The debate continues

Following the last reply from Bruce Richards, I have emailed some questions of my own. And just in case you are wondering, I have forwarded this thread to all the flyline manufacturers! - Paul

Paul:

You wrote...

'Actually, two objects of the same shape and density but different weights will not fall at exactly the same rate. Wind resistance (or water resistance) is somewhat less for larger objects as they have less surface area to volume.'

I find this answer confusing. By this I *think* you mean that objects of a smaller surface area will fall quicker due to the decreased wind resistance. Surely objects of the same volume and shape fall at the same velocity, no matter what the mass, as Galileo is reputed to have demonstrated over the side of the Leaning Tower of Pisa?

Bruce:

This is a confusing one.

Most simply, if you drop two steel balls of different diameters, the larger one will fall very slightly faster. An interesting fact regarding spheres is that larger sperical objects have a higher volume to surface area ratio meaning that they have slightly less wind resistance, relatively speaking.

Obviously they have a larger surface, but relatively larger mass also,hence less wind resistance. This has little practical application anywhere, the effect is small, but there.

Objects of the same volume and same shape definitely do not fall at the same velocity unless they are the same mass. Envision two objects of same volume and shape, one being a round, inflatable plastic ball, one being a steel ball of the exact same size. Same volume, same shape, definitely won't fall at the same rate, unless in a vacuum.

I can't remember now what Galileo proved on the Tower, but that wasn't it! Again, don't fixate on this bit of trivia, it has little application or impact on anything related to fly fishing.

Paul:

Anyway I have a couple of questions of my own:

In the 'perfect' cast the loop (including leader) should straighten at exactly the same moment as it the flyline runs out of forward momentum:

To what extent does the forward taper play in the speed of the unfurling of the loop?

Bruce:

The front taper has a big impact on the speed of the loop. Again, back to the laws of physics.

To make this simpler at first, let's assume the the line is not being 'shot, the bottom of the loop is static. Also, let's assume casting in a vacuum, I'll add air later.

The law of conservation of energy accounts for where energy goes.When a loop is propogated, a certain amount of energy is contained in the moving part of the line. As the loop moves forward, the top of the loop is moving forward, the bottom of the loop is static.

'The law' says that the energy of the cast has to go somewhere, in this case, as the loop proceeds forward the bottom of the loop has essentially no energy which means the dynamic top of the loop has it all. Barring losses from wind resistance, if the top of the loop has more energy, but with decreasing mass, it must accelerate. Theoretically speaking, as the top leg of the loop gets very short, its speed will approach the speed of light.

Seems incredible I know, but it is mathematically correct. (Even with wind resistance we know the speed of the tip of the line can be very high as the 'whip crack' sound we occasionally hear attests. The "whip crack" means that the tip of the line has exceeded the speed of sound which is over 700 mph.

In the presence of air the speed of the tip of the line cannot approach the speed of light as the 'excess' energy packed into the loops top leg is sapped through friction with the air.

Now, about the taper. As the top leg gets shorter it gets lighter which causes the acceleration. If the line is tapered also it gets lighter even faster as it gets shorter.

Long tapers facilitate more rapid, earlier acceleration and, hence, energy dissipation through high wind resistance. Short tapers conserve the energy longer causing a harder, faster turn over right at the end.

Large tip diameters (mass) also keep the mass from reducing as quickly, again resulting in a later, harder turnover.

I know this may seem a little hard to believe, but check with a physicist, I have, it is correct.

Paul:

What is the correlation between the unfurling of the loop, the lack of friction of the running line through the rings and the velocity of the forward travel?

Bruce:

When shooting, the lower the resistance of the bottom leg of the loop (I include the line in the rod and on the ground waiting to shoot) the more slowly the loop will turn over. You can demonstrate this easily yourself. Make a long cast and shoot a bunch of line. Make the same cast again but grab the line and stop the shoot when the line a ways out. The loop will quickly turn over just when you grab the line and stop it. As soon as you stop the shoot all the energy of the cast is forced into the top leg which causes a rapid acceleration.

Paul:

Is it possible to design a flyline which unfurls slowly and yet shoots quickly?

Bruce:

Yes, take a cue from tournament distance casters. For the longest casts they use a shooting head about 55 ft. long and very fine monofilament as a shooting line. The loop doesn't exactly unfurl slowly, but because the head is long and there is very little resistance from the shooting line it takes quite a while for the loop to completely unroll.

The real key is the small diameter shooting line, it offers little resistance to the bottom leg which allows the loop to open at the slowest possible rate, which is critical to deliver a fly at long range.

Paul:

And lastly, the Scandinavians who cast for Loop are talking about the running line on some shooting heads being too thin, and 'overtaking' the loop. Frankly I can't see how this is possible as it is the forward momentum of the flyline which pulls out the running line. Surely what is happening here is that there isn't sufficient friction in the running line to make the loop unfurl?

Bruce:

I think there is always enough friction, besides the loop forms when the rod tip stops. The top leg pulls the bottom leg along, the bottom leg cannot overtake the top. If a loop isn't turning over properly a skilled caster will manually add some friction to the bottom leg to force it to turn over.

Many many thanks to Bruce for giving his time to this subject - Paul

Back to Peter Suttons comments