Many think that this cast is a saltwater cast and, indeed it is. It is also frequently used by fly fishermen using "shooting heads" often used for salmon and steelhead fishing. I find that it is also useful in more normal or traditional trout type fishing when you need to "shoot" your fly line into a stiff wind. This cast is very useful in casting longer distances more quickly. This cast develops more line speed quickly and helps get your fly on the water sooner. Lefty Kreh was a "master caster" on top of being one of the great "characters" in fly fishing (first video). Lefty's take on the double haul is that it "puts more energy in the fly rod". In my mind, that equates to the same thing as increased line speed...make the rod do more of the work!

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Lefty Kreh on The Double Haul Fly Cast | Fly Fishing | Hooked Up Channel

The Double Haul - Orvis, Pete Kutzer (YouTube)

How To Double Haul - by Simon Gawesworth (Rio)

Tips and Tricks for a Better Double Haul - by Tim Rajeff (Gink and Gasoline)