It is not very clear here if the chords form 60 degrees angle with each other or with a central line. Lets try both cases:
Let the angle made by chords with the central line is 'd' degrees.
Total puling (or forward) force on the ship = 2 x 8000 cos d
And, force = mass x acceleration
Thus, acceleration = Total force/mass
Case 1: chords make 60 degree angle with each other. Here, d = 30 degrees
Acceleration = (2 x 8000 cos (30))/6500 m/s^2 = 2.13 m/s^2
Case 2: Chords make 60 degree angle with the central line, here d= 60 degrees
Acceleration = (2 x 8000 cos (60))/6500 = 1.23 m/s^2.
Kindly note, if nothing else is give in question, we can assume that chords make an angle of 60 degrees with each other and case 1 is applicable.
Hope this helps.