Worm,
on drowners, think of chain length as adding that much more depth required to drown the animal, simple as that. If you hind foot catch a big male coon on a drowner slide wire rig, think that a big coon when stretched out from its hind foot (one with the trap on it) to the tip of its nose will run close to 3' anyway on just the big coon. I use tip of its nose as in the water if the drowner rig holds the hind foot down near the bottom where the wire is running, then the coon will have its head up trying to get air to breath, so at least the tip of its nose needs to get under water level also to drown the coon. Now, if your trap and chain add another 12" between the drowner wire and the coon's nose tip, well now you need 4 feet of water to drown it. Get a front foot catch and you now put the coon's nose much closer to the drowner wire underwater and now its tail will be in the air vs. its nose on the drowning rig.
Shortening the chain length will decrease the depth of water you need to drown the coon then, accordingly.
For beaver, which can run much longer in body length especially on very large adults, on a hind foot catch from wire, trap, chain, hind foot, and now to tip of the beaver's nose...well, you need some fairly deep water to pull off a complete drowning. Again, front foot catch and the depth need decreases by nearly the beaver's body length. But we all know from experience that the hind foot is the more desirable foot to nab in a trap for beaver to avoid wring off damage losses.
If deep water is not necessarily available at a good set location, it isn't always completely necessary to go for the full drown, either. As I mentioned, I rarely run drowners for coon at all anymore but that is mostly due to the locations I trap generally do not have deep water anyway so I adapted to holding them without a complete drowning kill. As the Bogmaster does with beaver, you can switch to bodygrippers and problem is solved irregardless of water depth, except if your state laws are similar to ours where #330s must be completely underwater, so I need at least that much water (10") to set a #330 in Nebraska...but if there are any beaver present that generally is not a problem finding 10" depth of water.
For coon, you can run drowners even out in shallow water, shallow enough even that it won't drown them. Just by holding them out in water of any depth will do a couple of things for you...it seems to reduce loss because the coon doesn't really have anything to grab onto to muscle out of the trap, and coon fur isn't waterproof like a beaver's is. If it is extremely cold out, you can actually kill them with hypothermia, but don't bank on that as coons are tough animals. On some small streams I trap the water may only be 10" deep in the center and maybe only 3'-5' wide. I can stretch a drowning wire across the entire stream, make a little knot/loop in the middle of the wire about midstream, and when I catch a coon it is held right at the knot, midstream, and in middle of whatever water is there. That trick works great on coon for me. Otherwise I just use earth anchors and anchor them right at the set sight, pocket or whatever set it was, and by using a #11 the coon is nearly always there waiting for me when I check sets the next day.
Good luck, Worm!
Getting ready for season yet? Its been hard to think about it here with temps hoving around 100 for so long now. I can hardly wait for a frost!
Jim