I just threw together a crude video
of our SJ21 deep reaching last weekend. It was more instructive before YouTube's compression got at it, but there are still some details worth noting.
TWA was about 160 degrees, tucked behind my left ear. You can see the jib clew flutter as the mainsail blankets it. But the jib luff is still drawing as is the main -- just around the 1 minute mark, you can (maybe) see the telltales show apparent wind just behind the beam: the boat is moving almost 4 kts in 8 kts true. I'll take it.
Boom is well out; could afford to go out farther. Jib cars shoved all the way forward; outboard sheeting would be better, since the inboard cars pull the clew in. The lazy sheet is very slack so its weight doesn't pull the clew in still more. I should have eased the halyard/downhaul and outhaul more; the mainsail is too flat for such light winds. (It was blowing 15-18 earlier.)
Vang is on, but not too hard. Yes, vanging flattens the bottom of the sail slightly -- but not vanging
permits the upper half to twist off, luff, and spill its air. That's where the good breeze is, and you want to snag it. Don't vang so hard you flex the boom, because that will pull draft out of the sail bottom. But if you bag out the foot and then fail to vang, all your lovely captured wind is gonna slide right up the sail and spill off the twisted leech. Vang just enuf to bring that top batten back into line. On deep headings -- once you are confident the aft portion of the sail is no longer lifting -- the leech line can be used to cup the fabric still farther, giving more drag power. (Make sure to ease it when heading back upwind, tho, or it will spoil the exiting air.)
Subtle, subtle technique: we can spend our whole lives learning to sail downwind well.
I intend to!
And preferred tactics will change according to boat type. The SJ21 is a light, flat-bottomed sloop that easily gets to hull speed in modest winds, even on working sails. On a heavier cruiser, you would probably sail deeper angles and pile on sail area.
I love this sport.