Katana Marblehead Design – Hull

Katana has the same waterline beam as Octave. Canoe body maximum depth has increased by 2.4mm. Maximum cross section area is unchanged, staying at a value that has proven optimal.  Moving some midsection area from the turn of the bilge to the bottom of the hull gives a midsection that returns to being as close as practical to a true semicircle.

Katana in red, Octave in gray

Shaping of the ends incorporates lessons on boundary layer behavior learned in other work we have done. This new knowledge has refined our analytic tools, reducing the margin of error.
Armed with higher resolution tools, a more linear pressure recovery could be engineered reliably. The newly resolved pressure recovery rate is achieved through straighter diagonals from the mid section to the transom, combined with aft sections that are closer to semicircular. 

Straightening the run aft makes pressure recovery smoother, placing less stress on the boundary layer. In practical terms, this means not asking the water flow to follow excessively tight curves toward the back of the boat because, by the time the water reaches the back half of the boat, much energy has been lost to friction in the boundary layer.

This concept is not new, but being able to quantify how much we can 'ask the flow to do' empowers us to identify the optimum values for the conflicting requirements we are trying to mediate.
A very simplified overview might go something like this:
On the one hand we want to bring the flow back together (from max beam/draught to a point on the centreline/waterline near the transom) to
1) Make the wake as small as possible - smoothly refill the hole in the water made by the boat and
2) Get as much 'push' as we can from the water pressure on the aft surfaces of the boat - since the surfaces are angled inward, the normal pressure that acts at 90 degrees to the surfaces has a component pushing the boat forward. This component would in an ideal world be the same as that pushing back on the forward parts of the hull, but in reality is less due to energy lost through viscosity in the boundary layer.
On the other hand we want to maximise volume in the stern to
1) Get as much support as possible from the stern wave,
2) Damp pitching,
3) Avoid flow separation and
4) Maximise power.
All the while we want to keep wetted area to a minimum...
So you can see how nailing down more exact values makes our design choices much clearer!

In most conditions this particular change as implemented on Katana is near neutral. It trades the power and support of firm aft sections for reduced drag.
But in specific conditions (namely low to medium speeds, very high speeds, and in waves) our updated analyses show a small but measurable gain.
The new aft treatment has the advantage of less wetted surface area, which is a bonus at low speeds. At higher speeds the risk of laminar separation is reduced. 

The new stern treatment has the effect of reducing prismatic coefficient. In order to maintain the high prismatic coefficient of our successful previous designs, the sections in the forefoot were made even firmer, adding volume with a pronounced ‘U’ shape that transitions smoothly into the semicircular mid and stern sections. 
The forward volume distribution has been revised with a less aggressive rocker profile but more angular sections in the forefoot. 

This treatment of the forward sections has several advantages: it increases resistance to bow-down trimming moment both hydrostatically and dynamically, it keeps the entry narrow at the waterline (by pushing volume down rather than out), dampens pitching, and moves the LCB forward (also a trend in the evolution of our designs).

Above the water, the forward sections remain vertical, with a peaked foredeck for clean wave piercing and to keep added drag to a minimum when over-pressed. 

Moving aft, the topsides are no longer vertical but instead flare progressively. 

Amidships the moderate flare provides additional support, smoothing the heeled waterlines and helping to locate the heeled LCB such that trim remains neutral or slightly positive with heel. 

At the maximum deck beam location there is a subtle inflection under the gunwale to enhance water shedding when pressed and in waves, keeping aft flowing water off the sidedeck.

Finally some flare in the topsides aft has been introduced, accounting for perhaps the single largest visible change from Octave. 
In fact the new stern treatment achieves a similar effect to the characteristic soft chine/tumblehome of Octave but does away with some associated minor penalties. 

Specifically, water shedding is now done by the hull/deck joint instead of the chine. The sharp edge and acute included angle are more effective, but are higher up, so the flow remains attached a bit longer than would be ideal. 
However, since the sections are more rounded, the actual distance along the hull surface between the two separation lines is only marginally greater than before. 

Also, the new sheerline is lower at the back, reducing the distance even further and doing away with some mass in the process (the sheerline is more steeply inclined, being the same height as on Octave amidships, and higher at the front). 

As always there are compromises involved. This aspect of this particular choice is a net gain in some conditions, neutral in others and possibly a slight loss in the particular circumstances when the previous arrangement was at its best. 

To tip the scales, the principal advantage of the new stern shape is enhanced pitch damping. Marbleheads are inherently susceptible to speed sapping pitching due to their deep bulb, tall rigs, fine ends and (obviously) their small size relative to common wind generated waves. 

Our updated tools tell us that the dynamic effect of horizontal area aft is smaller than previous results showed. 

This is consistent with a more accurate understanding of boundary layer behavior. 

So the best way to damp pitching aft (over the full range of speeds/conditions) is hydrostatically, by progressively increasing waterplane area as the aft sections sink.

In summary, the new boat incorporates several small but significant changes that are all consistent with new knowledge we have acquired through other work as well as feedback from prototype development.
Major values such as waterline beam, midsection area and prismatic coefficient have not changed. 
Management of the flow has been refined whilst still achieving a 1.5% reduction in wetted surface area and an increase in power to carry sail, especially downwind.

It is worth remembering that the differences identified through more accurate theoretical analysis tools are small. But they do exist. 

And each small change cumulatively contributes to race winning differences. 

Furthermore, a deeper understanding of aspects such as boundary layer behavior enables the designer to adopt a consistent approach. The parts can be designed to work better together taking into account realistic flow phenomena. 

Quite apart from fine numerical validation, meaningful gains were made by learning from real observations of handling characteristics and other aspects of behaviour by a number of different observers, through a deliberate and structured development programme. 

This is why we are now confident to embark on series production of Katana.

Marblehead Development

A sneak preview of our next RM design: Katana.

Katana is an evolution of Octave, incorporating improvements in several key areas.

The individual changes are small, but sufficiently numerous to cumulatively warrant a new designation.

This decision has been made with existing customers in mind as it will give them a clear option when placing an order. Those who have ordered recently were naturally briefed on the upcoming transition so they could make an informed choice based on the characteristics of the two boats.

As always, we make a clear distinction between development work that we carry out in house or in collaboration with like minded skippers, and commercial series production.

Committing to production involves significant investment in tooling on our part and requires a high level of confidence to guarantee a known performance profile to the customer who does not wish to risk investing in an unproven design.

The nature of our business is such that we are always developing and looking to the next performance gains. We must therefore be disciplined in structuring R&D with respect to value for money from the point of view of the customer. 
There are several key tests that we apply to a new idea as it progresses from intuition, to vague notion, to sketch, to virtual model, to quantitative analysis, to prototype... 
At each stage the value of the idea must stand up to tests which cover performance as well as reproducibility, cost, compatibility with existing items, durability, and especially the relationship between these key attributes.

Over the 18 years that we have been developing RC yachts, we have been careful to structure development and series production accordingly, and our repeat customers are a testament to the effectiveness of our approach. 
In competitive performance applications, risk cannot be eliminated, but it should be estimated and managed. 
There are always compromises to be made with respect to performance in different conditions and circumstances. We therefore make an effort to narrow the uncertainty so that we can inform the customer of the characteristics and suitability of each product.

It is fascinating to study the overlap between the passion for that elusive perfect design and the real world constraints of technology, cost, and commercial consistency. 

As I have stated previously, successful projects incorporate such real constraints in the design brief and in the project management process to create the best result in the real world.

Weighing the Options

As mentioned previously, the choice of tooling material and shape depends on the construction process of the parts to be moulded.

To decide on construction method we look at the desired properties of the finished product.

The hull can be thought of as a box girder that has to resist global bending loads and other localised forces at specific points such as stay attachments, beam junctions, foil housings and where the crew stands.

In a box girder the outer edges take tension and compression and the connecting faces work mainly in shear, preventing the load bearing edges from moving relative to each-other. 

This is an efficient arrangement because the corners are furthest away from the neutral axis so can be thought of as having the best leverage. 

The curvature of the edges also makes them less prone to local buckling.

The concept is similar to a truss such as you might see on a construction crane. 

The members that make up the long edges of the truss are substantial but the diagonal members are comparatively dainty. 

To build on the analogy, an A Cat hull relies on additional unidirectional fibres running along the turns of the bilge and the gunnels to take global bending loads efficiently. 

The panels between the four outer edges will have fibres running diagonally between the edges in a pattern similar to the diagonal elements of a truss.

Truss boom on an IACC yacht. To resist global bending loads, the long outer edges take tension and compression.
The connecting panels use diagonally aligned fibres to prevent relative movement of the edges.
Image credit unknown. 

Where forces are applied at a mechanical connection point such as a stay attachment or beam junction, the load path can be resolved locally with additional reinforcement and possibly a bulkhead or ring frame. 

Where the load is hydrostatic or hydrodynamic, panel stiffness needs to be considered more globally. 

In both cases, if the panels are inherently stiff, then less additional support is required for a given deformation.

Panel stiffness is therefore important to global stiffness as well as to maintaining the local design shape. 

Thickening a panel increases its stiffness. 

For reasons similar to those governing material distribution in a truss, the material furthest away from the neutral plane of the panel works most efficiently. 

This is why a comparatively weak material such as foam or a low density material such as honeycomb can be used in the middle of the panel in conjunction with strong/stiff materials such as carbon fibre for the skins.

So stiff is good and thick is stiff. 

Thickness is best achieved using sandwich construction. 

This brings us to our first major decision: what core material to use in the sandwich. 

The two candidates are foam and honeycomb…

A Cat RANSE

Graphic visualisation of hull wave height around a candidate shape.
These simulations are very intensive in terms of processing power, so must be used selectively to keep time frames realistic.
Fortunately we are in good hands. More will be revealed soon.

A Cat Update

Just a quick progress report for those of you who are regular followers.
Design work is going well with some very interesting insights already in the bag.
A promising hull concept has been identified and tests have started on a family of variants.

The opportunity came up to run some more advanced simulations than we had originally hoped for.
This will add four weeks to the schedule but will give even greater confidence in the final design choices.

While the design work continues we have been evaluating options for tooling and construction methods as well as choosing suppliers for materials and parts.

The first choice regards which parts and stages to machine using CNC/CAM technology vs. traditional pattern/mould making and hand finishing.
This decision is about striking the right balance between machine time cost and labour cost.
Interestingly, the optimum strategy will differ depending on local labour rates, competitiveness in the CNC/CAM market, and the complexity of each part.

Early in the project we decided that investment in tooling is warranted where it will reduce the time required to assemble/finish each boat to the desired tolerances.
Though the exact shapes have not yet been finalised, it is safe to assume that the foil tooling will have a non planar geometry requiring high precision (fine tolerances) whilst being difficult to build using traditional methods owing to the lack of a flat reference plane.
Finally, it makes sense to include in the tooling certain details to optimise beam junctions, stay attachments, and fitting mounting features, again to reduce time spent hand finishing each boat.

With all the above considerations, and given availability of competitively priced CNC/CAM service providers in Australia, the numbers come out decidedly in favour of automated machining straight from the digital 3D model. This approach is consistent with our standard practice of fully modelling all assemblies before manufacturing.

Initially we looked at machining female tools out of solid material (alternatives included tooling board, modelling foam with machinable putty skins, or MDF with a glass skin).
This would eliminate the step of laying up female tools over traditional male 'plugs', but would have the drawback of constraining the temperature and pressure we could apply during the curing of the final parts.

Traditional composite female moulds laid up over computer machined male plugs seem like the way to go. They give the freedom to use prepregs at reasonable temperatures and pressures to obtain better compacted and more stable finished parts.

Integral in this decision-making process was an evaluation of different core materials that led to some interesting conclusions...

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.