*This post is a near-duplicate of a post I made on my other (now private) blog about triathlon swimming. But it’s of general interest (and probably nerdy enough) to re-post here.*

A posted video from the very informative Adam Young (Swim Smooth) emphasized the importance of swimming in straight lines during triathlon events to minimise the extra mileage that you would swim if you snaked or swerved your way through the water. Of course, this is true. Sighting poorly and thus not swimming parallel to the course adds distance. That’s simple geometry.

But how much difference does it really make? The narrator suggests “hundreds of metres”, but did he really do the math on this? Well, in an attempt to procrastinate thesis writing, I did. Here’s what I figured out.

First, we have to make some assumptions about the non-linearity of the swimming. To make the demonstration simple (and the math easy), I simplified it to a series of straight lines that do not directly progress towards the finish. Rather, I will assume, for the sake of ease of calculation, that a swimmer can swim about 100m in a straight line (whichever direction that may be). We also need to decide how far off-course this swimmer will go. So I envisioned a very geometrical scenario where a swimmer swims 100m to the opposite edge of the swim course, then corrects to the other edge for 100m, then back again, and so forth. This would be the equivalent of swimming in 100m straight lines between kayak paddle taps (we’ve all had one of these at some point). I calculated two scenarios: one with a 25m course width and one with a 50m course width.

This might seem like an extreme example, but it’s the same (distance-wise) as how you would travel if you swam on the same angles, just not for 100m at a time. So let’s just go with it. We’ll call it the ‘overcompensation nightmare’.

Now, while this might look like a LOT of extra distance, I was somewhat but not really surprised (and a little relieved, as I’m not the straightest swimmer) that the overall distance swam wasn’t that much further than a perfect straight-shot (also unlikely, but whatever). In the 25m course, a swimmer covers about 97m of course for every 100m of swimming, resulting in a total 1900m Half Ironman (HIM) course swim of 1960m. Not bad. This gets worse in the 50m angle example, where 100m of swimming only nets 87m of course, resulting in a total swim of 2180m: 280m longer than the actual race distance.

In both cases, there is extra mileage for sure. But consider these examples (especially the 50m) as worst-case-scenarios: they are drastically off-course, the never swim straight at all, and there’s no consideration of buoys or finish lines that would straighten people out as they approach. Don’t get me wrong. Even an extra 60m of swimming sucks on race day. You’re better off to just learn to sight properly. My only point here is that you shouldn’t concern yourself if you’re not swimming perfectly straight. The other point of the video was learning to breathe properly, which will keep you in a straight line. I’m not sure I can argue that with simple geometry, so I guess I’ll get back in the pool and work on my bilateral breathing.

Okay, but this also got me thinking…

Lots of racers start WAY off to the side of a triathlon swim course where the crowds are thinner and the resulting washing machine is on a more gentle cycle. I have often wondered just how much difference this makes in swim distance. So I played with that too.

I based this again on a hypothetical out-and-back HIM swim where the would-be course heads straight out for 950m, turns around, and heads back. I also assumed that, after the athletes reach the turn-around buoy, they swim straight back to the finish (950m). I did two calculations: one based on an athlete starting 100m off the straight-line to the buoy and a second based on an athlete starting 200m off the straight-line. And here’s what I figured.

Starting 100m off the straight-line course costs you…wait for it…5m. Meh. Starting 200m off the straight-line course costs you a bit more: 20m. Now, you don’t need to be a math whiz to see that this relationship between distance off the straight-line and swim distance is related in a highly predictable way. And, you can probably envision that these numbers change somewhat as the distance to the first buoy changes. Well, rather than do all the calculations, I thought I would just plot it.

The take-home message: if you want to start well outside the pack, do it. In the grand scheme of things, it doesn’t cost you that much. And if you can swim faster in the less crowded areas, bonus. Also, the farther you have to swim to the first buoy, the less it costs you to line up well to the outside. I can’t imagine really needing to be more than about 100m to the outside, which costs you between 10 and 20m, depending on the buoy. Again, meh. Most days, I’d rather not come out of the water with black eyes.