Suppose the ship arrives at point A at 9 am, the lighthouse at point C, and 1 1 arrives at point B in the morning.

According to the meaning of the question, the triangle ABC is an isosceles right triangle.

2AB^2=68^2

AB=34√2

So the speed of this ship is 34 √ 2/(11-9) =17 √ 2 (nautical miles per hour).