Does A Hydrofoil Really Work? (Explained For Beginners)

Your boat’s speed and lift will affect how it cuts through the water.

Many boats use hydrofoils to get the best speed and reduce drag.

But do they really work?

Here’s How a Hydrofoil Really Works:

A hydrofoil is a surface mounted beneath the boat. As a boat picks up speed, this surface generates lift and begins to rise, eventually raising the boat clear of the water. With the hull above the water’s surface and no longer causing drag, the boat’s speed greatly increases. If the boat maintains speed, it can ride up on the foils indefinitely.

What Exactly is a Hydrofoil?

A hydrofoil is a surface, usually a wing-like structure, that provides lift in the water.

They operate in the same manner as airfoils do on airplanes. This structure is mounted under the hull of a boat with struts or on the bottom of the centerboards on catamarans.

As the boat picks up speed, these foils lift the boat, and the hull is raised out of the water.

The foil moving through the water creates higher pressure on the bottom of the foil and reduced pressure on top; this is how the lift is generated. This lift continues to raise the boat until a balance exists between the lifting force and the craft’s weight, and a point of equilibrium is reached.

When a boat is foiling, the entire wetted surface of the hull is eliminated and no longer causes drag. The only surface causing drag are the foils with their much-reduced surface area.

This propels the boat too far greater speeds than could ever be reached with its hull in the water. This is because the sails generate the same power, but the hull’s drag is no longer working against it.

The hydrofoil can actually take several different shapes. It might be a simple wing-like surface, or it may resemble a bent arm. Some have flaps that are controlled remotely.

Some hydrofoils, particularly some of the earlier models, are designed to lift the bow of the boat clear out of the water rather than the entire hull.

There is a point of speed approaching 70 miles per hour where the acceleration levels off. This is due to cavitation on the foil, which bends the foil’s surface and keeps it from going faster. First identified in the 1950s, even modern hydrofoils struggle with this phenomenon.

Another problem with hydrofoils manifests if they are operating in waves. If the foil cuts through a wave’s surface and passes through the air, the lift is negated, and a negative attack angle is generated.

This causes the boat to crash back down when the foil hits the next wave.

Hydrofoils are also susceptible to being damaged. They are thin, and striking an object in the water can damage the foil or the attachment point on the hull.

What Types of Boats Can Use Hydrofoil Technology?

There are a variety of boats that can use foiling technology, both sailboats, and motorboats.

In most of these cases, the boat is designed to have the foil rather than adding the feature at a later date.

Hydrofoils are also generally expensive to develop and produce, so their use is not widespread.

The technology was developed originally in powerboats, both as a test of the concept and to push for greater speeds and records.

The militaries of various countries have used hydrofoils. The US navy developed several models, particularly the Pegasus class, but these were discontinued in the mid-1990s because they were not deemed cost-effective. Russia’s military has continued to use and develop hydrofoil technology, particularly in patrol or torpedo boats.

Several passenger boats, like high-speed ferries, have been built to use hydrofoils. These are expensive to produce and maintain, so they have generally appealed to a more wealthy clientele. These kinds of ferries are on the decline worldwide, though they are still in heavy service in Japan and parts of Asia and Europe.

Some of the most visual examples in the sailing community are the past several America’s Cup boats.

The previous two challenges used foiling catamarans, while the 2021 series uses a foiling monohull with a rotating wing.

Some lightweight, small dinghies have been successfully retrofitted with foiling technology, such as some Moth-class boats.

Various racing boats have been developed to use foils, both monohulls, and catamarans, from dinghies all the way up to 60-foot offshore yachts.

There are some surfboards and kayaks that are designed with hydrofoils. Some kite-sailing boards are using foiling technology. Many windsurfers have adopted foiling technology.

There are also new kinds of electronic personal watercraft being developed and sold. These are essentially platforms or skis that a rider stands on, and as speed is gained, it rises on the foil and carries the rider several feet above the water.

However, they are not designed for speed, just the experience of foiling and have a top speed of about 25 mph.

How Much Does it Cost to Have it Added?

Most hydrofoiling boats are built around the foiling concept rather than having it retrofitted.

There are exceptions, like the foiling moths. New foiling moths cost from $10,000 to $20,000.

Some moth sailors on forums who have added hydrofoils to their hulls say they spent as little as $1,000 on materials, but $5,000 seems to be the average cost based on the most recent posts by the more serious racers.

There are foiling kits that may be added to other boats, as well. A hydrofoiling kit to add to Lasers, made by the company FOILSZ, costs around $2,700 in the USA.

Adding a foil to an existing keelboat has not been done. The cost of retrofitting an existing is theoretically prohibitive, and these boats are almost all cumbersome compared to the boats designed to foil. It is cheaper (though still costly) to buy a monohull designed for foiling, given the cost and uncertainty.

Likewise, most foiling catamarans have been designed and built around the concept rather than being retrofitted. But there are some exceptions, like the A-Class catamaran. Many owners who race have installed their own foils, with a starting price of around $3,000 in most cases.

Other boats, like the Hobie 16 and Prindle 19, have seen models retrofitted with foils. Several companies make foils for this purpose.

HydroSail is one such manufacturer. Their smallest foiling kit costs#1,000 and is suitable for a Hobie 16. The next size up is suitable for a boat around the Prindle 19 and costs $5,000.

They also make a kit for much larger catamarans, and it costs $30,000.

Adding foils to a windsurfer will cost around $2,500 as an initial cost, though better foils cost more than this. Putting foils on a surfboard starts at about $1,200.

As with sailboats, most powerboats with foiling technology were designed and built to the concept, rather than having them retrofitted on the hull. This is because of the extensive rebuilding that would need to be done in beefing up the areas of foil attachment and the hull design being different and not necessarily conducive to hydrofoiling.

Some of the first retrofitted kits for small powerboats were called the Up-Right Hydrofoil Kits, built between the years 1961 and 1962 by the Up-Right scaffold company.

They did not sell enough to make a profit, though, and the product was discontinued. Since then, few retrofitted hydrofoil kits were made for powerboats, and none of them have been particularly successful.

Some hydrofoils attach to the bottom of outboard motors. These do not lift the boat out of the water as conventional hydrofoils do; instead, they are intended to stabilize the ride and help fuel efficiency.

Claims of increased speed are made, but most do not feel they actually make the boat faster. These are commonly listed for around $1,000.

How Stable and Safe is a Boat Using Hydrofoil?

Stability is a concern for hydrofoiling boats, as they become more top-heavy when they are riding upon the foils.

Observation of America’s Cup catamarans will reveal a bit of porpoising on the foils as they rock slightly back and forth while raised out of the water.

A hydrofoiling boat has a motion that is closer to a plane than a conventional boat. It has to contend with six ranges of motion, specifically surge, sway, roll, heave, pitch, and yaw. It can be difficult to control all of these, though electronic trim tabs on larger boats help.

As long as a foiling boat maintains its speed, it is a relatively stable platform. As mentioned earlier, coming out of a wave will make the boat dive and land hard back into the water.

As far as safety goes, foiling can be dangerous. Experienced mariners will handle one without much difficulty, but a beginner will not feel comfortable initially riding on the foils. The danger here is not great, though, as it simply means getting off the foils and getting the hull wet again.

There is a danger with foiling surfboards, however, if there are other surfers about.

A fast-moving foil that strikes a fellow surfer can hurt them very badly.

Is a Hydrofoil Really Worth It?

If you want to experience the fastest a sailboat can go, the answer is yes.

It will be an expensive proposition, but there is little comparison to the rush of flying above the water.

For the commercial hydrofoiling powerboats like ferries, the answer is proving to be no, as the boats and their maintenance are far greater than a standard, similar-sized boat. While there are still commercial hydrofoiling ventures in existence, they cater to smaller, increasingly wealthy clienteles, and they are finding it difficult to maintain profitability.

The expense and upkeep will probably not justify buying a boat with hydrofoils for the casual sailor or powerboater.

Sources:

Hydrofoil – Wikipedia

How a Hydrofoil Works – MIT

Hydrofoils: Expert Advice – Boat US

HydroSail Home

Hydrofoil Control

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