Multihulls are typically either catamarans, which have two similar hulls, or trimarans, which have a larger hull in the center and two smaller ones on either side. In either case they are typically much wider than the equivalent monohull, which allows multihull sailboats to carry no ballast, so they are typically faster than monohulls under equivalent conditions (see Nathaniel Herreshoff's "Amarylis", also 1988 America's Cup). It also means that multihulls are less prone to sink than monohulls when their hulls are compromised. There are also multihull powerboats, both for racing and transportation.
Multihulls' width is often an issue, especially when docking. They are also more expensive to produce than a monohull of the same length.
Multihulls are substantially faster than monohulls, because the absence of ballast reduces their weight considerably without reducing the amount of sail that they can carry, and because the water-line hull-length to width ratio is so large.
Unfortunately, it is common wisdom (among monohull sailors, at least) that in the open ocean, multihull craft are unsafe. If a storm or wave capsizes a small monohull, it may recover, if it does not broach and sink. The rigging will probably be severely damaged, but the crew will be able to jury-rig and reach a port. Multihulls can capsize but they rarely sink. Even most rescued crews (in races) have reported that they were unable to dismount the deck-mounted liferaft or emergency radio from the mass of broken, submerged rigging under the capsized craft.
Proponents argue, with some justice, that no careful captain ever finds himself in capsizing conditions. Most crews that have capsized in the open ocean found it an extremely traumatic event no matter what type of boat they sailed.
Proponents also argue that capsize is only one of many types of catastrophe that can befall yachts at sea. In other types (for example hull compromise by hitting submerged debris) multihulls are substantially safer than monohulls because they don't carry ballast and can therefore float even when severely damaged.
Multihulls are quite popular for racing, especially in Europe and Australia, and are somewhat popular for cruising in the Caribbean. They're not seen very often in the United States, although they're gradually getting more popular. Until the 1980s most multihull sailboats (except for beach cats) were built either by their owners or by boatbuilders on a semi-custom basis. Since then several companies have been successful selling mass-produced (by boat industry standards) boats.
Brand names include Hobie (small catamarans), Corsair Marine (mid-sized trimarans), and Privilege (large, luxurious catamarans).
see also sailing.