In recent years, the superyacht market has become more focused on greater performance by optimizing all aspects of a yacht’s design, engineering and construction.
The choice of the keel configuration is surely one of them.
It’s not surprising that the first three units of the brand new SW105 miniseries will each have unique keel designs to satisfy the requirements of three different owners.
To his end, we’ve asked Jim Schmicker, Vice President of Farr Yacht Design, one of the world’s
foremost designer of racing and cruising sailboats, to explain how the choice of the keel design has
specific benefits that make it the best choice for a particular owner’s needs.
Jim Schmicker Is Vice President and shareholder of Farr Yacht Design. The company is recognized as one of the world’s foremost racing yacht design studio, based on one of the most impressive winning results records ever compiled by a single company. For more than 30 years, FYD has been developing fast, custom and production cruising yachts. Southern Wind has collaborated with this reputable studio since 1992.
When approaching the cholce of a keel, an owner should be aware that each of the options has advantages and disadvantages
but all of them are designed to deliver excellent performance and achieve stringent stability targets while maintaining a similar displacement.
The simplest keel option for construction and installed systems is the fixed keel. The choice of draft for a fixed keel Is decided by balancing upwind performance against reasonable access to ports and anchorages. Reaching and downwind performance is the strongest feature of the fixed keel so long as sufficient stability is achieved. The keel is fabricated out of mild steel plates that are formed and rolled into the correct shape. Considering cost, portions of the keel, such as the leading and trailing edges of the fin, can be CNC machined and the rest hand-faired or the entire keel can be machined. This construction method results in a simple and light structure. Given the shallower draft compared to lifting or telescopic keels a heavier bulb Is necessary to achieve the target righting moment. However, the light fin construction helps to mitigate some of the relatively greater bulb weight. Attachment to the hull is entirely below the cabin sole which facilitates a variety of choices of interior layout with no constraints on either the accommodation or machinery spaces.
Fixed Keel showing Bolt Pattern and Internal Construction
The lifting keel is a popular choice for superyachts of this size. The ability to raise and lower the keel allows access to ports and anchorages with limited water depth while the deep maximum draft achieves excellent upwind performance. The keel construction is complicated with hydraulic cylinders housed internally to the keel, PLC systems, locking pins to hold the keel In the raised position and adjustable bearing pads to ensure tight tolerances and no movement of the keel In Its trunk while underway. The high number of moving parts and complex hydraulic control systems have associated installation and maintenance costs. The keel trunk takes up significant space In the accommodation but with clever integration with other aspects of the interior its impact can be diminished. The keel is typically constructed out of high strength carbon steel plates welded together and CNC machined to an extremely high level of accuracy. As such, advanced foil sections can be used which results in higher lift to drag ratios being achieved. The lower portion of the keel fin, below the hull in the raised position, is tapered to improve lift efficiency, optimising the amount of surface ares and reducing drag.
Lifting Keel with Tapered Lower Portion Showing Hydraulic Cylinders, Trunk and Bearing Pads
The telescopic keel combines some of the benefits of the fixed keel and lifting keel. It achieves a similar amount of draft adjustment as the lifting keel with only minor intrusion into the interior. The upper, fixed part of this design is installed partly inside the hull but mostly outside and below the hull surface. The lower, moving part retracts into the upper part and incorporates a foil-shaped shell that slides over the outside of the upper part. Similar to the lifting keel, the telescopic keel is a complex installation with a high number of moving parts and hydraulic systems with associated costs. The fin is typically constructed out of high strength stainless steal plates welded together and CNC machined to an extremely high level of accuracy. The un-tapered planform shape required to house the hydraulic cylinders and structure supporting the lower part results in higher surface area, The fin components have a relatively higher weight and center of gravity.
Telescopic Keel with Un-tapered Lower Portion Showing Hydraulic Cylinders, Internal Structure and Shell
Each of the keel designs has specific benefits that may make it the best choice for a particular owner’s needs. In terms of draft, both the lifting and telescopic keels achieve shallow draft (3.15m to 3.65m) without compromising performance as a result of their heavier fins and associated structure. The fixed keel requires an acceptable amount of draft (in this case 4.5 meters) for reasonable upwind performance while still allowing access to the owner’s preferred ports and anchorages. A fixed keel has a much lighter fin and associated structure weight. For the same displacement, it achieves the highest righting moment because the keel has the deepest center of gravity as a percentage of Its draft. A secondary benefit of the fixed keel Is less heeling moment because the sideforce it generates is acting closer to the surface of the water so the fixed keel version operates at a lower angle of heel.
The telescopic keel has the best combination of performance, harbor access and disruption of the interior. Its disadvantages are greater wetted surface, volume outside of the hull and maintenance costs. With specific reference to the Southern Wind SW105 project, because similar displacement was a design requirement, the performance differences between the first three units is not large. However, the deepest maximum draft (5.6m) of the telescopic keel produces the best upwind performance, as a result of its lower induced drag.
The lifting keel (at 5,15m draft) has the next best upwind performance while the fixed keel is strongest in power reaching conditions. For performance versus rating the lifting and fixed keel versions are essentially equivalent over a balanced race course with the advantage going to the lifting keel for more upwind-downwind oriented races and to the fixed keel when the reaching content Is greater. The telescopic keel, with its slightly less efficient keel shape, comes in a very close third place behind the other two options. The initial cost of the fixed keel is the least of the three and ongoing maintenance costs will be less than these of the lifting and telescopic options.
KEEL CHOICES FOR THE FIRST THREE SOUTHERN WIND 105’S
The Southern Wind 105 Is the newest addition to the SWS line of luxurious, performance, blue-water cruising superyachts. The first three yachts constructed will each have unique keel designs to satisfy the requirements of the three owners. The overall parameters of a superyacht of this size, the necessary draft for reasonable upwind performance and the owner’s requirements for keel draft for access to his preferred ports and anchorages have led to fixed, lifting or telescopic keels being viable options.
Desire for an advanced keel design with maximum upwind performance without any significant compromise to the interior layout and saloon space.
Best performance for both racing and cruising and no requirement for a specific minimum draft. The 4.5m draft is designed to achieve the low leeway angles desirable for racing combined with high sailing stability.
Best performance combined with a minimum draft requirement of 3.1m Is the strongest driver of the keel design. Intrusion into the interior is apparent but details of the trunk design allow light across the saloon and avoid a complete separation of the two sides of the yacht.