SLP (Straight Line Path) of the Rod Tip
Few things in fly casting are as misunderstood as the concept of the “straight line path” (known in casting-geek-speak as “SLP”) of the rod tip during the cast. Indeed, overly simplified or overly optimized diagrams of SLP have been wreaking havoc on casters’ imaginations for decades. While Possessing SLP is often thought of as a good thing for much of casting, what it really looks like is often shrouded in some mystery. Fortunately, high-speed video and motion-capture technology have made SLP easy to see…
Below is an image taken from a high-speed video session (at 500 frames-per-second), showing SLP in a cast with 10 meters of line out of the rod tip (thanks to my friend, Grunde Løvoll, for the video from which this pic is taken).
Various aspects of the cast, including the infamous SLP of the rod tip. Rod is a Sage TCR 590-4.
Below is an image compiled from the 3-D motion-capture system used at the Fly Casting Institute (at 200 frames-per-second), showing SLP made with a different rod and caster (caster is me, rod is a Sage XP 590-4).
Additional info: After I originally posted this, I exchanged a few e-mails about various aspects of casting with Dr. Server Sadik, who is both an engineering prof at Montana State University and part of the original MSU/FCI motion-capture crew from back in 2004. In our back-and-forth, Server mentioned that he had some good SLP still shots from that original mo-cap session. He forwarded them to me over the weekend, and I thought that I would post them as an adjunct.
One unique thing about the mo-cap system is that it allows for the rod and line markers to be tracked clearly and accurately as they move over time. This is great for visualizing SLP, as well as how much the rod tip moves during SLP (and all the way through unload into counterflex), versus the rod butt (it’s a huge difference). So…without further rambling, here are a couple of still frames from Server, showing all the tracked data (caster is me, rod is a Sage XP 590-4):
Above: Very long, slightly climbing SLP.
Above: Somewhat shorter SLP, with tracking showing rod unload into counterflex. Note how much the rod tip moves relative to the rod butt over the same period of time…
As you can see with all of these images, SLP lasts for only a portion of the cast, then the rod dips downward away from SLP, through “rod straight position” (or RSP, in casting-geek-speak) and into counterflex, as the nose of the loop is being formed. This dip away from SLP at the end of the cast is normal, and the line loop is the ultimate result.
There is a lot to say about SLP and its relationship to the cast, and some of that may indeed show up on on FF&W going forward. But for now, I just wanted to post some data that allows casters to get a better mental image of what is really happening (or not happening) up there in 3-D casting space. If you want to see it all in motion, play the video from this post.