Information Wants to be Free: Sandettie Lightship and the English Channel

Information Wants to be Free: Sandettie Lightship and the English Channel

(tl; dr — 10 years of English Channel weather data, in a single CSV file. And some fun charts.)

Weather can turn on a dime in the English Channel, and the dreams (and finances) of English Channel swimmers often turn on the weather.

sandettie lightship
Location of Sandettie Lightship in the English Channel

The most important source of information about that weather is a 156-foot lightvessel called Sandettie, which serves as both a floating lighthouse and a weather station. Here’s a nice photo.

Sandettie collects a variety of important meteorological data – air and sea temperatures, wind speed and direction, wave height and period, humidity, and barometric pressure. These data are then fed back to the UK Met Office, who publish the most recent 24 hours’ of observations on their website.

Anything before the last 24 hours are what the Met Office call “chargeable data” — at the rate of £6800 per 10 years, per two elements (e.g., air temp & sea temp). According to the today’s exchange rate, that converts to no less than $11,575 USD.

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Most popular threads in the history of the Marathon Swimmers Forum

Most popular threads in the history of the Marathon Swimmers Forum

Since launching in March 2012, the Marathon Swimmers Forum has grown to 935 verified members who have contributed 13,195 comments in 796 discussion threads.

Of those 796 threads, the most popular by far (as measured by page-views) was the discussion of Diana Nyad’s purported swim from Cuba to Florida in September 2013.

The top 25 threads in the history of the Forum are as follows. Relative page views for each thread are shown (indexed to the Nyad thread = 100).

  1. 110 miles, 53 hours: Questions for Diana Nyad [100]
  2. Best Goggles for Open Water Swimming and Triathlon [34.6]
  3. 10km training schedule – 3 months out [11.1]
  4. Waterproof MP3 players for swimming [9.4]
  5. Diana Nyad’s Directional “Streamer” [7.9]
  6. USMS OW SANCTIONING [7.6]
  7. You might be an open water swimmer if … [6.1]
  8. Recommended Products [5.8]
  9. The Animal Set Thread [5.8]
  10. Swim Videos [5.2]
  11. training for a 5km swim [5.2]
  12. Manhattan Island Marathon Swim 2013 [5]
  13. What is an appropriate weekly mileage for completing marathon swims? [4.6]
  14. Sunscreen?! [4.5]
  15. Warming up after a cold water swim [4.3]
  16. Driven [4.2]
  17. Open Water Blogs [4.2]
  18. Diana Nyad’s epic swim [4]
  19. What type of Garmin is recommended for tracking distance and time of open water swims?


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A better SWOLF formula

A better SWOLF formula

SWOLF (“swim golf”) is a drill that measures swimming efficiency. A SWOLF score is your time (in seconds) on one lap of the pool, added to the number of strokes you took. Lower scores = Higher efficiency. SWOLF is a fuzzy, indirect measure of efficiency, because stroke count doesn’t necessarily reflect effort. In my view, the most precise definition of SWOLF is that it identifies the most efficient stroke count for a given level of effort.

I originally wrote about SWOLF in April 2012, and the post has become – by a wide margin – the most widely-read in the history of this blog. In a subsequent post a month later – “Stroke Count Games” – I described how SWOLF doesn’t quite capture the most efficient stroke count. At least for me, using stroke cycles (number of strokes divided by two) produces better results.

I wondered if this was true for other swimmers, so I asked any interested readers to send me their own data, using a test set of 8×100. Three readers sent me their results.…

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Marathon Swimming Rules Survey: Results and Analysis

Marathon Swimming Rules Survey: Results and Analysis

Marathon swimmers talk a lot about rules – what should and shouldn’t be allowed during a swim – but as far as I know, there has never been any systematic study of what marathon swimmers actually think, as a matter of public opinion.

Perhaps most would agree that goggles are OK, and fins are verboten… but what about swim streamers and stinger suits? Or drafting off the escort boat? If you only read blogs and forums, you might assume the most vocal opinions represent the majority. But do they really?

Earlier this month the SBCSA launched a survey to find out. Over 25 days, we received 175 responses from around the world.


First, a Summary of Findings (TL/DR). Click any of the following links to skip directly to the relevant section.

I. We received responses from a representative sample of marathon swimmers – current, former, and aspiring.

II(a). Marathon swimmers agree on basic channel-rules attire: traditional porous textile swimsuit (including jammers), goggles, one latex or silicone cap, ear plugs, and nose clips.…

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Google search trends and open water swimming

Google search trends and open water swimming

Google has a fun tool that lets you visualize trends in search queries submitted by its users. Google is often the first place people go to find out more about a given topic, so it’s a powerful measure of the public’s “interest” in that topic. Below are a few Google Trends graphs related to open water swimming.

Is open water swimming “growing”? 

search term: “open water swimming”

Some observations:

  • Interest in open water swimming is highly cyclical, with summer peaks and winter troughs. (Obviously.)
  • Two big “spikes” corresponding to the Olympic Games in 2008 and 2012.
  • Aside from the seasonal cycles and Olympic spikes, the peaks and troughs do seem to rising slightly over time.

What about two sub-genres of open water swimming: marathon swimming and triathlon swimming?
search terms: “marathon swim” (blue) vs. “triathlon swim”

As expected, triathlon swimming is consistently bigger than marathon swimming. One exception: the surge of interest associated with the London Olympic 10K marathon swim.


What about the Triple Crown events: English Channel, Catalina Channel, and the Manhattan Island Marathon Swim?…

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What is the speed advantage of a wetsuit?

What is the speed advantage of a wetsuit?

Everyone knows wetsuits help keep you warm in cold water. Lesser known among the general public (but well-known among triathletes) is that wetsuits also make you swim faster! The buoyant neoprene in a wetsuit floats a swimmer higher in the water, decreasing drag and thus increasing swim speed.

But how much faster is a wetsuit? I’ve heard various rules of thumb: 10% speed increase; 4-6 seconds per 100m; 1 minute per kilometer. I’ve also heard various caveats: it depends on the swimmer’s skill (better swimmers benefit less); it depends on the swimmer’s body-type (naturally floaty people benefit less); it depends on the quality of the wetsuit (you get what you pay for); it depends on the fit of the wetsuit; and so forth.

So the answer is: It depends. Because I’m usually disinclined to let things go at “it depends,” I decided to conduct a field experiment. Reef & Run, which I’ve written about previously, provided the perfect laboratory. Almost every Thursday evening between June 21 and yesterday, August 23, I swam one mile in the ocean at East Beach in Santa Barbara.…

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Stroke count games

Stroke count games

What’s the fewest number of strokes you can take for a single length of the pool? (No streamlining past the flags; no more than three kicks per stroke.)

I can get down to 8 strokes per 25 SCY, but it’s tough to sustain for more than one length. 9 strokes per length (SPL) I can do pretty much indefinitely – but it’s incredibly inefficient. The inefficiency is readily apparent: a huge dead spot in my momentum as I glide (glide, glide…) after each stroke. The Swim Smooth guys have a term for this: Overgliding.

I swim most efficiently between 13 and 15 SPL, depending on pace. 13 for channel/marathon pace; 14 for “threshold” pace (from the mile up to about 5K); 15 for 200/500 pace. For an all-out sprint, I’ll add one more stroke (16 SPL).

Experienced pool swimmers have an intuitive feel for this… but what if you don’t? Is there a formula to identify the most efficient stroke count for a given pace? This question led me to try the following set:

8×100, as fast as possible, with about a minute rest between each.…

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