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AKA: Proportions - dry fly
The physics of dry flies are quite simple either they rely on  buoyancy (displacement) to simply float or they rely on the materials used to be light and  resist water  penetration and consequently to be supported on the surface tension of the water.

Booby beetles and foam beetles are good examples of flies that rely on displacement to float. Simply stated the principal of displacement states that an item will float if the weight of water displaced by the item is more than the weight of the item itself.  So for this class of fly we rely on  materials such as  booby eyes  and closed cell foam  bodies to be  lighter, when combined with the other materials in the fly, than the weight of water that they displace. Proportions for flies that rely on displacement are limited only by the imagination of some fly tiers and hopefully by what is being imitated and how the flies fish. The best way to see if a displacement fly will float is to put it in a glass of water or jar with just a very small drop of detergent in the water. Put your hand over the top or put the lid on and give the lot a good shake. If the fly floats then you can expect it to float when fished.  If the fly sinks then better  go back to the vice rather than hoping it will float better when you fish it.

Tags and duns are good examples of flies that rely on the materials used in their construction to be light and  resist water  penetration and consequently to be supported on the surface tension of the water. Also the materials used in construction of these flies have been carefully selected so that they don't hold water if accidentally drowned. The materials used in this class of dry fly needs to be of a nature, that if accidentally drowned the water can be shed off the fly as the fly is false cast or if the fly is soiled then it can be cleaned and dried and will then float again.

If you take one of these flies and do the displacement test above, drown it in a glass of water then more often than not they won't have enough built in buoyancy and will sink. If it is tied with the correct materials you should however be able to then wash the fly, shake the water off it, apply floatant and it should then float again.

Acceptable proportions for dry flies that rely on the materials used to be light and  resist water  penetration and consequently to be supported on the surface tension of the water have evolved over time and the guidelines that I apply are set out below.

A	Hooks This is a representation for a standard dry fly hook. The shank is twice as long as the gape of the hook is wide. The size of the fly is determined by the width of the gape of the hook. I prefer a dry fly hook with a 1X shank. This means that the shank of the hook is the same length as it would be for the next larger size hook. For streamers and nymphs I like 4X shanks. That is the shank is equivalent to a dry fly 4 sizes larger than the gape of the hook I use. For most flies thread is generally tied in at the 95% position and then wound in touching turns to the 0% position before proceeding to the second step..

B	Dry fly hackles Quick-link

C	Dry fly tails Quick-link

D	Dry fly tags Quick-link

E	Dry fly wings The relationship of the wing on a dry fly is very close to its surrounding hackle. It doesn't matter if the wing is made out of calf tail hair, deer hair, hackle fibres or synthetic, as a general rule of thumb upright wings and spent wings on dry flies should be at least as long as the longest barbules on the hackle and up to another 50% longer. Quick-link

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