A ketch is a common rig for cruising sailboats. It has two masts: a traditional mainmast as on a sloop, plus a smaller mast in the rear of the boat, called the mizzenmast. Technically, the mizzenmast must be mounted forward of the boat’s rudderpost to be a ketch; if the mizzen is mounted further aft, behind the rudderpost, it is considered a yawl. The mizzenmast is typically smaller on a yawl than on a ketch, but otherwise these rigs are similar.
A ketch therefore uses three primary sails: the mainsail and head sail, as on a sloop, plus the mizzen sail aft. A ketch may also use a spinnaker.
The fact of three sails does not necessarily mean that the sail area is larger than on a sloop of the same size, however. Sail area is usually planned by boat designers based on the boat's size, displacement (weight), and hull shape and configuration - not the number of masts or sails. This means that the mainsail and head sail of a ketch are generally smaller than on a sloop, but the mizzen sail roughly makes up the difference.
A boat's overall sail area is planned based on typical wind speeds in the range of 10-15 knots. If a ketch had larger sails than an equivalent sloop, it would heel more (making sailing less efficient) and be more likely to capsize.
So it's not the sail area that matters when deciding between a sloop and a ketch, but differences resulting from having that area spread over three rather than two sails. When the wind is stronger, for example, and all three sails are not needed, any combination of two can be used to different effects.
Continue for the benefits and disadvantages of sloops vs ketches.