bad vibes

[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!

[Jim Fulton]

If you are feeling the vibrations in dead calms, could they have been generated by propellers? Maybe outboards on small boats that are not readily visible? Or could there be some other source not on your boat?

Jim Fulton