A long piece of timber, flat at one end, and round or square at the other, used to propel a vessel through the water. The flat part, which is dipped in the water, is called the blade; and that which is within the board is termed the loom, whose extremity, being small enough to be grasped by the rowers, is called the handle. To push the vessel forwards by this instrument, the.rowers turn their backs forwards, and dipping the blade of the oar in the water, pull the handle forward, so that the blade at the same time may move aft in the water. But since the blade cannot be so moved without striking the water, this impulsion is the same as if the water were to strike the blade from the stern towards the head; the vessel is therefore necessarily moved according to the direction. Hence it follows that she will advance with the greater rapidity, by as much as the oar strikes the water more forcibly; consequently, an oar acts upon the side of a boat or vessel like a lever of the second class, whose fulcrum is the station upon which the oar rests on the boat's gunwale.

In large vessels this station is usually called the row-port; but in lighters and boats, the row-lock. Oars for ships are generally cut out of fir-timber; those for barges, out of Dantzic or New England rafters; and those for boats, either out of English ash or Norway fir rafters. See Boat.