Loop width control is a recurrent topic in casting instruction. Several aspects govern loop width, but “matching casting arc to rod bend” is what instructors use the most; so “if your loops are too wide narrow your casting arc” is the usual fix we offer.
Let’s say that I am bass fishing with a popper knotted to a short, stout leader, followed by a bass taper #7 weight line. I want to cover a very promising spot, just a small hole among low hanging branches, which asks for a side cast with a narrow loop.
First try… my line crashes against a branch; I need to narrow the loop considerably. So keeping everything equal I decrease my stroke angle… and the loop fails to straighten. What has happened?
Think of you starting your car. You accelerate by pushing the gas pedal and your car increases its speed. After covering a distance of, say, 20 meters, the car has gained some speed. If you keep pushing the gas pedal the speed attained after covering a distance of 50 meters will be even higher. That is, for the same force the longer the distance over which that force is applied the higher the attained speed.
So back to rod and line in the example above, if you decrease your stroke angle you have decreased the total distance your rod tip has travelled; now you are applying the same force –pushing the pedal equally– along a 20 meters long distance instead of a 50 long one; the result, obviously, is less line speed.
Well, if by decreasing stroke angle I fail to unroll my line completely I obviously need more rod tip speed to fix that.
So —with the narrow loop I need in mind— I keep that same reduced stroke angle but increase the acceleration applied to the rod to gain that lost line speed due to having decreased the distance covered by the rod tip. But, hold on one second! In that way I will get a tailing loop!
More acceleration applied to the same mass results in more force –F = m.a–, and more force results in more rod bend, and a basic principle of casting mechanics says that we have to match casting arc to rod bend! So although I started that cast thinking of decreasing the casting arc, actually I am forced to increase it to get a good loop!
So on the next try I increase the force and also increase the stroke angle to accommodate a bigger bend in the rod. The result of increasing the force and widening the angle is similar to pushing the gas pedal further down and covering even more distance: much higher line speed… and my popper curves to the left hooking a branch, as a result of an overpowered cast!
Fellow fly casting instructor Walter Simbirski has summarized it all on a recent comment to this article:
We use more arc to generate more line speed but we need to use more force in order to maintain slp over the larger arc but with the extra force we need less arc to generate the line speed but if we use less arc we get a tailing loop so we need to increase our casting arc but now we have a large loop so we need to increase force to flatten the rod tip.
Seems pretty simple to me. Why are you guys struggling with this?
Reducing casting stroke angle to narrow your loops doesn’t work in isolation. No wonder that beginners have a hard time in controlling loop width, as it is a question of very fine-tuned adjustments –very subtle indeed– more than just varying casting arc.
Playing with loops by changing the casting stroke angle is fine; but play with force as well. And specially, with stroke length too, as this element is key and, however, it is the less understood of all them. But this is stuff for another article.