1509 shallow pool effect

Back in the 18th century Benjamin Franklin documented his observation that vessels in shallow water move slower than vessels in deep water (with all other things equal between the vessels).


As the vessel moves forward it is pushing against the water in all directions, pushing water molecules out of the way so the vessel can fill that space ahead. Molecules of water form pressure waves which move outward from the vessel on all sides which we observe as waves on the surface. It also sends pressure waves downward toward the bottom. These waves cannot be seen, but they can be felt. If close enough that wave will hit the bottom, bounce up and push against the vessel, causing it to move slower in the water.

Possibly some of you have noticed that, as swimmers, we have the same situation in pools with shallow depth.

Here are the shapes and depths of the three pools I have used in the last 3 weeks while on a visit to my home state of Oregon USA.

1509 shallow pool

Pools A and B are actually at the same athletic club. Pool A is indoors and Pool B is outdoors. I naturally gravitated toward the outdoor pool when it was not occupied in the morning, but would have to switch to the indoor pool within the hour, once the kids showed up for daily swim club.

So, a couple times I was in the middle of stroke-counting sets when I used my rest interval to switch pools then resume the set inside. It was interesting to note that my stroke count was about 1 SPL lower when I moved to the indoor Pool A. After the first time I switched and noticed the difference, I wondered if that outdoor pool was slightly longer. The outdoor pool is heated to  maybe 1-2 C (2-3 F) warmer than the indoor pool and slightly too warm for my comfort – so I must admit that factor played a small part in how ‘energetic’ I felt, but still, after testing it a few times it was obvious there was an echo effect happening, slowing me down in the pool with a more shallow bottom.

A few years ago during a visit and a few times during this recent trip I swam in Pool C, which went from shallow to deep for more than half of the length. A few years ago, while doing 4×25 sprint sets with strict SPL x Tempo combinations I distinctly noticed the stroke count difference between lengths – I would take 1/2 to 1 strokes more swimming back to the shallow side wall than when I swam toward the deep end. I did scores of repeats over a few weeks to feel confident the depth changes were affecting SPL.

Have you ever observed this in your own pool?

Why bring this up?

Well, first of all, it is just interesting hydro-physics and I know there are a few geeks out there reading this who like to think about this stuff too.

Second, when comparing your SPL from one pool to your SPL in another pool, you should take into account the differences between the depth of the two pools even if they are the same length. You may feel surprised by achieving an exceptionally long, or exceptionally short stroke while visiting a new pool one day and wonder what happened. The depth of the pool could be a significant factor.

And, it is quite common for public pools (in the USA, at least) to have a shape something like Pool C to make room for a diving board at one end. If you are a precision data geek then you may appreciate knowing that dramatic changes in pool depth can affect your SPL going one direction versus the other.

Also, this is one of the reason why those TI Height/SPL Charts (on our Resource Page) which show suggested SPL ‘Green Zone’ by height (or wingspan) are an estimate of our optimal SPL range, not a rule we must conform to. There are many factors that need to be taken into account when dialing in your personal SPL expectations.


If you are interested in more on the topic of performance pool design here are a couple links describing the pools used for the 2008 Beijing Olympics and the 2012 London Olympics. The consensus of these two videos says that 3m depth is ideal for a minimal echo off the bottom of the pool.

China’s Olympic Swimming Pool: Redefining Fast

Science Of The Summer Olympics: Designing A “Fast Pool”

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