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Saturday, November 16, 2013

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.

Saturday, October 19, 2013

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.

Saturday, October 12, 2013

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...

Tuesday, August 27, 2013

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.

Thursday, August 8, 2013

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.

Tuesday, August 6, 2013

Orange on Blue

Some more images before a detailed update...

Friday, August 2, 2013

While You Are At It...

Katana M in orange Durepox

Saturday, July 27, 2013

Version 2.0

Details to follow...

Tuesday, July 9, 2013

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.

Sunday, June 23, 2013


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...

Thursday, June 13, 2013

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.


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.

Sunday, June 2, 2013

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...

Tuesday, May 21, 2013

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...

Saturday, May 18, 2013


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.

Tuesday, April 30, 2013


As already mentioned, we have spent lots of time on the water recently, with different foil concepts, testing, evaluating and tuning. Some tests with Paradox sailing alone and some in the company of other known fast A Cats. It is safe to say that we are getting a handle on key issues and how they will be addressed on the production boat.

Two boat testing session
As a general observation, we are in unexplored territory for this class and arguably for this type of boat at this scale simply because the relative effect of foil setup on overall performance is much greater when the foils are working hard enough to support most of the mass of boat and skipper most of the time (and all of it some of the time).
Put simply, when the foils are doing most of the work, getting the settings right is much more influential than if they were only helping out a little.
In hindsight this should be no surprise. Ask any 'Mothista' about the effect of a small fraction of a degree of foil angle and they will say it is like night and day. When the foils are the only part of the boat actively interacting with the water (in the case of Paradox the hull may still be 'skimming' but our measurements tell us it is supported by the foils, not by the water) their effect is dominant.

The saving grace is that the correct setup is mostly related to crew weight and remains constant for different conditions. Once the correlation is understood, it should be easily duplicated.
Now that we understand the (far reaching) effects of main foil shape, toe-in and rake, the key is getting the right amount of 'lift share' so that the sterns are supported by the rudder winglets, but a step back can still raise the bows up sufficiently to 'pop' the boat up onto the foils.
This is a function of some combination of winglet Angle of Attack (AoA) and winglet area.

Lets say we want X amount of lift from the rudders such that they will support enough weight to keep the sterns 'flying' but not so much that the stern cannot be made to sink somewhat when the skipper takes a step aft.
We could obtain the desired lift with small winglets at a big AoA or with big winglets at a smaller AoA.
Assuming aspect ratio can be optimised in both cases, the lowest drag solution will come down to the chosen foil section - and the lift coefficient (Cl) it is happiest at.
However the choice will also have an effect on stability: If the AoA is larger, then the boat will trim down further before the winglet goes through a neutral AoA and begins pulling down to restore the desired pitch attitude.
In reality having the winglets actually pull down will only happen in rare 'extreme' situations. However it is a helpful way to visualise the dynamics at play.
Simply reducing the AoA with bow-down trim is enough to introduce a stern-down restoring moment.
The vital part is the rate at which this moment increases since its rate of change is key to stability in pitch.

Optimum ride height with skipper not all the way aft and correct lift sharing by the rudder winglets.
As the bow pitches down, rudder winglet AoA decreases.
In fact we found that the most stable setups tend to takeoff 'stern first' and stay level or slightly bow-down in flight.
The boat happily sits in this attitude when set up correctly. Notice the absence of wake other than spray.
We are cristallising a useful map of how this foil system works and how it can be exploited.
It appears that performance is good when it is set up correctly.
The most impressive aspect has been the utter predictability and controllability of some foils (more than others) when pushing hard downwind. That is definitely an aspect of the brief that was met successfully.

What remains to be proven is whether the gains are exploitable around the course.
One finding for example has been that with some foil types there is quite a significant benefit in raising the windward foil when sailing upwind.
In a close racing situation this can only be exploited if the system to raise and lower the foils is extremely easy and fast to use with minimal distraction.
We have therefore experimented with a series of mechanical solutions and it seems the last iteration meets the criteria.
In short it uses elastics to raise the boards automatically and a single line with significant mechanical advantage to lower them. More detail on the evolution of these mechanical systems will be released later.
As already mentioned, this is a problem that some of our competitors will also have to solve as they adopt 'S' foils with outward inflection at the top that alters dihedral angle as a function of foil vertical position.

Having explored this development path we will only adopt in production a system that is reliable and easy to use without distracting from 'keeping eyes out of the boat'.
If we are not satisfied that such a system can be engineered (meaning that, after friction is overcome and single line operation achieved, the burden on the skipper is still judged to be excessive) then we will change the foil system so that it does not need to be touched during racing.
This may involve changing foil shape and/or finding a compromise toe-in setting that is optimised for always having both foils down.

Referring to the design brief for Paradox, the final balance to be struck must be in favour of best achievable speed around the course.
If a certain setup cannot be sailed at a high percentage of its potential for a large percentage of the time around a course (by a 'mere mortal'), then a slightly compromised variation that is more exploitable will be more competitive.

With this in mind, simplifying the boat is vitally important and what we are learning now will inform the choices for the next prototypes and the production setup.

Tuesday, April 23, 2013


Lots of testing in the 'off season'. We completed some very productive sessions over the past few days and have more planned. As some of you may have observed, we went through some different concepts to the initial S foil setup. We are working on several areas for both numbers and feel, testing initially alone, then against other As.

The first area of investigation is properly understanding the behaviour of the foil system as the relationship varies between main foil and rudder foil force.
Rudder foil force can be manipulated by raking the rudder (tuning) and altering rudder winglet area (permanent change). The latter also influences aspect ratio which in turn modifies the overall lift-to-drag ratio at different angles of attack.

Aside from qualitative information such as visual observations and feedback from the skipper, we identified the need to work methodically through the range of possible combinations to gain quantitative 'proof' of how any concept performs.

The outcomes we are interested in are performance and stability (in that order).

We know that we can make the foils work much harder than with a conventional geometry (meaning they can be set up to carry a much higher percentage of boat weight without loss of control upon 'takeoff' - Takeoff simply being when that percentage reaches 100, keeping in mind that ride height is intended to be minimal).

What we don't know yet is whether this mode is faster in terms of VMG to the bottom mark.

The trade-off between hull drag and foil drag is a fascinating, subtle and complex one given the A Class rules. It certainly seems to be less clear-cut than in some other classes.
So we are working through a range of settings for different values of rudder area to correlate our measurements with theoretical predictions.

We started with (gen 2) rudder winglets with Xcm 'chopped off' and tested the full range of useful rudder rake angles. Then repeated for the same rudders with a bit more length chopped off...
At the same time we monitor the 'load share' of the main foils and the behaviour of the platform as a whole.
Obviously we want to nail the minimum foil force required for stability (as that corresponds to the smallest drag penalty) then ascertain the best combination of area and angle to achieve the desired force.

A positive finding is that once set for crew weight little adjustment is required. Fine tuning is achieved by stepping forward and aft on the gunnel. Since rudder winglet angle is always positive in normal conditions (the exception being an incipient nosedive when they can go beyond neutral and start pulling the sterns back down), trimming the bow down neutralises their effect and thus reduces induced drag.
Similarly there are various options for foil immersion (connected to dihedral) and rake (connected to up/down lift) when sailing upwind.

The second area of investigation is the optimum transition point/s between different modes. Such as between sailing 'conventionally' with moderate heel to reduce immersed (windward) foil area and flat with the traveller down to get both foils working and carrying the entire weight of the platform together with the rudder winglets.

Finally we are looking carefully into the 'human factors' or interface issue.
Some additional complexity is acceptable if the reward is increased performance. However we are working hard to simplify the systems to make them easy to understand and to use effectively with minimal training.

As previously described, foil rake is (after some fine tuning of the setup) easily adjusted with a single line.
Foil immersion is a bit more tricky as the foil should be able to be raised and lowered remotely.
It remains to be seen whether vertical adjustment will be deemed worthwhile in terms of the cost/benefit trade-off between the workload of making the adjustment and the performance reward.

Vertical adjustment is predicted to only be required in sub trapezing conditions (less than 100% Righting Moment).
It is also worth noting that at least two other manufacturers have released information to the effect that they will also be incorporating dihedral change through vertical foil adjustment.
It will be interesting to see whether the predicted gains on offer can be realised in the real world during close racing.
In an ideal world the windward foil would always be fully up when sailing upwind/not foiling, but the constraints of a singlehanded boat make the practical 'bancability' of that option rather finely balanced.

Thursday, April 18, 2013

FAQ: Optimum Altitude

Q: "You seem to be just 'skimming' above the surface. Why not fly higher?"

A: Altitude control in Martin Fischer's foil concept is supposed to come from the change in foil curvature just below the hull exit point. As the boat rises, the radius of the part of the foil immediately under the water surface changes progressively so that the immersed portion of the foil gets more vertical. This gradual reduction in dihedral of the wet part of the foil reduces the vertical lift component and encourages ride height to settle.

The position of this change in foil curvature determines ride height.
It would be possible to position it further below the hull. However the span under the inflection would have to remain the same as it is sized to provide sideforce when foiling. Therefore the whole foil would have to be longer. This would make the span excessive at sub-foiling speeds so would bring a drag penalty upwind and in light winds.

Actually, to be technically correct, such additional span at the top (aimed only at increasing ride height when foiling) would need to be 'washed out' to stop it making significant sideforce. If the additional top part did provide sideforce, it would effectively reduce overall dihedral angle: Its sideforce would subtract from the contribution to sideforce by the rest of the foil. So the vertical component of the remaining span would also shrink...

Q: "Would flying higher have any advantages?"

A: Flying higher would require some additional foil span that would add only drag at sub-foiling speeds. In a racing context this penalty would be present more than 50% of the time.

Spray drag is an interesting consideration: Though it looks messy, the spray being thrown up by the foils does not cause additional drag when it hits the hulls. The reason is that energy had already been transferred to the water in the spray when it was directed upward by the foils. If anything, redirecting the spray down and back returns some energy to the hulls.
Think of the exchange of energy in terms of equal and opposite reactions: When you push water up and forward it pushes you down and back which slows you down. When you push it down and back it pushes you up and forward, a form of energy recovery.
So spray only adds drag if it strikes the front half of the boat while moving back. If it strikes the back part of the boat while moving forward it can be ignored...
The ideal solution would be to fence the foils to suppress/redirect the spray in the first place, but this is not practical since the foils must pass through the bearings at the hull surfaces.

It may be that in future it will pay to foil all the time as rigs get more powerful, sailing techniques develop, materials get stiffer and our understanding of hydrodynamics evolves.
If that happens then considerations such as wave clearance and amplified shifts in the CG due to heel will come into play.

It should be noted that, as long as two foils are being used, the dynamics of heeling to windward are not analogous to those on a Moth. If it were possible to fly on the leeward foil only (as the AC72s are doing) then flying higher might allow some windward heel which may have some advantages. If that is the case then foiling higher still could amplify those advantages.

Finally there would be a tradeoff between raising the rig into better wind and losing some end-plate effect from the water surface.

But in the A class, with current technology and within the present rule restrictions, it appears that it does not pay to foil all the time. The long slender hull combined with a low displacement is very efficient at low speed and even more so when foil assisted. The limited sail area and constrained foil horizontal span also contribute to make foil assisted sailing the most attractive option at intermediate speeds before stability becomes an issue.

The initial solution chosen for Paradox is therefore to make the necessary compromises for what is effectively a foil assisted boat that can transition fully onto the foils and become dynamically stable when certain conditions are encountered. With the initial Fischer S foil solution full foiling is proving just too expensive in terms of drag.

This is an example of how a clear brief is vital in guiding the assessment process during development: The brief called for a boat that could be pushed hard through being dynamically stable as the foils begin to generate enough lift to support 100% of mass.

Regardless of whether that goal has been achieved (still being evalusted), the bottom line is that overall drag around the course is what matters.

We will continue to experiment until this crucial value has been reduced below that of other designs.
At the same time the drag reductions must be exploitable: the boat must be simple and intuitive to use so the single-handed skipper can look 'out of the boat' and concentrate on the race.

Now we are working to establish the best settings to get the most out of the first generation concept.
The next step will be to assess whether the configuration is in fact faster around the course in a wide range of conditions.
This includes straight line speed, maneuverability and ergonomic aspects such as ease of handling and making adjustments.
After that we will play with different concepts and draw some informed conclusions.

We will continue to be open and honest about our findings and to share the process as we learn more along the way.

Tuesday, April 16, 2013


We tested over the weekend with reduced horizontal surface area on the rudders and the results were interesting though more work still lies ahead.
Stability is unaffected but the boat is more responsive to changes in longitudinal trim.

In the following sequences Tom Stuchbery is deliberately 'provoking' the boat with aggressive steering inputs to get a feel for how it responds.
Existing foil assisted A Cats with C foils would continue in a 'pitch-up' feedback loop until the foils stall, making it very hard for the skipper to stay in control.
The pitch up is initiated by a slight downward component in the steering force generated by the rudders (this is present due to heel and is independent of any T, L or + foils on the rudders).
The pitch-up feedback loop is not just due to pitch instability. It is due to heave instability inherent in C and J foils: Even if the C or J foils are combined with rudder winglets to give pitch damping, heave instability remains because all the lift is generated at the bottom of such foils (this is where the horizontal area is concentrated) so the lifting area stays fully immersed. Therefore an increase in ride height does not cause a decrease in lift so it is not automatically corrected.
Our S foils differ by having the horizontal lifting part close to the hull so that this area immediately decreases as ride height goes up (the lifting segment of the foil immediately comes out of the water as ride height increases).
Paradox responds less 'wildly' to upsetting trimming forces.
One drwaback of having the lift just under the surface, however, is that the foils ventilate quite easily. A problem that could be solvable by optimising foil section and/or adding boundary layer devices to keep the flow attached...

To be clear, these are handling issues, not performance issues.
In most conditions C and J foils can be managed by setting them up so that their lift does not exceed the weight of the whole boat.
As long as the hull takes some weight (even if just the stern is 'skimming'), heave and pitch stability are not an issue.
However if such speeds are reached that foil lift exceeds boat mass (and corrections are not made such as partially raising the foils to reduce dihedral angle), then a loss of control will be inevitable.

Our future testing will be aimed at weighing the drag penalties associated with maintaining stability, and determining whether they are worth accepting in normal racing situations.
Right now we are foiling but not claiming definitively that this is faster than foil assisted sailing in the A class. That remains to be seen.

To exploit the gains, one must understand the way the foils work. The boat must be kept flat so both foils can work together. The traveler seems to work best slightly lowered to direct the sail vector forward.

We are confident that we can regain our upwind superiority by incorporating an automatic toe-in adjustment in the foil bearings.
The aim remains to simplify the systems and evaluate whether Martin's ingenious foil configuration is exploitable around the course.

As already mentioned, we are also exploring other configurations that have most of the advantages of the S foils but require less 'retraining' to exploit.

Our focus is sharply on getting around the course as quickly as possible. That is the basis for every design choice.

It is important to keep an open mind so testing will inform our understanding regardless of the attractiveness of each initial theory.

The process is about testing the theory with a view to refining it to gain an understanding of its validity.

Tuesday, April 9, 2013

More FAQs

This is the second post in response to questions we are receiving frequently, mostly in connection with design choices on Paradox and how they may compare to developments seen elsewhere.

I have added 'FAQ' as a label so in future these posts can be filtered out by those (fellow sailing nerds) who are interested...

Why Ls on the rudders instead of Ts or '+' s?

Here are some of the considerations when designing complex foils made up of more than one surface/span.


A single bent foil has no intersections so there is no interference drag (strictly speaking there is still some interaction between the pressure fields, but it is much smaller since the transition is very gradual).

Crossing two foils is very costly in terms of drag because of the way the pressure gradients combine and interact.
Basically, the low pressure peak near the main foil leading edge combines with the corresponding similar peak on the intersecting second foil and the two amplify.
Since flow speed is related to pressure, this spike in the pressure distribution is also a radical change in flow velocity.
Accelerating the mass of any fluid involves an expenditure of energy (F=ma) that comes from the total kinetic energy of the boat which is therefore diminished... In other words redirecting water around an intersection between two bodies is draggy. The tighter the included angle the worse the drag penalty.

In some applications intersections are unavoidable, so to minimise the damage designers arrange them with intervening bodies that basically smooth out the transition by spacing the working sections of the intersecting foils apart (in three dimensions) with surfaces locally orthogonal to their respective spans.

An assortment of Moth horizontal T foils with junction bulbs.
The bulbs smooth out pressure peaks and may even be designed to create destructive interference:
A high pressure area in the bulb can be made to coincide with a low pressure area in a foil.
The two pressure fields cancel in a way not dissimilar to the waves behind the bulbous bow of a ship.
Image source : 
Inverted gull wings on F4U Corsair meet fuselage orthogonal to its local surface,
minimising junction drag without the need for fairings.
Landing gear is placed at the kink so it can be shorter for a given prop clearance.
Image source:
T foils are less penalising than '+' foils as only three bodies intersect instead of four.
In some applications + foils are warranted when other advantages are sought. Examples of this are 14' skiff rudders where the distance from the foil to the water surface (the stern wave) is critical. Also foil assisted multihulls optimised to have the windward rudder winglet exit the water at small heel angles. Though in the latter case a different area distribution is usually a better solution.

T tail bulb visible on an Ilyushin Il-62. Image source:
Another way to minimise interference drag where intersections are unavoidable is to stagger the two foils longitudinally. Especially if the foil chord dimensions are different, this can help to make sure that the pressure peaks on the two foils do not coincide. This solution requires a good understanding of the operating envelope of the foils because the pressure peaks do move around with varying speed and AoA.

Horizontal tail surface staggered ahead of vertical. Image source

For relatively lightly loaded applications, an L is structurally much more efficient since the fibres are continuous across the two foils.
In theory a T can be engineered with very little bending moment if the horizontal foil is symmetrical about the vertical. However, on a boat that sails with heel and leeway, the load will not always be identical for both sides. Any difference will impose a bending stress on the junction which will have to be engineered accordingly.
It is possible to engineer the junction to withstand the uneven forces however, for a given material/construction, an L will always be lighter and cheaper to build accurately.


For a given span, the L solution allows the rudder to be placed further outboard, leaving the horizontal foil as an uninterrupted span (all the way in to the centreline exclusion zone in the A Class).
Placing the horizontal foil entirely on the low pressure side of the leeward rudder (the one that does more work to resist leeway) actually increases the efficiency of the rudder, partly offsetting the drag penalty associated with winglets at upwind speeds, when they are not vital to longitudinal stability.

Blended winglet on a commercial airliner.
Image source:

The bend at the bottom of our rudders is not 90 degrees. The primary reason is consideration of three dimensional effects to do with stability. There is a coupling of rudder sideforce and vertical lift, tuned to help maintain stability at all times, especially when bearing away.
As a secondary benefit the leeward winglet remains horizontal even when heel angle exceeds hull cant angle (when the leeward hull is heeled to leeward).
Since the winglets are not horizontal when the boat is level, the rudders are legal when pulled right up behind the hull.
Even if optimum area turns out to be much smaller than expected, angling the winglets allows a longer span and thus a higher aspect ratio for the same area.

Practical Considerations

We found that it is easier (still not easy but easier) to shed seaweed from L rudders than from intersecting T rudders.
The debris has some chance of slipping off the end of the L rudder, while it is much more constrained on a T or + arrangement.
Using Ls combined with cassettes has several advantages such as constant compensation, precise control over winglet AoA, and the ability to partially retract (and now reverse) the rudder while maintaining efficient steerage.