bad vibes

[gstej]

Hello,

We have a Pacific Seacraft 37 that is new to us. At the dock we are able to feel subtle vibrations below that we assume is harmonics (vortex shedding)? We are wondering how to confirm. I cannot feel or hear any vibration in the mast, although the halyards when tight do vibrate in a breeze. It is felt off and on, day and night. Not clear if it is related to windspeed.

I should mention that we experimented with a large fender up the mast as well as wrapping the mast with halyards in a spiral, neither of which seemed to help. There have been two occasions where even in near calms I have felt the vibrations below, and even standing on the dock next to our slip. We have done many, many experiments to attempt to find the cause (shock cords on shrouds, different docklines, moving halyards, playing with the topping lift and mainsheet tension, etc.)

When we first got the boat we ordered new rigging that matched the original with the exception of the lower terminals that are Sta-loks. After our mast was stepped we had a local rigger come out to assist us with tuning. It is a masthead rig with an inner forestay and running backs. Everything is 1x19 stainless and correctly dock-tuned (prebend, in column, slight rake) except the jibstay which has the furler. I am not confident that the jibstay tension is correct but in an effort to deal with the assumed harmonics I began tensioning it. I put the loos pt-3 on our backstay (already tensioned to ~12%), and monitored it with each turn on the forestay, stopping when I saw a turn in the forestay start to tension the back. I worry that I have overtuned the jibstay but have no method to confirm its tension short of removing the furler entirely. (Our jibstay has a profurl "classic" furler).

Our current settings per loos pt-3

capshrouds (9/32") both 17 (11%)
forward lowers (1/4") both 11 (12%)
aft lowers (1/4") both 7 (8%)
babystay (1/4") 2 (our running backs are slack)
backstay (9/32") 19 (12%)
jibstay (5/16") unknown

Is harmonics the only likely cause of the vibrations below?

Thanks very much in advance.
g

[Ian McColgin]

On a day when it's bad, have a person below to listen while you walk around grabbing one stay at a time and sweating it a little or even shake it slowly to break any vibration. This might lead to one culprit.

I have never tuned to gauge settings, only to sailing performance, so my personal observation of humming I've witnessed as being caused by an overly-tight stay or two may not fit your case.

It's not completely impossible for an antenna installation to translate it's sound down an aluminum mast which, like a good horn, can both amplify and change the pitch.

G'luck

[gstej]

Appreciate the quick response. Yesterday checking things in a breeze yesterday it seemed the topping lift was vibrating most. Pulling on it stopped the vibrations nicely, so today took it out to see what happens going forward.

Thanks much!

[Brion Toss]

Hello,
If the problem is the lift, little streamers, spaced along its length, might generate enough turbulence to stop the vibration, If not, then you could have a case of the dreaded vortex sheddings, which are not typically responsive to tune, Be in touch if the problem persists.
Fair leads,
Brion Toss

[gstej]

I have had some time to experience the problem continuing with the lift fully slack.

I should clarify we hear no audible sound, and in fact it is subtle enough that my wife cannot usually recognize it. I myself can sense it clearly each and every instance, and of course lie awake fretting about metal fatigue. When we first discovered the issue, I tried to sight up the mast and have never been able to visually see any movement in the spar (which is quite robust).

Have experiment with rig settings (currently 12% all 4 lowers, 15% caps and backstay) but no change. (Judging by sight we have maybe 2 inches of rake aft. Mast is in column.)


Thanks

[Brion Toss]

Hi again,
Lovely puzzle. If you have eliminated the lift pendant and any other running rigging (I once had a problem with very tight lazyjacks), then that probably means it is vortex shedding.This is not usually affected by rig tune (more on that later), and the oscillations from a fine buzz might not be visible as you look up the mast. If the fender trick didn't work, please try either a line attached to a main luff slide. This slide lives, unattached to the main, on top of the sail. It should go up and down with the main. Attach the main halyard to it, and a separate line that is something over half the length of the mast. Hoist the slide with the halyard to about halfway up. Take the second line aft to the cockpit and tighten it. Experiment with different heights. Report back.
As for tune, if it is going to affect this problem, it will be by imposing bend. See what happens if you take up on the forestay (inner stay), ease off on the aft lowers, and take up on the forward lowers. Go for 12% forward and 10% aft. Should get at least a couple of inches of bend. This will be a better tune, regardless.
Fair leads,
Brion Toss

[gstej]

Apologies for the long absence but I wanted to provide an update on this. It seems that most of the problem has gone away by simply positioning our main halyard a couple inches away from our mast.

We were able to go sailing and do some more homework. I believe our forestay may be loose as he bottom toggle appeared to go slack for a moment bobbing down a wave. We do not have a baseline for our forestay tension and had been paranoid about overtensioning, but after observing our last sailI think it needs more tension. We have a furler but are unsure the best method to determine the correct tension to add short of removing the furling drum.

I thought I accomplish this by viewing mast rake and tensioning the backstay, but at this point I am still a bit confused how to proceed.

[Brion Toss]

Hi,
And thanks for the update. I am glad that the halyard positioning seems to have done the trick, though if you are then subject to mast slap, you might try an aft-leading component of some sort.
The forestay issue is a separate item, most likely. Do you mean the inner stay, which most (aggravating) usage has as the stays'l stay? Or do you mean the jibstay, which goes to the masthead? If the latter, determining tension is a simple vector problem: you measure tension on the backstay, and correct for the relative angles of the two stays; nearly all backstays form a greater angle to the mast than the jibstay, so a given amount of tension on the backstay will result in greater tension on the jibstay. If you skipped too many trig classes to figure this out, I am available for consultation. Either way, the entire point is to arrive at a desired luff sag on the jib, which I can also help you with.
If you meant the inner stay, tension is provided by efficient runners (very rare) or aft-led intermediates (all too common, markedly ineffective), as well as by the mast stiffness. Slack on either stay is never a good thing.
Fair leads,
Brion Toss

[gstej]

I meant jibstay. too long ago, Mathematics was my field, so I owe it to myself to work through the trig.

Thanks again!

[gstej]

Well, more that three times now I have felt the vibrations in dead calms.

I now think vortex shedding is unlikely. More than anything the vibrations do seem to coincide with unusually high tides, so I am convinced it is related to water flow and I am overly sensitive (or paranoid?)... unless there is a better theory to account for this.

What great forums, thanks all!