A circular cylindrical buoy floats in sea water (relative density 1.026) with its axis vertical. The buoy is 3m in diameter, 3m high and has a mass of 3000kg. The centre of gravity of the buoy is on its central axis 1m from the bottom face. If a mass of 500kg is placed centrally on the top face of the buoy, find the maximum height of its centre of gravity above the bottom of the buoy so that the buoy may remain in stable equilibrium.