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).
Why?
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.
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”
© 2015, Mediterra International, LLC. All rights reserved. Unauthorized use and/or duplication of this material without express and written permission from this site’s author and/or owner is strictly prohibited. Excerpts and links may be used, provided that full and clear credit is given to Mediterra International, LLC and Mediterraswim.com with appropriate and specific direction to the original content.
Hi Mat,
I think I mentioned this phenomenon in Sweden after my stroke count from the BTS had inconsistencies for timings of lengths. In some pools I can almost feel myself being pushed back from or meeting greater resistance when swimming into shallow end. My favourite 50 meter pool is 2.5 metres throughout and is much nicer to swim in – I clock a steady 40 to 42 (depending on my physical condition at the time).
Furthermore in the 4 lane 25m pool I coach in there is a significant current flowing from deep to shallow caused by the inlet being in the middle of the shallow end. The result for me is 19 to 20 SPL shallow to deep end in Lane 1 and 15 to 16SPL deep to shallow. In Lane 4 the current flows the other way to confuse things further.
Equally important thing is the water temperature. In the sports pool, the water is colder by 3 or 4 degrees Celsius. If you warm up in the recreational pool 30 degrees and then walk into a sporting swim with greater comfort and ease. This may be due to the fact that the Cold = higher density = more displacement … what do you think? sorry for the spelling mistakes but my English is not high. I think you will understand 🙂
TI coach Adrian from Poland
Hi Adrian, good to hear from you. There are certainly physiological factors to consider if two pools being compared are not equal in all details except for shape. No doubt, when I went to the indoor pool that was slightly cooler I simply feel more energetic and I know it has some psychosomatic effect also – which is the reason I prefer cooler water. The change in density of water by temperature would have scientific significance for sure, but it may be an insignificant factor at a 1 C temperature difference. Also, pool chemical composition would have an effect also. At two different fitness club facilities they have the exact same pool distance and bottom shape, but one is standard chlorine and the other is a special chloric-saline pool and I was wondering how that chemical difference might affect performance. But without the pools next to each other I could not compare.
I absolutely feel more buoyant in a pool that may be only 2 degrees colder. I’ve often wondered about this. Am I more buoyant or am I just feeling the water more because of it’s colder temperature.
Now this has got me thinking. I wrote about buoyancy differences in https://mediterraswim.com/2014/11/22/neutral-buoyancy-point/. The test that I proposed might work as a not-perfect, but close-enough way to test the difference of buoyancy in pools of different temperature if the differences were big enough to be detectable. One would have to have perfectly still water and hold breath long enough to have the body come to complete rest and then have a way to measure where the water line is on the head. But it would be hard to put the body in identical position and condition for each test, on two different days. One should probably weigh in before the test also, to see that body mass was the same.
I’m no physicist (struggle even to spell it) but at zero degrees water is pretty much solid and 100 degrees it’s gas, so I would think 2 degree difference should make a bit of a difference. That said when I swam in a pool at 20 degrees (Celsius) a while back I was not really aware of being much more buoyant than my favourite pool at 27.
I think we are talking viscosity here aren’t we? I found this but don’t understand it
http://www.engineeringtoolbox.com/water-dynamic-kinematic-viscosity-d_596.html
It’s clearly measurable but to what extent it affects a human’s body position in the a water We need a scientist I think now!
http://www.vaxasoftware.com/doc_eduen/qui/viscoh2o.pdf
or this?