Sailboat - Hulls

Hulls

Traditional sailboats are monohulls, but multi-hull catamarans and trimarans are gaining popularity. Monohull boats generally rely on ballast for stability, and usually are displacement hulls. This stabilizing ballast can, in boats designed for racing, be as much as 50% of the weight of the boat, but is generally around 30%. It creates two problems; one, it gives the monohull tremendous inertia, making it less maneuverable and reducing its acceleration. Secondly, unless it has been built with buoyant foam or air tanks, if a monohull fills with water, it will sink.

Handbook of offshore cruising: The Dream and Reality of Modern Ocean Cruising. http://books.google.ch/books?id=NB4uFQuUlnEC&pg=PA37&dq=catamaran+cruising+offshore#PPA36,M1. "... the weight of a multihull, of this length, is probably not much more than half the weight of a monohull of the same length and it can be sailed with less crew effort."

Multihulls rely on the geometry and the broad stance of the multiple hulls for their stability, eschewing any form of ballast. Indeed, multihulls are designed to be as light-weight as possible, yet maintain structural integrity. They are often built with foam-filled flotation chambers and many modern commercial trimarans are rated as unsinkable, meaning that, should every crew compartment be completely filled with water, the hull itself has sufficient buoyancy to remain afloat.

This absence of ballast also results in some very real performance gains in terms of acceleration, top speed, and maneuverability.

• The lack of ballast makes it much easier to get a multihull on plane, reducing its wetted surface area and thus its drag.The absence of drag improves wind precision, giving it its great handling.
• Compared to a monohull, acceleration to top speed is near-instantaneous.
• Reduced overall weight means a reduced draft, with a much reduced underwater profile. This, in turn, results directly in reduced wetted surface area and drag, yielding higher top speeds.
• Without a ballast keel, multihulls can go in shallow waters where monohulls can't.

There are some tradeoffs, however, in multihull design:

• A well designed ballasted boat can recover from a capsize, even from turning over completely. The Swan 65 Sayula II won the 1973-74 Whitbread Round the World Race after doing a 180 degree capsize in the Southern Ocean. Righting a multihull that has gotten upside down is difficult in any case and impossible without outside help unless the boat is small or carries special equipment for the purpose. Several round the world racing multihulls have been lost after they capsized.
• Multihulls often prove more difficult to tack, since the reduced weight leads directly to reduced momentum, causing multihulls to more quickly lose speed when headed into the wind.
• Also, structural integrity is much easier to achieve in a one piece monohull than in a two or three piece multihull whose connecting structure must be substantial and well connected to the hulls.

All these hull types may also be manufactured as, or outfitted with, hydrofoils.