Here is an alternative way to tie an improved loop knot (as inspired by Edward Truter) to a crab fly. It is very useful when targeting reef fishes in shallow water, the type of situations in which most flies snag the bottom.
I used it on coral reef as well as on limestone-bedrock reef in the Socotra Island Archipelago and was quite surprised when I noticed its efficacy. Where flies typically got snagged on every retrieve, they snagged one in every four to five retrieves with this knot. It is also strong and although not really attached directly to the hook, the Velcro crab patterns that I combined it with never came off while casting or stripping etc.
I believe the knot is meant to be used when fishing with a floating line. The retrieve should ‘lift’ the fly at the ‘correct angle’ off the reef substrate to prevent the hook tip from snagging rock, shellfish or coral. To decrease the odds of getting snagged, combine this knot with Pete’s centre-weighted, epoxy Velcro crab pattern (http://feathersandfluoro.com/?p=6469); furthermore, Pete suggested a foam back for these crabs (and other crustaceans – also for natural, more neutrally buoyant presentations), which will ensure an ‘upright’ (hook tip pointing upwards) presentation and retrieve (see: http://feathersandfluoro.com/?p=8876).
Remember, there is no fly that NEVER snags, but there are many tricks that may contribute to snag less often. I believe the three tips above, 1) tie crabs centre weighted with dumbbells and cover them with epoxy, 2) use a foam layer for the carapace, and 3) use the snag-less knot, will all aid in a more successful presentation on reef and rocky substrates in shallow water. I hate using a weed-guard in my flies, I have found that it does influence the hook-up rate on fish (resulting in a poorer hook-up rate), but a nylon weed-guard may also be added for those fly-impassable rocky substrates.
Note that the knot described above also works on Charlies, Clouser Minnows and other patterns to some extent. The knot’s snag-less ability improves with thicker line (although somewhat related to fly weight) and the full potential of the knot is seen when used with lines ≥0.5 mm diameter (0.5 mm = 31 lb fluorocarbon of Sufix Zippy leader material). The strength of the knot lies in the way the tag end (loose end) is returned through the original overhand knot so that it always enters the overhand knot where it previously exited.
Snag-less knot tying demo:
1) Start with a simple overhand knot with a relatively long tag end:
2) Return the tag end through the overhand knot the same side it previously exited:
3) Wrap the tag end three times over the main line following its natural curve:
4) Thread the tag end through the centre of the overhand knot, again the same side it exited previously:
5) Tighten the overhand knot first, then pull the tag end tight:
The traditional way to tie this knot to the hook eye:
The snag-less method of tying:
Note how the nylon lifts this metal object in the upright position due to the angle of the knot: