Rig vibration during sail and at dock? - Page 2 - SailNet Community
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post #11 of 24 Old 06-13-2011
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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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post #12 of 24 Old 06-14-2011
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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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post #13 of 24 Old 06-14-2011
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Quote:
Originally Posted by puddinlegs View Post
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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post #14 of 24 Old 06-15-2011
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Quote:
Originally Posted by RichH View Post
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.

;-)
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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post #15 of 24 Old 06-22-2011 Thread Starter
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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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post #16 of 24 Old 06-22-2011
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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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post #17 of 24 Old 06-22-2011
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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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post #18 of 24 Old 06-22-2011
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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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post #19 of 24 Old 06-22-2011
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Quote:
Originally Posted by cruisingdream View Post
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...ZZZZZZZZztzzz z


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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Last edited by Sublime; 06-22-2011 at 02:46 PM.
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post #20 of 24 Old 06-22-2011
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Originally Posted by Frogwatch View Post
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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