Boat 1 Hull 1

The first of the black stuff in the A Cat hull moulds...

Parts

Machined fittings for the A Class cat in anodised aluminium, stainless steel, plastic and bronze.
Contact us for more info or for your custom CNC machining requirements.
Carbonicboats can take you from an idea or sketch, through 3d modelling, to finished components in any quantity...

Another Rubicon

Chris Woods, John Fisher, and Roger Paul, have been building the latest incarnation of our Rubicon 10 Rater under license.
They have added their own touches in deck layout and fit-out, and the project is moving along nicely.
Production of our Marblehead Katana in house is underway.
Our IOM is next on the list as, for the time being, it is only available from licensed builders in Europe who are struggling to keep up with demand.

Jigsaw

Blogging about the new A Cat should resume soon after a pause enforced by transport times.
Getting everything in one place across this big country is a great logistical challenge...
A Class Cat mould making has now started in earnest. Rolls of the black stuff visible too.

Another Teaser

More Angles

Some more views of the A Cat shapes in far away Mareeba...

Glossed

The process and resin we are using for our Katana Marblehead are optimised to obtain the best ratio of resin to reinforcement fibres.
In practice this means minimising the amount of resin that cures around the fibres which are themselves a fixed quantity determined by the weight and number of layers of carbon fabric put into the moulds.

The hull and deck skins are vacuum bagged to draw out entrapped air and any resin that is not closely in contact with the fibres.
The upside is a stiffer laminate because the matrix is effectively more tightly packed with reinforcement.

It also means that the fibres appear extremely close to the surface of the finished material simply because there is no surplus thickness of transparent resin to give the visual effect of 'wetness'.
This is what modern composite parts look like. It is a sort of satin finish with perhaps a utiliterian or militaristic 'stealth' quality to the surface finish.
It is interesting that the actual surface is smooth and glossy, replicating the mould surface which in this case is CNC milled then polished to a high gloss and waxed. However the gloss has no 'depth' because the fibres are densely packed microns below the outer surface.

For those who prefer the 'old fashioned' glossy look showing off the fibres encased in an amber like transparent glossy surface, we offer the no cost option of a tough polyurithane varnish which is applied by specialists at YachtMod.
The aesthetic coating is kept as thin as possible thus minimising the associated weight penalty.

As always, the choice is up to the individual. Some prefer one look and some the other. Some want to save every last gram and others are willing to sacrifice a tiny sliver of weight for the sake of beauty. It must be said that the difference in performance is too small to measure so either option should be competitive.

The varnished option offers the possibility to wet sand the boat periodically which is considered good practice by most skippers. Cutting the surface back with 1200 or 2000 grit paper can get rid of any scratches that may accumulate with use and decontaminates the surface from any dirt or oil.
If done correctly it leaves a low sheen (yet another look) that allows water to form a thin coating over the surface without beading.
There is an argument that this characteristic prevents air bubbles from sticking to the surface and acting as trip-turbulators thus possibly delaying the transition from laminar to turbulent flow until somewhere further aft.

However if conditions are sufficiently rough to cause enough pitching to introduce air onto surfaces below the waterline, then the oncoming flow will be turbulent anyway.
Specific tests on this aspect of boundary layer behaviour are few and in my opinion inconclusive.
As long as the surface is smooth and free of contaminants such as road film or dust, drag will be close enough to the practical minimum.

Volume distribution, foil shape, rig positioning, weight distribution, stability and sail trim have effects greater by orders of magnitude.
In a tight class sailing skill will be the key.
The design that best complements the class rule and can be sailed fast with greatest ease will give the sailor the winning edge.

Transit Time

No new interesting images this week since parts are on the road and other projects tick along in that phase where a lot of work takes place but the visible differences are small...

For now, here is a video of the Aptec CNC machine at work on the A Cat rudder plugs.

Thanks to all our regular followers for the great feedback so far. Stay tuned and keep them coming.

Milestone

Machining of plugs is complete. Next week they will leave the Aptec facility. After a few days in transit lamination of the female moulds can begin. Our decision to invest time and resources on highly detailed plugs should hopefully make the next steps more efficient.

A few words on what you can see in the images:

The beam curvature evolved from structural considerations.
Initially the obvious choice seemed to be a simple 'arch' shape (as viewed from the front), possibly with tightening curvature approaching the hulls.
However such a shape has the drawback that any deflection increases the distance between the hulls.
If you imagine a straight beam deflecting, you can see that the ends move closer together.
With an arch shape that is intuitively strong, the ends move apart instead. That is why gothic cathedrals have flying buttresses...

With properly engineered beams, the deflection in normal conditions is small but it cannot be zero. Pushing the hulls apart decreases toe-in of the foils, reducing the angle of attack of the leeward loaded foil and increasing that of the windward (likely surface piercing) one.
With angled or curved foils this also reduces the amount of vertical lift available.

So our chosen solution is a compromise to combine global platform stiffness and water clearance.
The downward turn in the ends makes the junction with the hulls more normal (closer to a right angle) minimising drag when the junction is splashed by waves.
It also maximises clearance near the leeward hull, where it is most needed.
Beam size increases toward the centre as bending moment increases.
The plugs include extensions at the ends where the beams plunge into the hulls.

Finally the box shapes you can see on the hull plugs are to house inserts that determine whether a hull is a port or starboard one.
The machined inserts include detailing for beam junctions (when making inboard hulls) and foil cases (when making outboard ones).
These features are obviously different for each half of each hull so there are four sets oncluding blanks where the hull being made has no features in that area.

And Now for Something Completely Different

Recent consultancy virtual modelling work.
Once again, we are not allowed to show much given the nature of the project (UAV with defence applications)...
There are definitely interesting parallels with foiling sailboats, especially with respect to dynamic stability.

Piercing Insights - Part 3

We saw in Part 1 and Part 2 that generalised statements about the handling qualities of ‘wavepiercing’ bows miss the point that bow profile is a reflection of sectional volume distribution, which is a much more useful indicator of design priorities.

Multihull bow sections have recently tended to carry volume lower down rather than above the water ‘in reserve’. Maximum buoyancy is available at smaller bow-down trim angles.
These shapes are inherently slab-sided so tend to come together in upright stem profiles.
Such sections, combined with peaked foredecks designed to shed water easily, characterise modern bow profiles, though there is still considerable variation in the details.
Generally it can be said that such shapes behave more lineally: Gone is the sudden ‘tripping’ effect generated by a wide flat deck suddenly becoming submerged.

So far we have concentrated on downwind bow burying conditions. However the choice of bow shape must also take into account more common cases.

Image source

Straight Line Sailing

As we saw in our look at the A Cat state of the art, multihull volume distribution must consider the doubling in displacement of the leeward hull as the windward one leaves the water. As flying a hull has become more common, limiting immersion or ‘sink’ of the leeward hull has become more important. Another reason for more U shaped sections.
In this respect, the decision must take into account the relative importance of wetted area and cross sectional area, values that can to some extent be traded.

As average speeds have increased, prismatic coefficients have grown, making the ends fuller.
This has led to very interesting findings about the sharpness of forward waterline endings. The concept of a fine bow ‘cutting’ the water has been replaced by more sophisticated ideas that have more in common with aerofoil leading edge theories. Fuller bows have evolved into more bulbous elliptical entries that are less sensitive to changes in the angle of the oncoming flow.

Image source

Pitching

Dynamic periodic motion is a complex subject but the simplified rule of thumb is that the damping effect of fuller extremities is greater than their contribution to pitching.
This is the one context where ‘wavepiercing’ is an apt description. Meaning the upper part of the bow does not contribute to pitching moment as there is no upward component to the hydrostatic pressure. On the contrary, there is a small cancellation with the lower part of the bow. Interestingly, stern flare helps with pitch damping as the dynamics there are slightly different.

Conclusion

Modern bows come in a variety of profiles reflecting different design choices in section shape.
Rather than bundling all these types into one sweeping category and then drawing generalised conclusions, much understanding can be gained by looking a bit more carefully at the underlying section shape.
This should enable us to make a good educated assessment of the priorities driving the design choices in each individual case.

Piercing Insights - Part 2

As we've seen in previous posts, conventional hulls resist bow down trimming forces by immersing more volume forward.
This shifts the centre of buoyancy forward.
If the centre of gravity remains stationary or moves aft, the resulting separation gives a bow up righting moment.

Tornado style ‘conventional’ raked bow profiles indicate flared hull sections.
Meaning the sections get wider moving up, resulting in more volume at the top of the bow.
Such additional volume in the upper part of the hull is what we mean by ‘reserve buoyancy’.

Compare the two IOR maxi bows in the foreground with 
the modern VO70 bows in the background. The hollow profile of the red bow (Steinlager II) reflects progressively widening flare in the topsides. In some cases the bow rake is made less extreme by ‘cheating’ the natural intersections of the two hull halves with a variable radius between the two surfaces. Image source 

Downwind

Conventional bows seek to marry a fine waterline entry with extra volume that only becomes immersed when needed. 

Inherent in this mechanism is a need for significant bow down trim in order for the reserve buoyancy to take effect. 

On a conventional multihull this is not a problem: As the bow is pressed down, the boat will keep sailing horizontally along the surface as the bow immerses. The additional volume going into the water at the front will shift the CB forward and a new equilibrium will be reached. A few degrees of bow down trim has no adverse effect.

Boat trims bow-down and reserve buoyancy gets to work. Image source

The limiting factors in this case are bow freeboard and additional hull drag due to the progressively blunter entry of the trimmed immersed shape. 

Freeboard limits the absolute amount of reserve buoyancy available. Hence the use of 'ski jumps'.  

Additional drag limits acceleration which in turn affects apparent wind (accelerating downwind reduces pressure in the rig, relieving bow down trimming moment).

Multihull evolution has seen reserve buoyancy move down, progressively closer to the normal water level. Meaning sections have developed from being ‘V’ shaped to more ‘U’ shaped. It is no coincidence that this trend occurred at the same time as the advent of angled/curved foils.

Imagine a conventional Tornado style hull with angled or curved foils. 
When reaching at speed the foils would be providing significant vertical force helping to keep the bow up and reducing effective displacement. 

Now imagine this hypothetical boat encountering a gust: The rig would power up and press the bow down. Since the reserve buoyancy is some distance above the water, bow down trim would initially increase to bring the reserve buoyancy into play. 

But at the same time the bow down trim would reduce the angle of attack of the foils, possibly even bringing it below neutral. 

The boat would not tend to follow the water surface. Instead it would want to follow the chord line of the foils. This would create a feedback loop where bow down trim would increase bow down trimming force…

Having reserve buoyancy low down in the bow sections makes it immediately available. This is desirable when pitch attitude is critical such as on foil assisted boats.

When things get out of shape, the wide flat deck of a conventional hull abruptly increases drag at the very point where reserve buoyancy runs out. 

A carefully shaped ‘upside down’ bow brings the water flow back together cleanly above it, giving the boat a better chance of accelerating and shedding water to allow the bow to pop back up. 

Image source

But this is not the exclusive preserve of radical inverted bows. 
More moderate shapes such as the Boyer MkIV A Cat still benefit from this effect. 

This brings us back to the premise that 'wavepiercing bow' is too generic a term to be indicative of behaviour or performance.
The vertical location of the maximum section width is the feature that tells you the most about the design priorities of a particular boat. 

The bow profile is an indication of this vertical volume distribution.

In the final Part 3 we will look at the more subtle considerations of straight line sailing and wave induced pitching. 

The New Black

Katana Marblehead specimens just out of the moulds. More to follow shortly...

Piercing Insights - Part 1

We are receiving many questions about the pros and cons of so-called wavepiercing bows.
There seems to be much debate among sailors, partly fueled by unsubstantiated claims from manufacturers.

As our regular followers and clients know, at Carbonicboats we do not make dogmatic proclamations about what our products will do.
Instead we explain the reasoning that leads us to each design choice.
We aim to demystify the principles at work, acknowledging that in most cases there are tradeoffs involved.
Sharing the process is a way to communicate our passion for the art of design.

So let’s look at modern bow shapes.
Unsurprisingly, the ‘piercing vs conventional’ debate is founded on a false dichotomy.
The obvious visual character of a bow profile is in fact almost incidental.
It is driven by something more subtle: The distribution of volume in the cross sections.

Each cross section shape naturally comes together to give a characteristic bow profile.
To understand how this works, look at the following illustrations.
The port and starboard hull halves are shown in red and green respectively, and the transverse section lines are in yellow.
By extending each hull half past the centreline, you can clearly see that the way the two sides intersect defines the centerline profile of the bow.

So now when you look at a bow profile you will be able to read the section shape.
It follows that wavepiercing bows are not just conventional bows 'chopped off' with the excess freeboard removed.
Instead they are simply a consequence of a particular section volume distribution.
It doesn’t make sense to say that they are inherently more or less susceptible to burying as is being claimed by certain parties.
The question instead becomes: ‘what are the pros and cons of different section shapes?’
That will be the subject of Part 2

Getting Closer

More progress on the A Cat hull plugs in Far North QLD...
The stable and durable yet relatively economical material chosen requires sealing of the final release surface. Skilled hands are needed to do so without altering the machined shape. A final pass in the machine ensures the shape has remained true before painting with a thin coating specifically formulated for moulding applications.
At the same time the beam, foil and other detail moulds are being machined next door.