When do marathon swimmers slow down to fuel up mid-race?

(Photo: Patrick B. Kraemer, EPA)RIO DE JANEIRO — Marathon swimming is a two-hour race on the open water where there are all kinds of variables from waves to water temperatures. But one of the biggest tactical decisions for a swimmer is when to fuel up during the race.

If you watched the women’s 10-kilometer race on Monday, won by Sharon van Rouwendaal of the Netherlands, you might have noticed little boats surrounding the pack with cups on the end of sticks extended out for the swimmers to grab. Inside those cups are a variety of calories and electrolytes and whatever else is helpful to get through a brutal race.

Think of it like a pit stop in auto racing: A swimmer needs to decide…