destructive' harmonic. It was not the supporting cable that had the harmonic, it was the deck structure that failed at the open box beam structure / girder system (deck). Can happen in cranes/gantries, unsupported spars, bridges and any structure especially with elastic high aspect ratio structure such as poor aerodynamic design as was typical in (high) wing support spars and landing gear of early aircraft --- all dependent on the natural frequency of the combined structure or of individual sections or 'parts'.

Very complex subject; but, usually easy to figure out 'forensically' to affect correction to a spar:
in order -
1. Change the elasticity / 'spring effect' / 'stiffness' via IE4 of the structure by 'bowing', etc. which simply change the natural harmonic exitation frequency.
2. change the mass (functionally trivial) of the mast, etc.
3. remove/attenuate the vortices. (usually not possible on a boat mast) other rotating the boat to gain a different incident angle of airflow to and from.
Its really immaterial if the vortex shedding is acting on the mast or the wire when the MAST is vibrating in an 'excited' state as changing the 'frequency' to a non-excited state is always the first attempt at a 'cure'. Pre-bowing the mast changes the 'stiffness' of the 'spring' thats oscilating; hence, changes the natural frequency of oscillation.

 

[MedSailor]

The classic example of an harmonic inducement failure was the infamous Tacoma Narrows bridge whose vortex shedding was the inducement of the 'destructive' harmonic.

If only someone had nailed a rubber chicken to the side, it all could have been avoided....

MedSailor