A submarine moves horizontally in sea & has its axis 15m below the surface of water. A pitot tube properly placed infront of submarine & along its axis is connected to the two limbs of a U tube containing mercury. The difference of mercury level in two limbs is 170mm. If sea water is 1.26 times as dense as compared to normal water & take specific gravity of mercury 13.6. The speed of submarine in (m/sec) is