Physics FOR Fly Casting – The Einstein Series

Straight Lines Rule

Short Version

Cast in straight lines with straight lines.

The most efficient way to apply a force is in a straight line.

  1. Looking down on a caster from directly above (drone view) the forward cast and the back cast need to be in straight lines – at 180deg to each other. ie Good tracking.

  2. It also means the rod tip should travel, relative to the horizontal, (caster viewed from side on) in a straight line for as long as possible during a cast. This is the venerable Straight Line Path which casting geeks talk about a lot.

  3. Deviations from the straight line in either plane mean that force is both not going where you intended it and, worse, is going in directions at least partly opposed to your intended direction. ie It’s not helping you and it’s fighting you.

  4. Same deal with the fly line – it needs to be kept as straight as possible or part of the force you apply will be spent taking up slack or following curves in the line instead of moving the fly line in your intended direction.

  5. Newton’s second law of motion states that when a mass is accelerated the Force needed will equal the mass multiplied by the extent of the acceleration. F=ma. The tricky bit is that F means a net Force or acceleration of a mass in a single direction. If you are casting with force going in multiple directions the Force in your intended direction will be reduced by force going in any other direction. The net Force is with you. The rest is against you.

  6. This explains why if you cast like a windscreen wiper instead of a spear thrower your casts don’t go very far.

  7. Straight lines rule, ok?

Casting in Straight Lines

Pretty much anyone who has read or heard much about fly casting will have come across the 5 Essentials of Fly Casting by Bill and Jay Gammel. (Don’t worry I’m not going to do yet another recitation and commentary on the 5 essentials.) I’m going to call them “rules” instead of “essentials” because it fits nicely with the “laws” of physics. It has been said (and I agree) that four of the rules are really there to assist with the fifth rule. For some reason that rule normally appears as number 3:

“The rod tip must follow a straight line path.”

To get the most from this rule you need to consider the path of the rod tip from two different angles. First, the side on viewimagine you are watching a caster from off to the side of their casting arm. What you are checking out is the path of the rod tip. If it moves vertically, up or down, it has deviated from the Straight Line Path (SLP).

You also need to consider the path of the rod tip as seen from directly above the caster – imagine you have got drone footage. Now you are watching the path of the rod tip to see if it moves horizontally to one side or another of the SLP. Ideally then, the rod tip will travel in a straight line vertically and also in a straight line horizontally. In both cases the closer we are to straight lines the more efficiently we will be casting.

Some physics to back that up. What is the shortest distance between two points? Easy question, it’s a straight line. What is the most efficient way to apply a force? Same answer in a straight line.

Let’s take a practical example to demonstrate the point. Say your car has broken down and a friend has agreed to give you a tow. Common sense will/should tell you to attach the tow rope as near as possible to the centre of the friend’s car at the rear and to the centre of your car at the front. If, for example, the rope was instead attached to one corner of one car and the diagonally opposing corner of the other car that would be dumb. The cars will go down the road trying to pull each into line. This is nature’s way of trying to even things out and apply force in a straight line.

Back to fly casting. The tip of the fly rod tows the fly line. Looking from above, the caster’s rod tip and fly line should travel from front to rear and back again in a straight line or single plane. Deviations to the left or right are known as tracking errors. They are bad because they steal force from the cast in two ways. One, force applied by the caster is used up by that sideways line being pulled back into the intended direction of the cast. Part of your casting stroke is used up pulling the line sideways instead of straight at your target. Two, when a fly line goes a bit sideways, instead of straight, more line surface area gets exposed to the air causing more drag force to be applied to the line. Drag is also a thief of casting force whose exploits will be covered in more detail later in this series.

The Straight Line Path

As casting geeks we can get very intense about something called the Straight Line Path (SLP). It is, of course, the path of the rod tip we are talking about because basically where it goes the line goes. In this instance we are talking about vertical rod tip movement as seen from the side. Deviations from this straight line involve fundamentally the same problem of things stealing casting force as we were talking about looking down from above the caster. This time, however, let’s delve a bit deeper.

To explore the complexity thrown up by the SLP we need to discuss a law of physics. Some time after an apple supposedly fell on his head Isaac Newton came up with three laws of motion. It’s the second one we are interested in here. That law is about accelerating an object and states that the Force required will always equal the mass of the object multiplied by the extent of its acceleration. The formula is F=ma.

Seems simple enough. In our earlier example the mass was a dysfunctional car and it needed to be accelerated from standing still up to the speed of the tow vehicle which will be providing the required Force. However, gravity is pulling down on your car and on your friend’s car so the Force required has to overcome the effect of gravity on the two cars. It also has to overcome mechanical resistance from all the metal bits that have to turn before both cars can be accelerated up to the right speed. If you were silly enough to attach the tow rope diagonally some force will pulling both cars sideways and some will be pulling them forward. So, the Force required to get your car moving forward and up to speed is a net Force – in this case it is the net Force required to overcome opposing forces of gravity, mechanical resistance and that sideways stuff.

Newton’s second law is (always) about net Force in a single direction. For fly casters the SLP is that single direction. Where we want the cast to go is where the SLP is aimed.

Back to towing your fly line, the net Force required to make it travel in the intended direction of your cast has to overcome gravity and air drag. You also need enough force to move the body parts involved in making the cast but let’s not get anal about all of the forces working for and against us. The most important thing to remember is that net Force implies a single or resultant direction of acceleration.

If the rod tip always moved in a perfectly straight line life would be easier and simpler but sadly the SLP is a just a handy ideal. In reality you and the rod tip move in a mixture of curved and straight lines. We pull the rod along in a straight(ish) line (called translation) and then we rotate it (rotation). A casting instructor will tell you you to rotate late in your casting stroke, to delay rotation as much as possible. Why? Because the more we rotate the rod when we should instead be translating it the less distance the rod tip moves in a straight line. At the extreme, our casting strokes will look like the line is attached to a windscreen wiper instead of to the end of a spear thrower (Woomera or Atlatl). What immediately separates a novice caster from a seriously good caster is the amount of time the rod tip is made to stay moving in a straight line during the casting stroke. The one thrashing about, making big fat loops and not getting anywhere much is the novice. Don’t laughthat was nearly all of us when we were starting out.

The SLP is not some precious aesthetic. It is rather the quintessential expression of force being applied efficiently in a straight line. What happens when the rod tip deviates/rotates away from a straight line too much and too early in the casting stroke is a) Force isn’t going in our intended direction and b) much worse than that, any force that isn’t going where we want it to is actually going in an opposing direction.

Consider a forward cast. Let’s say we are making like a windscreen wiper and we rotate the rod tip through 90 degrees from just back behind the vertical to just before the horizontal. When we start everything is cool but by the time we reach the 45 degree mark about half the force is going forward and the other half is going toward the ground.

Casting With Straight Lines

Let’s consider a Pick Up And Lay Down cast. Imagine ten metres of your fly line is lying on the ground in a perfectly straight line out in front of you. Your rod tip is held low and the rod is in a straight line with the fly line. As soon as you start to lift the rod all of the fly line starts to move. Why? Obviously, because there is no slack in the line.

If there was a big loop of slack in the fly line out to one side when you started lifting the rod the fly end of the line would not move until nearly all the slack was taken up. The tow vehicle won’t move the towed vehicle until the tow rope comes tight.

The Force required to move (accelerate) the whole section of line (the mass) up and back behind you is a net Force – net of whatever it takes to pull the line straight. When you are false casting you might get a big line sack on a back cast. Your next casting stroke forwards will have to take up the slack before the line goes where you want it to go. Ideally, you want to be towing a spear of line in both directions. It doesn’t help when your spear turns into a spaghetti noodle.

A straight fly line being moved is a line under tension. Contrastingly a pile of spaghetti line lying still on the ground is a pile of slack line. Line tension is a useful thing we might talk more about later but for now a line under tension is the straight line that we want to cast with so it’s the opposite of a wavy or slack line which we don’t want to cast with.

If you read through the thread in the following link it might make your head hurt but it shows that although in reality we don’t often cast with perfectly straight fly lines it is undeniably helpful to keep our fly lines as straight as possible.

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