The ship's average speed on the trip to Vancouver was 18 km/h (kilometers per hour). This can be calculated by dividing the total distance traveled (which is the same for both trips) by the total time taken for both trips. The total distance traveled is 24 km, and the total time taken for both trips is 15 hours. Therefore, the average speed for the trip to Vancouver is 24 km / 15 hours = 18 km/h.