Australian Sailing - March 2011
The YuuZoo Big Boat Racing Team undertook upgrades to their maxi yacht in preparation for the Rolex Sydney-Hobart Race. Dario Valenza explains the science.
The upgrades to the YuuZoo maxi were about optimising the underwater lift-producing appendages and refining the rig, as well as introducing the ability to add mass when sailing upwind in waves.
The boat was originally designed to comply with the then compulsory 10 degree static heel limit, which restricted the influence movable ballast could have on overall stability. Because of this limit, the keel was only allowed to cant moderately and therefore (still being close to vertical when canted) contributed significantly to the production of underwater lift or lateral resistance.
Chief designer Alex Simonis of Simonis-Voogd commented: “With the rating limit gone, the larger yachts got an edge and YuuZoo needed work. The original design allowed for the canting keel angle to be increased, together with a larger rig to be carried. However with this came the problem of dealing with the lift requirements to keep leeway under control.
“The small, forward, lifting canard with trim tab which handled this problem so well in 2004 (when the boat won line honours) could no longer do the job as the help from the keel foil was now reduced to hardly anything: with a canting angle of 55 degrees plus 20 degrees of heel, the keel foil is at 75 degrees from horizontal.”
The brief was to come up with an arrangement which would enable the boat to sail upwind with zero leeway while being able to do away with redundant appendages when reaching and running.
The design team looked at twin asymmetric boards as seen in the Volvo 70s and Open 60s, but became convinced that this solution was not appropriate for a narrow boat with a more widely focussed performance envelope. Also considered was a larger board with a trim tab but the question was whether the structural constraints imposed by a trim tab would restrict our choice of foil section and result in an overly thick/heavy foil. The trim tab hinge line needs to remain close to straight unless heavy and complex compromises such as multiple spans are considered.
Alex Simonis: “To settle the debate, an in-depth study with advanced CFD (Computational Fluid Dynamics) was undertaken, integrated with FEA (Finite Element Analysis) to loop the hydrodynamic and structural elements of the design.
“With the initial models set up, the early CFD runs confirmed that this was indeed the way to go. The question was now how to generate the required lift, either through a board with a trim tab or the alternative idea which was suggested, to fit a type of ‘gybing’ board as seen on some advanced dinghies. Basically a gybing or ‘tipping’ board allows the fin to settle upwind at a positive angle of attack in such a way that it already generates a fair amount, if not all, the required lift without the hull making any leeway.”
Gybing boards in skiffs are generally set up to "flop" across of their own accord. This is achieved by creating a bearing arrangement such that the centre of effort (CE) or centre of lift of the foil is ahead of the pivot point. As the angle of attack (AoA) increases, the foil wants to rotate such that the angle increases further and so on… You can visualise this like an over-balanced rudder where the shaft is too far aft along the rudder chord.
On the YuuZoo maxi the AoA of the foil can be controlled by means of a hydraulic ram. The design team came up with a novel and very elegant bearing arrangement involving a "cassette" that houses the foil and turns within the watertight box or case. The bearings are positioned so that the foil is semi balanced and retains a slight tendency to self-centre as a safety feature.
The structural arrangement is very efficient, reacting the foil loads within 600mm of the hull floor and thus maintaining a low centre of gravity and limiting protrusion of the blade above deck when retracted.
The key to the design is having the ability to rotate the foil and retract it with infinite adjustment. Under normal upwind sailing conditions in moderate breeze, the foil will be fully lowered and set at a moderate AoA so that it generates most of the side force required to balance the sail side force with zero leeway – thus minimising hull drag as the hull will track true. In close tactical situations, the foil can be rotated to increase AoA. This will increase the lift.
Since the foil is forward of the mast, the extra lift will shift the underwater balance forward and create weather helm. If the helmsperson reacts by turning the wheel (helm down) and increasing the AoA of the rudder, the net result will be that the boat will climb to windward “crabbing’ along at negative leeway. Naturally there is a drag penalty involved so this is not the ideal configuration for best VMG (Velocity Made Good) but could be handy when there is a need to climb away from a competitor or shoot a rounding mark.
As the boat bears away onto a reach, the foil is gradually retracted so only the bare minimum required is in the water at any given time.
According to Alex Simonis, “Back in the ‘virtual tank’ (CFD models) the gybing board was tested against the fixed fin with trim tab and it was found that both solutions could work nearly equally well. There were certain sailing conditions where the trim tab would do the job better than the tipping board and vice versa but overall there was little in it.
“The FEA loop of the design gave the final answer in favour of the tipping board as the required strength and deflection limits could be better achieved with that.
“The gybing board is unorthodox and to the best of my knowledge not really applied before in a yacht of this size and concept, but that is what the Yuuzoo BBRT and Ludde Ingvall (the owner) stand for: getting there innovatively without always following the route that others do. The solution certainly looks good on paper, but the proof of it will be in the proverbial pudding.”
Other upgrades include optimisations to the sailplan and the addition of water ballast. Of note is a new furling foresail with inflatable battens that can be pressurised from on deck once the sail is unfurled. The battens are then deflated for furling. The new sail wardrobe is from Quantum while the forward foil upgrades and related deck modifications were carried out under contract by Boatspeed.
After a tough Sydney-Hobart race which saw YuuZoo withdraw after a multitude of incidents, none related to the modifications, the boat will continue her summer sailing program with coastal racing and continued crew training. Development will continue, hopefully unlocking the extra performance from the upgraded configuration.