Understanding Sub-Cavitating Hydrofoils And Use On Pontoons, Tritoons, and Power Catamarans

 

Benefits and Challenges

VARA® Foil Hydrofoils are the profile of wings that create lift in water.

Hydrofoils are not new. The technology continues to improve each year. Understanding how they work is key when determining if one is in your future.

Water is approximately 800 times denser than air, depending on temperature, density, and altitude, etc. This means a short hydrofoil can produce incredible amounts of lift for a given speed versus a much larger aircraft wing. The major difference between an airfoil and a hydrofoil is the phenomenon called cavitation. If a spot in the flow pressure becomes lower than the vapor pressure, the water flow can boil locally and form (cavitation) vapor bubbles. When vapor bubbles are transported in the flow to an area with higher pressure then they collapse. If this collapse occurs near a rigid surface, very high-pressure fluctuations may cause erosion of the hydrofoil surface.

As you may know, this cavitation effect regularly happens on boat propellor blades that are mounted too high in the waterstream, tilted too high while underway which can also produce signs of pitting on the surface. Efficient hydrofoils must be designed with respect to the onset of cavitation, whereas the boundary layer flow is less significant.

1. Speed Range

There is a speed limitation called the “Wall” that occurs when subsonic hydrofoils develop air bubbles at maximum speed/lift. At this point, they create more drag than lift.

Sub cavitation/subsonic flow hydrofoils work best when water flows smoothly around them without the formation of vapor bubbles known as cavitation. At higher speeds, cavitation and ventilation reduce lift, increases drag, and can damage the hydrofoil surface. Typical efficient speed range is between 10 and 60 mph depending on the hydrofoil design. Generally, a subsonic hydrofoil will form vapor bubbles and the downstream water will actually boil at ~ 50–60 knots.  The design features of our VARA® Foil hydrofoil section have been tested to speeds between 67-70mph with out onset of this vapor boiling, which is also known as the wall described above.

VARA® Foils are subsonic hydrofoils that perform extremely well in the ~20 to 60 mph range.

However, a standard VARA® Foil, not the larger span Offshore Series offered, has an efficiecnt wheelhouse speed range between ~18 and 48 mph of true boat speed, because of it’s much larger span or foil width.

2. Hydrofoil Load and Size Sensitivity

Hydrofoils must be carefully matched to the boat’s weight and balance. Too much load on a given hydrofoil thickness and/or chord length will cause a hydrofoil to stall or ventilate when it draws air down from the surface. If the hydrofoil size or span is too small for a given boat, it will not create enough lift for the hydrofoil to engage. For this reason, the one-size-fits-all hydrofoil designs are inferior to a custom-made hydrofoil that is calculated and built for the actual boat needs. Custom-built VARA® Foil designs are extremely desirable for the performance boater looking for the “best” experience hydrofoiling along with an ease of learning the new handling and feel associated with a hydrofoil.

Very heavy and slower vessels (i.e. double decker pontoons, displacement catamarans, large tritoons with water absorbing strakes) won’t achieve maximum efficiency using hydrofoils as their lift requirements become difficult to achieve. VARA® Foils can be installed on some of these challenging boat designs, but customer communication with the design team is imperative to manage expectations.

3. Sea State Limitations

Hydrofoils need clean water to be most efficient. Traveling in aerated disturbed water or following a boat is inefficient for hydrofoil boats. Keep the water flow clean like geese traveling south, or a formation of jets searching for clean air in a delta formation.

Hydrofoils work best in moderate seas and medium chop. In heavy waves, the hydrofoil may breach the surface or ventilate in the troughs between the waves, reducing efficiency and passenger comfort. You can see in these images how ventilation is more common in rough water. A hydrofoil will always seek the path of least resistance, which in this case will be the surface of the water, or the trough of wave which supplies air onto the foil surface rendering it inefficient.

Ventilation of a T-Foil shown above is simulated in the towing tank and on the right is at full-scale speeds in a lake testing facility.

4. Creation of Drag at Low Speeds

Hydrofoil technology is purely mathematical when it comes to lift, drag and speed gain. At displacement or takeoff speeds, hydrofoils actually create drag rather than produce lift. Hydrofoils become efficient once enough speed is achieved to generate enough lift required to raise the hull of the water. One mathematical rule to apply to achieve speed gain is that the amount of wetted area added with a hydrofoil wing must be smaller than the area of hull lifted out of the water it produces. For example, if you install a standard VARA® Foil hydrofoil that has 4 square feet of wetted area, the wetted area it lifts from the water must be greater than its size of 4 square feet plus a small amount. This small amount is a proprietary mathematical rule affected by the design and size of a given hydrofoil, the strut system, and the efficiency of the hydrofoil shape. VARA® Foil designs maximize lift with decreased drag using this calculation.

5. Structural Demands

Hydrofoils take very high axial, bending, and torsional loads that must be used in conjunction with strong strut system and mounting points. The cost and engineering complexity are high but crucial in that damage from striking floating debris or shallow bottoms can be catastrophic.

You can see here that the patent-pending and professionally-engineered VARA® Struts and mounting rails are extremely durable.

6. Cavitation vs. Ventilation

Cavitation happens at high speed when the local water pressure drops below the vapor pressure causing bubbles that collapse violently on the hydrofoil surface.

Ventilation occurs when air is sucked down from the surface along the strut onto the hydrofoil – interrupting its lift capability.

Both effects drastically reduce the performance and can damage the hydrofoil. Thin, axial- loaded strut designs outperform standard single hydrofoil strut designs for all the above reasons. The patent pending, axial loaded VARA® Strut system is thin and very strong similar to bicycle spoke designs. This design results in an extremely strong product, minimal ventilation effect, more lift and less drag as compared to standard strut designs.

7. Efficiency Tradeoffs

Designing a hydrofoil that is superior across the speed spectrums, crew weights, water conditions encountered, etc. is unachievable and requires tradeoffs. Compare it to matching a car tire to ALL conditions that you travel. One type of tire does not always perform well in dry, wet, mud, snow, slush, high speed, high inclines, and off-road conditions. At PHA, Inc., our design team custom matches the VARA® Foil type to the conditions you most often travel in in conjunction with your most important goals.

It is important to understand the following differences. Hydrofoils optimized for low-speeds are usually thicker and generate more drag at higher speeds but produce more lift.

Hydrofoils designed for higher-speed are usually thinner creating less lift. These foils can be prone to cavitation and may require more precise control.

A VARA® Foil kit is designed to match the needs and desires of the specific boater. Customers complete a Post Checkout Form with questions focused on boat type, dimensions and infrastructure along with motor specifications to best accommodate their goals.

8. Cost and Maintenance

The addition of a hydrofoil is considered a great “upgrade” for pontoons, tritoons, and power catamarans. Although the upgrade has an initial expense, it can be easily justified with better performance, quieter/drier ride, lower fuel cost, and less wear and tear on the engine. Depending on how much the vessel is actually used, the cost may be recouped in a couple of years.

As with any marine product, routine inspection and maintenance is very important. The hydrofoil surface and struts must remain smooth, free of damage, fouling, or corrosion to ensure maximum efficiency.

9.  True Data-Driven Results

PHA, Inc. has carved out an important niche with our unique and patent-pending design. Our customers are extremely satisfied and their results are well documented. For example, Chris and Donna increased their speed from 23-mph max to 41mph max with a 140 hp Suzuki motor after installing a VARA® Foil Kit. (Click Here to Watch ).

The overall benefits of adding one of our VARA® Foil hydrofoil kits include

1) increased maximum and cruising speeds of vessels in the acceptable speed range,

2) a quieter ride by pushing less water,

3) a more comfortable ride,

4) less wear and tear on your power plant resulting in lower maintenance costs,

5) huge savings on fuel. It really is a no-brainer for boats that meet our criteria.

6) Less wake for skiers and jumpers

The initial cost outlay of our “best-in-class”, adjustable hydrofoil can quickly be recovered in fuel savings while also upgrading your boating experience after you make the smart decision to install the technology. Our system is unique and stands out when comparing to the competition. Note two very important features of our VARA® Foil hydrofoil kits including:

1) the position of the hydrofoil is adjustable allowing for x-axis front-to-back changes, y-axis up and down depth changes, and z-axis AOA or angle of attack wing changes

2) each kit includes a VARA® Plate which mounts to your outboard anti-ventilation plate.

This pitch control plate allows the boat driver to adjust the bow attitude dynamically while moving by using the outboard trim tilt switch resulting in steady flight. Watch this video to see how the VARA® Plate is mounted. This video will show the VARA® Plate and it’s basic use.

We just announced our newest Gen 3 VARA® Plate designed for the 80-85 mph speed range for larger and faster boats. It is shown above in a twin motor application. Twin motor applications DO NOT NEED trim tabs with our VARA® Plates installed. You can easily adjust side to side trim by use of motors. If one side is to high, lets say from the wind, you may trim that motor up a bit while trimming the other motor down a bit. Extremely efficient and saves you from trim tab purchases, maintenance, and constant adjustment.

For more info call us at 727-455-5576 or via email at mike@pontoonhydrofoil.com.

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Website: pontoonhydrofoil.com

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Pontoon and Tritoon Speed Gearheads

Thanks to these online sources: Flosur.com and Max Haase

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