Yacht designers work hard and use exotic materials to save just a few pounds when designing a racing yacht. Imagine saving thousands of pounds and ending up with a less expensive boat that is safer and faster. Using hydrodynamic force instead of gravity to produce righting moment is an innovation that offers these benefits and more.
Ballast weight has been used to stabilize sailboats throughout history, from ancient times to the modern racing yacht. The very first approach was to stack stones in the bilge or bottom of the boat. Effective, but not efficient, this approach remained basically unchanged for thousands of years. Ballast stones were used in ships from before the time of Christ to the fast clipper ships in the nineteenth century. Over the last 150 years, refinements in keel design and ballast location have improved stability and performance. Fixed keel designs evolved from full keels, running the length of the vessel, to fin keels typical in sailboats today. Ballast moved from stones piled inside the hull to deep high aspect keels ballasted with lead outside the hull. The modern ballasted fin keel serves two functions. Because it has a center of mass well below the hull, the modern keel with a ballast bulb does a much more efficient job of keeping a boat upright than internal ballast. The second function of the keel is to act as an underwater wing, or in technical terms, a vertical, hydrodynamic lifting surface which keeps the boat from slipping sideways when sailing up wind. Sailboats cannot sail directly into the wind; however, the efficiency of the modern fin keel allows boats to sail much closer to the wind than sailboats could 100 years ago. Refinements of the fixed fin keel continue with the addition of trim tabs, fixed wings and ballast bulbs. The latest embodiment is to cant, or rotate the keel to maximize the leverage the ballast bulb has against the force of the wind. However, the tradition of using the force of gravity has remained the primary driver to counteract the force of the wind.
A number of different open ocean class sailboats use canting keels. The “Volvo Open 70” class best exemplifies the design challenges associated with the canting keel. VO70s use a 10,000 pound lead bulb at the end of a thin 14 foot long blade. Hydraulic rams are used to rotate the keel up to 40 degrees from vertical, generating approximately 100,000 Newton Meters of righting moment. My interest in canting keels began with an offer to crew on, what was at the time, one of the fastest monohull sailboats in the world. This thoroughbred racing yacht had a canting keel. I was very excited to have the opportunity to sail on a racing yacht that represented the state of the art in naval architecture and marine engineering. My excitement, however, was short lived. The vessel suffered a failure of the rudder post, not the keel, before the race which put the boat in dry dock and out of commission. I later learned that, although very fast, “canters” were also very fragile. Even the yacht that I was scheduled to race on had a history of mechanical breakdowns and only finished one of the four races it had started because of structural failures. In fact, mechanism and structural failures were very common with the entire fleet of canters. Seeing the boat out of the water was an eye opener. Standing next to the huge keel and ballast bulb gave me, for the first time, an appreciation of the magnitude of the powerful forces that have to be managed to swing these massive loads.
There are many issues that sailboats using canting keels must conquer. Though reliability will inevitably improve over time, these are some issues that cannot be eliminated through better engineering or improved design:
- The hydraulic rams need to work against a significant mechanical disadvantage when canting the keel and ballast bulb. The hydraulic rams need to generate at least 25,000 lbs. of force just to hold the keel in place when canted.
- A large structural framework is required to support the reaction loads and isolate the hull from the forces generated by the large hydraulic rams.
- Ballast weight is a large component of the gross weight of the vessel. Righting moment requirements are great when sailing up wind but the same ballast weight is carried when sailing downwind even though it is not needed.
- The 10,000 lbs. ballast bulb represents over 60 percent of the overall weight of the 16,000 lbs. vessel and is suspended 14 feet below the boat on a very thin, high aspect keel. The inertia of the bulb needs to be overcome when changing course, generating huge torsional loads on the keel. Many failures resulted in the bulb snapping off the keel.
- Canting sailboats require the addition of canards or dagger boards to replace the loss of the primary underwater lifting surface, adding significant complexity and underwater surface area to the vessel. Lowering and raising the dagger boards need to be coordinated with motion of the keel when tacking or jibing.
- The variety of extreme dynamic sea conditions make it very difficult to build in sufficient design margins. Insufficient design margin is a major factor behind canting keel mechanism reliability and subsequent safety issues.
- The canting mechanism requires the use of dynamic seals. Catastrophic failure and subsequent keel damage could cause seal leakage to be large enough to endanger the vessel and crew.
- The electrical power needed to generate the hydraulic pressure for the rams is significant and difficult to generate on a sailboat. It takes approximately ten seconds to tack or jibe with a canting keel.
Even with these issues, sailboats using canting keels retain the status of being the world’s fastest ocean going monohulls. Standing next to the boat, I wondered if there was a better way to generate righting moment other than by moving lead ballast weight alone. I had an idea for a simple mechanism to control a wing attached to the bottom of the keel. This wing would use the flow of water, or hydrodynamics to generate righting moment. Some ballast is required for initial stability, but a significant percentage of ballast weight would be eliminated by using hydrodynamic force to supplement the gravitational force from the inertial mass of the keel and ballast bulb. The wing would be attached to a shaft at an obtuse angle, so as the shaft rotates, the angle of attack of the wing would pitch down. The wing would be rotated straight back when sailing down wind and rotated forward when sailing up wind. The weight of the ballast bulb could be reduced by up to 50 percent with a small penalty of additional drag from the wing.
Since I had not seen or heard of anything like it before, I did a patent search on the concept and filled an application with the US Patent and Trademark Authority. I was granted an international utility patent on the concept in 2009.
Performance and safety advantages of this concept include:
- The same righting moment 100,000 nt. m. as the canted ballast bulb at 15 knots with two meters additional wetted area form the wing. That would be the maximum wetted area penalty if you were to replace all of the ballast weight. Of course the actual wing area would depend on the percentage of ballast replaced by hydrodynamic force and the weight of the wing.
- The keel is fixed with no loss in lifting surface. There is no need for supplemental dagger boards.
- The reduction in overall weight reduces the overall wetted area and drag.
- Time to tack and jibe is reduced by 50 percent.
- No seals are required to seal the actuator shaft. The shaft tube can be extended above the water line, similar to a centerboard trunk. The wing and shaft could completely break off the keel and not endanger the vessel or crew.
- Since rotating the wing does no work to lift the ballast weight against gravity, power requirements are reduced from 289 (10 seconds to tack a canting keel) to 29 horse power (5 seconds to tack a wing keel)
- Like winglets on an airplane wing, the horizontal wing on the bottom of the keel reduces the effect of tip vortices. Turbulence from the wing tips reduce lift, the plate effect from the wing increases the effective aspect ratio of the keel.
- Fore and aft hull trim is automatically accommodated as the wing rotates. The center of mass of the wing is aft when sailing downwind and moves forward when sailing upwind, eliminating the need for water ballasting to prevent pitch polling when sailing downwind.
- Stresses are confined in the keel. No internal structural framework is needed to support the loads. The drive motor can be mounted to the keel bolts. Hydraulics are not needed as loads are small enough to rotate the wing with a gearhead DC motor.
- Righting moment is a function of speed. The faster the boat goes, the more force is generated by the wing.
I described the hydrodynamic lifting blade concept to a number of prominent yacht designers. Dr. Len Imas, Associate Professor at Stevens Institute of Technology’s Davidson Labs (who did the original analysis and tow tank testing on the Volvo 70 design), along with other researchers at the lab, took the time to explain the structural engineering challenges to me in detail. Being able to meet and talk with some of the most respected scientists and designers in the industry was, so far, the most rewarding aspect of this project. They all agreed that the idea had merit. They also agreed that the approach could theoretically provide the forces required, make the boat faster, safer, and even less costly. However, the design would have to be proven before an investor would spend the money necessary to build a racing yacht using my keel concept.
My project was intended as an initial proof of concept for the hydrodynamic wing. To test the concept, I built a remote control functional model capable of being fitted with either a canting or hydrodynamic keel. I was unable to have access to a fluids lab so I built my own open flow bench using a rebuilt 4 horse power Jacuzzi pump. My high school physics teacher supplied the instrumentation and data acquisition software. I was able to measure up wind and downwind drag, along with the righting moment, as an angle of heel, upwind. This was a comparative test only, as the model was too small to generate results that would apply at full scale.
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