BC = 60 nautical miles,

In addition, the meaning of this question shows that:

AB = 36 nautical miles

AC = 48 nautical miles

From the above data, we can see that:

AB :AC :BC = 3 :4 :5

That is, the triangle ABC is a right triangle, where the angle cab = 90,

Because the course of ship A is 50 degrees east-north,

So the course of B ship after departure is 40 south by east.