Hooke's Law and Fly Fishing guest post by Zedd

In order to serve a broad range of expertise and skill levels, I'll be inviting guest posts from time to time.  Here's the first by Zedd:

Whilst out fly-fishing this weekend I got to thinking (a dangerous thing, wrought with mischief to be sure) that maybe we should take lessons from the largemouth bass. Unlike the vaunted trout, a fish known for becoming dogmatic in its eating, the largemouth doesn’t really care about its chow. If it moves, makes noise, and generally appears to be an organism big enough to digest, the largemouth will hit your lure. This got me to thinking about fly-fishing in general.  What does it mean to ‘fly fish’?

I define ‘fly fishing’ as use tools (fly rod, fly line, leader, tippet, fly reel, etc…) which allow you to use lures too light for typical momentum casting such as with spinning or bait-casting equipment. This definition helps me to think of the fly-fishing equipment as a whole as opposed to a series of individual ‘categorical casts’. For example, fly-fishing has literally dozens of types of casts; back casts, forward casts, side-arm casts, bow cast, vertical casts, false-casts, roll casts, curve casts, S-casts, ad infinitum. I think it’s best to do away with naming the cast, and learn what  happens when I apply different techniques during the cast. A wholistic  approach if you will.

First, what ‘happens’ during a fly-cast?  Instead of the weight of the lure pulling the line off the reel, the weight of the line is pulling the fly along for the ride. The most basic means of this happening is sometimes called the bow cast, where the fly is carefully pinched between two fingers, pulled back against the fly rod-causing it to bow, and then released. The action of pulling the fly back causes the fly rod to flex. In reality, the fly rod is a material that exhibits both rigidity and elasticity, a spring:

In mathematical terms (Hooke’s Law):     F=-kx

During the compression phase (pulling the fly back) the rod stores mechanical energy and a force is generated in proportion to the length of the pull (X). Upon release, the rod de-flex’s and releases its force in a torque like manner; that is it rotates in an elliptical movement, increasing speed, and imparting this rotational torque into the fly line in the form of centripetal force, shooting the fly line and its rider, the fly. Fly Fisherman call this storing of mechanical energy into the fly rod as ‘powering up the rod’.

This may seem a little technical, but it is the rotational torque part that is important. Fly fisherman say by way of good fishing, ‘tight loops buddy [ma’am]’. This is a reference to the importance of learning the fly rod technique of casting with a tight loop. The ability to create tight loops is the first important skill a good fly fisherman needs learn. Remember the rod tip will de-flex in a curve? Centripetal force teaches that the rod tip will tend to want to impart energy into the line in a continuous and constant cycle throughout the energy release. The effect is that when the rod is held in a stationary manner, the loop will become large rather than tight. The trick with casting with a fly rod is to first learn how to move your rod tip as it releases its force in such a manner that the rod tip stays in a fairly straight line. The straighter the stroke of the casting rod tip, the tighter the line. To be sure, there are many times when you will want to make a broad round stroke, thus imparting a curve, or even multiple curves of different widths into the fly line. However, it is important to start off with the basic ability to cast a tight looped line, and this is only accomplished by casting in such a manner as to keep the rod tip in a constant straight line as it unleashes its force in a rotational manner.

This sound tricky and it is. Still, remember there are millions of fly-fisherman the world over who have mastered the technique. Once this stroke is learned, experimentation with manipulating the rod will teach you how to make the loops and casts needed for an infinite variety of presentations your favorite bay, lake, pond, river, or stream will offer you. The wholistic approach to understanding the what happens when will lead to a more instinctive cast without having to memorize and implement a variety of individual methods.