@Holcim PRB Sailing Team
In the previous article, we followed the evolution of the submerged appendages that we will find on the slower boats starting in the first groups of the Global Solo Challenge. This second article continues the analysis of the faster designs that will set off in the last groups.
The increasing need to produce high-performing and fast hulls with a reduction in the overall weight of the structures pushed designers to take certain concepts to further extremes. The digital revolution accelerated the design and production processes through the use of CAD (Computer Aided Design), FEM (Finite Element Method) structural analysis, and CFD (Computational Fluid Dynamics) analysis. These tools allowed for the widespread use of carbon fibre and lightweight alloys, optimizing weight, stiffness, and hydrodynamic efficiency.
Rudders evolved into thin, deep blades with reduced drag. Keels now have long wing-section profiles that end with torpedoes bulbs built with high-density metals, minimizing the wetted surface and concentrating the ballast lower, obtaining good righting moment and excellent performance both upwind and off the wind.
The natural evolution of the torpedo keel is the canting keel, a concept that is both simple and complex to implement. The fin is no longer fixed to the hull but is able to oscillate to the right or left, changing position to adjust the trim of the boat. The advantages of this solution are obvious: the righting moment of the hull increases without additional weight, sail area can be increased. Hull design lines are less constrained by the stability of shape and are optimised for hydrodynamic efficiency.
The disadvantages are the high production costs, management of the hydraulic systems and the delicate set-up. Software-controlled operating systems can control heeling angles of up to 45 degrees.
As the angle of inclination of the keel increases, the efficiency of the fin decreases. For this reason, canting keel boats introduced “daggerboards”, retractable fins with an asymmetrical profile that are lowered into the water to replace the lost function performed by the vertical fin of the keel.
In some cases, we can see additional complementary appendages called “canards”. Fixed fins with a variable incidence that work in combination with the primary rudder to compensate for leeway and to steer the boat.
The ultimate expression for this type of design is the Imoca 60 Class, wide and flat hulls that need two rudders. When they heel, the midline is out of the water, and a single central rudder would not work. Two smaller rudders, inclined on the vertical plane, work optimally. The leeward blade always remains submerged and is almost vertical. The windward rudder remains out of the water reducing hydrodynamic resistance.
The new generation of ocean racers has adopted “foils“, complex winged appendages submerged with variable thickness and chord profiles.
There are various shapes (such as “L” shape, “C” shape, “triple inclination” etc.) with different characteristics and elements that are constantly evolving because they are crucial for the performance boost of modern hulls.
Essentially, foils increase the righting moment without weighing down the boat. High-modulus and high-strength carbon fibre have made it possible for designers to build them even for racing yachts of considerable displacement. Speeds of up to 40 knots are not uncommon, generating significant stresses.
In the Imoca class, boats that adopt foils do not use daggerboards or canards. Foils perform a dual function: the vertical part that exits the hull counteracts the leeway. The horizontal section generates vertical lift increasing righting moment and “lightening” the boat, which at times comes almost entirely out of the water, drastically reducing resistance to advancement.
Another characteristic is the increase in more rounded and powerful bows to counteract the possibility of nose-diving or digging and offering less resistance to impact with the water. Fuller bows also have an aerodynamic stabilizing function. Imoca rules do not allow for T-foils on the rudders. Boat movement can be very violent and “bouncy”, generating huge loads and structural stresses during the fall phase.
Finally, we come to the extreme of the America’s Cup AC75s. The keel no longer exists, at least not in its traditional conception. The goal of these boats is to sail in “full foiling” mode, constantly out of the water.
The submerged Y-shaped foil arm is positioned to leeward, under and outside of the boat, terminating with a double wing equipped with flaps that provide hydrodynamic lift and righting moment to keep the mast always vertical. The other windward foil arm remains out of the water as ballast generates a downward force.
The central T-shaped rudder provides manoeuvrability and lifts the stern of the hull. It also controls the pitching of the boat together with the foil flaps.
There is a “skeg” (although it has little to do with the rudders’ skegs) commonly called “little keel” to limit leeway during the displacement phase during takeoff and acceleration, making the transitions smoother.
AC75s are designed for inshore racing in protected waters and could never take part in the Global Solo Challenge. We probably will not see IMOCAs with foils at the start either, at least not in the first edition. However, it would not be a problem if this were the case. The format of the event makes all participants potential winners taking into account each boat’s performance profile and expected optimal circumnavigation time.