Foiling vs. Floating; Understanding the Differences Between Hydrofoil Boats and Other Boats

A displacement boat represents one of the oldest types of hull designs. It moves through water by pushing it aside, unlike planing boats that ride on top. These boats are known for their efficiency and slower speed. In marine design, a common rule associates the speed of a displacement boat with its length, often expressed as hull speed. This estimate indicates the maximum speed a displacement hull can efficiently achieve without increasing resistance and fuel consumption significantly. For instance, a 6-meter boat can reach about 6 knots, while a 7-meter boat can achieve slightly under 7 knots.

Beyond hull speed, increasing speed demands significantly more power, becoming inefficient as the boat climbs the stern wave instead of slicing through water. A distinguishing design feature of many traditional displacement boats is their narrower and shallower hulls. Canoes and some rowboats are prime examples, benefiting from longer waterlines which enhance their efficiency.

Traditional displacement boats typically rely on conventional propulsion methods like paddles, oars, and sails, although they can also be motor-driven. Examples include most sailboats, barges, houseboats, and other slow-moving vessels.

Planing motorboats come in various shapes and sizes, each tailored for specific purposes. They are equipped with different engines for leisure and commercial activities. As a planing boat accelerates, its flat or semi-V-shaped hull forces water downward. According to Newton’s third law (for every action, there is an equal and opposite reaction), this action creates an upward force, lifting the hull above the water’s surface.

This mechanism allows the boat to cruise at higher speeds than a displacement boat. However, it also requires a significant amount of power. For example, a 25-foot powerboat uses 15 times more energy per mile than a standard car. Planing boats gained popularity only after powerful car engines were adapted for marine use following World War II. Today, this type of boat dominates the market, ranging from fishing boats to cruisers and racing boats.

Fast waterborne transport, with high power needs, is among Earth’s most polluting and costly transportation modes.

Hydrofoil technology isn’t new. Hydrofoils blend some of the best aspects of both displacement (efficiency) and planing boats (speed). However, they operate on a unique principle that allows them to achieve greater efficiency and speed.

Their roots trace back to the early 20th century, with inventors like Alexander Graham Bell and Casey Baldwin experimenting with surface-piercing designs. During World War II, military forces of several nations utilized experimental hydrofoil boats for their speed and agility. In the post-war period, hydrofoil designs have found applications in various fields, from military to water sports.

The secret to their effectiveness lies beneath the water. Hydrofoil boats generate upward force as they accelerate, offsetting the weight and allowing the hull to rise above water. It reduces drag, and enables the boat to maintain high speeds. This principle of fluid dynamics, similar to an airplane’s takeoff mechanism, distinguishes hydrofoil boats from their conventional counterparts.

Passengers in hydrofoil boats experience significantly reduced noise and vibrations, offering a more comfortable and less disruptive journey.

Hydrofoil boat hulls significantly reduce water resistance and enable higher speeds compared to traditional hull designs. Large traditional displacement boats cut through water while smaller pleasure boats utilize hull shape and engine power to partially lift themselves for reduced drag.

V-shaped hulls are designed to handle rough waters more effectively than flat-bottomed hulls by slicing through waves. This provides a more comfortable ride at higher speeds.

Hydrofoil shapes have changed over time. Early 1900s hydrofoil designs, pioneered by Italian engineer Enrico Forlanini, primarily utilized ladder frame hydrofoils. They mimicked the shape of a ladder and allowed the boat to create lift proportional to the speed. Ladder frame designs found their way into some military prototypes and other use cases throughout the 20th century, but never became commonplace.

Hydrofoils vary in shapes and sizes, mainly categorized into two families: V and T-shaped foils. V-shaped foils generally form a V or U shape under the width of the boat in order to pierce the surface of the water, rising above the water surface when foilborne. This allows for self-stabilising when it comes to rolling movement.

T-shaped foils, are fully submerged beneath the water, typically featuring a horizontal wing rather than the curved wing seen in V-shaped hydrofoils. This allows them to be less affected by wave action, and, therefore, more stable at sea. They are also more efficient as they cause less drag. However, T-shaped foils are not self-stabilising. The angle of attack on the hydrofoils must be adjusted continuously to changing conditions. It requires the intervention of sensors and onboard computers. The Candela C-8 electric hydrofoil boat harnesses this technology.

Within these two families, many different variations and designs exist for a variety of purposes. America’s Cup class sailing boats use diverse designs to improve turning, speed, and stability, while leisure boats often prioritize designs that improve seakeeping and comfort.

Manual and automatic hydrofoil retraction are increasingly common in modern hydrofoil boats, offering captains more operational flexibility. This also allows for easier maintenance and less concern over reefs and shallow areas. Boats like the C-8 showcase this retractable capability.

Foil boats achieve higher speeds and superior performance relative to engine power, even in choppy waters, due to efficiency. By reducing air and water resistance by up to 80%, hydrofoils enable faster speeds and enhance stability against wave action.

This reduction in drag ensures a smoother and more efficient operation, making hydrofoil boats superior for navigating various water conditions.

One of the key advantages of hydrofoil boats is their ability to maintain higher speeds in rough weather, as they simply fly over the waves. This is why the US and other navies pioneered the use of fully T-shaped hydrofoil ships, leading to fast patrol boats such as the Pegasus-class that could cruise at 48 knots. Hydrofoil boats are capable of:

• Lifting above the water surface at a speed of 11-22 knots depending on hydrofoil wing profile and other factors
• Achieving and maintaining higher speeds than conventional boats, given the same power
• Maintaining top speed even in less than ideal sea conditions
• Reducing drag significantly

This technology allows hydrofoil boats, including the fastest production sailboats, to excel in speed and efficiency.

As conventional planing boats accelerate, the drag on their hulls increases due to increased water displacement and surface friction. Since hydrofoil boats lift above the water surface, increasing their speed can have the opposite effect: they become more efficient at high speeds compared to slow displacement speeds. This is due to the hull having minimal contact with the water, allowing for less friction and thus, less drag. It allows hydrofoil boats to travel at high speeds while consuming less energy, making them more economical with a smoother ride.

Hydrofoil boats offer significant benefits in terms of fuel efficiency:

• Reduced drag, resulting in lower fuel consumption
• Up to 80% reduction in energy use compared to regular boats

Hydrofoils, especially fully submerged variants, can significantly increase stability of a boat. Since the hull of the boat has less contact with the waves, the result is less reaction to wave conditions. More specifically, hydrofoil boats control heave, roll, pitch, and yaw.

Partially submerged V foils remain stable in roll and maintain flight height automatically. On the other hand, fully submerged foils require an active control system to stabilize.

Hydrofoil technology has been used historically and is expected to gain widespread adoption due to its benefits in speed and efficiency.

Military:

Throughout the Cold War, hydrofoil technology was explored due to their advantages in speed, silence and stability for military vessels. The United States and the Soviet Union heavily invested in hydrofoil technology, resulting in vessels like the Boeing Pegasus Class and the Soviet Sarancha class. These boats were nimble and had a higher average speed than other ships while having superior seakeeping.
Though many of these ships were commissioned from the 60’s to 80’s, few remain in service today. The Italian Sparviero class ships served both the Italian Navy and the Japanese Maritime Self-Defence Forces. Some former Soviet vessels also remain in service in various modern countries.

Sports:

Over the years, hydrofoils have become increasingly popular in water sports, particularly among those seeking greater speed. Hydrofoil craft have frequently held the world water speed record, with Paul Larsen’s Vestas Sailrocket currently holding the world sailing speed record.

Leisure & Commercial Activities:

Hydrofoiling is increasingly used commercially and recreationally, notably in surfboards, even electric ones, enabling surfing on calm waters. The expanding market includes E-water bikes for pedaling over water and hydrofoiling kayaks for faster speeds, offering a new water experience.

On the other hand, hydrofoil technology has also found its way into maritime transport. For instance, it is well-suited to ferry services where high speed and good comfort is essential. Electric hydrofoil ferries like the Candela P-12 boost stability, efficiency, longer battery range, and faster speeds compared to non-foiling counterparts. Tourist destinations such as southern Italy, which rely on tourism, utilize hydrofoil vessels to provide a quicker alternative for traveling between coastal locations.

Hydrofoil vessels have unique capabilities for navigating various water conditions:

Shallow Water Navigation:

Modern hydrofoil boats can navigate shallow waters effectively by raising their foils, reducing the risk of running aground. Foils can be retracted to prevent damage in these conditions, allowing safe operation without the need for deep water.

Deep Water Efficiency:

In deep water conditions, modern hydrofoil boats extend their foils fully. This seamless transition between shallow and deep waters enables modern hydrofoil boats to access harbours and an ease of maintenance without limitations faced by their older counterparts with fixed foils.

The future of boating looks promising with advancements in hydrofoil technology. The maritime industry envisions hydrofoil technology becoming standard across vessels, including ferries, due to energy savings and speed enhancements.

The hydrofoil market’s growth will increase availability and affordability for recreational and commercial users. Electric hydrofoil technology, spearheaded by companies like Candela, is emerging as a key trend in the boating industry. By harnessing electric power, hydrofoil boats can achieve high speeds while producing zero emissions. This minimizies their carbon footprint and reduces environmental impact. Electric propulsion systems also offer quieter operation compared to conventional engine boats, minimizing disturbance to marine life.

When investing in a hydrofoil boat, one requires considering the cost. Hydrofoil ticket prices are usually higher than traditional ferry services due to fuel consumption, especially with turbine-powered foilers like the Boeing 929 Jetfoiler. However, passengers find the premium price justified for faster travel times and superior comfort compared to standard ferries. With efficient, electric-powered foiling ferries such as Candela P-12, operational costs would actually be much reduced instead of increased.

Hydrofoil boats demand an initial investment, often incorporating carbon front foils, hulls, and decks. However, they provide long-term operational cost savings through energy efficiency and reduced maintenance. For instance, the purchase price of Candela C-8 electric hydrofoil boat, starts at €330,000 excluding optional features. With operating costs 95% lower than fossil fuel boats, buyers save thousands on fuel, offering long-term savings potential.

Foiling boats exhibit both impressive functionality and sustainability. Their designs range from Soviet Raketas’ 1950s spaceship aesthetics to sleek, modern styles reflecting advanced technology. These designs often resemble racing cars or jet fighters, emphasizing a futuristic look aligned with speed and efficiency.

Many modern hydrofoils use carbon fiber to achieve light weight and a rigid structure.

The market for recreational and commercial passenger hydrofoil vessels is still young, with many companies beginning to offer these products. Also many of them are electric as the increased efficiency now allows for great performance off a reasonably sized battery. For the first time, hydrofoils are challenging conventional hull designs.

Hydrofoil boats aren’t available in every category. However, for higher-end powerboats, they offer comparable performance to conventional planing speedboats but with significantly lower operational costs, approximately 95% lower due to their efficiency.

How do hydrofoil boats achieve high speeds?

They achieve high speeds by generating lift with underwater wings or hydrofoils. This lifts the hull above the water’s surface and reduces drag which allows the boat to reach higher speeds.

What makes hydrofoil boats more fuel-efficient?

Foil boats are more fuel-efficient due to the reduction in drag from the hydrofoils. This leads to faster speeds and up to 80% reduction in the consumption of fuel/energy.

How do hydrofoil boats provide a smoother ride?

They do so by elevating the hull above the waterline, reducing waves and turbulence for improved passenger comfort. This also creates minimal wake, leading to a smooth ride with reduced disturbance to the marine environment.

Are hydrofoil boats versatile?

Yes, they are versatile and can be used in various applications, from ferry services to fishing, navigating both shallow and deep waters seamlessly.

Are hydrofoil boats expensive?

Since foiling boats are often constructed from carbon fiber and feature advanced foil control systems, they fall into the premium category of boating. However, once purchased, they offer long-term cost savings through fuel efficiency and reduced maintenance. For example, the purchase price of Candela C-8 starts at €330,000, excluding optional features. But it costs around 10 euros for a full battery, providing 50+ nautical miles. A conventional planing boat with two outboards would cost 200 euros to run the same distance – or 20 times more expensive to run.