Off-Topic Aerodynamic Question

[reidh]

We are looking for help solving a debate going on at work, unrelated to our business.

The arguement is as follows: Many swimmers do not shave their legs until just before a big meet, using the logic that training with hair on your body is like resistance training. Shaving it allows you to go faster for the "big meet".

Although I am mechanical and not an aero engineer, I guessed that having hair could actually break up the boundary layer on your legs/arms when swimming. Thus, smooth arms/legs would actually increase drag.

Can anybody with more than just an undergraduate fluids class shed some technical light on this subject?

Thank you.

 

[IRstuff]

At that level, most such effects are psychological, so if it makes them THINK they can go faster, they probably will.

If they were really pursuing such effects, the simplest thing to do would be to swim with extra weights or towing a weight.

TTFN

 

[rb1957]

yeah, psychological reasoning is pretty important; cyclists often do the same, for what ou'd imagine is very little difference (but then what's the difference between 1st and 2nd? ... not much time ... so a small advantage might be significant.

almost on topic is waxing racing boats. sailors used to wax their hulls to a glassy shine. somewhere someone researched this, and proved that a diamond-like shine isn't necessarily the best ... it depends on the wax, and the adhesive angle of water droplets. some waxes produce a very nice shine, but cause the water to "stick" to the surface, increasing the drag of the hull.

 

[magicme]

with all the turbulence, wakes and bubbles the swimmer is embedded in, plus the transient motions of his/her body it's hard to imagine a boundary layer building up anywhere.

i'm speculating that body hair would trap water or bubbles next to body parts and would have to be dragged along with the swimmer. with hair removed, there would be no entrapment.

sound credible?

magicme

------------------------------------
"not all that glitters is gold"

 

[IRstuff]

A couple of summer Olympics ago, there was a big todo about something called "shark skin," or something to that effect that was supposedly better than sliced bread.

TTFN

 

[reidh]

Intersting link about the sharkskin swimsuits. Not much evidence supporting the claims, but intersting none the less.

http://www.findarticles.com/p/articles/mi_qn4158/is_20000317/ai_n14282652

Reidh

 

[crysta1c1ear]

I'll pass on the difficult question of whether shaving increases or decreases drag, and just make a comment about the idea of shaving specifically for a race: is it a good idea.

I heard commentry for wimbledom where a tennis player wasn't allowed (by his coach) to cut his fingernails during a tournament, for fear of upsetting his balance and judgement.

I knew a very keen swimmer once. He was simultaneously a member of several swimming clubs in different countries so that business travel would not interfere with his training. He would talk about a term used by competition swimmers: the feel of the water. The idea is that with sufficient training and practice, one gets a feel for exactly what the water is going to do. Anyone who has swum a lot can identify with feelings that some days the water is thick and slugish and other days it is smooth and flowing.

Swimming crawl, swimmers breath in with their mouths below the waterline because they take air from a dip in the bow wave made by their head cutting through the water. Sometimes, swimming faster can take less effort since less head rotation is required if a bigger bow wave is made from greater speed, or if the dip in the wave and breadthing hole are perfectly aligned.

Swimming breast stroke, beginners will expend lots of effort bobbing up and down to breath. Better swimmers will swim faster just by having a flatter body position in the water: the legs kick more horizontally. But the legs are kicking in the wake of where the body has just been. If the legs are too low, there is the bobbing inefficiency. If the legs are too high, (kick too flat in the wake,) and heels will come out of the water and that's inefficient too.

From the crawl breathing and breaststroke kick examples, you can see that this feel of the water is the key to swimming fast; being one with the water that surrounds you.

So I think shaving for a race is a bad idea.

Swimmers are not going their fastest when they race. When they practice they will warm up with small paddles on their hands and then use bigger paddles to develop muscles when training seriously. Swimming without the paddles is slower.
So I don't see much point to only shaving legs for races, unless the idea is to avoid looking like a transvestite.

A Swiss friend of mine was expecting his countryman to get an olympic gold medal once. The Swiss swimmer shaved his head bald for the olympics in an attempt to be more aquadynamic. I think he came third. That doesn't prove anything!

 

[bitstar]

let's think about it. there are a lot of backgrounds to do that, also that psychological effect crysta. but in fact there is an improvement at least aerodynamical if u shave ur legs. hairs generate turbulence and turbulence produces resistance. but also with shaved legs u have an instationary flow displacement.
so for example they position the straight seam of ice-skating suits to supply a controlled flow displacement.
SUM: resistance(legs with hair) > resistance(legs without)

to do it only before a big meet is like a train effect. similar to running with weights and during a contest without. and i cant imagine that u can't change your feelings for the ambience fast. i've tested it. psychological maybe but it isn't so much important for the basic problem.
and the other thing is maybe the inflammation of skin so that a lot of athletics do it only before a big meet.

bitstar

 

[Surestick]

"cyclists often do the same, for what ou'd imagine is very little difference (but then what's the difference between 1st and 2nd? ... not much time ... so a small advantage might be significant."

Cyclists shave their legs primarily for reasons other than aerodynamics.
-Easier to clean in case of a crash
-Easier/more comfortable to get a massage after a race
-Makes muscles look more defined which provides a psychological advantage.
-All the other cyclists do it.

For races where aerodynamics are considered more important, generally ones where drafting is used less, (time trials & velodrome) you see skin suits being used.

 

[bitstar]

you're right surestick,
these are the most important reasons for cyclists. you see there are many different ways to discuss that issue.there isn't only one way to solute this topic...
yes skin suits...but sometimes the cheaper version or considering the heat developement or comfort is to shave the body...
it is individual, phsycological, medical, aerodynamical...