A ketch is a type of sailing vessel with two masts. Originally, ketches were fishing vessels; the term "ketch" comes from the same root word as "catch". Their versatility and ease of use has made them popular, particularly in Northern European waters, where their sail plan allows them to react quickly to varying wind conditions.
The two masts of this type of craft are called the main and mizzen masts. The main mast is located toward the bow, and is taller than the mizzen mast. The mizzen mast in a ketch is located aft of the main mast. The exact placement of the mizzen mast is what defines a ketch. If the mizzen mast is toward the stern of the vessel, aft of the rudder post, the vessel is a yawl.
The most common rig for ketches is three or four fore-and-aft sails. The largest of these is the mainsail, which is set on a boom running aft from the main mast. The smaller mizzensail is set in the same way on the mizzen mast. One or more sails may be set on the stay running between the top of the main mast and the bow of the ship; these are called jibs. A ketch which is not rigged for jibs is called a cat ketch because of its similarity to another type of sailing vessel called a cat or catboat.
The characteristic sail plan of the ketch originates from its use as a fishing boat. Once the craft had reached its fishing ground, the mainsail could be furled and the mizzen set so that the head of the boat would come up into the wind. Since the swell of the sea comes from the direction of the wind, the boat could easily ride the waves without having to risk being taken broadside-on by a wave and capsized. This thus freed the crew to concentrate on fishing.
In addition to being used as fishing vessels, ketches saw naval service in the 18th century. Bomb-ketches were small vessels which carried heavy mortars used to bombard enemy shore positions. Because of the large weapons forward, the masts of these craft were located further aft than most vessels of the same type. The resulting balance and sail distribution problems made them clumsy and hard to handle, and the design was phased out, with bombardment being carried out by larger vessels. However, the term "bomb-ketch" stuck and was sometimes used to describe bombardment vessels, even vessels that were not ketches.