I had intended to write this post back in June to follow up on my earlier posts regarding why the line or fly slaps the water behind the caster. I was happily sidetracked with a discussion on the role of the reel’s mass. As the summer winds down, I’d like to tie up some loose ends.
For many casters, the fly or line slaps the water behind a caster because the rod is accelerated late in the stroke. I think that there are two typical ways that late acceleration causes the fly or line to hit the water behind the caster. In this post, I would like to discuss one of them, and in a later post, I will discuss the other. I will have a third follow-up outlining how the fly line can contact the water that is not related to late acceleration.
In the video below, we will watch a fairly new caster. The caster is several feet above the level of the water, and there is a tailwind which makes it difficult for the fly line to straighten out in the back cast.
Although the video is of a pickup (and not a back cast with an aerialized line), it illustrates several important points.
To help analyze the video, I have marked the rod tip position every 20 frames (Figure 1). The distance between dots is proportional to the rod tip’s velocity. Smaller gaps indicate lower velocities and larger gaps mean higher velocities. When the distance between dots changes, it indicates periods of acceleration or deceleration of the rod tip.
Period “A” starts when the caster begins accelerating the rod in earnest to initiate the back cast – once the entire fly line is moving on the water. It ends when the fly rod becomes vertical.
Period “B” starts when the rod becomes vertical, and it ends when the rod tip reaches its highest point in the stroke.
Period “C” is the remaining stroke after the rod tip reaches its apex.
To isolate the horizontal movement of the rod tip, I have moved the rod tip positions throughout the stroke into a row at the bottom of Figure 2. I have removed the last 40 frames (three dots) of the stoke since the tip is moving forward instead of backward.
To isolate the vertical movement of the rod tip, I have moved the rod tip positions when it is moving upward (Periods “A” and “B”) into a column on the left of Figure 2. Similarly, I have moved the rod tip positions when the tip is moving downward (Period “C”) into a column on the right of Figure 2. The last three dots are slightly faded since they are not depicted in the row showing horizontal movements.
During the first 20 frames (2 dots) of Period “A”, there is more vertical acceleration of the rod tip upward compared to horizontal acceleration rearward. Subsequently, the vertical velocity of the rod tip remains similar over the next 40 frames, and the horizontal velocity of the rod tip increases. This results in a high trajectory initially.
Then, towards the end of Period “A”, the rod tip appears to decelerate both horizontally and vertically. Although I did not mark the position of the first guide of the rod through the pickup, the velocity of the first guide doesn’t seem to slow down after the initial acceleration over the first 80 frames of Period “A”. Consequently, the slowing of the rod tip velocity appears to be a result of the rod bending.
Once the rod has fully flexed to the mass of the line (approximately when the rod becomes vertical – orange dot and line), the rod tip begins to accelerate again. The acceleration now is predominantly rearward and no longer upward.
The apex of the rod tip (green dot) occurs after the rod has achieved a vertical position (green line).
To maintain the anonymity of the caster, it is not possible to see that the caster is primarily using the wrist and elbow to rotate the rod. The bicep muscle is effectively lifting the rod’s center of rotation. As discussed in a previous post, this upward movement allows the rod tip’s apex to occur after the rod has reached a vertical position.
Once the rod tip reaches its apex (beginning of Period “C”), the rod tip continues to accelerate primarily in a rearward direction. However, the direction of the acceleration changes as the downward component overtakes the rearward one.
Figure 3 illustrates how the rod tip’s path and acceleration form the loop.
There are approximately 180 frames between the point when the caster initially accelerates the rod and when the rod tip reaches its apex. Once the rod tip starts moving down and until the rod is stopped, approximately 120 frames have elapsed.
As a result, the downward acceleration of the stroke in the latter parts exceed the upward acceleration of the earlier parts. Not only is the acceleration greater, but it also occurs over a greater distance.
Vertically, these opposing forces pull the loop apart resulting in a widening loop that progresses rearward. The wind, a loop that is not aerodynamic, and the lack of rearward velocity prevents the line from straightening out.
When the caster starts the forward stroke and the fly finally starts moving in response to the forward movement of the rod, the fly is pulled downward along the path of the line. (See Figures 4 and 5.) The fly hits the water at a location predicted by the orientation of the fly line.
One might think that the fly should be pulled towards the rod tip. But, this occurs when the line between the fly and the rod tip is straight and taut.
Force and momentum at the rod tip act on a tiny portion of the fly line immediately after it. Then that next section’s momentum or energy acts on the next tiny portion of the fly line. And so on and so forth. The rod tip’s force and momentum do not act through the air to affect the fly. They act along the fly line. And the more slack is present, the longer it takes for the fly to follow the rod tip – instead of the line.
In the same way, as this caster’s rod tip moves forward, it slowly pulls more and more line along the direction of the line. As the fly and the line’s velocity increase (with additional help of gravity), their momentum towards the water increases, even after the line becomes taut to the rod tip and much of the line finally begins to move in the direction of the rod tip, the fly’s momentum still causes it to strike the water before moving forward with the rest of the line.
The caster didn’t drive the loop into the water during the back cast. The late acceleration downward created a big loop that left a substantial amount of line angled towards the water. And as the caster came forward, the fly was yanked down to the water – along the path of the line.
Glen Ozawa, OD
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