Skirt the eddy; hit the meander

Reading about the Marion-Bermuda Race this year brought back memories of another Marion race in which I participated.

We took off from Buzzard’s Bay expecting five days of pushing the boat, and we knew there were three distinct navigational challenges that could help us win or, if mishandled, possibly have the opposite effect. There was a warm eddy north of the Gulf Stream, a sharply inclined southbound meander at the main body of the stream and a cold eddy spinning counterclockwise on the other side. All three features were within reasonable distance of the rhumb line. The trick for the navigator (me) was to maneuver towards or around these features using only dead reckoning and celestial navigation.

We were just a couple of days out on the evening of June 21st, hard on the wind on a starboard tack, heading south or a bit west of south, against a west-southwesterly breeze. Our strategy was to pass to the west (upwind) of the wrong side of the clockwise-spinning warm eddy and then bear away toward a designated point (37° 20′ N, 70 26′ W) at the top of the meander just where it turned southeast. We would ride the meander for 50 to 100 miles and then steer a course for the favorable side of the cold eddy.

By dinner time our DR position showed us south of the warm eddy. I didn’t request a course change for the meander target until I had a good star fix after dinner.

A. Using 1999 almanac data, what would have been the starting time of evening civil twilight at our DR position of 38°30′ N, 70°30′ W? The sky was clear for sunset so I knew we would get stars. I shot three lines of three different stars, the best ones of which are presented here. Times were in GMT, there was no index error, and we used a height of eye of 10 feet.

B. Determine the fix position for the following star sights taken from the same DR position?

Star Time HsArcturus 0015 68°28’Spica 0017 40°20’Vega 0018 32°44′

By the time I had worked out my sights it was well into the evening watch. As requested, I woke up the skipper so we could discuss the position, its probable accuracy, and plans for the next leg. We decided to bear off a bit and head south toward the target position at the top of the elbow.

C. Given a variation of 15° W, and assuming no deviation, what was the course to steer on our magnetic compass from the fix position to the target point?

D. At a speed of 7.5 knots would we get there before or after sunrise?

We were rocketing south on a close reach with easy steering conditions and increasingly warmer temperatures. Finally water temperature topped out at 81° That’s when we knew we were there. We turned farther off the wind to follow the meander, always probing with the thermometer, adjusting course to the right or left depending on water temperature, and I finally felt justified in collapsing in my bunk. At dawn there were no other boats in sight. It was a good feeling.