Surface Currents and Swim Speed
evmo
SydneyAdmin
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?
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
Am I misunderstanding your question?
Regards,
Jim
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.
loneswimmer.com
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.