Physics FOR Fly Casting – The Einstein Series

 

Other Casts and Other Stuff

Introduction

In this Episode we will look at casts other than standard overhead casts, hauling in a bit more detail, how to be with the Force and lastly we will check out some common casting faults and see what physics has to say about them.

On a Roll

We all know that roll casts and overhead casts are different. For present purposes, the essential difference is the extent to which the line being cast is aerialised and allowed to turnover on the backcast. For an overhead cast it’s usually all the line. For a roll cast it’s only ever a part of it. As we will see that is a very important difference.

The setup for a forward roll cast can be thought of as the equivalent of an overhead back cast – regardless of whether we are making a static or a dynamic roll cast.

Spey casts, by the way, are simply roll casts preceded by a repositioning of the line. I won’t give them separate treatment.

All roll casts employ a D loop as well as a normal overhead style loop during the delivery part of the cast. They come in two basic varieties – a static roll and a dynamic roll cast. The fundamental differences between the two types are how much line goes into the D loop and whether the setup (or back cast) involves static line or moving (dynamic) line – hence the names.

With those preliminaries out of the way let’s get down to it. Everything covered in the series to date applies to roll casts – straight lines rule even though we are knowingly using a curvy line. The rod is still a flexible lever. A fly line does what it does. We can still make waves and mends and so on. There is, however, a salient difference between a good roll cast and a good overhead cast and that is the inherent inefficiency of a roll cast. We have to accelerate the line harder to make a roll cast than we would if we were making a cover at the same distance with an overhead cast.

Let’s unpack that by considering the amount of line or mass that we are accelerating with our net Force in the intended direction of the cast. In the ideal overhead cast our casting stroke is moving 100% of the line for 100% of the distance over which we make the stroke – no slack, no tracking errors – fully compliant with the Straight Lines Rule. That’s seriously cool efficiency, seldom if ever achieved, but that is still the ideal scenario we aim for and the closer we get the better we cast.

In a roll cast we can never achieve that level of efficiency because we can only apply direct Force to the line in the top half of the D Loop. The rest is having a snooze on the ground or water. It’s a bystander waiting to be picked up by the passing parade. However, because the roll cast is inherently inefficient it means that our casting technique needs to be right at the top of its efficiency game to compensate for the inbuilt losses. We absolutely don’t want to add to them unnecessarily.

Static Roll

To look at this in more detail let’s consider a static roll cast of say 15m. We draw the line towards us, reposition the rod and pause. How much line will be in the top half of the D loop (the rod leg) when we start the delivery? I don’t know of anyone who has measured this. Optimistic guesstimate? (Depending on the caster’s reach and elevation, the rod length and how far back the caster reaches.) It’s unlikely to exceed 5m or 33% of the line. That, folks, is all we’ve got available to accelerate. That’s actually the tow vehicle and the rest is the broken down vehicle being towed away progressively. In this case it is as though we tow the tow vehicle and it tows the towed vehicle.

Back to F=ma. We are working directly on only a third or less of the mass we want to cast. Five metres, or less, of fly line has to be accelerated hard enough to tow the other ten metres of line. Only by increasing acceleration can we compensate for the decrease in the mass we have to work with. Only then can we apply the Force we need to cast all of the line as far as we want it to go.

I hate static rolls. Rarely use them when fishing. Never use them if I can avoid it. Time, distance to target and the room behind me would all need to be in short supply.

Dynamic Roll aka Jump Roll

In comparison a dynamic roll cast operates with more live line and less dead line. For a dynamic roll of 15m it’s even possible to invert the proportions and get as much as 10m (66%) of line in the upper half of the D loop towing 5m (33%). Even half the line or 7.5m would be a 50% increase in efficiency compared with 5m. Bonus!

The more live line we accelerate directly, the more efficiently we can apply Force. More mass being directly accelerated means less mass being towed or indirectly accelerated. We also want to minimise any resistance by the towed line. That is why we should minimise the amount of line on the water by having an anchor with just enough to hold during delivery. Surface tension in the water, which holds onto the line, is only our friend when we don’t see much of him.

I like dynamic rolls. Use them a lot when fishing. No time wasted by false casting. Useful as a pick up cast before an overhead cast. Useful in other ways.

Tension in a D Loop

Did you notice line tension wasn’t mentioned and/or wonder why only the top half of the D loop was talked about? Have you heard that tension in the D loop “loads the rod”? Excuse my bluntness but you really shouldn’t listen to that crap. It won’t help you and it just ain’t so. For donkeys years people went on about how there is tension in the line between the anchor of a roll cast and the rod tip. That tension, bless its venerable socks, supposedly made it possible to cast because it loaded the rod.

Back in “What a Rod is and Does – Really” we canned rod loading. Some years ago Aitor Coterón, another member of my Advisory Group, kicked tension in the D loop out of the park. Check out these two vids and you will see what I mean:

https://vimeo.com/26829591

https://vimeo.com/39907238

They show that during a roll cast, even on a very low friction surface, the anchor stays put until late in the cast when it finally starts to slip. That could only happen if there was very little tension in the bottom half of the D loop.

Blown and Misaligned Anchors

When we blow the anchor it’s not so much line tension that we lose but some of precious net Force in the direction of our cast. How? Because the part of the line that slips backwards goes in the opposite direction to our cast so it exerts an opposing force. That adds to the inefficiency of the cast and we can ill afford the cost of Force theft.

Anchors that are not aligned with the direction of the delivery cast likewise steal Force. They are the on water equivalent of an aerial tracking error and offend against the Straight Lines Rule.

Lifts and Pullback

As with overhead casts you will often see good casters employ a small lift and some pullback as the finishing flourish to a roll cast. As explained in the previous episode this could tighten the rod leg, narrow the loop, enhance propagation and maybe even increase fly leg speed. I use often them with roll casts and they improve my results.

Hauling – Why, When, How

Hauling has been mentioned a few times in different sections. Let’s pull some of that together and look at hauling in a bit more detail. Hauls are very useful to fly casting in a variety of ways – not all of which are relevant in a discussion of physics for fly casting. In this instance, however, I think it is ok to stretch the boundaries a little because hauls are important enough and not widely understood well enough.

Why Haul

Among other things, hauls share the Work done on the fly line between the rod hand and the line hand. Both hands can now contribute to accelerating the fly line. That means we can use less Force applied to the rod and thus a shorter stroke than we would in a non-hauled cast achieving the same line speed and distance as a hauled cast.

Put another way, using the same stroke distance as we would for a non-hauled cast we can get greater line speed and distance, with or without shooting line. That means better control. Effort is the enemy of control and control is accuracy’s best friend.

Hauling can give us tighter loops because the stroke length can be reduced and rod flex (bend) and counterflex (bend the other way) will also be reduced. Less of a windscreen wiper arc during rotation will mean narrower loops. Tighter loops mean less opposition from drag.

Hauling can even save time and false casting when covering a fish.

Try this. In a practice session pick a target at what for you is a medium/long casting distance. That is, at a distance you can reach but not too easily. Imagine your target is where you would put the fly to cover a fish. Cover your target fish alternately with hauled and non hauled casts. If you can’t notice a difference then change to a more distant target. If you still don’t notice a difference it could be your hauling technique needs a bit of polish! Going the other way, try practicing without hauls for a substantial part of the session. If you are a good at double hauling, losing it for a while will probably expose faults in casting technique that hauling otherwise made up for.

When and How to Haul

Hauling accelerates the line if we do it before loop formation. Ideally then you would haul so as to optimise the combined line acceleration produced by both hands. That would mostly likely be when the rod, after translation, is rotated at the end of the cast. This is also sometimes referred to as the “power snap”.

The usual advice from casting instructors is to “haul late” and that is good advice – as late as you can actually, before the rod tip slows enough to allow a loop to form. Remember that hauling before loop formation means you are hauling on what will soon become the fly leg.

If you haul much too early before loop formation the rod tip can dip and rise again. This can create tailing loop problems. The tip dips when haul force is applied and rises when the haul is finished. That might not be such a problem if the rod hand was putting in some extra bend during the haul, another reason why we want to haul and reach peak rod acceleration at much the same time.

If you haul at least partly after loop formation then you are now pulling back on the rod leg instead of forward on the “fly leg”. This will enhance loop propagation velocity but diminish loop travel velocity. You will get zippier turnover but less distance.

The text books and videos will tell you to use short hauls for short casts and longer ones for longer casts. They will also tell you to use a down/up motion so the line you pull down is returned up while there is enough speed and energy in the line to avoid creating slack during the return.

Newtonian physics has no problem with that advice. However, when you have acquired good hauling technique I would encourage you to experiment and improvise. Use different contributions between rod and line hands to take your shots. Use different haul speeds and haul lengths and see what happens. Personally? I vary these things when working to produce just the tempo and combination of movements I want for the type of cast I want to make. A gentle and considered dry fly presentation is not the same as snappy nymph shot to a fish which just drifted obligingly within reach. Different fish, different folks, different strokes, different hauls. Find what works and feels good for you.

The Force And How To Be With it

A couple of years back I wondered if anybody actually knew how much force it took to propel a fly line so I asked. I asked WTF, What’s The Force? The answer was that the force applied at the rod tip to produce a measured casting distance is roughly 1 Newton for each 4m of the cast. So a 20m cast needs about 5N of Force during the rotation phase. This is a surprisingly small amount.

To put that in somewhat more accessible terms, one Newton is about 100g of weight. Say my shopping is in the car boot and the two bags weigh 5kg in total. I lift them out, exerting a force of about 50N. That same amount of force applied by the rod tip would theoretically enable me to make a 200m fly cast.

The force required during translation is even more ridiculously small being about 1N (100g) for a 20m cast. It doesn’t matter whether the numbers I have just quoted are precisely correct. The point is that fly lines weigh very little so it takes very little force to move them and that is both good news and bad news.

The good news is we can cast a long way with not much Force at the rod tip. If we measure the Force at the rod butt instead of the tip, the Force is about twice what it is at the tip but it’s still far smaller than we typically feel it is because most of what we are feeling is the mass of the flesh and bones that we move in order to accelerate the rod butt and rod tip.

The bad news is far less obvious, seldom discussed and rarely attended to. It comes in two parts. First part, in making a 20m cast we actually apply a relatively small Force in the direction of our cast, say about half a kilo’s worth at the rod tip. Accordingly, the kinetic energy we put into the line is also relatively small.

Second part, and here comes the crunch, since we don’t put much energy in, even small losses of energy (out) can have very costly consequences. Force thieves like tracking errors, slack, drag, mistimed hauls and so on aren’t just pinching a few energy scraps from the table, they are making off with the family silver.

I suspect our underestimation of the energy-out losses is aligned with both the overestimation of energy-in required to make a cast and a deep instinctive reliance on more force to go further. The combination is toxic to casting excellence.

Now, I know, it’s possible to put extra effort into a shortish cast and still get a decent result but that doesn’t disprove my point or restore faith in extra effort being the answer to extra distance. Yes, you can overpower a short cast and make it lay out faster but try that with progressively longer casts and it will inevitably end in tears. Soon enough, previously hidden problems with our casting technique will become ever more serious and apparent and we will end up with collapsing loops and piles of spaghetti at an embarrassingly short distance from our feet.

If power was the simple answer all the winning competition distance casters would be shaped like body builders and they aren’t. Keep your eye on Maxine McCormick, a casting phenomenon. As a teenager she is dominating accuracy competitions. If she keeps at it for another ten years and gets into distance competition. Look out. Obviously, the secret of her success is not strength, it is efficiency.

I’m not saying power is completely irrelevant but rather that a long cast won’t work with extra effort alone. What it needs instead is extra purity of applied force so that net Force isn’t depleted too heavily by losses to Force thieves. This in fact is what underpins almost all aspects of good casting technique and if it doesn’t then it’s the technique model that needs adjustment. Trust me, there is far more to be gained by preserving net Force than there is in trying to increase it with extra effort. Straight lines will always rule and efficiency will always trump exertion.

Casting Faults – Mechanical Failures

A great deal of the traditional ideas and methods for teaching fly casting seem to be concerned with correcting faults. I don’t want to go too far down that track, especially because I believe it’s better to teach people to fly cast than to teach fly casting to people. However, I would like to demonstrate how understanding the mechanics of casting can help with a few common faults. It’s by no means all we need know but it’s useful.

Heaving

When I’m out fishing I often get to watch other people cast. The two most common things I notice are fat loops and overpowered casts, often in combination. To fully understand the many problems that heaving (overpowering) causes we would need to draw on knowledge about human movement, biomechanics and neuroscience. Hmm. A little beyond the scope of this project, but let’s see what we can do with the physics stuff – well, mostly.

Applying casting force efficiently means we optimise the net Force and minimise the forces applied in other directions. As we have seen, the required casting forces are surprisingly small so it is very easy to overpower – to heave – without realising it. Heaving leads to early rotation and/or over rotation which violates the Straight Lines Rule, yet again. Heaving leads to tailing loop troubles. It adversely affects accuracy. There are lots of reasons why we shouldn’t heave. Ok, got it, but why do we heave? Simple question with complex answers, most of which have nothing to do with physics. Having spent years assiduously trying not to heave, at all, I now think I know some of the reasons why we do it.

First, when we need to throw things further and/or faster we instinctively throw them harder. More power is our default solution for all kinds of problems – it’s the get-a-bigger-hammer syndrome. After a lifetime of confirming instinct while playing sport, skipping stones or expressing frustration with hand tools it becomes a very hard habit to break.

Second and related to the first reason, I think part of the heaving problem comes from a lack of feedback. Imagine you are given a golfball and a ping pong ball and asked to throw each of them into a hoop lying on the ground 10m away. No practising, one shot for each ball. My money is on the golfball. Unless you are used to throwing ping pong balls, ie your systems are calibrated for the task, they will be harder to throw with the right amount of force than something much heavier like a golf ball. Very hard to judge how hard to throw something so light we can barely feel its weight in our throwing hand.

Most fly lines don’t weigh very much. We want to be able to feel some resistance, to feel the rod towing the line. What happens when we don’t feel that? We speed up trying get back in touch. Results? Lumpy acceleration, force going all over the shop, casts descending into complete failure. Slack in the fly line often creates this sort of problem.We can carry a lot of line in the air provided the sequence of false casts keeps us in touch. When it goes wrong and we start chasing slack we start to overpower and it becomes very difficult to recover.

Heaving is a problem in itself because it leads to lots of opposition forces which steal from our precious net Force in the direction of the cast. I suspect it is also a problem because we do it to try to compensate for other shortcomings in our technique. Here it becomes a fault combining with the other faults to make an even bigger mess.

Fat Loops

We throw wide loops when we make the rod tip travel in wide arcs instead of straight lines. In other words wide loops are evidence that we have broken the Straight Lines Rule and haven’t used our casting force efficiently. The usual suspects for too much rotation are not enough translation before rotation, too much wrist movement during the stroke and not stopping the stroke high enough. Wide loops also present more surface area of line to the air so drag is increased, further diminishing efficiency.

Now we have an inefficient casting stroke that isn’t putting the fly where we want it. The answer? Of course, more force, right? Nope. Heaving will probably make things worse rather than better.

Yes, it’s true that sometimes we might choose a wider loop – say downwind casting or casting multiple flies that love to tangle. However, that’s not an excuse for fat loops. It’s a choice made by someone who can vary their loop sizes at will.

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