All casters have the inclination to test their ability by distance casting. It is often used as a measure of their competence against other casters or to check their personal skill development.
The amount of kinetic energy that is present in a fly line, at the point when it is launched during the cast, will determine how far it travels and how efficiently it turns the fly over. This is assuming that the bottom leg of the casting loop remains on the same straight line path as the one which the fly-line has been pulled along (known as the 180 degree principle for those that are familiar with this term). More kinetic energy may be required for casting directly into the wind or for turning over a light, bushy fly on the end of a long leader.
The launch point is the moment when the fly-line just begins to turn over. This occurs just before (approximately 10 milliseconds) the rod straight position (RSP) and is evident because there are no forces acting on the rod-tip, otherwise the fly-rod would not be straight! After the launch, the caster cannot transfer any more kinetic energy into the top leg of the fly-line and is only able to influence the loop shape and how the fly-line lands.
So this is why it is important to have continuous acceleration of the rod-tip from the beginning of the casting stroke up to the stop (maximum rod flex). It is often referred to as constant acceleration but like the straight line path it is an imaginary concept which is unlikely to be achieved in practice, although it can have its uses for visualisation and making in-depth studies of casting. After the stop, the unflexing fly-rod continues to accelerate the rod-tip but the rate of acceleration slows down up till the launch point and importantly the velocity at the rod-tip continues to increase so that it peaks at the launch point. this means that the fly-line will be launched at maximum velocity and hence the maximum amount kinetic energy will have been transferred (Ke = ½mv² Joules, where m is the mass of the fly-line and v is the velocity of the rod-tip).
It is possible to introduce more line speed by pulling on the line with the non-casting hand (hauling), towards the end of the casting stroke, and this will transfer even more energy into the fly-line at the rod-tip. Importantly, for optimum effect the haul should be applied as quickly and as close to the stop as possible. Maximum kinetic energy in the fly-line is one of the key requirements for distance casting. Later on we will consider other contributory factors.