Dan Stadtlander, our friend from the Bermuda 1-2, found the flaw in my earlier complaint about things coming unscrewed at the boat club. Specifically, he said:
 "If the bolts are vertical wouldn't the rising waters force the timber up against the bolts and force them up? Vibrations set up in the timber and bolts by wind and surf can then vibrate the fasteners causing them to walk up. But I do like the idea of a mischievous spirit playing with your bolts and nuts!"
Thanks, Dan, for the cleansing analysis. I'm sure there is a similarly simple explanation for the shackles becoming unscrewed (without mentioning the fact that I could have wired them in place.)
But I, too, did like the thought of mysterious forces at work.