I got 112 miles north.
Took me a about 12 minutes to get thoughts straight, and 2 mins of calculations afterwards.
x = distance boat travelled north.
y = distance from northest point back to ship's final place after 4 hours
ship travelled 128 miles, that is 64 vertical and 64*root3 horizontal.
so, we get a rt angled triangle.
(x + 64)^2 + 64^2*3 = y^2
and x + y is distance boat can travel, which is 320 (80 * 4)
solving, we get x = 112.
If you can't change the people, change the people.