I agree with Montanan. The forces generated by a sea anchor from a vessel surging backwards in a storm and coming up against the sea-anchor's rode can be extreme.
For that reason I would only ever deploy a large sea anchor if there was limited sea room.
Some one time friends of mine in an Oro sat out a storm using a sea anchor for two days. In that time they moved over forty nautical miles, some of it caused by ocean currents, the rest by dragging a sea anchor. They deployed it from the bow using chain bridles.
So--you need plenty of sea room. The more the better. If there is plenty of ocean downwind, then the series drogue is a better option than a sea anchor--it is less strain on crew and vessel and it is a softer ride.
However, if you MUST deploy a LARGE sea anchor, you have to think of it as an immovable object. That means you have to have some sort of spring between it and the vessel. Most will opt for a long length of heavy nylon twist-laid rope
, at least two hundred metres of it because nylon has a good stretch factor, and provided that splices have many tucks, can be spliced to stainless or galvanized thimbles without too much trouble.
I do not recommend bringing nylon through fairleads UNLESS they are roller-gated. I have done it, but where I did so I encased the nylon in a plastic sleeve designed to protect the rope
I like to splice loops into my nylon lines and rig those loops with thimbles. One splices in a length of nylon with the ends of the loop facing the direction of the sea anchor or bollard--whatever is mooring
the vessel. Splice in one end ahead of the other--that makes a stronger splice and smaller diameter than a cat's paw loop. It is an old sailing-ship man's ploy seldom seen these days, but it makes a good way of setting the quarter of a vessel into a seaway, or of runnning a bridle, (I like to use chains to the vessel for my bridles if using a sea anchor on a nylon rode. Chains sag and do not flick up out of the water
easily, do not chafe, and take a moused shackle without any problems. They are quickly rigged, and if the main part of the chain is enclosed in a plastic hose, will not scratch paintwork.)
Water-cooled nylon, not rubbing on anything, does not fail unless its breaking load or splicing factor is exceeded.
Having said all this--I never used my sea anchor in earnest either. I deployed it from time to time for practice--but I always managed most unpleasant weather
with a conical drogue made up of three or four vehicle tyres of different sizes spliced together and towed like a cone--which tyres when separated also served me as fenders.
The only thing one has to be concerned with when using a drogue from the stern, is your rudders. They are subjected to a lot of strain as seas rush past them from the wrong direction--if the drogue is being towed. It is a good idea to lash the helm
midships and just let the vessel run--if you have a clear run of course.
Which brings me to something so obvious one ought not to need to say it--but as soon as a serious storm is forecast
, abandon your course if necessary, and get your vessel into such a position that it has as much sea room as possible. More vessels run ashore in storms than are ever sunk by storm driven waves in deep water