How to Stay on Foil
Why would an airplane company design a ship? In 1962, the USS High Point launched as a demonstrator vessel for the capabilities of hydrofoil ships in the US Navy. This ship was designed by the Boeing corporation, and Boeing would go on to design a few more hydrofoil ships. Ironically, Boeing was a logical choice; they brought skills critical to hydrofoil design. More specifically, they understood hydrofoil control. Every hydrofoil vessel needs it, even sailing hydrofoils. This article discusses the problem of hydrofoil control and several solutions.
A hydrofoil vessel behaves like a fighter jet, not a boat. When a fighter jet cruises steady, it needs to control six motions, six degrees of freedom (Figure 1‑1):
When we talk about stability of motion, we need some method to control each of these six motions. If the jet gets a disturbance in any motion, we need to ensure that it reacts to stop that disturbance and return to steady flight. Some fighter jets do this with careful shaping of the wings, and others utilize advanced flight control systems. But in either method, controlling these motions requires continuous small adjustments.
Ships need to control the same six motions. In a conventional displacement hull, the shape of the hull automatically controls most of those motions through hydrostatic forces:
- Heave (hydrostatic forces)
- Roll (transverse stability)
- Pitch (longitudinal stability)
- Yaw (lateral center of effort, plus rudder)
That only leaves surge and sway, which are partially controlled through the rudder and through propulsion adjustments. However, all bets are off with a hydrofoil. We lose the benefit of hydrostatic forces and need to invent new methods of control through careful engineering.
We most often see active control systems on fully submerged hydrofoil configurations. (Figure 2‑1) These are the most efficient configuration, but they depend on complicated computer monitoring. A computer controls flaps and ventilators to makes continuous small adjustments. Design of these control computers require detailed knowledge of the physical systems and a strong background in controls theory. They far exceed any simple task. That advanced control system is part of the reason that Boeing was a reasonable choice for designing a hydrofoil craft.
The alternative to an expensive computer is passive control systems. By careful design of the foil shape and angle, we create a system that maintains stable motion of the vessel, without any computer monitoring. However, these passive systems create a conundrum of coupling between several degrees of freedom. Notice how the same design features control stability in multiple degrees of freedom.
Surge stabilizes nicely through propulsion control. The vessel still generates significant resistance as it runs through the water. Even up on hydrofoils, that resistance balances the forward thrust to ensure surge stability.
Sway requires canted hydrofoils for passive stability. Imagine the ship suddenly shifts to port. That sideways motion changes the angle of water flow across the hydrofoils. The port motion increases the angle of attack on the port hydrofoil, and higher angle of attack means more lift. The opposite happens to the starboard hydrofoil. Add the two foils together, and you achieve a correction force to stop sway. A perfectly horizontal foil never reacts to these velocity changes; the canted foil was the key.
The angle of the canted foil also controls the center of effort for our sway reaction. (Figure 3‑1) Ideally, we want the sway center to focus near the center of gravity. Otherwise, we generate problems that couple the sway force to roll stability.
Passive roll stability relies on the partially submerged hydrofoil. (Figure 3‑2) As one side rolls down, more of the foil enters the water, increasing its surface area. More surface area increases the lift generated on that side. We get unequal lift between port and starboard foils, yielding a righting moment for roll.
The angle of the foils also affects the roll center. The roll center equates to the metacenter in a displacement ship. Just remember that the roll center must remain above the vertical center of gravity in all vessel loading conditions. Without that critical positioning, the vessel capsizes.
To passively control heave, we need partially submerged hydrofoils. Whenever the vessel runs on foil, part of the hydrofoil sits above the water. If the vessel heaves down, more of the hydrofoil enters the water. This increases the surface area of the foil and generates more lift. That extra lift reacts to the heave motion and pushes back up. The reverse happens if the ship heaves up. This natural balance depends on the partially submerged hydrofoils.
Partially submerged foils also add to pitch stability. For pitch stability, we combine the partially submerged foils with careful selection of the longitudinal foil position. Figure 3‑3 shows typical foil configurations. The critical decision rests with placement of the foils relative to the center of gravity.
Many vessels favor the Conventional configuration, where the foils follow the same dynamics as a modern airplane. In this case, the forward foil provides the majority of the lift. The aft foil may only function as pitch control. In some cases, the aft foil actually pulls down, just to maintain pitch control. The relative longitudinal positions of these foils control their sensitivity to pitch, with critical implications on pitch stability.
Yaw stability still requires a rudder. The canted hydrofoils also push laterally to ensure yaw stability. They act similar to the keel on a sailing vessel.
There is a third option for hydrofoil stability. Design the vessel for partial support by hydrofoils. In this case, the hull never fully leaves the water. The hydrofoils carry part of the weight, with the remainder taken by the hull. (Figure 4‑1) This system was fairly cunning; if the hull never leaves the water, we still get all the stability of conventional displacement hulls. (Same benefits apply to planing hulls.) This allows a simple solution to get some reduction due to hydrofoils and avoid all the complexity of ensuring stability.
Figure 4‑1: Example of Foil Assisted Catamaran 
Hydrofoil ships have more in common with aircraft. They require detailed planning to ensure stable control of vessel motions. Options include active control, passive control, or partial hydrofoils that rely on hydrostatics for control. And just like aircraft, these complex control systems require forethought for the risks involved and the demands of the control scheme. When properly engineered, hydrofoils deliver impressive speeds, plus a lot fun.
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