Final answer:
To draw a vector diagram of the velocities of the boat, represent the boat's velocity with a vector pointing upwards and the wind's velocity with a vector pointing leftwards. Draw the two vectors starting from the same point.
Step-by-step explanation:
To draw a vector diagram of the velocities of the boat, we need to consider the boat's velocity and the wind's velocity as separate vectors. Since the boat sails due north at 20 knots, we can represent its velocity with a vector pointing upwards. The wind blowing the boat due west at 15 knots can be represented with a vector pointing leftwards. To draw the vector diagram, we simply draw the two vectors starting from the same point, with one pointing upwards and the other pointing leftwards.