7₁ knot

A knot is a way of measuring how fast something is moving through the water. 7₁ knot is when something is moving through the water at a speed of 7 knots and 1/10th of a knot. It's like instead of saying "I'm running at 10 miles per hour," you're saying "I'm running at 10 miles and 1/10th of a mile per hour." So, 7₁ knot is just a very specific way of measuring speed.