The everyday problems faced by swimmers in their local swimming baths don't often require a detailed
scientific explanation: raucous teenagers with beachballs, toddlers
with bladder control issues, the exit of the 'Ultimate Crazy Tornado
Experience' depositing screaming sliders right in the middle of your
breast stroke. madbeachcomber, however,
noticed something a little more subtle at work...

Whilst at the local swimming pool doing lengths, I noticed it
seemed easier to swim from shallow end to deep end than the other way
round. Is it my imagination or is there a logical explanation... or is
it because I was going downhill..? 

So what's the explanation? Three theories caught the eyes of the
SExperts.

1. Turbulent Times?

Perhaps, thought Zubeneschamali, it was
all to do with turbulence:

There may be some effect on speed swimming from deep to shallow
water, just as the outside lanes in a competition pool are slower
because of turbulence from the walls.

Not a bad theory, but as Zube himself pointed out: 'I think you'd
need a stopwatch to measure it'.

The theme of turbulence was expanded on by Advocatus Diaboli:

It's a wave-drag effect, where the swimmer's bow wave interacts
with the bottom of the pool as well as the surface, rather than
dissipating as it would in deeper water¹. [It occurs when the water is] shallow
enough for the wave to reflect and impinge on the swimmer. This means
the swimmer is trying to do additional work against him/herself in
order to go at the same speed, so swimming is either slower or feels
harder.

The effect is also noticeable in boats: on tidal rivers, even
when there is no net current flowing, a boat will often overtake a
similar one around the outside of a bend, just because it is in the
deeper water. Rowing over the Fulham flats
on London's tideway is notoriously like
rowing through treacle.

Overall, the opinion of the SExperts seemed to be that these
effects are almost certainly present, but are perhaps unlikely to make
a noticeable difference to a swimmer in a small pool. BouncyBleepImZentrum summed it up nicely:

Well there should, I think, be generally freer flow of water at the
deeper end, because the sides and bottom are fixed and generate most
resistance to the flow. Much the same as a river flows faster in the
middle than at the banks.

But, is it a significant effect in a swimming pool? I'd be
surprised. Moreover, I don't think it'd explain why swimming in a
particular direction was more difficult, just a different area.

Reluctant to leave this theme, other SExperts waded in with their
own variations:

Could it be anything to do with wave interference? In coastal
regions, wave amplitude increases as a wave approaches shallower
waters. A dramatic example of this is a tsunami. As you swim towards
the deep end the water is less turbulent, so you can go faster? Come
to think of it, this effect is probably tiny in a pool...
- Ste

There is another possible explanation, and it's much simpler,
too². As an ex-collegiate swimmer it's
something that I've had some direct experience with. It could be
simply the water return jets making a small, barely noticeable current
towards the deep end of the pool. Happens a lot with older pools of
varying depth. One of the main reasons competition pools are of a
singular depth and a separate diving tank is used for diving
events.
- JD

The final word on the subject of turbulence goes to Arnie Appleaide and some anecdotal evidence:

I've heard top-notch swimmers say that deeper water is faster,
because of the turbulence factor (less in deep water). But that could
just be typical racer superstition.

2. Taxing Technique?

Perhaps, thought Orcus, it was all to do
with differences in swimming technique when travelling from deep to
shallow, compared with the opposite direction. Perhaps, a swimmer is
more likely to look at the bottom of the pool when swimming into deep
water than when moving into shallower depths:

Putting your head down means your feet move upwards and you present
less surface area drag to the direction you are swimming - hence it is
easier to swim and you go faster too. Most people breathe wrong and
stick their heads out of the water making swimming far more of a
labour than it need be.

3. Trivial Tricks?

The third theory put forward by the SExperts was that is was all
simply some sort of illusion. Traveller in
Time thought it might be a trick of the light:

The deep end is more brightly illuminated then the shallow end. It
is more easy to go to the light then into the dark. Just the feeling
you get, but it does work

Alternatively, Blatherskite the Mugwump
suggested that it was all to do with depth perception:

You're measuring your speed with the only visual reference you
have, which is the bottom. The bottom is further away in the deep end,
so it appears to move less in relation to you. The bottom is closer to
you in the shallow end, so it appears to move more in relation to
you.

TiT, however, wasn't convinced:

Assuming you are doing a breast stroke, normal vision would be the
water down and in front of you. [This would make] you think you are
slowing down when it gets deeper. The statement was: 'easier to swim
from shallow end to deep end'. Whereas this optical illusion would
suggest the other way around: to advance faster near the shallow
end.

And the Winner is...

In all likelihood, the answer is some combination of all three.
There may be more turbulence in the shallow end, but it's unlikely to
make a huge difference. Unless you have the technique of an Olympic medallist there will always be some
improvement you can make to ease your passage through the water. And
maybe, just maybe, it's all in the mind...

This article was based on a conversation at the SEx forum - where science is explained.

Why not pop over with your own questions? The pick of the bunch
will feature in The Post's next
issue.

SEx Education Archive

Danny B.

21.09.06 Front Page

Back Issue Page

¹Wave effects are also
why it's faster to swim underwater - so whales only surface long
enough to breathe.²The 'explanations' in this forum tend to get arcane,
sometimes even positively esoteric!