#swaystability

https://youtu.be/AAlUzidUByg    

1.0 The Joy of Hydrofoil Sailing

“The feeling for me of foil is like flying.  When you start gaining speed on a small plane, and then instantly take off and leave the ground.  It’s very similar on a boat” -Jimmy Spithill, Skipper Oracle Team USA 2014 No discussion of hydrofoils is complete without addressing their application to the 2013 America’s Cup yachts.  Catamarans screamed across the ocean, flying on innovative hydrofoils. (Figure 1‑1)  The crew constantly struggled to balance the foils against the wind and extract every last point of speed.  It made excellent drama and energized the sport.  But with all that excitement, we sometimes forget how the crew jeopardize their lives in every race.   https://www.youtube.com/watch?v=5-16cxaFjpw Figure 1‑1:  America’s Cup Hydrofoils 101   DMS engineers seek to preserve life while pushing for new innovation and greater achievements.  This article presents an engineering perspective on the America’s Cup hydrofoils of 2013, with options for improvement.

2.0 Control of Sailing Hydrofoils

Hydrofoils work fine on sailing vessels; we possess the knowledge to design a stable sailing vessel with hydrofoils.  (See article on hydrofoil control.)  The roll stability under hydrofoils reacts similar to a normal displacement yacht.  Under wind forces, the yacht heels over.  This changes the angle and submersion of the hydrofoils.  Various systems of control react and adjust the foil forces to balance the heeling moment.  On a tack, the vessel switches direction of heel, and the foil on the opposite side takes control.  The opposite side foil . . . We encounter a stability problem with racing hydrofoils in the 2013 America’s Cup.  These yachts target extreme performance.  In pursuing those extremes, the yachts include the ability to retract the windward hydrofoil from the water.  It improves the speed, . . . and it eliminates any passive stability in the foil system.  By all rights, the yacht should fall off foil at that point. The only thing that sustains foil flight in this unstable condition is the crew.  They carefully balance the heeling moment of the wind against the hydrofoils.  This is equivalent to balancing on a high wire, while someone keeps shaking the wire.  While running at 40 – 60 knots.  Used in this fashion, sailing hydrofoils are inherently unstable, always trying to fall off of foil.

3.0 Risk of Sailing Hydrofoils

The challenge of sailing hydrofoils adds to the fun, but as an engineer, I focus on the large increased risk to life.  Sadly, those risks already progressed beyond theory.  While training for the 2013 America’s Cup, sailor Andrew Simpson was killed.   The yacht capsized, and Mr. Simpson became trapped under the solid sections of the yacht.  Emergency divers failed to locate him in time.  The vessel owners prepared for the potential of a capsize with a myriad of safety features : Helmets Body armor Knives to cut free of rigging Emergency oxygen bottles Underwater training for all crew Emergency divers always ready As an engineer, this tells me that the racers were not reckless; they cared about safety.  But those preparations were not enough.  I accept that extreme sportsmen demand new challenges, but I can’t accept that a sport warrants jeopardizing your life. Max Sirena added: “The boat is basically too powerful. At the same time, this is our sport. This is a risk we take.” Beyond all other requirements, ships must protect the lives of their crew and passengers.  The current designs for sailing hydrofoils disappoint in this area.  But we don’t throw in the towel and forget about hydrofoils.  Engineering gives us the tools to work the problem and develop new solutions.  Maybe the vessels protected against the wrong challenge.  We need to understand the fundamental risk of sailing hydrofoils.

4.0 The Problem of Speed

The basic problem is the speed inherent to hydrofoil sailing.  If a small dinghy capsizes at 5 knots, the consequences are minor.  But capsizing at 40+ knots carries a much higher chance of injury.  As a heuristic comparison, Figure 4‑1 indicates fatality rates, compared to ship speed.  Although this was based on pedestrian collisions with motor vehicles, it shows much more severe danger above 30 knots.

Figure 4-1: Indicated Fatality Rate with Speed Based on Pedestrian Fatality vs Speed In Motor Vehicles  

5.0 Design for Capsize

The combination of instability and extreme speed mean too high of a risk.  No one wants to remove the hydrofoils.  Members of the racing industry desire that challenge of the hydrofoil instability.  In that case, we need to accept and anticipate more crash stops and capsizes.  If we want yachts designed like fighter jets, give the crew the protection of a fighter jet: Five point harness restraints in cockpit Fully enclosed cockpit to protect against capsize Watertight cockpit with emergency air supply Safety systems that engage even if crew are unconscious Bottom escape hatch to prepare for capsize Failsafe design in the sail rigging At DMS, we do not see engineering as an obstruction to new challenges.  Engineering gives us the tools to try new ideas and evolve them into safe technology.  With the proper planning and risk management, we can implement hydrofoil sailboats in a safe manner.

6.0 References

How to Stay on Foil

https://youtu.be/KJpkN_EFpzI     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.

1.0 The Problem of Control

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): Surge Sway Heave Roll Pitch Yaw 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.

Figure 1-1: Six Degrees of Freedom  

1.1 Ship Motion Control

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.

2.0 Active Control Systems

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.

Figure 2-1: Fully Submerged Hydrofoil with Active Control System  

3.0 Passive Control Systems

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.

3.1 Surge Stability

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.

3.2 Sway 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.

Figure 3-1: Diagram of Sway Stability Reaction  

3.3 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.

Figure 3-2: Force Diagram of Roll Stability  

3.4 Heave Stability

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.

3.5 Pitch Stability

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.

Figure 3-3: Typical Hydrofoil Configurations   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.

3.6 Yaw 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.

4.0 Partial Hydrofoils

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. https://youtu.be/Ss0CDKhPEb4 Figure 4‑1:  Example of Foil Assisted Catamaran  

5.0 Conclusion

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.

6.0 References