Surface Currents and Swim Speed

evmoevmo San FranciscoAdmin
edited October 2012 in General Discussion
I'm writing a blog post about the effect of surface currents on open-water swim speeds.

The math is pretty basic with pure head or tail currents - you just add the velocity of the current (positive or negative) to the swimmer's speed, to get net forward progress.

More interestingly, currents affect swimmers of different speeds differently. A strong head current may neutralize most of a slow swimmer's forward progress, while hardly making a dent in a fast swimmer's progress. A strong tail current, contrastingly, tends to mask differences between fast and slow swimmers (e.g., MIMS).

But what about cross currents? How do we model this? Fast swimmers are still affected less than slow swimmers (example: the straight-line paths of Stoychev and Grimsey across the English Channel, compared to the typical S-curve), but by how much? In the English Channel, cross-currents reverse every six hours, thus neutralizing the net effect, but what about a persistent cross-current?

Any physicists or hydro-scientists out there?
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Comments

  • Maybe I am looking at this too simply, but the impact of the cross current on a swimmer can be broken down into two components, or vectors. There is the part that acts perpendicularly to the body and one that acts parallel for any given angle to the swimmers direction. You would need to know the current speed and angle to the direction of the swimmer. If velocity is V, then the parallel current is V*cos(angle) and the perpendicular current is V*sin(angle). Swimming into a head current is just a variation of this as the angle is 0. thus V*cos(0) = V*1=V.

    Am I misunderstanding your question?

    Regards,
    Jim
  • evmoevmo San FranciscoAdmin
    No, I think you understood me - that's the perfect, blog-friendly answer I was looking for. Thanks @Jim_Pape!

    I have heard anecdotes... I wouldn't even call them "data"... more like "feelings"... that crabbing across a cross-current is more/less difficult depending on how high a swimmer rides naturally in the water. I forget which is which. But I think it's less relevant to my current purposes.
  • loneswimmerloneswimmer IrelandCharter Mem​ber
    When trying explain Trent's and the English Channel currents for the non-swimmers who might read my articles about him, I just created this quick simplistic image. Don't know if this is what you mean.

    image

    loneswimmer.com

  • Hi @evmo, the anecdotes regarding the difficulty depending on how high you are in the water I'm having a little trouble understanding. Just thinking about it for a short bit, there are 3 main factors effecting the swimmers direction. The direction and speed of the swimmer, water and air. One could argue that the higher you are in the water, the more you are effected by the air speed and direction, but considering how much denser the water is, I would expect the water to be the dominant factor. On the other hand, the air speed (wind) does effect the surface conditions, which I would guess would be a thin layer of water close to the surface. So maybe there is something to it after all!
  • Jim_PapeJim_Pape Member
    edited October 2012
    @evmo I was interested in your statement regarding difficulty swimming in currents depending on how high you ride in the water. A swimmer in the water is swimming in the top 1-2 feet of the water surface. So while water currents can vary by depth, I assume that the narrow layer that a swimmer would be in would be rather uniform. Air currents can effect the surface speed of the water and I was curious as to the impact of this current on a swimmer. Of all places, I found a description of these currents at a fishing site... anglerworld.com. In this article they state:

    The speed of wind generated currents is about two percent of the speed of the forming wind. At fairly low wind velocities, the current may only reach 1.3 percent of the wind speed. Thus a 10 mile wind creates an almost imperceptible water movement. A strong 35 mile an hour wind still creates currents that are less than one mile per hour (mph). Current speed increases as wind speed increases up to a critical point where surface disturbance and the conversion of wind energy to wave motion starts to reduce flow. Researchers at Lake Mendota, for example, found winds over about 20 mph decreased current speed. The apparent effects of wind are generally more consistent in large lakes than in smaller ones.

    The full article is here: http://www.wmi.org/bassfish/articles/t178.htm

    The Lake Mendota article published by Shulman and Bryson of the University of Wisconsin called "THE VERTICAL VARIATION OF WIND-DRIVEN CURRENTS" where they talk about the frictional influence of wind on water currents. I did not really study the article, but glancing over it on table 2, they describe a 5-9.9 mph wind creating a water current of roughly 5cm/sec (~0.1mph) at a depth of 290cm (about a foot), so it would seem consistent and may well be where anglerworld's #'s came from.

    Full article here: wap.aslo.org/lo/toc/vol_6/issue_3/0347.pdf

    The final effect of wind on a swimmer would be the higher you are out of the water, the more of a sail effect you would have, and a stronger wind could effect your speed. Not sure how big of an impact this would have though. I would be interested in what others thought about this force.

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