Tracy&James | Thursday, 13 December 2018

If you’re a regular board reader you might be aware of a thread from some time back discussing the ‘lift’ generated in a the loop of a fly-cast, a well known journal article proposed that this ‘lift’ was highly significant in achieving the desired distance etc. The paper beautifully explained the physics behind the lift and the mathematics allowed for the effect to be quantified. This was obviously peer reviewed and published in a respected journal, a really nice study that has been quoted on internet forums many times since. It’s obviously complete bollocks though (I think I’ve said that here before)!

The thing with papers such as the ‘lift’ one is that predictions can be drawn from them.  These predictions might not be obvious to the authors and certainly not to the peer reviewers who probably don’t know one end of a fly rod from the other.  The conclusion of the paper in question is that lift is generated in the plane of the fly cast, as such a prediction is that if this plane is rotated to the horizontal, the lift will manifest itself as an acceleration of the line off to one side (the force no longer having to counteract gravity).  If you throw some figures into the equations that give a reasonable answer for a vertically orientated cast, then the same figures produce a predicted sideways movement of many metres once gravity is out of the frame.  Now Paul has said he can’t tell if his side casts deviate from the aim point (I suspect to be purposefully argumentative Smile), I can tell you, however, that mine don’t end up missing the target by metres.  For me, this failure to match reality calls the whole paper into question.  On top of this, countless slo-mo videos and photographs showing that the fly-leg is launched upwards, not perfectly horizontally as assumed in the paper, suggests the apparent extended flight time of a fly cast is caused by a far more mundane reason.  Sorry about effectively repeating an earlier FP.

In my work I often review data from gas-gun experiments.  A gas-gun is essentially an air-rifle on steroids, lots and lots of steroids (imagine in terms of a 10m long barrel).  They are often used to study shock-physics phenomena and work by firing a large projectile into an instrumented assembly held in a target chamber that is sealed to the end of the barrel.  The gun barrel and the target chamber is evacuated, this means that the pressure difference either side of the projectile (driven by a high pressure helium gas charge, which is explosively released) is huge.  As such, large velocities can be achieved and the results are generated in the absence of air, which us scientists like a lot (one less thing to think about).

None of the experiments have anything to do with fly casting though (mostly it’s for material characterisation or more interesting stuff like space impact studies.  I used to have an inch thick piece of steel through which we’d punched a hole with a piece of plastic about the size of a pea.  You can imagine how fast that was going, but anyway I digress).  However, sometimes the experiments are held in place with strapping, often nylon.  Late on in the experiment, after the intended data from the collision has been captured, occasionally these straps snap.  If we’re lucky enough to have a fast framing camera pointing at the experiment (our frame rate might be as high as 1 million images per second), these breakages can sometimes be seen in the background.  I’m probably the only person that gets excited about seeing this background information, because when one end of a strap breaks it can propagate uncannily like a fly loop.  You can then (in a lunch break of course) measure the velocity of the free end and the loop using the specialised software available.

So where’s this going?  Well there’s another paper that predicts a fly-lines’ acceleration in a vacuum.  Any guesses as to what I make of that one?

I predict that I will be making some 120ft plus casts with a #5 weight this weekend, especially given the wind forecast.  I may be wrong though.

Have a good one whatever you’re doing.


James casting at WC ST27