special trade passenger (STP) ships Cross-channel, coastal and harbour ferries Luxury and cruising yachts Sail training and multi-masted ships Recreational boats and craft – rowed, masted and motorised craft Special-purpose vessels – weather and research vessels, deep sea survey vessels, and icebreakers. Submersibles – industrial exploration, scientific research, tourist and hydrographic survey. Warships and other surface combatants – aircraft carriers, destroyers, frigates, corvettes, minesweepers, etc. Some of these are discussed in the following sections. Inland vessels Freshwater shipping may occur on lakes, rivers and canals. Ships designed for those body of waters may be specially adapted to the widths and depths of specific waterways. Examples of freshwater waterways that are navigable in part by large vessels include the Danube, Mississippi, Rhine, Yangtze and Amazon Rivers, and the Great Lakes. Great Lakes Lake freighters, also called lakers, are cargo vessels that ply the Great Lakes. The most well-known is , the latest major vessel to be wrecked on the Lakes. These vessels are traditionally called boats, not ships. Visiting ocean-going vessels are called "salties". Because of their additional beam, very large salties are never seen inland of the Saint Lawrence Seaway. Because the smallest of the Soo Locks is larger than any Seaway lock, salties that can pass through the Seaway may travel anywhere in the Great Lakes. Because of their deeper draft, salties may accept partial loads on the Great Lakes, "topping off" when they have exited the Seaway. Similarly, the largest lakers are confined to the Upper Lakes (Superior, Michigan, Huron, Erie) because they are too large to use the Seaway locks, beginning at the Welland Canal that bypasses the Niagara River. Since the freshwater lakes are less corrosive to ships than the salt water of the oceans, lakers tend to last much longer than ocean freighters. Lakers older than 50 years are not unusual, and as of 2005, all were over 20 years of age. , built in 1906 as William P Snyder, was the oldest laker still working on the Lakes until its conversion into a barge starting in 2013. Similarly, E.M. Ford, built in 1898 as Presque Isle, was sailing the lakes 98 years later in 1996. As of 2007 E.M. Ford was still afloat as a stationary transfer vessel at a riverside cement silo in Saginaw, Michigan. Merchant ship Merchant ships are ships used for commercial purposes and can be divided into four broad categories: