The making of the Hobie Wild Cat

Dr Yann Roux of K-Epsilon on the development of the Hobie Wildcat F18

With the F18 World Championship taking place in Erquy, France this week, so it seemed a good opportunity to look at how Hobie's new Wildcat F18, launched last year, was developed:

Left: K-Epsilon's Dr Yann Roux and right designer Martin Fischer

At present the Italian, French, and Dutch national F18 titles are held by crews sailing the Hobie Wild Cat. At the Eurocat 2010 regatta held last in May in Carnac, seven Wild Cats finished in the top 10 and their “constructor’s title” is looking good too at the Worlds Mitch Booth, Olivier Backes and Micha Heemskerk in the top 10.

Left: K-Epsilon's Dr Yann Roux and right designer Martin Fischer

The development of the Hobie Wild Cat was a joint effort of Hobie Cat Europe, their sailing team, K-epsilon for the CFD RANSE simulations and Martin Fischer who was in charge of the design. After a development and prototyping period of about six months the series production was launched in early spring 2009.

The design brief given by Hobie was very simple: Designing a boat that can win the F18 World Championship. Despite the existence and success of the Hobie Tiger, design work started from a blank sheet of paper and there was no constraint to use existing parts from the Hobie production line for the new boat. Several design meetings with top sailors like Jean Christophe Mourniac, Mitch Booth, Darren Bundock and Glen Ashby were held. They gave valuable input on many aspects, and this collaboration certainly helped improve the final version of the boat.

To race in the F18 class, boats must fit into a simple box rule that basically specifies maximum length (5.52m) and beam (2.60m), maximum mast length (9.10m) and sail area (21.00 sqm upwind plus 21.00 sqm for the spinnaker) and a minimum weight of 180 kg, ready to sail plus specifications on building materials and some additional dimensions. This relatively open rule has led to an intense development and refinement of the boats over the past 10 years. Therefore it was clear at the beginning of the project that a serious development effort had to be made to get a boat that cannot only beat the existing ones, but that can also stay competitive for several years.

Hull

Right from the beginning it was decided to run a full set of CFD simulations for the hull development. K-epsilon in France was commissioned to run these simulations using a state of the art free surface RANSE Navier Stokes code.

In general on major regattas an up-and-down course is sailed. We therefore focused for the tests on a boat speed range of 10 to 12 knots (typical upwind speed) and 16 to 20 knots (typical downwind speed).

Over the last years F18 hulls have become significantly wider to be able to push harder downwind. The hull width was therefore an important research area for us. We tested shapes with varying hull widths. The results of the RANSE simulations showed that there is a drag penalty especially upwind if we were going for very wide hulls. After discussion with Hobie and some of the top sailors we decided to go for a moderately wide shape. The idea behind this was that many regattas are decided on the first windward leg, so being quick upwind pays in general.

A very distinct feature of the Hobie Wild Cat are the 'spray rails' or 'strakes' in the front part of the hull. Initially all hull candidates that we tested had smooth front sections without spray rails. These hulls are so narrow that the bow wave does not break. Instead it sticks to the hull surface which increases the wetted surface and hence the drag. The strakes create an obstacle for the bow wave and forces it to detach from the hull. This detachment is not free of charge but comes with some parasitic drag. Above a certain boat speed this parasitic drag is more than compensated by the reduction in wetted surface; below this threshold the parasitic drag of the strakes is too costly to be compensated by the reduction in wetted surface. We used the CFD RANSE simulations to position the strakes such that at low speed, when the spray rails are costly they do not – or only occasionally – hit the water, and only at higher speed, when they are advantageous, do they come into play. For the development of those strakes a relatively sophisticated CFD RANSE programme was absolutely crucial. We never would have dared adding strakes to the boat without such simulations.

Appendages:

For the rudders and daggerboards we used the DAG wing section, which was especially optimised for the use on sail boats. It is a section with a pronounced hollow at the back. The difference compared to most other hollow sections is that this one was designed for a fully turbulent boundary condition and not for a laminar one. Wing sections that rely on a laminar boundary layer may show rather poor performance if the incoming flow is already quite turbulent. In that case the transition from laminar to turbulent boundary layer is very early and the rear part of the laminar section does not work as it is supposed to do. On small boats sailing at relatively high speed in rough conditions (wave induced turbulence) laminar sections are therefore in my opinion not appropriate. Today there are five different versions of the DAG section (mk-I to mk-V). Different DAG sections have been used on the Capricorn, Wild Cat, Groupama 2 and Groupama 3, Sodebo-Maxi, Banque Populare-60 and on the BMW Oracle Racing trimaran.

For the daggerboard plan form we used shapes with a very high aspect ratio. Such appendages are highly efficient, but they also stall more easily at low speed. Thus they require proper steering especially during manoeuvres and in starts. This point was discussed with Hobie but their design brief was very clear: designing a boat for top level competition. So we went for these more demanding appendages. At the beginning many sailors were sceptical about this choice, but after one year of racing it seems that we took the right decision.

On a multihull the part of the windward rudder that works depends on the heeling angle. Thus it may happen that the compensation of the rudders changes as a function of heeling angle. This is a very undesirable feature since it makes for a change of the sensation at the helm when the heeling angle changes. Designing the plan form for the rudders we paid a lot of attention to ensure that the rudder moment and hence the sensation at the tiller is not affected by heel.

Mast

The mast for the Hobie Wild Cat was designed from scratch. The goal was to obtain a real wing mast with a long chord length at the upper limit of the F18 rule, but maintaining sufficient lateral stiffness. At the same time we wanted to reduce the fore-aft stiffness to facilitate depowering of the main sail in strong winds.

With subtle changes of the position of the maximum width of the mast, the local surface curvature and the wall thickness we obtained a mast that is slightly stiffer sideward and slightly softer fore-aft than most other masts used in the F18 class. A full 3D finite element structural analysis was run at the end of the mast development process to verify buckling resistance.

Sails

As for any other race boat the sail development programme is an ongoing process. The principal partner of Hobie Cat Europe for the sails of the Wild Cat is Ullmann / Italy. Since the launch of the boat they have constantly been in contact with the Hobie Cat sailing team to further improve the performance of the boat. Recent race results are the best testimony for their success.