If you send a straight line from the end of the boat to the mooring you've completed a right triangle. One of the trig relationships we have is that the sine of an angle is equal to the ratio of the length of the side opposite the angle to the hypotenuse of the triangle. That is, . For your problem the opposite length will be the distance from the end of the boat to the mooring while the length of the boat is the hypotenuse. This works out to