All the running rigging that passes through a fairlead and which would be anywhere between inconvenient and disasterous if lost up the mast or overboard. Except of course spinaker sheets (but I don't have them anyway).
Figure of eight knots pulled good and tight and a sensible distance from the end.
As I'm usually single handed I have slightly longer than normal genoa sheets which I join together across the cockpit by tying each in a figure of eight around the other that way they are never out of reach.
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I find figure of eight knots work loose on jibsheets. I've now started to use the very simple overhand knot (half a reef knot) but with an extra twist i.e. over and under and over and under again. Isn't it difficult describing a knot without a diagram? When you've tied the knot you should work it up tight and it will make a biggish lump. It doesnt come undone but it can be undone when necessary
Agree figure of eight knots on halyards sheets and reefing lines with the exception of all spinaker gear. No stoppers on spinaker halyards, guys and sheets so in an emergency you can cast off everything and let it go. Never had to do it but did get close in a cross channel race at night on one occasion.
If a figure 8 is a little small I will use an oystermans stopper, Same as afigure 8 with an extra turn round the top of the 8. Tie it slipped if you think it might need to be got rid of in a hurry.
Last year sailing round the South coast we started off using figure of 8 knots which kept shaking out despite their tightness (this gets worse as sheet thickness increases) so ended up with the simpler but much more effective double overhand. No problems at all since
Twice I've lost spinnaker sheets and guys as a result of their being accidentally allowed to fly, and then the special quick release shackle at the clew has shaken loose. Cost approx £120 each time.
So now I put on stoppers. The sheets/guys are sufficiently long to allow the spinnaker to fly right forward if released: that has been done in a big broach and it works. Otherwise I would certainly have lost more.
I agree, the oysterman's stopper is ideal presenting as it does three rim parts which are quite symmetrical. It is so easy to tie as well. An alternative figure of 8 is one tied in the doubled end of the rope, a favourite of violin makers when the holes have become worn! It was also recommended some years ago by the then Department of Transport as an alternative for the bowline in modern and slippery man made fibre ropes as it can be tied with a loop.
As you say, the figure eight has to be tied up the line a bit from the end - which is sensible anyway to give a tail to hold if the knot goes hard up aganst a block, jammer, or whatever. Maybe not doing so is the cause of the problem some have.
In the over 45 years I have used them from dinghies to keelboats I cannot recall one ever coming undone.