Our last boat would swing quite a bit at anchor when the wind would pick up over about 15-18 kts. I've measured the swing angle as pushing 180 degrees.
I tried a bunch of tricks suggested here in a previous thread, and by others, including:
1) Use a two line snubber rather than a single line
2) Use an asymmetric snubber
3) Tie through the hawse pipe on only one side so the boat presents at an angle (extreme version of the symmetric snubber)
4) Hanging fenders off one side of the boat (yup, someone suggested that)
None of it made any difference.
I then found an article that seemed credible that explained what was happening. The basic dynamic is that the boat's natural center of rotation (the point around with it rotates if you pull sideways on one end) is BEHIND the center of force for wind hitting the boat. The result is unstable, like an arrow with feathers in the front rather than the back. Now this is all about the wind effect and doesn't take current into consideration. In my experience, current will almost always dominate wind.
So when a boat swings, here's what happens:
- As soon as the boat is anything other than perfectly into the wind, the point the wind force is acting on is forward of the boat's rotation center, so it wants to spin the boat around, and it does.
- But, as the boat rotates, more and more of it's side profile presents to the wind, and the center of windage shifts aft. The more house structure you have aft, the faster the center of windage will shift aft.
- As soon as the center of windage moves aft of the rotation center, it now wants to straighten the boat out, which it does. The anchor rode is also pulling from the bow, and that further encourages the boat to straighten out.
- As the boat straightens, momentum causes it to overshoot and the whole process repeats itself in the other direction.
The solutions offered were two fold, and you can find elements of them in all the remedies and tricks that people have suggested.
Remedy 1: Shift windage aft. This is why a small sail rigged aft helps, as does hanging fenders off the side (see, the person who suggested it wasn't crazy). Canvas "curtains" on aft railing, or enclosed aft decks could all help too. On some boats it might be possible to reduce forward windage, but I suspect that would be rare. Our last boat, with a high bow and forward pilot house, has lots of forward windage, so it makes sense that it would swing a lot.
Remedy 2: Move the anchor rode attachment point further forward. This has the effect of moving the boat's rotation point forward, and moving the relative location of windage aft. Unfortunately, this is impractical on most boats unless you want to make some permanent modification. At least I couldn't think of any practical way to rig a spar to move the anchor attachment point forward.
Hopefully this give everyone the basis for evaluating remedies. It helped me quite a bit, and was a significant factor is switching from a forward pilot house, to an aft pilot house boat.