That is a smart question, ships don't sail by miles, they sail by knots , one knot is 6076 foot , and each mile is 5280 Foot, so we work it out like this : .
General speed law is : Speed = Speed * Time
X = 180 / 18 = 10 Miles / Hour as general speed
And converting the numbers, we find that :
(10 Miles/Hour * 5280 Ft) / 6076 = 8.68 Knot - ship's speed in the return trip
Hope this answers clears your question;
Have a good day;