I mainly practice with a fast action 5wt 9′ rod and a long belly 5wt line (Barrio GT 125). Every now and again I like to switch to something else not just to break the monotony but to force adaption to a different gear setup and thereby fine tune sensory motor control. So, for a recent session I changed rods to a slightly longer and slower action 9′ 6″ rod.  (More about that rod here).  As well as the GT125  5wt line I took along an SLX “5wt” single hand spey line. The inverted commas are there because technically it is significantly heavier than the AFFTA standard range for a 5wt line which is in no way a criticism but rather a simple statement of fact.

During the first part of the practice session I warmed up with the GT125 going through the parts of the practice regime I considered would help to make the necessary adjustments. Then I moved on to making accuracy casts to 4 targets laid out at 10 foot intervals.

There were a few signs of things needing refinement with the most important being the size, shape and orientation of the loops – overhead and roll casting. They were just a tad too wide for my liking signifying that the efficiency of force application – amount and timing – was just a bit off. A little less effort overall and a slight delay in final rotation were the required adjustments.

Now it was time to switch lines and limber up with the SLX. These lines have a comparatively shorter, heavier head and as Vince Brandon pointed out to me in subsequent discussion, their taper profile is somewhat similar to a Wulff triangle taper – a line I have never owned or cast. I bought the SLX on something of a whim and have fished a couple of sessions with it on my fast action 5wt. On that rod I found it just a bit clunky at distance when overhead casting. On my 9′ 6″ rod it is a much sweeter line to throw.

The surprise came when the SLX was used for accuracy casting to the 4 targets. The loops were noticeably tidier, meaning tighter and not just for the shorter range casts of 60′ and 70′.  What was that about? Wider loops for the GT125 and narrower ones for the SLX when initially deployed.

Sparing you most of the analysis I think there were two related things going on that shed light on the different results with different lines and made the heavier line initially easier to control. On the sensory motor side the additional weight increased feedback. When feedback diminishes, from slack or lighter line weight, we have an instinctive tendency to speed up the stroke and increase effort until we find what we are searching for – the feeling of effective effort. Unfortunately, we are also in grave danger of producing a negative outcome – we heave and when we heave we rotate too much or too soon or both. That’s why loops get wider – in extreme cases we see the effects of a windscreen wiper stroke.

On the casting mechanics side I suspect the different weight of the amount of line aerialised affected the amount of rod bend. Rods shorten when they bend. If the body movements are identical then for the same casting stroke the tip of a longer rod travels more distance, ideally in the intended direction of the cast. The tip of a shorter rod travels less distance. In summary, the length of the lever amplifies the movement of the casting arm. The detail of what the rod does as a third class lever is explained here. A longer rod performs more Work on the line because the Force is applied for a greater distance.

So in the present example a slightly shorter rod giving more feedback is somewhat easier to control and we need control to cast efficiently. On the other hand the Work done by the longer rod is greater. For the same movement of the casting arm we get more kinetic energy going into the line. From a caster’s perspective the efficiency sweet spots for the two different lines will be slightly but significantly different. Staying in those sweet spots  produces a noticeably different overall feel. For the longer rod I needed to “wait for it” just a bit longer and throughout the casting stroke.

The actual measurements slotted into the relevant equations are not where this is going. They might be interesting but they provide no feel or feedback to a sensory motor system which is what we use to control our casting strokes. The extra rod length is proportionally small and the difference in rod flexibility is probably also on the low side. However, those differences combined with different weight distribution in the two fly lines do produce significant demands for adjustment in order to maintain control and sustain efficiency. Making those adjustments is enjoyable and produces better overall technique. Try it and see what you think.