A dry fly should float as much as possible like a natural fly on the water. Often, however, the current will be faster where your line lies than it is where the fly lights. In that case the line moves faster than the fly and thus pulls the fly in an unnatural way. This is fly casting technique is called drag.
In order to avoid drag, you will need to cast a curved line. This need happens often in dry fly fishing and occasionally even in wet fly work. So you may see why every dry fly fisherman should learn to cast a line that curves either in or out from either side, and still place the fly accurately, let me point out a few examples.
Most anglers who have fished for Brown trout know their habit of staying in the bit of slack water just above big boulders. Each Brown is usually poised there with head upstream watching for food the current may float down to him. As you will be wading and casting upstream, you need to put a curve cast to a point a few feet above the big rock from the right hand side. That would be an in-curve from the right side. Some writers call this a positive forehand curve.
To many fly casters, a curve cast seems a very mysterious and difficult thing. As a matter of fact, practically all beginners cast curves all the time. The only trouble is that they can’t tell which way their line is going to curve or why. It takes a lot more skill to cast a straight line than a curved one.
The only problem then, in learning to cast so that your line lands on the water in a curve, is how to cast the kind of a curve you want, when you want it, and to make the fly land at the place you pick out for it to land. Fly casting technique does take skill.
Fortunately in most fishing situations requiring a curve cast, there is quite a lot of leeway for the cast to still work even if it doesn’t land just where you intended.
There are two ways of making a fly line light in a curve on the water. Both are done with side casts, tilted at an angle of at least 45 degrees to one side. With a side cast the line will land in a curve in every case where the power put into the forward cast is not exactly sufficient to straighten out the line, leader and fly.
If this power in the forward cast is more than enough to straighten the line, then the end of the line and leader will bend around beyond a straight line and make an L-shaped bend inward.
With a right hand side cast, this bend will be to the left. This is an in-curve from the right, or a positive forehand curve.
With a left hand side cast, when too much power is applied, this L-shaped curve will be inward—to the right. This is an in-curve from the left, or a positive backhand curve.
Positive or in-curves are the best to use when you need a fairly short L-shape bend in your line to either side. Here is the way you make them.