Where has my inertia gone?

Where has my inertia gone?

Tracy&James | Tuesday, 20 December 2022

As is customary, I will start this FP with a ‘physics alert’ – this one is for casters with an interest in the technical aspects of chucking fly lines, if that’s not you then I’d skip straight over to the board!

Just over a year ago I wrote a front page which detailed the results of a test I performed in order to estimate the maximum force I generate during a single handed cast.  It turns out that with a ST27 outfit I produce just in excess of 10N of force at the rod tip.

To determine this force figure I incorporated a ‘weak link’ between the fly line and the shooting line; if this then subsequently broke during the cast I reasoned that I’d exceeded the ultimate tensile strength (or breaking strain as many incorrectly refer to it) of the tippet material that I’d tied in to create the weak link (adjusted, of course, for the knot strength as determined from mechanical testing).

Yesterday I did some more testing which may be of interest in light of the ‘head length vs head weight’ discussion on the board.  The crux of this thread is whether longer shooting heads feel lighter when casting than shorter heads of the exact same weight.  I happen to be in the camp that agrees with this hypothesis (through experience), however it would be great to have some proof.  It was this search for evidence that saw me out on my usual casting field here in North Wales, in freezing conditions, trying to tie perfection loops in tippet material I could hardly see due to my eyes watering in the icy wind!


This time I chose to use a Salmon S55 outfit.  The main reason for this was that I felt I needed to be at the top of my game with the ST27 single handed shooting head in order to break the lightest tippet I have, however I’m just not there at this moment in time so I wanted an easier option.  As such, I thought I’d use the double hander and 55 gram shooting heads.  I decided to make up the shortest 55g head possible – from a lump of lead with the addition of some wire (taped in place) to bring it to the exact weight of the fly line I would also be using.  So essentially I was going to be comparing a point mass (the lead/wire weight) with a 65ft shooting head.

My method for this test was straight forward – I would start by seeing what strength of ‘weak link’ I could break by performing a forward cast, off the ground, with the point mass.  When I established the highest strength of tippet material that would consistently break, then I’d attach the 65ft shooting head with an identical link.  I should point out here that when I tied in 6 inches of weak  material I would also tie in 18 inches or so of a much stronger ‘safety leash’.  This ‘safety leash’ was to stop the lead flying off when the ‘weak link’ broke, or that was the plan.  As things turned out, on one cast I broke both the ‘weak link’ and the ‘safety leash’ at the same time and watched my lead sail into the distance never to be seen again.  However, at this point I knew what ‘weak link’ I could easily snap with my (now missing) 55g weight and I could then set about snapping the same strength link with the 65ft, 55g shooting head.  It was significant that I could notsnap it!  No matter how hard I tried, even to the point of going to extremes with my input, the ‘weak link’ now held firm.

So what’s going on here?  Why could I easily break the line with a 55g weight but not with a 55g line?  Clearly the force I can generate with each option is very different but why?

The first option is that this is down to me (I don’t believe this is the case).  Perhaps I’m subconsciously biasing the result by not trying hard enough with the fly line.  Now this is where hopefully some of you come in.  Science means nothing if one practitioner’s results cannot be reproduced by others.  Hopefully you’ll agree that what is described above is a very simple test to do.  It doesn’t matter what outfit you chose so long as you match the point mass weight to the weight of the shooting head.  So over to you…

The second option is that I genuinely cannot produce the same force when a fly line is involved.  This is where things get interesting as, according to Newton and his second law, F=ma, the force should be identical (assuming I’m doing my bit correctly).  However, let’s consider the mass, both ~55g in this case, and the real-time inertia.  With the lead weight hopefully it is obvious that as soon as I start moving the rod I feel the whole inertia associated with the mass, i.e. all of the mass is accelerated in the same direction at the same time.  Now, even with the fly line laid out perfectly straight on the ground behind me I simply cannot get it all to accelerate in the same direction in a single instance.  For example, whilst the line nearest the rod tip lifts upwards as the casting arc commences, the line nearest the leader drags through the grass for some distance.  There are also complications added by the elasticity of the line.  When the line is in the air, these effects are only exacerbated – I think the old adage that “there’s no straight lines in fly casting” is very true in practice (but not necessarily so in modelling).  So, although I had 55g of fly line mass to accelerate, at any given point (either off the ground or false casting) the full inertia of the line was not accessible and, as such, I could not break the same strength of line that snapped easily with a 55g weight.

Some years back Bernd posted a diagram that perfectly described his observations from watching many hours of fly casting, explaining how slack is inevitable.  To this day I see his beautifully astute artwork, which essentially predicts the result of my test (above) plus many other fly casting occurrences, largely disregarded in favour of other hypotheses based pretty much on masses that move as a single point.  However, the conclusion that I’m drawing from my testing is that point masses and fly lines are very different due to the accumulation of slack (and thus inertial differences) as highlighted by Bernd.  Also, as longer fly lines are always going to be prone to more slack than shorter ones, then I think the initial hypothesis that long heads will feel lighter than short ones of the same weight is a sound one based on empirical evidence. [Noting that the force applied equates to the caster’s feeling of weight].

In honour of Bernd I’m going to print out a picture of his dangly bits and put it on the top of my Christmas tree.  I shall admire them every day and hope that one day others will be able to differentiate dangly bits from bollocks.

Have a great week,