The Bay was a comfortable 67 degrees. The Coast Guard, kayakers and power boaters were standing by to pluck any luckless swimmers out of the water.

The crowd of more than 500 anxious swimmers who had gathered at Maryland’s Sandy Point State Park last June 8 grew suddenly quiet as Derek Orner stepped up to the microphone to give them the most important information for their 4.4-mile trip across the Bay.

It was the latest word about the tides. “If we get you in here at Sandy Point at slack tide, you’ll be pretty much swimming with that slack tide all the way across the Bay,” said Orner, a biologist with the National Oceanic and Atmospheric Administration’s Chesapeake Bay Office.

A huge cheer erupted from the crowd.

In the earliest years of the Great Chesapeake Bay Swim — a fund-raiser for the March of Dimes — it was confounded by a force beyond anyone’s control: tides and currents.

In 1991, it turned to near disaster. Instead of entering the Bay at slack tide, the 884 swimmers hit the water when currents were near their peak. Unable to fight the ebbing tide, hundreds of swimmers were swept downstream.

Only 164 made it across; the Coast Guard and boaters pulled most of the swimmers out of the water. One person was rescued three miles down the Bay. Fortunately, no one drowned.

The following year, things weren’t much better: Only 48 of 331 entrants completed the swim.

After reading about those events in the newspaper, Bess Gillelan, then the head of NOAA’s Bay Office, contacted swim organizers to let them know that NOAA was in the business of predicting tides, and offered to lend a hand.

That has resulted in a productive partnership in which NOAA helps set the time of the swim. This year’s event, for example, will start sometime between 7 and 7:30 a.m. on June 13.

“What it takes to make it safe to swim across the Bay is complex,” Gillelan said. “This is because of the basic physics of the Bay.”

Tides are the result of gravitational force, mainly the moon tugging against the oceans, creating a bulge of water which it pulls along as it orbits the Earth. That bulge is the high tide. Its strength varies based on different factors; when the moon is between the Earth and the Sun, the gravitational pull of the moon and sun reinforce each other, creating stronger tides. Conversely, when the moon and sun are on opposite sides of the Earth, their gravitational pulls counteract each other, making the tides less severe.

In the Bay, though, it gets much more complex.

While the primary movement of tidal waters stems from the Bay’s mouth, a smaller tidal source comes from the Chesapeake and Delaware Canal, so the Bay gets tidal influences from both its north and south ends.

Complicating the picture further is the length of the Bay: It’s just long enough to experience one full “wave length.” When one high tide is reaching Havre de Grace in the upper Chesapeake, the next high tide is just beginning to enter near the Chesapeake Bay Bridge-Tunnel. At the same moment, there is a low-tide in the middle of the Bay, near the mouth of the Potomac.

As the tidal bulge moves up and down the Bay, it is also influenced by the width of the Chesapeake, the shape of the coastline, and the configuration of the bottom. Meteorological influences, such as wind, can also affect tides.

These fluctuations are of intense interest to shippers. Heavily loaded freighters often navigate the shallow Chesapeake with little ground clearance. As a result, NOAA provides “real time” tide monitoring information from monitoring stations in several parts of the Bay.

The closest of those is several miles from the Bay Bridge. So for several years, NOAA Chesapeake Bay Office personnel, with support form the University of Maryland, the Maryland Department of Natural Resources’ research vessel Discovery, and the NOAA National Ocean Service, worked to fine-tune the local tide prediction by towing an Acoustic Doppler Current Profiler — which bounces sound off the bottom to help measure water movement — through the area the day before the swimming event.

They discovered that because the depth of the Bay varies from its west side to east side at the Bay Bridge, the effect of a tide is felt on the west side before it is felt on the east side.

As a result, if the race is timed to start during a short period of “slack” water between the high and low tides, swimmers can follow that slack water across the Bay.

And that is why this year, swim organizers used NOAA’s advice and decided to start the swim at the crack of dawn.

The measure of success can be seen in last year’s numbers: 536 of the 549 swimmers who entered the water, finished.