In a naval exercise, two ships travel towards each other on a straight line beginning 90 nautical miles apart. One ship travels at 40 knots and the other travels at 50 knots. The exercise consists of a boat travelling to and fro between the ships transporting men and materials according to a set clockwork precise plan. The boat starts its voyage having been launched from the faster ship, and it sails at a speed of 70 knots. As soon as it reaches the destination ship, all three vessels come to a stop immediately. When the boat has done its job, all 3 vessels again start moving at the same speed. The boat leaves the slower ship, turns around and sails back at the same speed. When it reaches the faster ship again, the process is repeated and so on and so forth till the time the two ships meet each other. The boat consumes 0.5 barrels of fuel per nautical mile travelled either way. How many barrels of fuel are consumed by the boat?