I use the main halyard, attach this to the chair with a bowline.
THEN attach the shackle to the chair in a different point if possible.
I always use a safety - if somethings fails your dead or in a wheel chair, it's no hassle using a safety, just make sure that line is tied of, so that you can't at least hit the deck.
I always see boats with so many crew, yet they still dont use a safety, it's so easy and it could safe your life.
I don't use a knot I use the shackle on the main halyard. If I've any spare crew I'll also hook on the topping lift and have someone tail that round another winch as a safety line.
But next time I let the spi halyard go aloft!! I'll use the ladder I got at a jumble. You put the top on the main halyard and pull it up putting sliders in the track as you go. I'm sure it'll feel safer than being ground up on a winch.
some dinkle on a neighbouring boat let a ladder fall on me once, result ( shame ) he wasn't on it.
If you do go the ladder route suggest tying boiler suit or similar? to the topmost rung to avoid sideslip, saves all the scarring on the mast too. Oh and doble lash it.
Use a trace figure 8 plus attach the shackle separately and safety line if possible/available. One other point that has not been mentioned - if going aloft while underway (esp in rough conditions), attach a halyard to the base of the mast then grind it tight. Attach the bosun's chair / climbing harness to this with a carabiner (or something else that will slide up and down easily). This way, if you lose your grip you won't swing too far away from the mast.
I always attach twice (well at least on the one occasion I've been up!), bowline on the main then also used the shackle as a secondary point of attachment back onto the chair.
Trouble is now I've got a wire halyard so the bowline wont be so easy! Maybe use the Spinnaker I suppose - it doesnt get used for much else at the moment (big flappy sails in bags no good for crew of wife and two young kids!)
I would not attach both lines to the bosuns chair. Problem if it tips up or you fall out. I always put safety line on my person ,round my chest and under my arms . Yes I know it would be uncomfortable but if its needed the alternative would be far worse!
Yes, I usually put bowline on main halyard onto bosun's chair, then wear an independent harness, attached by another bowline in spin halyard. Probably all the shackles are clipped on too. Both halyards through clutches and main to a winch. Take up slack on the spin halyard to make sure you don't have too far to fall. I expect I'd be less thorough if I did it often.
Harness, fixed halyard to deck, two prussock loops...safety rope if ones spare tied on.
I've never liked bosuns chairs having been brought up climbing in a proper harness, no doubt there are some good ones but if your swinging around at sea I like something quite snug.
Thats the do it yourself single handed approach, its very quick. and you know your safe as you do everything. I've seen people almost dropped when the person on the winch gets distracted, someone comes out of hatch and they're in the way.....make sure they let you down slowly with no jerks, the shock loads are many times your weight and thats what could cause a break.
There is a reason that rock climbers and mountaneers use a trace figure 8 (with a half hitch chaser) instead of a bowline - it will not come undone accidentally; a bowline can (as proven at the cost of several climbers' lives, I am told).
I appreciate that a bowline is very unlikely to come undone especially while it remains under load - but on this issue I have been converted to the trace figure 8.
I also agree with TigaWave that a climbing harness is much more secure than a standard bosun's chair - although much less comfortable, unfortunately.