Hemingway once remarked that one of his novel's charcters, Lady Ashley, had "a body with curves like the hull of a racing yacht." There are very practical reasons for the subtle and sensuous curves in a boat's hull lines; and like any machine, a truly effective design always turns out somehow to be...beautiful.

Displacement monohulls have a tendency to tip over, or heel, under the pressure of wind on the sails. This tendency is counteracted by carrying heavy ballast weight in the lowest possible part of the keel, which provides a "righting moment", or a resistance to this heel. But this is not enough. Hull shape also plays an import role in hull stability.
Consider the cross-sectional shape of a traditional sailboat hull, the "wineglass section". Cut a sailboat at right angles to its length and the resulting exposed surface is the wineglass, widest at the deck, curving in a circular fashion to the waterline, and then the curve reverses radius and curves inward rapidly to where it is almost parallel to the water surface at the keel. The keel corresponds to the stem of the wineglass, and drops straight down to the bottom. The ballast is concentrated in the keel, and is as far away from the hull as practicable, providing as much leverage as possible to prevent the hull from capsizing in a blow. The farther away from the hull, the more effective the ballast is, but also adds to the depth of water required to float the boat, so in this, as in any design, some compromise must be reached depending on the expected conditions the boat will encounter during use.

As the boat heels, gravity provides a greater and greater counter-force which tries to keep it upright. In addition, at increasing angles of heel, the boat's weight forces more and more of the curve of the hull into the water, and the bouyancy helps to keep the hull from falling over. The wind is being forced to push the bulge of the hull (called the "bilge") under water and it's bouyancy resists this force. Since the hull cross section is curved, this counter-force is initially negligible, but increases rapidly as the angle of heel increases. At a certain angle (usually designed so that the waterline length of the boat is at a maximum, around 45 degrees) the force is maximized. Sailboats sail best when heeled over, the boat is almost sailing on its side, and the new waterline defined at this angle is maximized as well. The longer a boat is, the faster it can go all else being equal, a consequence of how water resists an object moving through it. A properly designed hull has a longer waterline and a higher speed when fully heeled over than when upright. This is why sailboats are such dogs under power, and slide so effortlessly through the water under sail.

This type of hull has very poor initial stability, or resistance to heel. At anchor, or at the dock, even the slightest mass transfer on deck or light wave action sets the boat to rocking violently, giving any landsman passengers a feeling of instability and danger. But underway, the hull stability increases rapidly under heel, and at one point she will simply stop rolling over, the hull at an extreme angle, but held rigidly by the bouyancy of her bilge, the weight of her ballast at the end of a long lever arm, and the damping "shock absorber" effect of the rig. A good sailor can feel this point instinctively and will trim his sails according to wind conditions and the course desired to keep the vessel on this fine edge of top performance. This is where she is designed to operate. She may be heeled so sharply it may be difficult to walk on deck or get around below, but the boat feels safe, stable, and happy to her crew, even if the passengers are terrified. Except under extreme conditions, it is impossible to capsize this hull, and even if knocked down so the mast is in the water, the forces of ballast weight and hull bouyancy will pop her right back up. In the worst case, where a freak combination of wind and waves turns her completely up-side down, she will be unstable in this position and further wave action will roll her back over again to an upright position. Unfortunately, this usually means her delicate rig will be a shambles, or swept away.

This is how the traditional offshore racing yacht is designed, but there are other considerations. The hull shape and proportions mean there is very little usable space below for a given size boat, and the required deep draft keeps her away from many shallow water cruising grounds. A hull designed this way can deal with almost any weather with a small crew, but it is cramped, restricted in its coastal operations, and very uncomfortable to spend any time on, even at anchor. Most boats make some design concessions to this ideal depending on their intended use (bay racing, offshore racing, bluewater vs coastal cruising, recreation, trailerability, family use, cargo capacity, comfort, ease of construction, size of crew, etc.) or their intended market. For example, the boat I describe in my article is a "gunkholer", or a recreational craft designed for pleasure and comfort (but definitely not luxury!) and exploring local waters and short overnight stays. It is perfectly capable of handling some rough weather and offshore work, but it is optimized for exploring shallow water bays and estuaries and short coastal passages. As I mention, it's hull shape is designed to give it higher initial stability (important for comfort and liveability) at the expense of high final stability. The hull also can carry a lot of cargo without becoming unbalanced, an important consideration when "camping out".

The discussion above restricts itself to the section, but there are also design considerations about hull features in the profile and plan dimensions, too. All the subtle adjustments made to hull shape by the designer in the athwartships direction must be complemented by hull shape decisions made in the fore-and-aft direction. Perhaps I will discuss these in a future essay.

The final result is an almost incredibly complex three dimensional shape, available in multiple variations, that allows men to trust their lives to the almost infinite combination of conditions the ocean can throw at them. Thousands of years of experience and tragedy have gone into developing these principles, and today we continue to refine them with technology and science. We simply cannot afford to allow this accumulated wisdom to ever be forgotten.