1) The channel is a semicircle in the cross section with the diameter of 13 m and the radius of 6.5 m. It is filled up to the upper level. 2) The boats are submerged 3 ft into the water. 3) The boat has / (the boats have) the width 2x.
Find the maximum possible x and then 2x (assuming that the boat is centered along the channel axis).
x == 5.77 ft. 2x = 2*5.77 = 11.53 ft.