Hull speed is the maximum speed a displacement hull can travel through the water. The mathematical formula is: find the waterline length of the hull; take the square root of that number and multiply if by 1.34. That’s the hull speed. A boat with a waterline length of 29 feet will have a hull speed of 7.22 knots. Multiply knots by 1.15 to get miles per hour. Speed of 7.22 knots becomes 8.3 miles an hour.

Hull Speed in Knots = 1.34 x √Waterline Length in feet.

Multiply the square root of the waterline length in feet by 1.54 to get the hull speed in miles per hour.

But what about when the boat is in water that’s moving? Add or subtract the speed of the water to the hull speed to get the speed along the track. The speed of the water, i.e., the speed of the current in which the boat is traveling, can add or subtract a significant amount of speed to a boat. If, for example the boat above with a hull speed of 8.3 mph is in a current going the same way of 10 miles and hour the speed of the boat over the earth will be 18.3 miles and hour. Unfortunately when the water is going in the opposite direction the boat will be moving backwards at 1.7 miles and hour over the earth. Even going full speed forward the boat will be moving backwards.

What about when the current is moving at an angle to the desired course? Trigonometry is needed to find the speed of the boat over the earth and it’s one of the problems faced by sailors at sea. A GPS can give an accurate measure of the speed over the earth but the GPS has no idea of the speed of the current or the speed of the boat through the water. It doesn’t need those speeds to show the speed over the earth. Current is the same as wind to the pilot in an aircraft. Pilots always take care to know the speed and direction of the wind but sailors can’t get the speed of the current except where some agency measures and publishes it but rest assured. The water is moving and it’s affecting everything near it.

What happens when the wind is blowing the boat off course. It must be turned into or away from the current and the speed over the earth will be lower. Trig will be needed to calculate the speed but a GPS device can provide the correct speed and heading without the math. Either way, the speed is different because of the wind.

Hampton Roads, Va to Lisbon, Portugal is 3,582 6 miles. Greta Thunberg spent 20 days, 480 hours travelling on a 48 foot yacht, a catamaran which calculates to 7.46 mph… Pretty slow when a Boing 767 travels at 610 mph. Greta took 480 hours when she could have done it in 5.87 hours on a 757. That’s 82 times slower on the Million Dollar 45 foot long catamaran La Vagabonde sailboat.

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