The following is from a comment I made in response to a thread that was discussing the effect of dynamic pressure (pressures generated by movement) in water on watches. I may have gotten a little spicy in my remarks.

The people here who talk about additional pressure due to movement under water (i.e. dynamic pressure) don't know what they're talking about, and the people who claim that 50m WR is not enough for pool activities based on very general guidelines and "recommendations" probably have no personal experience that demonstrates otherwise.

In real-world applications dynamic pressure is negligible under most conditions with regards to human aquatic activity, and I will prove it here.

See this article, particularly the section with the example (about 2/3 of the way down the page). Using the equation (velocity squared) = (2 x dynamic pressure)/density, note that in order to obtain a dynamic pressure in water that is just under 0.1 atm (10 kPa or 10000 Pa, i.e. an additional 1m WR requirement), water flow has to be nearly 4.5 m/s (which comes out to almost 10 mph), which is absurdly fast and humanly impossible even at surface-level swimming. This is assuming that arm speed in water is equal to overall swim speed (but arm speed in water is actually likely to be less than overall swim speed because the legs are also providing propulsion), which may not be the case for other aquatic activities. This also suggests that at surface-level swimming i.e. zero hydrostatic pressure (note that total pressure, which is what is indicated by WR ratings, is static + dynamic pressure), just 1m WR is required to withstand the dynamic pressures generated by the superhuman swimming speed of 10 mph.

Doing the math with regards to my own lap swimming, the only time I do anything at serious speed is my 100m freestyle sprint in the middle of my swim, which at my fastest takes me around 1'15" (or 1.25 minutes) in trunks. This comes out to roughly 80m/minute (or 1.33 m/s), which converts to just under 3 mph, which is half that of the fastest unassisted human swimmers. Plugging this into the dynamic pressure equation (to solve for dynamic pressure, not velocity), we use 1.33 m/s for v (or 8/6 for more precision) and get a dynamic pressure of 888.89 pascals (Pa). Note that 1 atm = 101325 Pa, so the dynamic pressure I generate in my 100m freestyle sprint is roughly 0.0088 atm. With regards to a WR rating, my sprinting speed has added 0.088m to WR requirements.

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