The evolution of hull and keel forms – Part 2

Schooner sails over the ocean waves in the bay near a harbor.

The “Great Exhibition of the Works of Industry of All Nations” took place in London in 1851. This first “world expo” was inspired by the French exhibitions held since 1798, especially the very successful one organized in Paris in 1844. However, unlike the “Exposition des produits de l’industrie française” that focused only on the French industry, the British conceived their exhibition as a platform where countries worldwide could display their accomplishments. Britain’s industrial leadership was at its apex, and one of the main British targets was to prove to the rest of the world its superiority.

On the Great Exhibition occasion, The British Royal Yacht Squadron (RYS), yacht club founded in 1815 and located in the Isle of Wight, invited the recently created New York Yacht Club (NYYC) to visit them. The NYYC responded positively, and assuming that racing against the British was to be held at some point, they formed a syndicate to build a yacht to sail to England, make money competing, and show off some of the US shipbuilding skills. The yacht, launched on May 3rd, 1851, was christened America, and compared to the British yachts of the time, her hull was shallower, featured a hollow bow, and carried its maximum beam farther aft.

In fact, after having dominated hull shapes for hundreds of years, the fish form analogy had finally found a challenger in the wave-line theory proposed in the first half of the 19th century by British shipbuilder and scientist John Scott Russell. Considered by many as the first theory of naval architecture, it argued that the energy expended in creating waves when the hull pushes its way through the water surface was the primary source of ship resistance. It also explained that this wave-making resistance could be reduced by hulls whose shapes complemented the wave profile and proposed that hulls with hollow and concave bows with their point of maximum beam located aft amidships were the solution.

The wave-line theory was wrong in its assumptions. Indeed, it slowly faded away during the last quarter of the 19th century after William Froude developed his laws of ship resistance based on experiments carried out in test tanks. Yet, the theory was surprisingly right in its practical application. Several yachts designed based on its principles had already been launched before 1850. They all proved to be faster than their traditional “cod’s head and mackerel’s tail” counterparts.

In Britain, despite the evidence, the wave-line theory remained largely ignored still for some decades. Critics stated that hollow bows would make the hull suffer from a lack of floatability forward and that it could submerge under certain circumstances and not be able to come up again. But things were much different in the USA. There, boat builders with a more practical approach to design started to be replaced by a new generation of yacht designers with a much more theoretical approach to naval architecture. George Steers, the designer of America, was one of them.

The race between these two different yacht design conceptions would finally take place on August 22nd, 1851. The British invited the Americans to take part in their annual race around the isle of Wight. As the RYS rules only allowed club members with their self-owned yachts to participate in their races, they created a new competition, the R. Y. S. £100 Cup, open to anyone from any nation. Eighteen yachts entered the race, 15 started, and only five finished under the time limit. America arrived first, followed by the British Aurora, who took about 8 minutes more to get to the finish line. The America’s Cup, the oldest trophy in modern international sport, was born.

America schooner in 1910
America was a gaff-rigged two-mast schooner. This picture was taken around 1910. She was finally scrapped in 1945.

But was indeed America a better design? America‘s waterline length was 87.4 ft, and her tonnage 170, and those of Aurora were 57 and 47, respectively. Even though it started to become evident that larger yachts can sail faster, the race was carried out without any time allowance or handicap system that would have allowed the two boats to be compared (hopefully) more fairly. Whatever the case, America won the race, and the boat and her lines instantly acquired a worldwide reputation.

After the defeat of 1851, British yachts started slowly to change. Bows became more hollow and concave, and the point of maximum beam gradually shifted farther aft. Still, for almost the rest of the century, their proportions remained under the influence of the Thames Measurement and other rating rules that penalized beam. These rules, which were well-suited for merchant vessels, produced deeper and narrower hulls when they were applied to racing yachts.

Due to their narrowness, British “plank-on-edge” designs suffered from poor form stability. Their hulls were easily heeled, thus, compromising their ability to carry sails. Yet, the deep down located ballast offered a considerable amount of weight stability and positive stability range. Consequently, these boats rarely capsized, and if so, they didn’t remain in that position for a long time.

Once it was clear that longer yachts could sail faster, handicap systems started to consider as well the waterline length of the upright hull. As a result, overhangs designed to extend the hull’s waterline length as the yacht heeled became almost ubiquitous. They aimed at bending the rules making the yachts capable of a higher speed without being “noticed” by the measurement.

On the other side of the Atlantic, Americans considered their victory as the ultimate validation of their hull form. In search of higher speeds, shallower and beamier yachts were launched. But the concept was soon stretched too much:

On one side, the “skimming dishes” based their stability in the form of their hulls (form stability). Due to the beamy hulls, they featured higher initial stability (stiffness) and thus higher capability to carry sails in lighter winds. Nevertheless, once the heeling angle exceeded 30 degrees, the yacht was prone to capsize, and if so, it was unable to come back upright. These yachts, although fast in light winds, were unsuitable for cruising.

Likewise, bigger initial stability or stiffness meant higher accelerations in rolling. These accelerations put higher stress on the rigging, the hull, and the crew. Consequently, the American yachts often ended the races in worse shape and had a more limited lifespan than their British counterparts.

Finally, their leeway angles could sometimes be quite important when sailing upwind. In fact, a moderate wind could easily push their very shallow hulls sideways despite the use of a centerboard.

As the century quickly advances to its end, progress in the field of hydrodynamics and new rating rules will make these two different visions eventually converge …


Do you want to read more articles like this?


Some of the links shown below are affiliate links and we may earn a commission at no additional cost to you:


The evolution of hull and keel forms – Part 1

Until the 19th century, the art of yacht design had been mainly influenced by tradition, economic and commercial motivations, understanding of materials and their availability, and genuine or semi-scientific developments in hydro and aerodynamics. Boats had been evolved through a long but reliable process of trial and error. But now, rating rules, which most of them had nothing to do with real scientific knowledge, started to play a significant role in hull design on both sides of the Atlantic.

What is the Froude number?

The Froude number is a dimensionless number used to quantify the influence of gravity on the motion of a fluid. It is the ratio of the inertia forces to the gravitational forces related to the mass of water displaced by a floating boat.

Introducing the hull

The hull provides a volume to house accommodation, machinery, supplies, and cargo. It has to be seaworthy enough for the routes the yacht will sail and provide the lowest possible resistance to forward movement. It has to resist the heeling forces generated by the wind’s interaction with the sails and have a large resistance to sideways movement to reduce the sideways drift to leeward of the desired course. The hull determines most of the yachts’ main attributes: stability, resistance, seaworthiness, maneuverability, and load-carrying capacity.

What the hydrodynamic resistance is and why it matters

The motion of a sailing yacht through water requires energy to overcome resistance. It is essential to know the mechanisms behind the generation of this force that works against the movement so that we can make the resistance of a new design match the project’s overall performance goals.


Go to Courses


Go to Merch

Send us a Message