The evolution of hull and keel forms – Part 1

Classic pilot cutter sailing

As soon as early humans began to explore new territories for hunting and trading, they found more and more natural water barriers standing in their way. Some of these barriers were easily crossed, but others like big rivers, seas, and oceans remained just unsurmountable.

Swimming was not an ordinary skill, and our ancestors started to use different materials such as reeds, logs, and inflated animal skins to help them float while crossing the water. But the risk of drowning was not the only challenge they faced in their short passages. Sometimes they carried goods that had to be kept dry. Some other times, the waters hid predators patiently waiting to claim their prize.

Early watertight structures in which sailors and their goods were kept somehow safe and dry emerged gradually during thousands of years. They were often unique to the societies that originated them. These boats slowly evolved through the ages from riverboats to sea-going capable craft. For millennia they were mainly used for fishing, trade, and military purposes. Some of them were able to sail downwind, but they all shared an awful performance when sailing upwind.

Although the earliest representations of a boat under sail was found on an Egyptian vase dated from 3100 BC, it is only in the 17th-century Netherlands that the development of sailing for pleasure took place on a large scale. There, large vessels used by Dutch government officials, called “jachts“, started to be converted to leisure craft.

Cod's head and mackerel's tail hull
Cod’s head and mackerel’s tail hull

However, the first sailing competition ever recorded was the race between the English King Charles II and his brother James along the Thames River in May 1661. Charles, who won the race, had been previously exiled in the Netherlands. When he returned to England in 1660 to become king, he brought a 52-foot “jacht” with him. He later adapted the hull form of the “jachts” to the English deeper waters by deepening the hull. The word “jacht” was rapidly anglicized as “yacht.”

Moving forward, the year 1720 saw the creation of the first organized yacht club: the Water Club of Cork in southern Ireland. It was followed by the first English club in 1775, the Cumberland Fleet, which later became the Royal Thames Yacht Club

Outside the British Isles, the first European club, the Royal Swedish Yacht Club, was established in Sweden in 1830, soon followed by the Société des Régates du Havre, created in France in 1838. In the United States, the first club was the New York Yacht Club, founded in 1844, and in the Netherlands, clubs were set up in different regions in 1846, 1847, and 1851. 

By the second half of the 19th century, sailing had established itself as a sport worldwide, and Germany (East Prussia), Spain, Italy, and Norway inaugurated their first clubs in 1855, 1870, 1879, and 1883 respectively. In Finland, the first club was founded in 1861 upon approval of its rules by Czar Alexander II in St. Petersburg. At the time, Finland was called the Grand Duchy of Finland, and it was part of the Russian Empire.

During the 18th and 19th centuries, sailors and designers found their main inspiration for yachts in working boats like pilot and fishing craft, which were much admired for their seaworthiness and endurance in any type of weather. Hull design was still largely based on intuition. As in the centuries before, one of the most popular hull forms, at least until the first half of the 19th century, was the “cod’s head and mackerel’s tail” configuration. They displayed under the water very full bows and finer hulls toward the stern. The belief, not supported by any scientific evidence, was that underbodies shaped like a fish operated more efficiently. The excess of fore buoyancy also helped the vessels to rise over the waves and to stay much dryer. Although this hull shape would eventually be discarded in surface ships in favor of more performant hull lines, it will ultimately find application, many years later, in fully submerged bodies such as submarines and aircraft.

In the 19th century, the need for winning races constituted the single strongest motivation behind the development of the study of hull forms. Class rules and the concept of rating were also created to allow different yachts to compete against each other on equal terms. Increasingly elaborated rating formulae were developed to cancel out, as much as possible, yacht differences by assessing a yacht speed as a function of its length, beam, draft, sail area, displacement, etc.

Until then, 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.

As the century unfolds, two clearly distinctive approaches to yacht design were made evident:

In England, the Builder’s Old Measurement first (since the 17th century), and from 1855 the Thames Measurement rule, also known as Thames Tonnage, were used. These rating rules were originally conceived for calculating the port dues, and their formulae only considered length and beam but not displacement. In fact, they heavily penalized beam, and as a consequence, English yachts became very narrow and deep. These yachts, also known as cutters, but also as “plank-on-edge” boats due to their extremely narrow beam, based their stability on the ballast located as low on the keel as possible (weight stability), and their hull lines followed the “cod’s head and mackerel’s tail” shape with their maximum beam situated fore amidships. Today we tend to define cutters based on a specific rig configuration (i.e., a yacht with a single mast and two or more foresails), but at that time, the term referred only to the hull form. 

Thames Measurement Rule Hull
Hull designed to the Thames Measurement Rule.

In the United States, yachts were developed based on fishing sloops (here again, the name referred to their hull form and not to their rigs), which had very beamy hulls and very little draft due to the locations where they had to operate. The main focus was to reduce the displacement rather than the beam. These type of boats, also known as “skimming dishes,” based their stability on their wide hulls (form stability), the weight of the crew, and on the mobile ballast (e.g., sandbags) they carried. Unlike their British counterparts, they had much of their beam aft and displayed hollow bows. The centerboard, an American innovation patented in 1811, which directly descended from the leeboards used by the Dutch “jachts”, was also used to minimize leeway when sailing upwind.

American broad and shallow sloop
American broad and shallow sloop.
It can be appreciated a hollowed bow and maximum beam located aft amidships.

These two different world views on yacht design would be confronted in 1851 when the Royal Yacht Squadron of Great Britain offered a Hundred Guinea Cup for a race around the Isle of Wight …


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    The evolution of hull and keel forms – Part 2

    British “plank-on-edge” suffered from poor form stability. Yet, the deep down located ballast offered a considerable amount of weight stability and positive stability range. These boats rarely capsized, and if so, they didn’t remain in that position for a long time. American “skimming dishes” featured higher initial stability. 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.

    Points of sail

    The points of sail refer to the course a boat is sailing in relation to the wind. They are defined in reference to the wind angle, which is the angle between the course sailed and the direction of the wind. The following terms are used: close-hauled, close reach, beam reach, broad reach, run, dead-run.

    Surface waves

    Surface waves are created by disturbance forces applied to a specific area. It can be either the wind, a surface vessel moving through the water, an earthquake, landslides, a splash, gravity (as it is the case for tides), or any combination of them. When studying ocean waves, we consider them to be the superimposition of single sinusoidal waves with different frequencies and amplitudes.

    Wind triangle

    The wind experienced by a fixed observer (true wind) is not the same wind an observer on a moving yacht will feel (apparent wind). The wind triangle helps us understand what the true and apparent winds are and how they are related.

    Frictional resistance of the hull (calculation)

    The frictional resistance is a force that is generated when the underwater parts of the boat and the water around are in relative movement (the boat moves through the water, or, in other words, the water passes down the boat). Therefore, it is a hydrodynamic force, and as a resistance, it works in the opposite direction to the movement, trying to slow the boat down.