Swimming around Manhattan, tides are (almost) everything

Just over a week ‘til the Manhattan Island Marathon Swim! Hard to believe it’s already upon us.

Sometimes people ask me if I have a “goal time” for the swim. That’s an interesting question. As anyone who’s spent much time in open water knows, the relationship between time and distance is somewhat complicated; even moreso for marathon swims.

MIMS is a different beast, though. I’d go so far as to say that MIMS times are pretty much meaningless - as an indication of speed. The typical winning time of 7 hours, 30 minutes works out to just under 59 seconds per 100m. So: world record 1500m pace, 28.5 times in a row. In MIMS, the tides are king - perhaps moreso than any other major marathon swim.

How important are the tides? Think of it this way. My ultra-marathon pace is about 2.3 knots. A world-class marathon swimmer? About 2.6-2.7 knots. The slowest swimmer in the MIMS field? Maybe 1.6 knots. Why am I describing swim speeds in terms of knots? Because that’s how river currents are measured. Now, guess what sort of current we will encounter when we round the southern tip of Manhattan and enter the East River? 3.1 knots. The river will be moving faster than we will be swimming.

It won’t always be 3.1 knots - that’s just the max flood tide at the Brooklyn Bridge, at 9:52am. But you get my point: MIMS involves navigating three separate rivers at different points in the tide cycle. At the start (from South Cove until we hit the East River), we’ll actually be swimming into a slight head current. The Harlem will max at about 1.4-1.9 knots. In the Hudson, an ebb tide of about 2 knots (max) will carry us home to the finish.

How important are the tides? You could take the best marathon swimmers in the world and, if you dropped them in one of the rivers at a particularly disadvantageous point in the tide cycle, quite possibly they would be unable to finish the swim.

And that’s not all! The currents don’t affect everyone equally - it depends on where you are in the river (i.e., how fast or slowly you’ve swum up to that point). For example, if you’re a fast swimmer, you may arrive in the Harlem River before the tide change - so you’ll swim into a head current at first. Slower swimmers arriving after the tide change will get a push from the outset. This has the effect of compressing the field (the trailing swimmers end up closer to the leading swimmers than they would in a slack current).

Same thing in the Hudson: the ebb tide doesn’t begin until 2:30pm - possibly after the leading swimmers arrive. And the tide doesn’t max out until just before 6pm - well after the leading swimmers have finished. In other words, the trailing swimmers get a better push coming down the Hudson, making their finishing times seem “faster” relative to the winner.

And that’s not all! Sometimes the tide change arrives early - or not at all. Sometimes (especially in the Hudson), a strong south wind will blow against the ebb tide and whip up a nasty chop - in effect, neutralizing the current assist.

The point is: Who the heck knows what MIMS times mean? The only thing that matters is the order of finish. All this is a round-about way of saying: I don’t have a goal time. The rivers will do what they will do; my job is to keep swimming until I finish.

There’s a saying among English Channel swimmers (attributed to Alison Streeter, I think), that “even a paper cup will eventually wind up on French soil.” I don’t know if that’s literally true - but to the extent it is, it’s probably even more true of MIMS. You don’t have to be a fast swimmer to finish MIMS. The tides will carry you. The important thing is to keep swimming until you get there.

So, if you ever hear anyone talk about their MIMS “time” as if it indicates anything about swimming speed - now you know to ask: “So how does that compare to a paper cup?”

Posted 09 June 2011 in: analysis Tags: MIMS