Multihulls: catamarans and trimarans.

Catamarans and trimarans

Wikipedia defines a catamaran as a boat with two parallel hulls of equal size, and a trimaran as a central hull with two outrigger hulls. Both are multihulls, as opposed to the single hull of a dinghy or keelboat.

Stability without ballast

A catamaran gets its stability from the wide spacing of its hulls rather than from ballast, according to Wikipedia. The same source describes multihulls as lighter and faster than monohulls, but harder to tack and harder to right after a capsize.

The tacking point shows up in training material too. The US Coast Guard Auxiliary notes that catamarans, like heavy boats, are more prone to getting stuck in irons (stopped head to wind), and that tacking in light air is harder. See tacking and gybing.

The trade-offs

UKSA, a UK sailing school, describes catamarans as stable and roomy, good for families or groups, and fast. Wikipedia lists the drawbacks of catamarans:

• More length is needed to carry the same payload as a monohull.
• Two engines and two rudders add expense.
• They take up more marina space.
• They are harder to tack.

Wikipedia also gives figures for how little cruising catamarans heel and how shallow they can draw. That claim rests on a single source, so we have not repeated the numbers here.

Beach catamarans

Wikipedia describes beach catamarans as built for two crew and launched from a beach. They usually have a trampoline, a rotating mast and a fully battened mainsail, and performance versions use trapezes. UKSA's example of a catamaran for beginners is the Hobie Cat.

North Sails, a sailmaker, says full-batten mainsails suit large roaches (the sail area outside a straight line from head to clew) and are typical on multihulls and on boats without a backstay. The Formula 18 is an example of a formula class, where only a few key specifications are fixed, according to Wikipedia; see one-design and Olympic classes for how class types differ.

Cruising catamarans

Both main US training bodies have a catamaran course:

• ASA 114 Cruising Catamaran covers multi-day liveaboard cruising on 30 to 45 ft auxiliary catamarans, and requires ASA 104 Bareboat Cruising first.
• US Sailing lists a Catamaran endorsement in its adult certification ladder.

As a first boat to own, Practical Sailor suggests a monohull rather than a catamaran, noting that few people start on a cat. It recommends trying both through a charter or a sailing school before deciding. More on that in buying a boat and chartering.

Multihulls at the top of the sport

Multihulls race at the Olympics, in professional leagues and offshore:

Multihulls also race on handicap. US Sailing lists a Multihull certificate among the ORC certificate types, and the RYA says mixed fleets of dinghies, keelboats and multihulls can race together using Portsmouth Numbers. See handicap systems.