Regarding sea anchors: Sea anchors have gained a rather negative image for many, many generations. Before the advent of high tensile, light weight and rot resistent materials, sea anchors were almost impossible to carry, deploy, and retrieve. Worse perhaps, their difficulties caused them to be too small for the job when finally needed.
In my opinion, a sound sea anchor and rode coupled with chafe free attachment to the boat, is the last line of defense in truly severe conditions. The best source of information regarding sea anchors of which I am aware is a book, the The Drag Device Data Base. This is a collection of true-life reports regarding deployment of sea anchors and drogues. Drogues work over the stern; sea anchors from the bow.
Personally, I have deployed a sea anchor only three times. On no occasion could the conditions be considered "extreme". Deployment and recovery were easy and fast.
This experience leads me to think that the sea anchor should be so rigged that it will sink when not loaded. The head of the sea anchor (top of canopy) should be rigged with a buoy and enough cordage so that the unloaded, sunken sea anchor will be well below the recovering yacht and any of her appendages. I have employed braided rodes and not experienced any rotation of the sea anchor. We are advised to use a swivel in the rode, near the sea anchor, which troubles me because swivels are notoriously prone to fail. They are heavy however, and will act to sink the anchor as desired. Perhaps the best solution, here, is a very expensive non-ferrous, fool-proof swivel! Economy be damned!!
Regarding the motion of the boat, and sailing around the anchor: On one occasion I was on a monohull and had no trouble with the motion. The other occasions were in a catamaran and she lay bow-to the wind very nicely.
About rigging: I have read that a sea anchor deployed to windward will tend to create a slick in which the sea does not break. I have not experienced this. It is best to deploy the sea anchor so that both anchor and yacht are approximately in the same part of the wave systems. Clearly, this means an adjustable rode, sometimes quite long. In catamarans, in the interest of strength, a bridle is rigged between the roots of the forward beam between hulls. The rode is then attached at the apex of the bridle. We used this system, with about one hundred meters of rode, in a fourteen-meter catamaran with peak winds around fifty-five knots and seas of perhaps three or four meters. The boat lay beautifully, but obviously, adjusting the length of the rode would be a major feat, nearly impossible. Rigging suitably robust bridles would best be done in calmer conditions! In my opinion, the buoy supporting the unloaded sea anchor should not carry any retrieving line. Extraneous cordage in the area of the sea anchor is likely to become entangled in the "shrouds," rendering the sea anchor ineffective.
The PARATECH sea anchor seems to be state of the art, well-made and easy to deploy. These devices come in many sizes to suit most boats. I believe the admonition to carry and deploy a large as opposed to a merely adequate diameter sea anchor is good advice.
My wildest, or bleakest imaginings envision the yacht dragged bodily through the top eight to ten feet of breaking white water. Possibly worse would be dragging the vessel bodily through water which was not breaking. Such strains as these must be opposed by really robust landings on the vessel. A really big bow-cleat would not be adequate! Finally, the operator-manager-skipper-crew must be extremely vigilant regarding chafe. The loads will be great.
I would not go to sea without a suitable sea anchor and gear.
A few words concerning drogues: Drogues work from the stern and are deployed in order to reduce speed in extreme conditions. Drogues come in many designs from the very large and bulky to mere excess cordage, anchor rodes, sail bags, and anchors. One of their features is that they help the boat to steer when seas are breaking under the counter. Boat speed in these conditions is generated by windage in the rig and hull and by the slope of the sea. Clearly, the amount of load one desires in a drogue will vary; not much in modest conditions, increasing to rather a lot and always dependent on the boat, in really severe running. The bulky device mentioned above is said to be self-controlling: dragging harder and resisting more. A simpler and far more practical drogue is the SERIES drogue. This is a series of nylon cones worked into a long rode; the more resistance you want, the more rode you pay out. (I have not used these.)
It is arguably true that running off in hard weather is the easiest thing to do. This act is, however, accompanied by the threat of "pooping". It is preferable, in my opinion, to parry such large seas and winds with the bow rather than the stern. Hence I favor the sea anchors.
Hope this helps!
Best regards, Commodore
Warwick M. Tompkins Yacht FLASHGIRL Bay of Islands New Zealand