Boats, ships and offshore vessels are used to transport products, materials, and personnel on oceans, lakes, rivers, and canals. Some boat manufacturers and boat builders supply watercraft for leisure or recreational activities such as sailing, fishing, canoeing, kayaking, water skiing, yachting, and pleasure boating. Other boat companies produce offshore supply vessels, tugs and tugboats, barges, freighters, supply ships, and container vessels for commercial maritime applications. Shipyards and naval yards employ military and civilian personnel to build boats, ships and offshore vessels such as oilers and platform supply vessels. High-performance boats, cutters, cruisers, house boats, and ski boats are also available from suppliers of boats, ships and offshore vessels. Some boat manufacturers and boat builders specialize in the production of canoes and kayaks, day cruisers, deck and pontoon boats, fishing boats, inflatable watercraft and rafts, sailboats, yachts, or ski boats (bowriders). When selecting watercraft for leisure or recreational activities, specifications to consider include propulsion method, materials of construction, and storage space. Although canoes and kayaks are designed to be paddled or self-propelled, most fishing boats and rowboats are equipped with an inboard or outboard motor. Motors for boats, ships and offshore vessels are built specifically for maritime applications, and may be designed for freshwater or saltwater use. Tugboats, barges, and freighters are used in maritime trade and commercial applications include. Tugs or tugboats are designed to pull vessels that cannot or should not move under their own power, such as barges in a crowded canal. Barges are flat-bottomed boats built mainly for the canal or river transport of raw materials or manufactured goods. Although self-propelled barges are available, many vessels are unpowered. Freighters are cargo-carrying boats, ships and offshore vessels that are designed to traverse oceans, seas, canals, waterways, or large inland lakes. In North America, large lake freighters that carry iron ore are used to traverse the Great Lakes. Shipyards and navy yards are equipped with special facilities for military boats, ships, and offshore vessels (OSV). For example, most naval yards have one or more dry docks, enclosable basins that can be pumped dry so that construction, repair, or maintenance can be performed on the hulls of ocean-going ships. Platform supply vessels (PSV) are designed for the movement of military supplies, typically stored on deck in containers, to offshore installations. Other boats, ships and offshore vessels for military applications are also available.