Hydrofoils are Back! Let's Weigh the Pros and Cons
By: Captain Bill Jennings
Ride high. Ride fast. Ride smooth.
Could there be a hydrofoil boat in your future? With recent technical developments, foil boats offer new possibilities. As you may already know, hydrofoil boats are like regular boats, but have those strange wingy things dangling down below their bottom, that magically lift the boat completely above the water when going fast. Here are the details.
The first prototypes of hydrofoil boats date back to WWl, when even Alexander Graham Bell worked on their development. While several prototype models proved that the foil lift concept works, there were no specific needs for such a design of boat so development stalled. That is until research began again after WWll. Canada built a number of coastal patrol hydrofoils that had the speed to protect its extensive shorelines. In the 1970's, the U.S. Navy built six foil boats that were used for drug interdiction operations until scrapped in 1993. A more recent experiment in 2008, applying foils to a 24' Bayliner, re-ignited the interest in foils. With today's advanced computer control systems and specialized composites, successful hydrofoils are beginning to look more likely to become a popular choice. Numerous companies are now offering tempting designs in hydrofoil boats.
To seriously consider whether your next boat should be a hydrofoil, lets review how they operate and list their pros and cons. When we look at a standard powerboat today, we see that it floats on the surface of the water. When pushed, the shape of the hull causes it to ride up and skim along on the water surface, or 'plane,' with varying degrees of efficiency. Unfortunately, the wetted surface of a regular boat creates considerable drag when running, which in turn reduces efficiency. This drag is the main problem that foil boats can eliminate.
A hydrofoil boat supports a "wing" like structure under the hull. As boat speed increases that small wing creates lift. That lift reduces wetted surface and drag. As speed further increases, the boat lifts entirely free of the water surface. Only the foil, or wing, remains in the water. This is exactly the same as an aircraft lifting off a runway as the speed of laminar flow over its wing increases.
There are two types of hydrofoils: fully submerged and surface piercing.
Fully submerged foils are just that, and they are relatively unaffected by conditions on the surface. They do, however, require an automatic control system to maintain flying height and smoothness of ride, because without computer feedback hydrofoil boats are unstable in pitch. This 'autopilot' computer system controls how the foils move to cause the boat to take off, fly smoothly, and land safely. Height sensors in the computer, such as a gyro, or an inclinometer control foil movement for lift and stability. This computer adjusts the foil angle up or down within a five degree movement each way. Surface piercing foils operate in much the same way, but their lift is provided by 'V' shaped foils that run partly above the water surface. We see these used mostly on commercial vessels.
The power to push these two types of foils comes from three types of power. Propellers, water jets, or wind. I won't discuss the latter here, as the system is highly technical and primarily used on America's Cup racing sailboats. Water jets and propellers, as we see everywhere today, are well understood and readily available to match different foil horsepower applications.
The reason we are seeing a renewed interest in hydrofoils is because they can now safely offer several advantages to some pleasure boat applications.
Here are some hydrofoil "pros":
1. In most cases, foils will get you on a plane faster and allow you to stay on a plane at lower speeds. 2. From the many tests that I have read, and some that I have conducted, foils substantially improve fuel mileage. 3. Hydrofoils will decrease the motion index. Less porpoising means improved passenger ride and comfort. 4. Foil boats are not as affected by rough water as regular hull pleasure craft, and the larger the foil, the better the ride. 5. Whether foils can improve your boat speed seems to depend on a number of factors, but according to tests foils still present drag so a properly trimmed regular boat will probably run a similar speed to foils. Most test references to speed with foils refer to more passengers moved per fuel unit.
This list of foil advantages would be very convincing, if it were not for some of their disadvantages. Here are some hydrofoil "cons":
1. Driving a hydrofoil boat requires more skill than a standard runabout.
2. A strong power source is required, meaning more expensive engines.
3. If the foils are fixed your boat draft is greatly increased. If the foils retract or fold they take up space in the boat, or make the boat wider.
4. For lakes with shallow water, hydrofoils are not practical at all.
5. Bottom strikes while riding on foils can be catastrophic.
6 Hydrofoil pleasure boats under 40 feet are not considered ocean capable vessels. As wave height increases, hydrofoils must 'land' and cruise on their standard hull.
7. Should the foil lift out of the water over a wave, expect a hard water landing. 8. Hydrofoil vessels operate best within a specific speed range. Therefore, your speed choice with foils is not as great as it is with a standard boat.
9. When foil boats are running on the hull, rather than their foils, they have a great deal of drag produced by the foils themselves --- unless they retract.
10. Maintenance costs are high. Many commercial hydrofoil boats have been taken out of service due to the higher cost of operations and maintenance.
11. Because of the requirements for higher strength and added equipment, foil boats cost more. Too much so for the average pleasure boater.
If a real or perceived need for hydrofoils becomes commonplace, we may see a resurgence of foil boats. The key is identifying a specific purpose and advantage for such a design. But the appeal of foils to general powerboaters is still limited. Mr. Fisherman worries about a 4-foot draft when in many waterways. Mr. Performance does not want to be restricted by foil operational parameters. Mr. Weekend Boater wants a simple boat to drive. Mr. Practical will find the computer automated features on a foil boat to be excessive. Mr. Economical will find the initial cost too expensive, compared to standard boats that can provide the same boating pleasures.
So just as it was in the early 1900's, there still seems to be a limited application for hydrofoil boats. When you factor in the considerably higher up-front cost of hydrofoils over a conventional runabout, even more interest evaporates. For example, a 28-foot foil runabout currently sells for around $400,000 U.S.
While I always like to offer fact based advice from first hand experience about specific subjects, this is not always possible. Product advancements may still be in the works, while other products may be encountering new problems. And of course, personal preferences and requirements are also major factors. For boaters wanting to know more about hydrofoils, check out some of the papers and tests reported by the 60 year old International Hydrofoil Society.
As we are seeing several hydrofoil pleasure boat manufacturers appearing on new boat menus, the best advice I can offer is this: if you think you might want a foil, take one for a test drive. Nothing beats a first hand experience to determine if a hydrofoil boat is what you always wanted. #culture #tips