Seems to me there are two animals
in this room, a red herring and an elephant.
I'll address the red herring in this post:
I'm referring to the photo
of a boat rolling heavily while sailing downwind under spinnaker in moderate conditions. I'm not sure that it helps illuminate the situation of a boat close reaching, bareheaded, under conditions which have become too strong for a triple reefed main.
I personally think the fights which broke out among contributors regarding the wind direction arose from chasing an embedded red herring: I don't personally think the wind direction is a controversial aspect of the photo.
When a suitable (ie old school) boat sailing dead downwind gets the death rolls in moderate conditions as in the photo, the APPARENT wind direction at the masthead swings through a total of forty degrees or more with each roll, without any change in heading needed. This is because the masthead is effectively tacking downwind.
The spinnaker swings through a similarly radical angle, not just because the apparent wind is swinging, but also due to the influence of gravity. It's relatively free to swing, not being attached to a stay. (Modern, flatter spinnakers, coupled with modern hull
forms and underbodies are much less prone to these vices, which might help explain why people here are so perplexed by what they see.)
Most crucially, the sail swings because it is getting a good old rhythmical "one – two" from the vortex shedding series, coming to it from around the luff and leech of the main alternately.
It's this rhythmical shedding which is the root cause of the death roll sequence.
Things get truly unmanageable when the frequency of the vortex shedding happens to match the natural roll frequency of the hull
So my interpretation (having spent many happy hours on and around boats with this propensity) is that the boat is probably sailing Dead Down Wind.
Boats of this vintage tended to hold their heading pretty well even under this syndrome, and for the death rolls to initiate, the course has to be DDW or close to it.
(Otherwise all the vortices come off the leech (or the luff, if sailing by the lee)
Unfortunately the boats which can be steered actively so as to kill the rolls are also the boats which are least likely to get them in the first place. It takes a deep spade rudder, well aft, and a short-chord keel
. And someone on the helm who has the knack (probably a gifted dinghy
sailor) and bravery to burn ... It's much easier to make it worse than it is to cure it. You have to do something very specific and perfectly timed, but which is almost (but not exactly) opposite to what instinct suggests.