A Class Catamarans – A Look at the State of the Art Part 10

Having chosen a hull and foil geometry, the next task is to execute the carefully optimised shapes accurately and efficiently.

Class rules mandate a minimum overall weight of 75Kg for the complete boat with no other restrictions on material and shape above the waterline. 

Keeping weight at rule minimum is very important for performance as carrying additional mass is slow.

It is desirable to aim for a finished boat weight around 1Kg shy of the minimum to allow for

1)      Variations in weight between different rigs, and

2)      Inevitable repairs that may be required over the competitive life of the boat. These may result from collisions during racing, filling accumulated dings and scratches, or other accidents…

When the boat is new the weight difference is accounted for by ballast that can be placed centrally to minimise pitching.

It is possible to build boats well under rule minimum weight. The challenge however is to invest the mandated weight to best advantage, taking into account stiffness and mass distribution.

Overall platform stiffness is good because

1)      It maintains the designed geometry between hulls and foils under load, and

2)      It means less of the finite energy extracted from the wind is sapped by elastic deformation.

Similarly, stiffness of each hull

1)      Maintains underwater shape,

2)      Provides geometrically consistent rig support,

3)      Minimises resonant ‘wobbles’ when loads vary upon exiting waves.

Overall platform stiffness is mostly dependent on the stiffness of the crossbeams and their connections with the hulls.

Individual hull stiffness is determined by hull shape (mainly 'boxiness'), construction material, reinforcement choices, and internal structure. 

Achieving sufficient hull stiffness is challenging because of the long cantilever ahead of the front beam. This unsupported span typically amounts to half of the overall length, more on some recent designs. 

The hulls are also typically slab sided forward, with large flat areas that need to be carefully considered in terms of stiffness and local buckling.

In essence, each hull is a box girder (or squared tube) cantilevered in bending about the front beam and reacted at the rear beam. 

In the vertical plane the load is predominantly in ‘sagging’, with the forestay pulling up from part way along the cantilevered span, and the sidestay pulling up aft of the crossbeam. Mainsheet loads are passed to the back of each hull, adding to bending and introducying a shear/twist element.

In the horizontal plane there is an inward component from the stays and there are substantial hydrodynamic loads pushing the bows sideways (alternating both inward and outaward).

Most existing boats use horizontal stringers or ‘shelves’ along the middle of the flat topside panels to increase the moment of inertia of each hull side panel. Often the shelf extends inboard to ‘tie’ together the opposing hull sides.

Hull panel laminate also has to resist ‘bruising’ from the sailor kneeling/standing on the bilge during capsize recovery. 

Some degree of tolerance to ‘real world’ conditions is important. Light contact, beaching, and occasional rough handling should be considered without unduly compromising performance.

Since material choice is unrestricted, effective constraints are to do with

-          Stiffness for a given weight,

-          Longevity and ease of repair,

-          Material availability,

-          Construction (tooling) method and cost. Especially the relationship between tooling cost and individual boat cost.

Foam and honeycomb core materials are each used in competitive boats. The optimum solution changes with the relative emphasis placed on the above factors.

I will go into more detail on the pros and cons of foam vs. honeycomb core when discussing our choices for the new boat.

Beam junction loads are usually spread into the hulls by full bulkheads or ring frames that stiffen the hull shell locally.

Typical beam solutions include

-          Filament wound (or similarly mechanically produced) round tube, typically with greater wall thickness top and bottom to increase transverse bending stiffness,

-          Similar industrially produced straight tube but with a ‘D’ cross section rather than round,

-          Custom moulded curved beams made in open (two halves cured separately then glued together) or closed (bladder/slip joint) tools.

More on the merits of different beam construction and joining methods later.

I have posted before on the value of a well defined brief where class rules impose no apparent constraint. The A Cat is a great example of an open rule where choices have to be made within a broad rule space, so it is important to evaluate and prioritise solutions with an awareness of the desired outcome.

Complete freedom in hull shape, freeboard, sheerline, and detailing, allows great innovation. To be successful, the desired outcome must be clear, and priorities must be well defined.

Just to give one example: greater hull volume (width/height) at the sheerline improves stiffness but adversely impacts windage and drag in waves. A taller hull with a broader deck will be stiffer for a given weight but will have greater aerodynamic drag and more additional drag in waves.

It is a fascinating challenge to quantify the crossovers between the various factors being traded against each-other. A challenge we are thoroughly enjoying.