A definition of the perfect cast could be “the action of straightening the fly-line and aligning it with the direction of the forward cast by pulling it from the rod-tip, whilst providing it with sufficient kinetic energy for it to turn over, completely, after the rod tip has stopped moving”.
As instructors we are indebted to Bill and Jay Gammel who originated the Five Essentials of casting, including the concept of the Straight-Line Path. But do we consider their statement in its entirety?
The straight-line path is an imaginary line that the rod-tip travels from the beginning of the cast to the launch point, when the fly-line is released. In reality the rod-tip may deviate from the straight line due to the physical limitations of the caster and the response of the equipment, however, the closer it is to a straight line the greater the energy that is transferred into the line up to the launch point.
So how is the straight line achieved?
Often students will ask me where the straight-line begins and I reply, at the tip of the fly-line and so first and foremost, during the set up stage of the cast, the fly-line must be positioned behind the rod-tip so that as long a length as possible is lined up, straight, and trailing behind it. By doing this the pulling force of the rod-tip will be directed into the fly-line and so the energy losses due to random directional forces will be minimised, ensuring maximum energy transfer into the fly-line.
It is helpful to visualise the fly-line as being converted from a limp object into a straight, rigid, object that can be projected, in the same manner as a javelin or spear. Once the fly-line is aerialised, behind the rod-tip, it is laden with kinetic energy and so if the rod-tip is stopped abruptly the fly-line will carry on along the straight-line path that it is traveling. The distance that it rolls out will depend upon the losses caused by loop size, wind resistance, surface drag on the top leg of the fly-line, and the pull of gravity and so there should be just sufficient kinetic energy in the fly-line to make it turn over completely.
Next the casting stroke length and arc angle must be adjusted to ensure that the deflection of the rod-tip, due to loading, coincides with the imaginary straight-line path. This is dependant on the characteristics of the fly rod and also the skill of the caster. However, as a rule of thumb, the longer the fly-line, then the greater its mass and so the more it will load the fly rod. Hence, the rod-tip will deflect more and so the casting arc angle has to be wider so that the rod-tip starts and finishes at a lower level. Because there is a greater length of fly-line, in order to line this up behind the rod-tip, and as close to the straight-line path as possible, the acceleration phase must be increased accordingly. Furthermore, the launch speed of the fly-line from the rod-tip must be higher to transfer the extra amount of kinetic energy to cope with the longer amount of fly-line that is turned over.
The lowest point of the rod-tip determines the height of the straight-line path, above the water level, as it moves through the casting stroke. This occurs when there is a minimum chord length between the rod-tip and rod-butt, which is also the point of maximum acceleration of the rod-tip and maximum rod loading. Therefore maximum rod loading should be made to occur when the minimum chord length is perpendicular to the straight-line path.
To maintain the straight line path up to the launch point it is essential that at the end of the casting stroke the rod-tip is stopped as briskly as possible before it drops too far below the straight line. It may not be obvious but the stop should ideally occur immediately after maximum rod loading, which is much earlier than where casters stop intuitively, or are able to within their inbuilt reaction time. The counterflex occurs after the rod straight position and so this is the primary energy source for loop shape and not size. It does not make a useful contribution to the kinetic energy of the fly-line that is in flight because this has already been determined at the launch point, which occurs at the rod straight position and after this no further energy can be fed into the line, apart from that which causes loops or mends to occur. By stopping the rod-tip high will direct the fly-line compression, caused by the counterflex, into the fly-line rather than making unsightly waves in the lower leg of the fly-line.
When making longer casts it can be advantageous to throw the line back high and then drift back, following the line with the rod-tip, whilst the fly-line is turning over, and allow the fly-line to fall, under the influence of gravity, until it lines up with the straight-line path. Note that the straight-line path need not be horizontal to the water. It can be sloping up or down depending on wind conditions or the launch trajectory that is chosen by the caster. Often distance casters will prefer to cast upwards to give longer flight duration before the line falls due to the pull of gravity.
Deviation from the straight-line path will not only reduce energy efficiency but as most casters know it can cause tailing or open loops and tracking issues. Don’t forget that the straight-line rule applies to all viewing aspects including looking from above the caster as well as looking from the side. A useful tip for checking that the fly-rod is tracking a straight line is to observe the fly-reel and to make sure that its orientation (direction in which it is pointing) does not change throughout the casting stroke.
The elevation of bottom leg of the loop in fly-line will be determined by where the rod-tip stops after counterflex and the loop size will depend upon the distance of the rod-tip from the theoretical straight-line path. This is employed in Task 1 of the MCI Performance Test to vary the loop size.
Many casters have difficulty in roll casting 50ft of fly-line, which is Task 6 in the Master Fly Casting Instructor Performance Test. However, this task can be made easier by applying the straight path principal, even though this is a compromise because the fly-line cannot be fully extended behind the rod-tip. To overcome this limitation the caster should reach back as far as possible with the rod-tip, allowing at least part of the fly-line to be pulled along a straight line. By stopping the rod-tip high, at the end of the casting stroke, a relatively tight loop will be formed enabling a reasonable turnover to be accomplished.
This technique can be applied in all Spey casts because essentially they all end in a dynamic roll-cast and one of the most common faults in Spey Casting is not reaching back far enough, whilst circling up to form the D-loop.
Pulling and launching the fly-line in a straight line by necessity will mean that the rod-tip is being accelerated to maintain tension and that there is no slack, thus fulfilling two more of the essential requirements for a good cast.