A Hydrofoil for the Rest of Us

For many who are somewhat familiar with boating terms, the word, "hydrofoil" might conjure up an image of a vessel taking off like a bat out of hell such as in America’s Cup sailing event, or like high-speed passenger ferries such as the Victoria Clipper and her sister ship, the San Juan Clipper. The Clippers are said to be two of the fastest passenger boats in the Western hemisphere--not too shabby for ferries that carry 525 and 200 passengers, respectively.

Although it is correct that hydrofoils give these vessels their extra speed, hydrofoils are also used in other instances where the hydrofoil application is less dramatic and yet still advantageous, offering improved vessel handling, better fuel economy, and increased overall performance. In other words, you don't have to build passenger ferries or participate in America's Cup to benefit from a hydrofoil. But don't feel left out in the cold if you're not all that familiar with hydrofoils for "normal" boats--many boaters aren't.

A hydrofoil is a device with a cross-sectional shape similar to an airplane wing. This is mounted to the vessel under the surface. As the vessel with the foil attached moves through the water the flow across the foil will create lift. This lift can be slight or severe but the main advantage is that it reduces drag.

So, How Does a Hydrofoil Work?

How does a hydrofoil work? Be careful what you ask! The science behind the working of hydrofoils for boats is complicated because the shape, size, rigidity, and changing performance characteristics during different speeds, among other things all play a part in the design. And let's not even go near the mathematics...except for a bit of simple addition.

Suffice it to say that, at low speeds, the hull sits in the water and the hydrofoils are submerged, but as the boat's speed increases, the hydrofoils will create lift. When the boat gets up to the right speed, the lift produced by the hydrofoils equals the sum of the boat plus cargo weights, and the hull comes out of the water. Without the hydrofoils there would be drag; and the faster the speed, the more drag.

But because the hydrofoils have created lift out of the water the boat now has a more efficient way of cruising. Decreasing the drag contributes to more efficient use of the power needed for the boat's performance. And since hydrofoils for boats--as in, even the smaller pleasure boats--are available nowadays for the average boater, let's take a look at the outboard hydrofoil.

A great invention if ever there was one, an outboard hydrofoil kicks the boat out of the hole, works well at lower speeds, and is easy to install. At Go2marine we have hydrofoils such as the Sport Marine Tech 200 or 300, or the Doel Fin Hydrofoil Stabilizer from Davis Instruments, and the prices are extremely reasonable. There are alternate types of "foils" out there though...sort of.

How Does a Hydrofoil Work...with Trim Tabs?

Trim tabs are adjustable plates that are mounted at the transom of the boat. They're either automatic or controlled by a hydraulic or electric power unit that the driver controls. The trim tabs work to create lift to compensate for bow rise. The driver can compensate for weight distribution, speed, and water conditions, for instance by correcting for port or starboard listing.

Trim tabs are virtually a must-have for many people, but trim tabs are not hydrofoils and vice-versa. So, should you go for the outboard hydrofoil or trim tabs? Some boaters out there who have done so would say, "Get both." Although they are different, they are similar enough for both to work well for their respective tasks.

According to these folks, the hydrofoil helps to get the boat on a plane faster than the tabs, and as we've already seen, lets the boat plane at a slower speed. This is something tabs certainly will not do.

Plus, they say the hydrofoil prevents ventilation in high-speed turns. Meanwhile, the tabs allow the driver to correct for that annoying listing to port and starboard that often happens (correctly loading the boat before taking off may do much to alleviate this, however), and the driver can also push the bow down when needed to plow through heavier seas. And both 'foils and tabs contribute to a smoother, more stable ride.

Hydrofoils are not a new concept but only in recent years has their popularity increased. They are popular in the sailing community and power boaters are seeing the advantages, especially when it comes to combining a hydrofoil with trim tabs. So, if you're ready to give an outboard hydrofoil a try, or already have one and want to add trim tabs, take a look at what we have for you.