Background

A hydrofoil is simply a wing attached to the hull of any vessel moving through water to create lift which raises the hull out of the water as the vessel gains speed. Hydrofoils can be explained by thinking of it as an airplane wing, but water is the fluid which creates lift as opposed to air. As the vessel increases in speed, enough lift is generated by the hydrofoil to “lift off” the hull of the vessel from the surface of the water just as an airplane lifts off a runway to gain flight. This separation of the hull, either completely or partially from the water, reduces the boats drag which not only increases speed but allows for superior comfort in passenger boats. The lift created by airfoils is based off the principles of certain fluid equations: essentially that the air traveling over the longer, top face of the foil creates lower pressure than the slow air on the bottom face and “pushes” the foil upward. As well, as we all learned in middle school science class, a fluid is any continuous, amorphous substance and since water is a fluid the concepts of lift can be applied to a foil under water.


Towards the end of the 19th century the first attempts at hydrofoil assisted vessels were being made by some of the eras brightest engineers. Around 1900 a British boat designer John Thornycroft developed models of a boat with a single bow shaped foil attached to the hull with long stilt-like struts. By 1909 his company had developed a full scale 22-foot hydrofoil assisted boat, dubbed Miranda III, and latter models are credited with speeds up to 35 knots.

In 1906, an Italian engineer, inventor and aeronautical pioneer, Enrico Forlanini, successfully tested a craft powered by a 60 horsepower engine driving two counter-rotating air props on Lake Maggiore which reached a top speed of 42.5 mph. As well, Alexander Graham Bell dabbled in the hydrofoil design field and in 1908 began experiments with Casey Baldwin. Baldwin had studied the work of Enrico Forlanini and used his success as a beginning point for their designs. Between 1910 and 1911 Baldwin and Bell had met with Forlanini in Italy where they would ride together on Forlanini’s hydrofoil boat. Both Baldwin and Bell were astonished at Forlanini’s success, his craft had rapid acceleration, smooth riding described as smooth as flying, great stability, but most importantly it would achieve lift on every run.

It was developments around this era which set in motion the widespread application of hydrofoils. Baron von Schertel worked on hydrofoils for Germany during World War II, and after the war he established a company which designed many models of hydrofoils, Supramar. In 1952 Supramar gave way to the first commercial hydrofoil, the PT10. The PT10 was able to carry 32 passengers and could obtain speeds up to 35 knots. Its unveiling shocked the maritime industry in becoming one of the smoothest riding passenger vessels at the time and brought the advantages of hydrofoils to the forefront of the public eye. Research on hydrofoils did not let down, and by the 1960s many countries had hydrofoils included in their line of naval vessels. During the 1970s and 1980s the Soviet Union extensively experimented with hydrofoils, constructing streamlined passenger ferries. Such vessels sparked the “hydrofoil race” of the late 1900s and soon after the U.S. Navy deployed combat hydrofoil equipped vessels. From 1977 through to 1993 these ships displayed their advanced speed and handling over traditional ships while sacrificing no effectiveness of their wartime tasks. As well, the Italian Navy has employed 6 hydrofoils into the Nibbio class ships from the late 1970s, all of which were fully operational combat ships.



Hydrofoil

A hydrofoil shares the same basic design as an airplane wing or a helicopter rotor blade. The hydrofoil used in this experiment will have a flat bottom and an arched, tapered upper camber. The lift of a hydrofoil is directly the result of its shape (arch of the top face) and its angle of attack, if the hydrofoil were too deviant from the standard wing form then no lift would be generated, no matter how much velocity is applied.. The necessary component of the lift is the difference in velocity of the fluid flow field both above and below the centerline of the hydrofoil. When the flow on the top edge of the hydrofoil has more velocity than the flow on the bottom edge an area of low pressure is created above the hydrofoil and the high pressure region pushes the foil upward.

To create a hydrofoil we must understand its components. The basic dimensions of a hydrofoil are its chord length and camber height, which give way to the aspect ratio, or the ratio of the chord to the camber. Aspect ratios are used to “label” hydrofoils in that they are a measure of how long and slender a wing is from tip to tip. High aspect ratio wings have long spans which provide for efficient, stable travel, and low aspect ratio wings are short and thick, which increases drag and decreases handling.

Foils themselves have two distinct types, fully submerged and surface piercing. The surface piercing design can be pictured as a large V-shape, with the two outer edges of the foil remaining out of the water. This design is optimally used for calm surfaces, such as rivers or lakes, and is typically a slower, less efficient hydrofoil. Fully submerged hydrofoils however can be pictured as an airplane wing attached to two struts straight downward from the hull of the vessel, which provide for lift in much rougher waters as the surface disturbance does not interact with the flow zones of the foil.

Some of the principle advantages of hydrofoil ships, over all other monohull or alternative ship types are: (1) the ability of a ship, which is small by conventional ship standards, to operate effectively in nearly all sea environments, and (2) an improved ratio of power to displacement in the 30 to 50 knot speed range permitting economical operation at these higher speeds. The submerged-foil ship can maintain its speed and maneuverability in heavy seas while simultaneously providing a comfortable working environment for the crew. The ship's automatic control system (ACS) provides continuous dynamic control of the ship during takeoff, landing, and all foilborne operation. In addition to providing ship roll and pitch stability, the ACS controls the hull height above the water surface, provides the proper amount of banking in turns and all but eliminates ship motions caused by the orbital particle motion of waves. Foilborne operations only become limited as wave height exceeds the hydrofoil's strut length. Figure 4 shows operating data points for three submerged-foil hydrofoil ships in actual sea conditions. The data clearly show only a modest reduction in speed as wave heights increase. A hypothetical operating envelope is drawn to represent hydrofoils designed to have a 50-knot speed capability in calm water.

Besides a significant speed advantage, hydrofoils are more maneuverable and provide a more stable platform than conventional ships. Foilborne turns are accomplished in a banked (coordinated) fashion. This causes the centrifugal force required in turns to be provided predominantly by the reliable lift capability of the submerged foils rather than by the unpredictable side forces from the struts. Turn coordination enhances crew comfort during high-rate turns because the accelerations due to turning are felt primarily as slightly greater vertical forces rather than lateral forces. For example, a 0.4g turn is felt as only 0.08g vertical acceleration increase while the lateral acceleration is zero. Therefore, hydrofoil ships have design turn rates of 6 to 12 degrees per second, two to four times those of conventional ships, and they can maintain these rates in both calm and rough seas. This makes the hydrofoil ship a more difficult target for enemy missiles, guns, or torpedoes. The exceptional stability of the hydrofoil ship makes it a superior platform in which to mount surveillance equipment and weapons while maintaining crew comfort and proficiency.

Lift

Whether discussing airfoils or hydrofoils everything boils down to one princple, lift. Lift is a force generated by a fluid flowing around the surface of an object, this force is perpendicular to the flow of direction and in the case of our hydrofoil creates upward rise. Hydrofoils are designed to capitalize on the generation of lift while optimally producing minimal drag. Lift is commonly associated with fixed wing airplanes, however lift can be generated by propellers, helicopter rotors, sails on sailboats, spoilers on sports cars, wind turbines, or simply even your hand placed out the window of a moving car.

The most simple way to describe lift is to set the lift as a result of the downward direction of fluid flow by a foil. The wing exerts a force on the fluid which directs the flow downward and from the laws of motion we know the equal but opposite actions result in lift. This can be most easily observed by hanging your hand out the window of a moving car. Begin with your hand parallel to the ground, and slowly lift the leading edge of your hand, before long you will be fighting to hold your hand in the same position. This is a direct result of the winds force opposing the change in direction applied by your hand. This simplistic explanation of lift is known as deflection.

The more popular explanation of lift frequently encountered in basic sciences is a theory known as the equal transit-time theory. This states that the longer path generated by the arch in the upper camber face of the hydrofoil increases the distance the fluid must travel over the shorter, flatter bottom camber face of the hydrofoil. Moreover, the fluid traveling over the top of the hydrofoil must go faster in order to remain with the fluid flowing underneath the foil. Using Bernoulli’s Principle we result that the faster moving fluid creates lower pressure in the region above the foil and the pressure difference pushes the foil upward. Unfortunately, the equal transit-time theory is not completely accurate. It is true that the air moving over the top face of the foil does have higher velocity than the flow zone below the foil, but there is no definite requirement that the fluid particles must arrive at the trailing edge of the foil at the same time. Shockingly, the fluid flow zone on the upper camber face of a foil generating lift will always move much faster than estimated by the equal transit-time theory.

To truly understand the causes of lift we must understand the fundamental principles of physics, mainly: Newton’s laws of motion, specifically the second which states a relationship between the force on a fluid particle and its rate of momentum change; conservation of mass, implying the fluid is impermeable to the body of the foil; and an expression relating the fluid stresses to the properties of their flow. Explaining lift while considering all of the physics princples involved is not easily simplified. We will focus on explanations of lift based off Newton’s laws of motion and an insight into Bernoulli’s principle.

Bernoulli’s Principle

Bernoulli’s principle defines that for an inviscid flow (a set used for simplification of fluid dynamics and related equations, meaning a flow with low values of viscosity) an increase in velocity yields a decrease in pressure across the system of flow. This complex principle can be explained in termes of the law of conservation of energy. Take a hose and its spray nozzle for example, as a fluid moves from the wide hose to the constriction of the narrow nozzle a corresponding volume of fluid must move greater distance forward in the narrow pipe and thusly have a greater velocity. Simultaneously, the overall work done by corresponding volumes in the wider and narrower sections of hose must be expressed by the product of the pressure and the volume inside that hose system. Since the velocity is greater in the narrower pipe, the kinetic energy of that volume is also greater. As well, by the law of conservation of energy, this increase in kinetic energy must be followed by a decrease in the pressure-volume product of the system, yielding a decrease in pressure.

Fluid Dynamics

Hydrodynamics is a sub-category of fluid dynamics, which deals with the flow of fluid, that is the natural science of fluids in motion. Fluid dynamics is a widely applicable field of study, with real world studies including the flow rate of any fluid through a tube; such as the currently increasing rate of petroleum pipelines, predicting weather patterns and calculating forces on aircraft. To draw any conclusions using fluid dynamics it is typical to calculate many properties of the fluid, such as pressure, density, temperature and velocity.

The fundamental basics of fluid dynamics are the conservation laws, specifically of mass, momentum and energy. As well, it is accepted to assume for simplification purposes that fluids are a continuous system, rather than a system of discrete individual particles which would then interact individually. This is known as the continuum assumption. In addition to mass, momentum and energy conservation, a thermodynamical equation of state giving the pressure as a function of other thermodynamic variables for the fluid is required to completely isolate the problem, the perfect gas equation of state is an example of this thermodynamic equation.