The Gulf Stream (wiki):
Ben Franklin's map of the Gulf Stream drawn up
The Gulf Stream, together with its northern extension the North Atlantic Drift, is a warm and swift Atlantic ocean current that originates in the Gulf of Mexico and stretches to the tip of Florida, and follows the eastern coastlines of the United States and Newfoundland before crossing the Atlantic Ocean. The process of western intensification causes the Gulf Stream to be a northward accelerating current off the east coast of North America. At about 40°0′N 30°0′W, it splits in two, with the northern stream, the North Atlantic Drift, crossing to Northern Europe and the southern stream, the Canary Current, recirculating off West Africa. . . .
The Gulf Stream is typically 100 kilometres (62 mi) wide and 800 metres (2,600 ft) to 1,200 metres (3,900 ft) deep. The current velocity is fastest near the surface, with the maximum speed typically about 2.5 metres per second (5.6 mph)
The Charleston Bump: The gulf stream turns sharply eastward and around a submerged mountain of sorts almost directly east of Charleston.
Sailing Against The Wind: (Live Science) (also see MIT, )
HOW DO SAILORS MAXIMIZE BOAT EFFICIENCY?
The angle between the boat course and the apparent wind direction, b, is the boat's ANGLE OF ATTACK.
b = ea + eh.
The angle between the sail CHORD LINE and the wind direction, a is the sail's ANGLE OF ATTACK. If the sail points straight into the wind, there will be no airfoil shape, and no lift. The sail must be slightly angled The largest speeds are obtained while sailing as close to the wind as possible, while the sail chord is approximately co-linear with the boat's centerline. The sailor must turn the boat to follow the course, but alters the sail position (lets the sail out) to maintain the sail's optimum angle of attack.
The sailor may also change the sail's shape for changing wind speeds.
A thick airfoil generates more lift, but also more drag. If you subscribe to Bernoulli's theory, the increases are due to the higher velocity and lower pressure. If you prefer Euler, the lower pressure is due to the smaller radius of curvature . For the same reasons, a thin airfoil generates less drag, but also less lift.
The sail is "kept tight" in the shape of the thin airfoil at moderate to high wind velocities. Large lift is coupled with large heeling and the boat may tip over. When the wind speed is low, the sail is "let out" a bit to generate more lift, and thus more driving force. However, if the sail is let out too much, it will luff and force the boat away from the wind.
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On a sailboat, wind blowing against the boat at an angle inflates the sail, and it forms a similar foil shape, creating a difference in pressure that pushes the sail perpendicular to the wind direction.
It seems intuitive that sailboats, powered only by the wind, can travel easily with the wind at their backs, but it may seem impossible that they turn around and come home again, with the wind blowing straight against them.
But this reverse movement is possible because a moving boat's sail is shaped as an airfoil like the wing of a plane. When air moves over a plane's wing, from front to back, wind flowing over the top of the wing has to travel farther than wind flowing under the wing's bottom surface. This creates a pressure difference that lifts the plane.
According to "The Physics of Sailing Explained" (Sheridan House Inc, 2003), by Kent State University physics professor Bryon D. Anderson, this force from the sail's foil shape is combined with and balanced by other forces, including those of the boat's keel (the long thin piece that juts down from the bottom of the boat).
Together, the forces of drag, from the water, and the pressure from the wind against the sail itself push the craft forward. It moves at an angle opposite the direction of the wind, called windward in sailing terminology.
According to the American Institute of Physics' Physics Today magazine, the keel is especially important because without its balancing action, a boat would simply drift downwind.
Windward sailing also does not work if a boat is pointed directly opposite the wind direction, according to The Physics of Sailing. Wind has to be moving against the boat at an angle of at least 40 degrees for most vessels. Angling too sharply into the wind causes the forces on the boat to become unbalanced, and moves the boat sideways in the water.
A sailor intending to travel windward toward a point exactly in line with the direction of the wind will have to zig zag back and forth to reach its target. Using this "tacking" technique, and traveling at an angle as close to the wind's direction as possible, sailors can reach a point in any direction, regardless of the direction of wind.
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