HOW FLY CASTING WORKS

Straight top leg is key

When we cast a fly line, its most aerodynamic shape will be that of an arrow, i.e. straight.

The top leg of the line is moving fast, and the bottom leg is stationary and tethered to the rod. The fast-moving top leg folds over, forming the apex of the loop. The location of the apex is important, and the higher it is, the better (more on that).

Above is the line shape we strive for. The all-important part of this shape is the straight top leg. There are two parts to shaping a straight top leg.

1. The straight top leg is the result of smooth, constant acceleration.

2. It leads to a pointed loop when the stop of the rod rotation is abrupt and high.

1. Let’s look at „smooth, constant acceleration“. We start from standstill from A and accelerate smoothly to B where we come to a hard stop. As we accelerate at the same rate, the speed will be fastest just before the stop B. The same goes from the B to A part as the forward and backward casts are symmetric. The increase in speed needs to be smooth or linear.

2. The rod tip traveling from A to B must bend and trace out a straight line. If not, the path will be domed, and that does not lead to a straight top leg. When the rod reaches point B where we stop it, it is now straight (RSP rod straight position), and the top leg starts to move past the rod tip, and the loop forms. The rod tip will move past RSP because of its momentum and then rebound. The rod tip path after the casting stroke will determine the apex position.

Drag

The line is subject to two kinds of drag.

1. First is the skin drag from air moving parallel to the line’s surface. It’s relatively minor, and we can’t alter it.

2. Then there is form drag that a much greater impact on bodies in motion than the skin drag.

Minimizing fly leg form drag is key to efficient casting. The size of the loop is less important than how straight the fly leg is. The top leg is moving and the rod leg is stationary, so its position is relatively unimportant. However, the position of the apex of the loop is very important. After the line is past the apex of the loop, it stops moving. So, the moving part above the apex is subjected to the form drag. The three loop shapes above are seen from the side, and the red vertical bars depict the area of the line coming at you. That part of the loop is responsible for the form drag. It is easily seen that the loop at the top is in the most aerodynamic form.