Back in February this year I posted about The Meaning of Slack. The opening paragraph went like this:
“As part of the Einstein Series I wrote about the Straight Lines Rule and part of that was casting with straight lines which means fly lines having as little as possible slack in them. The reason to avoid slack is that taking up it is a Force thief. It steals from the net Force in the intended direction of the cast – forward or back – and we don’t have that much Force to spare. We don’t want to spend part of our stroke length taking up slack instead of powering the line out to where we want it to go. That’s the mechanics taken care of.”

I went on to explain that when I see slack creeping into my fly line I take it as a sure sign that my technique is starting to falter. In the extreme case the whole cast can fail and I have to strip in and start again. A review of the website stats shows that this post has had relatively few views – it’s in the bottom 30%. That’s a pity because I actually rate it as being one of my better and more insightful posts – in the top 10%. Duh, failure to connect!

Meanwhile down at the park practicing and out on the water fishing I have found the idea of line tension (the opposite of slack) to be one of the gifts that just keeps on giving. So I decided to have another go at communicating its importance because I know I’m not the first or only person to get the concept and understand how central it is to casting better.

After thinking about it for a while I wondered whether talking about “slack” as a fault to be corrected might be less effective than talking about tension as a strength and driver of improved casting. All casts, I was once fond of saying, are tension casts – even the ones we intentionally land with slack line. So let’s turn this around and ack-sen-chew-ate the positive. I’m also going to ease up on the mechanics or rather come at them in another way.

Here’s a practical and graphic demonstration of how much line tension matters. Cast or lay out 60-70 feet (18-21m) of fly line, preferably on some grass rather than on the water. Lock up your drag and keep the line length unchanged. Make sure the line is lying nice and straight. With your rod tip pointing down and your rod aligned with the fly line lift up and stop with your casting hand about level with your eye. Let’s be clear, I’m not suggesting that you to pick it up and aerialise it behind you as you would for a normal, full strength, PUALD cast. Just make a somewhat casual lift with no real effort and no real attempt to accelerate it hard. What happens? How much line was moved?

Next step, lay out the line again but this time put a bunch of wiggles in it or just one big curve like the large belly of a sagging cast, often a back cast. Repeat the lift with the same length of stroke and the same effort as before. (As an afterthought I wonder if doing this with eyes closed would control the natural temptation to power up and lengthen the slack line lift.) Again what happens and how much line has been relocated? Compare the pair.  If your experiments work like mine a there will be a noticeable difference between the results of straight and wiggly lines. Try both experiments again but this time using a snap cast. Use more oomph if you like. Similar results. Straight line – zippy snap cast. Lots of slack line – snap cast is a bit of a fizzer.

Moral of the story? It takes surprisingly little effort to move a lot of straight line and a lot more effort to move the same length of line when it isn’t straight. What’s going on?

I could say that it is about F=ma and net Force in the intended direction of the cast and bang on about (the) Straight Lines Rule again. I will say that the line tension I am talking about is not really concerned with opposing forces that create tension in a string. With a fly line only one end is tethered and the other end is free to move so the result is that in casting a fly line is not under significant “tension”. After the loop forms there will be some tension between the rod leg and the fly leg mediated via the loop and dependent on the timing and extent of line shoot. That tension is important because without it we could never complete a cast. Turnover would fail. However, all that is frankly consequential. It is dependent on having a fairly straight rod leg before the loop was formed. The rod leg has momentum and its kinetic energy which power turnover as the rod leg becomes the fly leg and goes back to being the rod leg. Being straighter provides more energy and momentum. And no, I’m not going to analyse the effects of friction when the line is pulled across the substrate, grass or water. Rod loading? Whispers of the devil.

The significant point is that straight fly lines move in the intended direction of the cast with considerably less caster effort than slack lines do. We all know this already, because we all know the felt difference between an overhead cast and a static roll or even a dynamic roll. In an overhead cast when we make the casting stroke we act immediately on far more live line (mass) than we do with a roll cast. The dead line which has to be towed by the live line is… well… “slack”. Of course knowing about it and appreciating just how significant it is are not necessarily the same thing.

To tell the same story a different way let’s think about motor vehicles, engine power, wheel traction and the effects both have on drive. On a dry tarmac road with good tyres you have good traction and can make good use of a powerful engine assuming you don’t just plant the foot and spin the drive wheels. Same vehicle but this time on ice or a greasy clay pan. All that power is now a problem and less traction means less drive.

In fly casting what we are after is drive, the more direct the drive the better. Straight lines, lines with tension, give your casting drive. Wobbly lines, lines with little tension and fat loops give you wheel spin. The nature of casting beast is that that “traction” is so, so precious and ultimately no amount of grunt will substitute for defective drive. Drive problems include the line sag or belly we often see, particularly with windscreen wiper casting strokes. Also included are failed or failing loops prior to the start of the next stroke. Tracking errors which create line hooks to the side are another though lesser culprit. In fact anything that diverts the line is a force thief and the small mass of the line means we have effectively precious little force to accelerate it with and any loss is surprisingly costly. As ever, efficiency is the solution and effort is usually part of the problem.

Now we can also reconsider why a tidy series of casts, forward and back, is required to make a decent cast of any great length. Good line tension facilitates increasing carry, makes shooting line less of a cost to turnover and generally sets you up nicely for the next cast in the opposite direction. Poor line tension does the reverse. Good line tension produces accumulating benefits. Poor line tension produces accumulating losses. Tension thrills, slack kills. Of course if you are typically casting <50 feet it won’t matter quite so much but if you are often casting >70 feet it will matter a lot more, exponentially so as the cast lengthens.

It is now part of my practice regime to focus on line tension and work to optimise it. Being smooth and optimising tension are parts of a virtuous cycle, each facilitating the other.