A World(s) of Learning - Part 1

One of the things I love about yacht racing is the ability to get objective feedback in testing and competition.

As with any experimental science, the feedback comes mixed with noise, and bundled with data that is correlated to, but may not be caused by, the variables being tested.

Part of the challenge is to decode this raw, dirty, but ultimately objective information and decipher the 'meaning' as relevant to the concepts being tested.

The channels available for collecting quantitative data are often limited by considerations of budget, time, and practicality. However qualitative feedback is plentiful.

Even in one-design sailing, the variables in settings and technique make the game fascinating. In development classes there are the same 'fine-tune' knobs to twiddle as well as coarser variations.

Remaining objective, being willing to set aside preconceived beliefs, and being able to adapt one's thinking are key to making progress. Cultivating the art of seeing the signal among the noise is an absorbing pursuit.

So in this series of posts I will share some of the impressive lessons gained at our first A Class Worlds.

Kinetics

Given the early and immature stage of foiling development in the class, the single most critical factor this time around was mastery by some of aggressive kinetic techniques able to momentarily force a high instantaneous flow rate over the foils.

A specific sequence was evident, having been learned and perfected both in parallel and in consultation by the top players. Here is an attempt at describing it:

1) Maximise boatspeed: Use a combination of energetic sail trim (pump) and steering for best speed through the water to unstick the windward hull (sail hotter if running downwind and foot off if sailing upwind).
Let the boat heel gradually through this phase, progressively transferring weight to a hiking position as the apparent wind increases causing heeling moment to grow.
The idea is to coax boatspeed to rise, regardless of heading, while keeping fore-and-aft trim level, to reduce foil angle of attack (AoA).
If bow up trim is excessive in this phase, the drag caused by the foils attempting to lift the boat prematurely will make it difficult to reach takeoff speed.

2) Once up to speed, steer down to reduce sideforce, using the trapeze to roll/flick the boat upright while stepping out/back.
Hardly any rudder angle is required since rolling the boat to windward helps it to pull away.
This step adds another burst of speed due to the dynamic effect of the mast rotating to windward.
When the boat is flat both foils will have maximum projected horizontal area.
Pulling away will have reduced the sideforce, unloading the vertical part of the foils.

3) Before the speed decays, step back to increase AoA on the foils.
The kinetic energy built up in steps 1 and 2 is converted to potential energy as the foils 'bite' and force the boat up.

4) Once 'popped', work hard to co-ordinate heading, trim and sail force to remain airborne.
In this mode hydrodynamic drag is so small that remaining fast enough to stay on the foils is relatively easy.
The challenge is to react in time to the inherent instability in heave of a 4 foil system (especially J foils).
The time available to make corrections gets smaller the higher the speed (the stronger the wind).

Balancing on Unstable Foils

Since there is no automatic decrease in foil force with ride height, the two stability 'controls' available to the skipper are speed through the water and AoA.

Speed can be manipulated by steering toward or away from the wind.
A small blessing is that the direction of 'salvation' coincides with where you want to go in terms of VMG: Downwind, as your speed threatens to increase, you progressively bear away. If done correctly you take the extra energy provided by a gust in depth rather than speed, preventing an increase in foil force that would lead to a 'launch' followed in short order by an undignified crash.
Upwind you sail higher when you need to reduce speed, taking height back once airborne.

AoA control is related to stability in pitch.
If the boat is unstable in pitch, you have to weight-shift fore-and-aft to control trim/AoA while also controlling speed and heel. Not an easy task!

This point is worth exploring: it is important to understand the distinction between stability in pitch (rotation about a transverse axis) and stability in heave (translation up/down of the whole boat).

J foils do not have stability in heave. There is no correlation between increasing ride height and decreasing lift. Therefore manual corrections must be made to keep lift equal to weight.
Our comma foils are predicted to be neutrally stable but we have not yet tested them applying the kind of kinetic techniques described in this post. We do not yet have sufficient information to draw conclusions so I will leave them aside for this discussion.

Stability in pitch is quite independent from stability in heave. It is simply the tendency to dampen out changes in bow up/bow down attitude.
To be strictly correct there is some coupling because, as ride height varies, foil area changes. But this change is small for J foils (part of why they are unstable in heave) and is largely canceled out by other variables such as speed and rig moment. So for the purposes of a conceptual understanding the two degrees of motion (pitch and heave) can be considered separately.

Stability in heave has been the subject of previous posts.
Most solutions to obtaining heave stability involve a drag penalty of some form.
Moths use a wand that senses the water surface and actuates a flap on the main foil, changing its camber and hence its coefficient of lift. This involves the (minor) parasitic drag from the wand 'spoon' and the flap hinge.

Multihulls can take advantage of asymmetric setups to obtain stability in heave.
The 'acute L' foil has proven to be the most effective solution in modern racing multihulls.
Since the horizontal foil has a component of lift to leeward, its drag penalty comes in the form of added induced drag because the vertical strut must produce extra sideforce...

In previous testing we have found the drag associated with obtaining stability in heave to be prohibitive. We will revisit this conclusion given that kinetic techniques now allow for much earlier takeoff than 'steady state' models predicted.

However stability in pitch is the 'low hanging fruit' and can be obtained with the right choice of rudder elevator size, section and angle.

The work we did on rudder setup gave us stability in pitch, relieving an overworked skipper from at least one variable.
With a pitch-stable but heave-unstable setup, only speed needs to be controlled to even out foil lift.

Prior to the Worlds, Glenn Ashby, Ray Davies, Peter Burling and Blair Tuke undertook an admirable testing programme in a relatively short but intense training camp.
I admire the rigour that they applied and the experimental methods they used.
One boat was always kept standard as a baseline. Rigs and skippers would be swapped on the test boats to isolate extraneous variables.
This allowed us to conclude with confidence that our rudder setup was the only one available that provided stability in pitch.
Overall drag was also lower but this is a secondary benefit since the exploitability of the boat was noticeably improved (meaning it could be sailed at a higher average percentage of available potential).

The key factor is the rate of change of elevator lift with pitch angle (AoA).
This is helped by having the elevator as far below the free surface as possible and minimising junction interference.

The design of the elevator foil was heavily influenced by my work with RC yachts.
Experience with this low Re application helped to develop a thin section with unusually straight exit runs.

Use of such a thin foil was made possible by efficient structural design and construction.
The last generation of rudders (current production spec) stood up well to incredibly punishing use.

Tough use and aggressive kinetics did lead to some very unexpected hardware failures that have informed the updated specs of our production items (more on that later).

Further Reading

For a good treatment of Kinetics, brush off High Performance Sailing by Frank Bethwaite (Chapter 23).

What It Is

Answer to the SA quiz about this pic:

Rapid proof-of-concept prototype of a system to adjust rudder winglet/elevator Angle of Attack in real time.

Actuated by twisting the tiller extension, it uses worm drives to rake the entire cassette on both hulls, maintaining rudder balance but adjusting the lift produced by the rudder foils.
It is also possible to adjust the rudders independently to get more lift on one side than the other.

This first iteration was a bit of a rush project in the lead-up to the Worlds.

The aim was to determine whether differential lift settings on different points of sail would give a net benefit.

The adjuster units had to be a self contained ‘bolt on’ addition so they could be easily removed without structural alterations to the boat.

Results are mixed and testing is ongoing. A more refined version has been made and is being tried now.

The project is by Carbonicboats with significant engineering input from Ben Guymer.

Stability Principles

Time to answer some questions about stability in pitch. 

Over the past few months of testing we have found some very interesting things worth sharing.

Stability as an Alternative to Active Management

Without going into the maths, stability in pitch has a strict definition in aircraft theory and is a requirement for what I would consider ‘sustained foiling’.

Meaning the ability of a boat to remain ‘balanced’ on foils without continuous corrections in the form of course changes, weight shifts, and adjustments to power settings (sheet tension).

It is possible for a boat to sail with the hulls clear of the water in spurts without being stable in pitch and/or heave. Even sustained ‘bursts’ of a few hundred metres are feasible. 

This is what is happening in the majority of As with C and J foils, as well as in the NACRA 17. 

When you compare these sporadic foilborne tracts (that are becoming more prolonged as sailors master new techniques) with the ‘rock steady’ sustained foiling of a Moth or AC72, the difference is obvious.

Stability at a Price

The interesting thing is that stability necessarily involves a drag penalty. 

Some stable setups also have additional drawbacks, such as the need to ‘tack’ the foils (retract the windward one as is necessary with the 'acute L' concept pioneered by ETNZ).

In the A Class, the drawbacks of stable foiling make the choice rather marginal:

-          Sail area and power are limited

-          The hulls have an extremely low displacement to length ratio

-          Simplicity is paramount as there is only one pair of hands on board

-          Maneuverability is a priority because racing takes place on relatively short windward/leeward courses.

On larger boats stability is vital for control. 

On a small boat such as an an A, the centre of gravity (CG), heading and sheet tension can all be altered very quickly in a coordinated way by the skipper: A step back, a pull on the tiller, letting out an armful of sheet… It can all happen in less than a second in response to a feeling in the inner ear. 

People learn to ride unicycles, so mastering a small unstable vehicle is not outside the realm of possibility. 

When the top skippers in the A Class today speak of learning to foil, they are referring to mastering the technique of prolonging their stints of balancing on an inherently unstable platform.

The evidence on the racetrack shows that this solution, when mastered, can be competitive since bursts of unstable foiling can offer gains compared to more conservative foil-assisted sailing.

The risks involved are higher because a mistake is more likely to end in capsize, but taking risks to win a race is nothing new.

Experimentation

As our followers know, we believe in sharing what we learn, explaining the reasoning behind our development choices and, always, following an objective evidence-based process. 

If theory disagrees with measured findings, then the theory must be revised.

In our early development of Paradox, we found that the original stable configuration brought unacceptable penalties in terms of drag and maneuverability. 

Stable full foiling was slower around the course than unstable ‘jumping’. 

In response we tested a few alternative configurations and came up with the current setup that is ‘just on the stable side of neutral’, but has much lower drag. 

We accepted a higher takeoff speed and more moderate ride height in exchange for simplicity, maneuverability and, above all, reduced drag. 

We started out 2013 with a deficit of boatspeed and ended the year with some outstanding upwind pace and a small edge downwind that we are confident we will be able to build on. 

Much will be learned at the Worlds and we will continue development after that.

The fascinating question in the A class at the moment is about striking the optimum balance between stability and drag.

Mental Model

The illustrations below aim to explain the key factors affecting pitch-stability. 

The simple way to think about it is this: A stable system will return to the initial state after being upset by an outside force.

When you consider a system made up of a main lifting foil, a rear foil, and a CG, it is easy to see that the relationship between these three objects will determine system behavior.

Think of the two foils as supports at either end of a plank. Then the CG is a person standing on that plank.

If the person stands right at one end of the plank, then the support at that end will be taking all his weight and the support at the far end will be taking almost no weight.

If the person stands exactly half way along the plank, then both supports will be sharing the weight equally.

Now imagine that the supports are not solid and immovable. 

Instead they are peculiar springs that can only push back so hard before giving out. 

The main foil has a higher threshold (maximum absolute lift) than the rear foil. 

The forward foil could take all the weight unassisted, but the rear one can only help up to something like, say, 35%. 

If the CG moves too close, the rear foil will at first attempt to push back harder. 

But eventually it will be unable to keep increasing its lift and will subside. 

Here some dynamic factors come into play: As it subsides, the ‘apparent’ Angle of Attack (AoA) changes. But we will ignore dynamic effects for now.

Key to understanding this system is the concept that as the AoA of each foil increases, so does the lift contributed by that foil.

When the whole system pitches up, lift will increase for both foils. 

The rate of change for each foil depends on initial loading (lift coefficient), section shape, aspect ratio and initial AoA. 

The lift generated by a foil will change a different amount when going from, say, 1 degree to 3 degrees, compared to when going from 4 degrees to 6. In both cases the change was 2 degrees, but, since the changes happened at different points on the graph of Lift Coefficient vs. AoA, the change in total lift force was not the same.

Now you can see that, all other things being equal, the relative angle of the main foil and rudder foil is very important to foiling behavior. 

The relative angle influences the differential in the rate of change of lift. 

In other words the initial setting will affect the difference in rate of change of lift as the whole system pitches.

When the CG is forward, the rear foil has a long lever arm and is therefore most effective at restoring neutral trim. It will naturally tend to restore level trim. 

As the CG moves back, the rudder foil must share more of the weight so it cannot be set to neutral.

Instead it will be sharing vertical load.

This reduces drag but makes the choice of rudder foil section and area crucial: Its rate of change of lift must be greater than that of the main foil if it is to maintain stability (Note that to make the rudder share vertical load, its AoA has to be increased relative to that of the main foil. If the rudder is left 'neutral' and the whole boat is pitched up, then the increase in lift for the main foil will tend to up ride height and the rudder will still want to restore level trim). 

The final complicating factor is the bow-down trimming moment exerted by the rig. 

This has to be taken into account when designing and setting up, but conceptually it does not alter the basic understanding of the system: adding a moment is equivalent to moving the CG so that it puts more pressure on the support that would be forced down by that moment.

We found that mast rake angle has an important effect on handling. Raking the mast shortens the lever arm between the drive force (green arrow) and the CG. It also vectors some of the drive force upward (blue arrow) 

Foil set at a 'cruising' AoA, rudder foil neutral (no AoA therefore no lift). CG is just far enough aft of the main foil to counter the bow-down moment from the rig (red arrow). Rudder foil is contributing only drag!

Same setup as above. When perturbed, lift on both foils increases. Since equilibrium was at zero rudder lift, this configuration is very stable: rudder foil has a lot of leverage to restore level trim.

When this same system pitches down, main foil lift drops to zero and then becomes 'negative' (pulling down). All along rudder foil force is increasing, exerting leverage to restore level trim.

The above three diagrams show the same foil setup as the first ones, but with the CG shifted aft. This represents what happens when you step back on an A that has small rudder foils sized/angled as 'pitch dampers'. It is obvious that the further back one stands, the more unstable the system becomes. The rudder foils have less and less leverage while the main foil has more and more.

A more stable setup uses the rudder foils to share lift at optimum trim. By selecting the appropriate rudder foil size, section and AoA, the rate of change of rudder lift can be made greater than that of the main foil. This setup is more tolerant of shifts in the CG location.

Racing Ahead

Lots to report with progress on many fronts.

One of our Paradox V2 A Cat prototypes (the orange boat AKA 'Glennis') has raced in several evens in Victoria, skippered by young Tom Stuchbery.
Feedback is extremely encouraging.
Overall we are now certain that the concept works well.
The boat is consistently fast, especially upwind where it can reliably switch between 'high' and 'low' mode to comfortably control every adversary so far encountered.
Downwind we have a less pronounced edge, but we are still learning about setup and technique to extend this advantage.
The results are gradually improving, just beginning to reflect our findings.
We have definitely come a long way in a year.
Your support has been instrumental as we had to overcome some serious setbacks in our supplier network thanks to very poor performance by certain contractors.

The last of the V2 prototypes (pictured below) has also come online and will become a regular feature at regattas around Australia before heading to the Worlds.

Our expectations about the Worlds are 'realistic' given both the short lead-up with the V2 concept and the fascinating pace of development in the class.
As usual, he who knows that the road ahead is long and challenging will be better equipped to successfully make the journey.

Several inquiries we have received include the question "am I better off waiting to see what happens with foil developments/rules before buying an A Cat or should I take the plunge now?".
Our answer is that Paradox has a versatile foil bearing system that lets you easily change foil configurations, accommodating a wide range of possible shapes with a view to preserving the value of the platform.

Paradox is a stiff boat with good volume, careful detailing and state-of-the-art construction.
The nature of a development class is that new things will, from time to time, come along and spread to be ubiquitous. In the A Class this process seems to be managed well. So get into it now and know that our boats are as 'future proof' as possible.

Finally, Carbonicboats is moving to a new facility, located in a rather special place.
Here we will be able to make everything in-house, using brand new equipment including a larger oven for curing prepregs.
This move comes in part as aresponse to being let down by contractors. But it is a great step in the growth of our capacity, enabling us to provide value to more people.
Our RC boats, airframe parts, and A Cats will all be made there, reducing our reliance on outside contractors.
More on our new home in early 2014.

Gray on White

V2 Paradox for a customer. Clean look with white hulls, gray grip and black tramp. Grip is by Raptor, paint by Durepox and tramp by Steve Brewin.

Keeping Busy

Testing, two boat tuning, regattas and finishing new As.
Weeding out weak links in the supply chain and beefing up QC.
Here are some pictures. Words will resume soon...

Testing Videos

Some moving pictures showing testing of Paradox V2.0.
The new foils are promising, with the boat feeling very lively and free.
Quantitative measurements back up the qualitative feedback.
As already mentioned, the principal benefit is ease of use: The foils require no intervention by the sailor.
Tacking performance is also better and handling is very forgiving.
As you can see in the videos, we are working through different specimens of the new foils, mainly to quantify loads and validate the structural specs.
We are also testing different toe-in angles, all less extreme than those that were required for the V1 foils.

Cultivating Simplicity

Paradox Version 2.0 Foil Concept

The new foil concept is simple, easy to use and low drag.
After lots of R&D to validate theoretical predictions, it seams that for most sailors the benefits of ease-of-use outweigh those of 'trick' configurations that need to be learned, tacked, and trimmed to suit changing conditions.

The brief for Paradox calls for a boat within the A Class rules that can win races.
This means getting around the course quickly, without overly taxing the skipper who has the only pair of hands on board. Straight line speed must be combined with good tacking and down-speed performance as well as predictable behaviour.

We will continue to test 'radical' foil geometries as part of long term development, but right now the best way to fulfill the brief is with our new production foils that you can see in the pictures below.

Their shape resembles an apostrophe (') or comma, rather than any letter of the alphabet.
They are polyhedral foils made up of three straight segments connected by two tight radii.

The working span of the foil is planar simply to minimise wetted area: A straight line gives the least frontal and wetted area for a given dihedral angle.
The foil goes from the hull exit point to the inward beam limit in a straight line which is shorter than any curve, whether C, S, or J shaped.

The bend just below the hull is to minimise junction interference drag: It makes the included angle between hull and foil closer to 90 degrees.

The inward bend near the top of the foil is a solution unique to the requirements of the A Class: Popular thinking in modern multihulls is to reduce dihedral angle as the foils are lowered.
The idea being that in light winds the leeward foil is pushed all the way down, increasing span and area while reducing the vertical lift component at the same time. This gives a high-aspect upright foil for light air sailing.
However this approach requires that the windward foil be retracted since the leeward one alone is deep enough to provide all the necessary sideforce.

Our foils take the opposite approach: they become more upright as they are retracted.
In light airs you sail with them both partially up, giving a pair of smaller vertical foils that are both contributing sideforce.
This means no need to 'tack' the foils every time you change direction.

As soon as there is enough wind to fully power up, you lower both foils all the way and leave them there for the whole race.
By putting the top 'handle' rope through different holes in the head of the foil, you can pre-set the 'max' dihedral angle to taste. A small change in foil immersion gives a relatively large change in dihedral.

Partially raised foil (shown red) is more upright than fully lowered.

Eliminating foil curvature makes the lifting surface efficient and gives a positive feel for the 'bite' that the foil has on the water.

Foil rake and toe-in are pre-set by our custom hull and deck bearings.
Since angle of attack in the horizontal plane is coupled with leeway angle, foil rake adjustment on the water is no longer necessary. Stepping aft to trim the stern down automatically increases both sideforce and vertical lift.

Combined with other revisions that will be described in later posts, Version 2.0 represents a return to a guiding philosophy of simplicity and ease of use.

This is a great example of the truism that profound simplicity is inherently much more challenging to design well than complexity.
To simplify a product, the designer must understand which elements are essential and how they can be combined, excluding the superfluous, in a way that enhances the user experience while maximising performance.

It is extremely satisfying to come full circle and be able to present a product that is simple not through elimination but through integration.

Lessons Learned

Our development journey has taken us in just under a year from the initial S foil concept to other ideas (from different camps) including various iterations of J and L foils adapted to be Class legal.
Throughout we kept an open mind, learning without prejudice from experimentation.

With L foils we achieved reliable stable flight and impressive top speeds. However the demands of these configurations, connected with the radical fore-and-aft positioning they called for, their need for active adjustments and, especially, their inherent asymmetry (requiring that they be tacked, jibed, and re-configured for different points of sail), made them demanding around the course. In a race situation, the straight-line gains did not justify the impositions they placed on the skipper.

The new foils will give stable flight at higher speeds because their area decreases lineally with increasing ride height. Takeoff speed can be lowered by increasing toe-in angle, however this will not give a net drag reduction.
The inherent efficiency of the long, slender, light A Class hull, combined with limited power and sail area, call for a middle road of stable foil assisted sailing with a 'late' transition to full foiling in an automated fashion.

We will continue to experiment, explore new ideas and share our findings while the production version of Paradox is out there getting runs on the board.

Orange on Blue

Some more images before a detailed update...

While You Are At It...

Katana M in orange Durepox

Version 2.0

Details to follow...

A Cat Rule - Is It Broken?

Many have asked me to comment on proposed changes to the A Class Catamaran Rule.
Questions fall in two areas: is a change necessary, and what should the revised restrictions be?

My position is that, as a manufacturer, our task is to work within the rules, not to influence them.
Therefore Carbonicboats will comment publicly when asked but will not favour either position or undertake any lobbying.

Generally, it should be noted that rule restrictions aimed at forcing a particular outcome (or at closing off a specific design space) invariably fail to achieve the initial intent if sufficient performance gains lie in that direction.

Like water pushing against a poorly finished dam, the forces of competition will ensure that designers and sailors find a way around, over, under, or through the obstacles imposed by specific rules.
The wording of a rule cannot cover every grain of the required 'barrier' in sufficiently fine detail (without effectively creating a one-design).

This is not a matter of refining the wording. Instead it is inherent in the nature of any rule framework. In fields as diverse as motor racing and tax law, the common fundamental problem is that rule writers cannot know the future.

When a rule is formulated, the experts involved consider developments that can be glimpsed on the horizon at that time. They cannot know what new permutations lie further into the future as a result of new technologies, new ideas, new combinations of concepts, new interpretations and new discoveries.

Prescriptive rules are always vulnerable, through their specific wording, to interpretations that satisfy their letter but not their intent. Such interpretations may or may not be conceivable when the rule is being written. They may be dismissed at time of writing but might suddenly become attractive to designers when new technologies appear.

At worst the result is complexity and expense to achieve an otherwise simple goal.
But at best this evolutionary process gives rise to new, superior solutions that would otherwise not have seen the light of day.
Ways around a rule need not be complex and expensive. They can be creative and innovative.
As long at the rule is applied fairly, consistently and as written.

The alternative is for rules to avoid prescriptive restrictions, instead defining general boundaries, chosen to incentivise the behaviours favoured by the rule makers.
Some of the vulnerability to exploitative interpretations remains, but 'wholesome' outcomes are more probable.
Of note is that every rule in the A Class except for Rule 8 defines dimensional limits. Only Rule 8 describes a particular type of feature.

The A Class is administered as an open development class with concessions to preserving popularity (number of participants) and closeness of performance. Part of the concern is a balance between innovation and preserving second-hand values.

Rather than applying a strict 'legalistic' system, the intent of the rule is often referred to when issuing interpretations. This may be fine as long as the context is not adversarial, and so far the outcomes have been positive. However departing from the letter of the law can only be defended under strict conditions. Doing so excessively removes certainty for all involved. Many members of the TC have high level experience in the America's Cup and Olympics, so I trust they know this well.

In light of respect for intent, it can be argued that a higher-level approach would work equally well.
For example, instead of mandating that appendages must be inserted from above, ease of use when launching and retrieving could equally be guaranteed by saying, as proposed, that the boats must float (upright) in knee-deep water.

If the intent is to open the door to foiling without compromising practicality, then thought should also be given to narrowing the exclusion zone between the hulls under the waterline.
The exclusion zone makes sense and should stay because it guarantees that the A will remain a true catamaran (this is analogous to monohull rules mandating a contiguous water plane and no transverse hollows).

However the width of the exclusion zone impacts the aspect ratio (and hence the efficiency) of any hydrofoils, whether horizontal, angled or curved. Narrowing the exclusion zone would give designers more freedom to balance induced drag against righting moment. This would invite people to explore a more widely varied range of foil shapes, meaning more experimentation (potentially including prototypes with more complexity) and thus more cost, at least in the short term.
But it would make it easier to foil efficiently and it would probably increase top speeds.

It seems to me that the debate is not whether or not the A should be a foiling class: If foiling is faster, then the Class will foil under the current rules. Evidence points to this being achieved with more complexity and less safety if a rule workaround is necessary.
The compromised foiling that will inevitably emerge may well increase the gap between club level sailors and those with more time and resources available to master the specific techniques required by rule-tortured foil solutions.

Instead the question should be whether the Class wants to maintain the current specific prescriptive restrictions or whether it wants to adopt a higher level rule that mandates a generic 'ease of use' goal instead, as proposed.

Carbonicboats (like other manufacturers and home builders) entered the Class knowing that restrictions existed on the design of appendages.
Our work has shown that foiling is possible within the current restrictions with some lateral, innovative thinking. We have not yet proven that full foiling is faster around the course but all indications are that it ultimately will be.

If tomorrow the rules were relaxed, we would modify our concepts accordingly.
In all likelihood the resultant updated product would be faster at a similar cost.

It is not our role as a company to favour either outcome. We must instead create the finest product within the framework in force on the day, in terms of design, ease of use and cost effectiveness.

As a designer my view is that a rule is simply part of the brief.
In the same way as cost and time constraints, rule restrictions should be seen as challenges integral to the design process.

Simply removing restrictive rules does not guarantee simplicity and cost-effectiveness. It may even increase costs and complexity as it widens the range of possible solutions.
On the other hand, defining rules at a higher level, closer to ultimate intent than to specific restrictions, usually guarantees healthier design outcomes.

This is a matter for the Class to decide.
Hopefully this post has helped to inform the reader on the implications of both sides of the argument.

What really matters is that the rules as written are applied consistently, giving everyone certainty about the design space and the freedom to innovate within it.

Update

With the A Cat Europeans and Australian Nationals coming up, it is a very exciting time in the class. Paradox will not be at these events as we are well into finalising the production design that, as you will see very soon, is a totally new boat with most key concepts re-visited.

Looking back at the brief, we had to make some honest assessments about the goals that had been achieved and the price we were paying in terms of performance and complexity of use.
With the same goals in mind, we revised our approach focusing on minimum drag and low-demand 'set and forget' systems.

Over the past few months we tested different concepts, using different ideas compared to the initial forward mounted S foils and max-span L rudders.
Our assessment is that the S foil solution involves too many compromises in this application.
Better all-round geometries exist that allow stable, easily managed foil assisted and full foilborne sailing where the skipper can push hard with confidence.

The hull shape has changed slightly to suit, the beams are tweaked and many engineering details are revised.

Indulging my obsession for elegant, beautiful detailing and top quality finish, even more fittings are custom and the overall package is even more refined.

At the same time the production process is being streamlined further to make the cost even more competitive.

Carbonicboats will have a presence at the A Cat nationals as a race day sponsor, supporting the class. It was a tough decision to sit out this regatta but ultimately it was a matter of resource management and rationalising priorities.

Given the lessons learned during the summer, it made sense to spend the winter getting the production boat sorted to offer the best possible product to our customers.

The coming summer will be upon us soon enough and it will bring a full calendar of racing that we thoroughly look forward to.

We want to be there with well prepared weapons and hope our fellow sailors will believe in us enough to put in more orders soon.

Our red 'periodic table' logo is there as we prepare

to get back into the fray very soon...

Alphabet Soup

In response to numerous requests, and with the 34th America's Cup almost upon us (short on teams but rich in technical curiosities), here is a 'back to basics' look at multihull foil solutions.
This post samples the main varieties of multihull foil, assuming some background sailing knowledge.

Angled Board

The simplest way to obtain a vertical component by just canting the foil lift vector.
This solution is extremely constrained in angle and span if a beam limit is to be respected when the foil is retracted. The same constraint also forces the foil exit point in the hull inboard toward the middle of the boat, moving the hull/foil junction closer to the free surface and reducing righting moment (because the centre of vertical lift moves inboard).

Every part of the foil span contributes evenly to vertical lift so, assuming enough foil angle is possible to lift the boat clear of the water, there is no stability in heave (ride height).
Tapering the foil planform and/or adding a vertical tip can help give some heave/vertical lift correlation.

Using two such foils together on a very wide platform such as Hydroptere (diagram below) can give heave stability by simply reducing immersed foil area with altitude.
But this arrangement is not practical in most classes racing 'around the cans'.

C, J, and L Foils

C Foil: Sideforce (to windward) is unevenly vectored to generate upward lift.
Vertical component is greatest near the bottom.
By tightening the radius, more extreme lift characteristics can be obtained regardless of beam restrictions.

On the practical side, C foils are easy to install because they fit in a constant-radius foil case.

C foils are unstable in heave: as ride height increases, vertical force does not decrease significantly. Given constant thrust, if lift is greater than total weight, the boat will rise until the foil ventilates or stalls, causing a crash.

C foils are helpful in foil-assisted sailing as long as they lift less than 100% of the weight of the boat.
Vertical lift can be 'dialed down' (without losing sideforce) by partially raising the foil.

J Foil: Similar to C foils but maximum lift remains available when a J foil is partially retracted (shown orange).
The lower part of a J foil stays ‘canted’ until the junction radius reaches the hull.
Unlike a C foil that becomes more upright as you pull it up.

J foils are also unstable in heave so are suited to foil-assisted sailing rather than full foiling.
They potentially have less drag when sailing downwind because their draught (and hence frontal area) can be reduced when vertical lift is still beneficial but less sideforce is required.

Both C and J foils can have high induced drag when set for max lift (raked - see last diagram below) because the lift distribution along the span becomes biased toward the tip. End devices such as winglets or washout at the tip help alleviate this but cause parasitic drag at other times and add complexity to the foil case design if the foil is to be fully retractable.

Note that tightening the transition radius on a J foil progressively, gives a ‘traditional’ 90 degree or 'open' L foil that is also unstable in heave.

'Acute L' Foil: A very elegant way to automatically regulate heave for full foiling on only one (leeward) foil.
First "stumbled upon" by the ETNZ design team, this idea is a great example of how rule constraints can push innovation by forcing competitors to think laterally.

As ride height goes up, the immersed area of vertical ‘strut’ decreases (lateral area is lost).
This makes leeway increase, in turn reducing the Angle of Attack (AoA) on the ‘horizontal’ foil.

To get your head around this, imagine what would happen if you made leeway extremely large (like 90 degrees): The horizontal foil would actually start pulling down!

Under normal conditions the change in leeway is small (say 5 degrees) but the component across the boat works to reduce the AoA on the horizontal foil, moderating lift to stop a runaway leap into the air.

So: boat goes up > lateral area gets smaller > boat starts slipping sideways a bit more > horizontal foil moves toward its own low pressure field > lift decreases > boat settles > lateral area increases > leeway decreases > vertical lift grows again... And so on until an equilibrium is reached.

The higher the inboard tip relative to the outboard root/junction, the closer the coupling between ride height (through sideforce) and vertical lift.

At extreme ride heights, the acute L foil begins to work as a conventional (powerboat) V hydrofoil: When the inboard tip of the horizontal foil breaches the surface, immersed foil area is gradually reduced regardless of sideforce.
This is helpful to avoiding a crash when pulling away to a near square run in reaction to a gust.
It is a good 'safety valve' in situations where speed (and lift) may be high but sideforce is small.
However it should be noted that the optimum condition requires the tip to remain submerged. Drag is much lower when only the vertical is surface-piercing and leeway moderates heave.

Combinations

With the basic components described above, designers have a kit of parts that can be mixed and matched to suit the particular application at hand.

The principal groups that can be seen when observing recent AC72 testing are described below in the order pictured above.

L Foil with Polyhedral: The bent inboard tip provides stability in the same way as an acute L foil. Kinking the horizontal foil reduces the junction angle between vertical strut and horizontal foil.
In a way similar to introducing a bulb or a radius, this decreases drag where interference effects are most prevalent.

The root of the horizontal is heavily influenced by the low pressure area inboard of the vertical strut so is less affected by leeway than the tip. It makes sense therefore to use the root to generate the bulk of vertical lift and exploit the tip for heave control.
The penalty is a bit more parasitic drag as there is more foil area for a given effective span.

The bent horizontal foil can also hug the hull more snugly when the foil is retracted, reducing drag when the windward hull is near the water.

Acute L with Kinked Strut: Bending the vertical strut enables some adjustment of the angle of the horizontal foil so that stability in heave can be fine-tuned.

A bend may also be necessary to stay inside the beam restriction if designers want to cant the strut inboard to get an effect similar to a C-L foil.

C-L Foil: Combines the heave stability of an acute L with some lift vectoring of the strut for lower overall drag. The cost is a shift inboard of the centre of lift which reduces righting moment.

S-L Foil: Similar objective to a C-L: more even lift sharing for lower overall drag.
But the inflection at the top moves the bottom outboard again, recovering full righting moment.

The S also fine-tunes the angle of the horizontal foil to adjust ride height and heave stability.
Often the intent is to have a deeper more upright foil for sailing upwind. At the same time as the strut becomes more upright, the tip angle decreases, giving up some heave stability. Upwind this is less critical since it is easier to maintain speed near constant by luffing up in the gusts (especially since it might not always pay to fully foil upwind. Instead an efficient foil-assisted mode may be preferred, leaving the hull to take care of heave stability).
Reducing heave stability unloads the vertical strut in sideforce because the leeward component of the horizontal foil lift goes away. So total lift-induced-drag is decreased.

The downsides are mechanical complexity at the bearings, a foil case that holds more water, and more friction when raising and lowering.
Bending the foil at the highly loaded area between hull and deck bearings is also structurally more demanding, especially on bigger boats.

And finally, a diagram showing how foil rake affects vertical lift:

Remember that heave stability is the tendency for lift to vary inversely with ride height.
For effective foiling it must be combined with pitch stability which is a bit simpler to obtain using properly sized T, + or L rudder foils.

On small boats such as the A Class, it may be possible to 'stay on top of' an unstable platform by actively managing weight placement and sideforce, countering in real time the continuous tendency to depart stable flight.

Like riding a unicycle this is difficult but humanly possible.
Until now this solution, though far from optimum, seems to be the best real world choice for racing around the course in the A Class, mainly due to rule constraints on foils.
The challenge for the future is getting stability with an acceptable drag penalty within the rule.

Bigger boats do not have the option of quickly shifting weight and aggressively trimming the sails, so true stability is important for safety and speed.

I hope this post has been informative for keen observers of the spectacular innovations on show in today's multihull scene.
Remember to look critically and skeptically at the physics when assessing how effective and stable various solutions might be.
Interesting times indeed.

Alea Iacta Est

Perth Radio Sailing Club reports on the launch of a pair of custom built Rubicon R10R specimens that have been quite some time in gestation...

Moving Pics

Another video of Paradox starting to properly exploit new stable foiling setups.
Stability is heavily influenced by the relative lift contribution of rudder winglets and main foils.
We now set up the rudders to provide some upward lift when the boat is at neutral trim.
If driving force from the sail increases, causing the bow to dip, rudder lift decreases as the rudder winglet AoA approaches neutral.
If trimming moment should keep increasing (this would only happen if a gust an order of magnitude greater than the average wind speed is encountered), rudder winglet AoA would become negative, pulling the sterns down.
When the setup is correct, crew weight can be placed surprisingly far forward. This is more efficient as it puts more mass over the main foils, reducing the burden on the rudder winglets which are smaller and so have to work harder to support a given weight.
Even though speeds are significantly higher than in displacement sailing, the feeling of losing the bow 'down the mine' disappears completely.
If anything the instinct to shed power must be reprogrammed as the limit is much, much further away. Easing sheet in a panic just causes ride height to momentarily increase and then settle again.
We are now confident that this mode is significantly faster in a straight line at least in winds over 10 knots.
The next question that must be answered is whether it is faster around the course when tacking and jibing are considered...

Pilotcam

A short video from a recent testing session.

Our understanding of the settings necessary for sustained stable foilborne sailing is steadily improving.

It is worth re-iterating the definition of stable flight with reference to the feedback loop that arises when external forces upset pitch angle and ride height.
A stable setup settles on an attitude and altitude without input from the crew.

We announced that Paradox could foil in a stable mode only when we were sure we had proven that it could.

Stability comes at some cost and we are open about the uncertainty regarding whether the benefits outweigh the costs. I believe we are close to finding an answer and I will describe our findings in detail in later posts.

Recent experiments by other manufacturers have shown that an unstable setup can be 'tamed': a well practiced skipper on a small boat can anticipate departures from the desired attitude and altitude given certain provisos, and make corrections, akin to balancing a ball on top of an inverted salad bowl.
In a racing context the conditions when this becomes unmanageable may not occur very often so overall an unstable setup can be competitive.
Think of it as riding a unicycle instead of a tricycle. Obviously humans are capable of learning to ride unicycles so the question becomes one of costs vs. benefits.
Exploiting an unstable platform is a muscle memory skill that can be learned 'by feel' with practice and is arguably more 'natural' once mastered than the mechanical and intellectual skill of adjusting foils for optimum trim.

In any case, our conclusions will reflect what we learned in testing. We will adopt the fastest configuration for getting around a course. Since we have been learning from our testing it will be different from the original setup.

A few notes on this video:

The discontinuities in the editing correspond to where Tom backed off to make adjustments using a control system that we would like to keep to ourselves for now.
The 'flight' was reliably uninterrupted for the whole run. The boat was safe and controllable throughout.

Looking at the wake carefully you can see the occasional disturbance due to ventilation of the surface piercing foil. This is an issue inherent in highly loaded surface piercing foils and we are experimenting with ways to mitigate it.
Fences are an obvious solution but have practical drawbacks as the foils must be retracted through the bottom bearing.
More promising options are leading edge discontinuities (cuffs) and boundary layer turbulators...
Better still is an optimised foil section with ideal pressure dstribution to prevent ventilation.