Wednesday, December 10, 2014


We have received many questions regarding the differences between ‘active’ and ‘passive’ foil systems for full foiling.
So here is a look at the principles with respect to performance.


An active system consists of a foil with variable camber or variable angle of incidence controlled by a sensor that measures heave position (ride height).
The input can be via a mechanical device such as a wand/float or an electronic sensor.

Usually the main lifting foil is fully submerged. In order to minimise the total lift necessary, the submerged foil should be angled to provide both vertical and horizontal force components.
The vertical component holds the boat up and the horizontal component resists leeway.
Moths achieve this vectoring by heeling to windward.
The setup we have been testing uses an open L (greater-than-90-degrees) with the active span being ‘drooped’ (inboard tip lower than outboard root).
By vectoring the lift from the submerged/active span, the vertical struts are not significantly loaded so surface-piercing effects are minimised.

It is interesting to note that where active T foils have been tried on catamarans the results have been less than promising because vectoring was difficult to achieve. Sideforce was provided by the surface piercing vertical struts. These got smaller with increasing ride height. Also their pressure field interfered with the main lifting foil degrading efficiency.

With twin Ts it may be possible to set the hulls up for differential ride height (set the neutral point on the respective sensors differently for windward and leeward foil) thereby encouraging the platform to stabilise at a heeled ride height. However the downside is that the windward foil will have a long span of submerged strut (since the foils are far away from the centreline, the difference in immersion from upright to heeled is large).

The concept we tested, designed by Dave Lister, showed promise by combining active heave control and lift vectoring with fully submerged lifting spans and minimum wetted area.

On an active control setup, lifting foil area does not change with heave. The submerged portions of the vertical struts get shorter but this has little effect on total lift. Instead lift is controlled by changing the lift coefficient of the main foil, either through altering angle of attack or, most effectively, through adding camber by deflecting a flap.

A flap alters camber and changes the angle between chord line (light blue)
and oncoming flow (dark blue)

This solution comes in different forms. Variations on V configurations rely on a decrease in immersed foil area with heave.
Other solutions such as the acute L/V rely on a coupling between heave and leeway such that increasing leeway reduces the effective angle of attack of the main lifting surface.
Where leeway values are very small, an L/V foil can also use a reduction in lifting area (inboard tip breaching the surface) as a last-resort means of limiting ride height.


Mechanically it can be argued that the overall complexity is similar: Active systems have swivels, pushrods and bellcranks that require significant refinement and must be looked after correctly. Passive foils require hull and deck bearings and means of adjusting depth and rake.
So ultimately the cost differences are minimal.
Active foils need some form of articulation built in (a shaft or flap) so they are more complex to produce. But they tend to be made from straight segments whereas passive foils tend to have curved spans so their tooling is more expensive.
Again, on balance cost is not a deciding factor.

Active foils with mechanical sensors tend to be at a disadvantage in light winds and marginal foiling conditions because there is a drag penalty associated with the control system.
In non-foiling conditions the sensors can be disconnected and retracted. But then no lift is available so any puffs would see the passive boat move ahead in foil-assisted mode.

Arguably the active setup is also heavier depending on where the sensors are located and how they connect to the foils.

So on a small cat the passive foil would have the competitive edge in very light winds.
The exact crossover remains a subject of investigation and will be found to depend on variables such as displacement/length ratio, sail area/wetted area ratio and the exact design of the foils...

Once foiling the active system requires less deliberate correction by the skipper.
This favours the less advanced sailor but probably makes little difference to the nuanced expert who is constantly making adjustments by muscle memory.

The crucial difference is this: An active foil can be smaller for a given takeoff speed because lift coefficient can be maximized when needed and dialed out when not required.
You can have an aggressively cambered foil on takeoff and a flat low-drag one at high speeds.

This is not impossible with passive foils. For example, the section used in the upper portion can have more camber than the one used near the tips.
But the compromise is more critical.
It is more difficult to have early takeoff and low drag at high speeds.

If the rules are tested and the A Class decides that active controls are not desirable, then passive systems will evolve rapidly and the problems will be solved.
Hopefully the decision will be an informed one based on a good understanding of the options rather than on prejudice and fear of the unknown.
In either eventuality the development process will continue to be fascinating.

Graphs from UNSW Team 1: Sam Paterson, David Kirkby, Byrce Edmonds,
Ashley Thornton, Felicity Kelleher, Nick Tenison, Syafiq Nazarudin 

And Team 2: Jarred Grimmond, Nay Myo Lwin, Stephen Narunsky,
Julia Shields, Tyler Steer, Hu Su

Friday, November 28, 2014


New Moth bow mechanism developed with Scott Babbage now available from SailingBits.
CarbonicBoats worked with Scott to develop a system with less friction, almost zero play, and built-in adjustment of gearing and wand length.

Four prototypes incorporating different shapes and bearing materials were created and tested before the production version could be signed off.
A great project for learning about the complex tradeoffs between mechanical efficiency, weight, reliability, repeatability of tolerances and cost effectiveness.

Milled from a single block of aluminium the new design takes out the wobble and play from other systems. With a larger diameter axle, and bronze bearings, the bow mech takes away the opportunity for unwanted movement that develops in other designs.

Incorporating adjustable gearing, adjustable wand and a large fast-point variation, you have a large range of adjustment to get you through the full range of conditions.

And for all those aero junkies, it incorporates and aerofoil maystick to reduce drag.

Available in Black, Silver, Red, Blue, Purple & Yellow.

Order yours now by going to:

Saturday, November 22, 2014


The nature of our UAV work is such that we don’t regularly get to share it with the public.
Confidentiality is often an important consideration in the aerospace game so it is exciting that on this occasion we can reveal images of a recent project.

This is the first airframe of the Cometa UAV designed by d3 Applied Technologies for surveillance and 3D mapping missions.
We worked closely with Gonzalo Redondo and the d3 team to develop tooling, details and fittings best suited for their design.

The beautifully integrated and streamlined airframe is demountable in the field so it can be transported in a compact package.
It can operate with landing gear or from a catapult with parachute landing.
The aerodynamic treatment, informed by in-house d3 CFD capabilities, is exquisite.

Wednesday, November 12, 2014

Checking In

Click here to read a Q&A Session with Martin Vanzulli who is doing a great job of keeping the A Class website up to date as well as running the Catsailingnews blog.

Tuesday, November 4, 2014

Wednesday, October 29, 2014

Higher Learning

Earlier today two teams from the University of NSW presented results of an inquiry into theoretical hydrofoil stability and performance.
The teams undertook to assess three candidate foil types on an A Class catamaran and investigate relative characteristics of lift, drag and change in lift with ride height/leeway.
A more detailed report is being prepared, but initial indications are in line with experimental observation.

Thanks to Dr Qing N. (Shaun) Chan for structuring the project.

More detail to come.

Saturday, October 25, 2014

Working the Angles

Our Paradox Version 3 A Class cat platform design is complete and tooling is underway.
The foil housing arrangement in the new boat is designed to accommodate virtually any shape with full interchangeability of parts using a new version of our proven hull and deck bearing system.

Now focus is on foil design.
The plan is to offer the boat with a foil package that prioritises ease of use. 
Design constraints were imposed to keep the overall arrangement symmetrical (so the foils need not be raised/lowered/trimmed at every tack or jibe) and to minimise part count.

An alternative foil package with flaps to control heave is being developed in parallel.
Owners will have the option of fitting either foil package depending on preference.
Full interchangeability is being implemented from the earliest stages of design.

Looking at the simple 'no moving parts' option, the most promising concept is the Z foil, itself a development of our 'comma' foils, in turn inspired by Hydroptere.

Studying the Z foil in detail reveals some interesting tradeoffs that the reader will appreciate.

Since the A Class has a maximum beam limit and an inboard limit for all immersed portions of the boat, there is a theoretical maximum available horizontal (projected) span.

To take advantage of the full available width, the 'working' part of any lifting foil should ideally start at maximum beam and end at the inboard limit.
This can be achieved in a number of ways including:
a) Mount the supporting strut right out at max beam.
b) Use a T foil.
c) Cant the strut outward so it exits the canoe body somewhere inboard of the hull maximum width, goes down and outboard until it hits maximum beam, then connects to the lifting span.

Option a) has the drawback of poor interference drag characteristics at the junction between hull and foil. Since the foil leaves the hull tangentially where the topsides roll into the 'shoulders' of the bilge, the included angle between the inboard face of the foil and the bottom of the hull is very acute.
Option b) could potentially be promising but it is difficult to overcome the drag of the T junction. The two free tips of the lifting element also give higher lift-induced drag.

Option c) leaves us with some interesting trades to make.
Moving the junction inboard gives better 'end plating' and less interference drag. 
These two factors also discourage ventilation when transitioning to full flight.
However moving the exit point inboard requires either more outward cant or more depth of the vertical strut to achieve the same span of working foil.

More outward cant means less draught and less overall foil area. But in some conditions the outward canted strut can generate downforce, negating some of the gains and adding induced drag.
Less outward cant means more draught and more overall foil area. But also more total lift.

Overall characteristics of lateral resistance (and optimum effective toe-in) are also affected by the above tradeoffs.
Cant angle of the upper strut also has an effect on the rate of change of effective dihedral with heave.
Which is a measure of heave stability (decreasing dihedral angle with ride height gives positive heave stability).

Surprisingly the best combination may well be to give up some horizontal span in order to limit outward cant and/or draught without moving the exit point too far outboard.

Other considerations are the 'droop' angle of the main lifting segment and the shape of the tips.
Interactions of these parts are quite complex as there is significant 'wraparound' of the pressure fields.

Fascinating as always.

Tuesday, October 14, 2014


Here is the first production item emerging from our development work in collaboration with Moth guru Scott Babbage.
Billet machined bellcrank now available from
The brief was to develop control system components that minimised play (manifested as slop/bumps in the foiling ride) while maintaining full adjustability.
More bits are under development and will be available soon.

Saturday, October 4, 2014


Here is an extended mix showing some early runs with experimental control system foil configurations.
Though there is still vast untapped potential, these sequences give a flavour of what is surely to come.
Everyone who tried it commented, through a persistent grin, that it is easy and feels secure.

As often repeated on this site, the goal is performance, not foiling at all costs.
Passive systems such as L/V foils give some measure of heave stability at the cost of some additional lift-induced drag. If properly designed they are competitive and manageable. The key is to design the system to work with the hull so that the highly foil assisted mode remains fast. In the right conditions and with the right technique the skipper can then push beyond a 99% lift share and transition to full foiling.
So far this has only been proven to pay downwind in flat water when fully powered up. But undoubtedly the profitable flight envelope will steadily grow, expanding to lighter winds and upwind.
By all accounts engaging this mode is hard work and requires finely tuned judgement to give net gains in VMG. But since gains are definitely available, it is a challenge to be relished.
V, comma, and now Z foils have improved performance, added a challenge and made the A Class safer to push hard, without taking away from the delicious responsiveness of this lightweight boat.

The difference between a passive system and a control system is that the latter is simply relentless. The boat will remain foilborne essentially until it stops, allowing for the skipper to look around, sit in, change gears and ride out lulls... All while the ride height is directly reacting to changing inputs.
So enjoy this first glimpse into just what is possible under this great class rule!

Wednesday, September 10, 2014

Bel Paese

Some great images of Andrea Ferrari racing his Paradox V2 in Italy

Monday, August 25, 2014


Another symbolic milestone: the last batch of Katana Marblehead hulls from the existing tooling is here.
The old moulds have been 'retired' and work will begin soon on updated tooling.
Deliveries are expected to start again in early 2015.
Get in touch for more details...

Saturday, August 2, 2014

Back to the Future

I have hinted in previous posts that the new home of Carbonicboats is in a rather special place.
Now the bulk of the preparation work is behind us, the Sydney International Boatshow is as good an occasion as any to reveal a bit more about our choice of this little known but fascinating location.

Image source
Cockatoo Island is a UNESCO World Heritage Site in the heart of Sydney. It is steeped in shipbuilding history. It was a place of great industrial production and innovation where many a destroyer with the proud 'HMAS' prefix was constructed and commissioned.
It was a key piece of infrastructure in supporting the Pacific Fleet during WW2.
Great technological innovation also took place there with notable presences such as de Havilland aircraft in the 1930s and naval submarines in more recent times.

Image source
After a period of limbo following the relocation of heavy industry, the island is being revitalised under the auspices of the Sydney Harbour Federation Trust. The many unique spaces it offers now play host to art exhibitions, concerts and film crews. A Marine Centre is also being readied with dry stack boat storage, food and entertainment.

Angelina Jolie directing Unbroken on Cockatoo Island. Image source
Such a place was a natural fit for our company.
Direct access to the water for testing, the convenience of being right next to Sydney city, security and relative privacy, are all great practical benefits.
But the significance of continuing an awesome tradition is deeply inspiring.
My greatest wish as a director of Carbonicboats is to foster a corporate culture where innovation and quality are valued highly. Being surrounded by the legacy of past creativity and being in the presence of similarly innovative companies will certainly help.

Friday, July 25, 2014

Think with a Twist

In response to an avalanche of questions about "how do you get your experimental foils with the lifting surface mounted at the end of a tube out from the top?" and "how big is a relatively small slot?", here are some illustrations:

First the foil is raked top-forward so the tube sits vertically in the slot through the hull.
Then the foil is rotated about the long axis of the now vertical tube so the main foil strut points inboard.
Finally the lifting surface is extracted through the slot.

This concept requires that the minimum clear length of tube is equal to or greater than the local freeboard of the hull.
Obviously if a boat were designed with this in mind from the outset and a shorter longitudinal displacement of the lifting surface were required, then the local freeboard could be reduced accordingly. In fact it would only need to be stepped down inboard of the slot.

The lifting foil is around 450mm in span (a bit more than the 400mm max legal horizontal distance because it has a tip-up angle) so the slot can be at a minimum around 380mm long. The width of the slot is equal to the diametre of the tube which on our prototype is 36mm.

More testing is needed, but initial indications are that stability is good, performance at foiling speeds is promising but the drag penalty at low speeds is significant.

Finally, and also in the spirit of sharing our development journey, an answer about the way we intend to extract from above an L/V foil with an acute included angle:
The smallest possible cassette would be one with a length equal to the span of the horizontal foil and a width equal to the chord of the foil. The foil would be rotated 90 degrees about a vertical axis once the cassette is raised.

The search continues, but so far the Class has seen only solutions that are either inventive but unnecessarily complex (cassettes, hinged foils, leeboards etc.) or limited in terms of performance (J and 'comma' or 'chevron' foils).
The latter are perceived by many to be an acceptable compromise and have in some cases turned opinion back toward keeping the Rule unchanged.
However the unexplored potential of 'true' foiling (as opposed to sometimes foiling on compromised appendages) remains vast. Exploring it is fascinating. Doing so within a now anachronistic rule makes it more challenging. But challenging quests can have surprisingly positive outcomes. So let us press on...

Saturday, July 19, 2014

Paving the Way

Lots to report as we continue to test over the winter... 
We are working on the next-generation Paradox A Class Cat design for the 2014/15 season, resolving the details for all new tooling to be created in-house. 
Now that our expanded production facility is operational we can tackle such jobs with confidence. This gives us more control than before when we relied on contractors for certain aspects of production.

The path we are taking is, as always, very empirical. Every idea is assessed for potential merit, tested objectively, evaluated, then either discarded or developed for the next round of testing.

The focus is on perfecting a foil package that will be a significant improvement on current designs. ‘Improvement’ in this case is strictly defined as the ability to generate better performance around the racetrack in most conditions. So ease of handling, maneuverability and acceleration play a role as well as outright straight-line speed.

We began this phase of R&D by prototyping a series of ‘acute L’ (AKA 'L/V') foils. These all shared a common vertical strut but had incrementally different horizontal chord, span, tip-up angle, and section characteristics. For testing they were inserted from below into simple straight (parallel-sided) cases. These cases are installed in one of our test platform (the orange boat nicknamed Glamorous Glennis) in the exact same position as the production ‘comma’ foils we used at the NZ Worlds.

Following are some thoughts on the testing process and the state of play in the Class:

The rest of the platform was unchanged but two candidate revised rudder designs were tested. More on the selection of rudder design in the next post. Except to confirm that all candidates fit without modification in our production rudder cassettes that have proven popular. For those of you who missed the previous related post, the 2014 version of the cassettes is pictured below. 
You will notice that the rake adjustment system has been simplified and construction beefed up to maximise stiffness.

Robust cassette assembly machined from billet available now.
Rod end/spherical bearings have been deleted and rake adjustment can now be easily done on the water
Continuing Foil R&D
Imposing the constraint of a straight vertical strut simplifies progress by reducing the variables to be accounted for. It also reduces production cost, allows us to use the full horizontal span permitted by the rule and makes fitting of the structural foil case very simple.
Relatively quickly we came to some definite conclusions regarding ideal tip-up angle, shape, and area for reliable stable foiling using the leeward foil only. Needless to say this configuration is extremely promising with upwind foiling and foiling jibes being a given. The key is the ability to use all the beam of the boat to generate righting moment. A marked difference can definitely be felt when the windward foil is out of the water and no longer pushing the windward hull up.

As an aside, the market has proved very hungry for this type of foil. Many customers want to retrofit their boat with the simplest, most cost effective package to just get out on the water and enjoy foiling.
Since racing in the A Class was always integral to our design brief, we have also devised a way to legally fit the final selected L/V foil in compliance with Rule 8. Perfecting this aspect of the concept will be the next step and hopefully the result will be relatively elegant. I say relatively because any solution other than inserting from below will be more complex than strictly necessary. But our challenge is to minimise the rule-mandated unnecessary complexity.

No Stone Unturned
Part of the test series is a radical concept that could potentially achieve two goals simultaneously: Firstly it could be inserted from above through a very modest slot/case with no complex cassettes. Secondly it could displace the horizontal lifting surface forward, increasing separation from the rudders, without affecting helm balance. 
A side-benefit is that the full horizontal span could be used without needing to put the vertical right outboard. 
Stability would still come from a tip-up angle (leeway coupling) exactly as for an L/V foil. This concept does involve a wetted area penalty (in the form of the area of the horizontal tube). It poses some structural challenges (flex in the tube and twist in the vertical foil) and it has a higher induced drag because it has more free tips exposed to the flow. Preliminary calculations showed that it had enough potential to warrant building a prototype for testing. We will know soon how it does in the real world…

In Parting
That sums up our status along the fascinating journey of performance development. 
Now to explain the title of this post: Observing competition in Europe we have been happy to note that the approach we took for the production V2 Paradox is now finding acceptance by other manufacturers.
Our concept of 'bent' foils (as opposed to curved) that exit the hull vertically then transition quickly to a straight span with pronounced dihedral has been emulated and refined to different extents (functionally the working portion of the foils in this concept is not dissimilar to that used successfully by Hydroptere).
Interestingly some newer designs have place the ‘elbow’ further down so that the hulls effectively sit higher when the foils are working in equilibrium. It looks more spectacular and arguably gives a bit more wave clearance, but the penalty is extra foil area - a compromise with respect to performance in displacement mode. The additional vertical span also delays the beneficial change in dihedral angle with foil retraction. This can be alleviated by raising the windward foil such that the lower bend passes above the hull floor when sailing upwind and in light airs, although getting the foil to locate properly when partially retracted requires engineered bearings rather than a simple slot. Our ‘bent’ foil designs (with rotating bearings) did this automatically, but our ‘autotack’ feature has yet to be replicated by others.

Our V2 production foils pictured at the NZ Worlds.
This concept of transitioning from a vertical exit to a Hydroptere style dihedral setup was a first in the A Class and has now adopted by others.
The upper bend in our design allowed the windward foil to adjust automatically to optimum dihedral when sailing upwind.
It is certainly great to see a move away from unstable J foils toward more stable (less unstable) arrangements. The guys at the Europeans are to be congratulated for some great performances that really extract much from well set up ‘four point’ arrangements. It is also great to see validated our findings that loaded surface-piercing foils require careful treatment of camber and entry angle to delay ventilation. Gonzalo Redondo of D3 and Mischa Heemskirk seem to have nailed that aspect of their foil design.

Interestingly the foils and beams on other designs have moved forward to closely match the positions seen on our production V2 boats. 

We were happy with the performance of our equipment at the NZ Worlds. But the next step will probably be ‘three point’ foiling. So that is where we are concentrating our energy now. 

There is yet another avenue we are exploring that has shown great potential in terms of safe, easy, reliable, fast foiling. More news on this and on our new testing centre in the coming weeks... 

Soon we will have to decide which way to go for the production boat. It may be that the market will continue to demand ‘unadulterated’ equipment in parallel with a competitive rule-legal version. So we will continue to offer both options.

The flattery of imitation is a great confidence booster, but pushing forward into the unknown is an even greater thrill.

Friday, June 27, 2014

The Internship

Two talented and enthusiastic young guys have joined us to lend a hand and learn.
Raphaël Censier and Sébastien Canva are students of the engineering school of ISMANS (Superior Institute of Materials And Advance Mechanics) in Le Mans, France.
They have made the trip Down Under to spend some time at CB.

They have already proven their talent with good work in some interesting projects.
Great timing as work in the new facility gradually begins.

Wednesday, May 28, 2014

Like a Swan

...Quiet on the surface but paddling frantically below, out of sight.
That is our situation at the moment as we are getting the new premises up and running.

Our factory A Cat test platforms are also 'in the shed', being fitted with new foil cases that will be used to validate a couple of full foiling configurations.

We have locked in the design for 'inserted from below' L/V foils and the tooling for these is ready.
Interestingly we have had huge demand from people wanting to install these regardless of the rule.

However we are also working on a rule-legal variant. More on that when testing resumes.

The immediate priority is to get our in-house capacity online so we can meet customer demand in terms of quality and time. For this we are creating an appropriate environment...

Where the magic will happen

Tuesday, April 22, 2014

Another Result

Using Paradox rudder blades retrofitted to his customised DNA platform, Mischa Heemskerk dominated the first Dutch event of the 2014 season at Muiderzand with 5 bullets out of six races.

Skilfully balancing in good foiling trim,
the instant expertly captured in a nice still image.
Remaining in this trim requires very active control.
Image copyright Jasper van Staveren
Report here:
Full results here:

This win further supports our findings regarding the effectiveness of the concept, shape, section and construction of our rudders.
Various skippers confirm that they offer better 'damping'  than any others on the market with consistently less drag and the ability to remain effective for longer (higher heave values) without losing grip.

However it is worth noting that the combination of even the best rudders with J foils (where both foils are used at the same time), while undoubtedly faster than non-foiling configurations, is a very compromised solution. And one that we are hoping will be greatly improved upon very soon thanks to the development and testing work we are carrying out now.

Using two foils halves the effective beam. In reality the leeward foil contributes somewhat more than half the lift,
but you can see that the windward one is always trying to raise the windward hull.
Leeway coupling has the effect of unloading the windward foil with increasing ride height,
adding a further element of non-linearity.
Since most lift comes from the bottom of the foil, there is no inherent heave stability.
This video gives a graphic illustration of the difficulty involved in sustaining flight on J foils:
The ultimate solution will undoubtedly be an L/V type concept with built-in automatic heave control.
Meaning a foil where total lift varies inversely with ride height.
And one where all the lift is generated on the leeward side, thus utilising the full beam of the boat to generate maximum righting moment, in turn giving maximum sail carrying power.
The variations we are testing now have the potential to fit within the current rules.

We urge anyone contemplating an upgrade to contact us before committing to J foils as these may soon be proven outdated.

Monday, March 31, 2014

Honing In

We are continuing our tests of L/V foils. The constraints imposed for this series are:
- Foils made of straight segments with minimum (hydrodynamically clean) transition radius.
- Total foil width not more than 400mm so that the option exists to insert the foils from above if Rule 8 persists.

The two main variables to explore are:
-Foil immersion (draft).
-Tip-up angle.

Findings so far have been very interesting. At the risk of employing an overused adjective, they have been fascinating.

In short the two variables listed above interact in a way very difficult to quantify but with a pronounced effect on handling.

Deep foil partially retracted.
Notice tip just beginning to breach the surface
As you can see in the above picture, we have made the verticals very deep so that we can explore a range of draft values. The goal being to fine-tune foil depth to one that gives an immersed area such that the range of values of lateral resistance yields the desired coupling between leeway angle and lift.

To obtain the desired range of values of lateral resistance, foil area has to be such that it gives enough hydro sideforce at full immersion (hull in the water) to counter maximum aero sideforce (driven by righting moment) with a relatively low Cl. Meaning small Angle of Attack/leeway.
But the area must also be small enough that it will require a high leeway value (large Cl but before the stall) to generate the same sideforce at optimum ride height. Thus permitting some sideslip to unload the horizontal before it breaches the surface completely.

However, if foil chord is such that draft must be very large to immerse enough vertical foil area, then leeway coupling will become too dominant as a means of heave control.
Meaning heave equilibrium will be reached long before the inboard tip of the horizontal foil reaches the surface.
Therefore, in situations where the speed is high but leeway is small, the boat will tend to fly too high. This in turn could lead to loss of 'grip' by the rudders/elevators.

So one of the interesting realisations to come from this testing is that chord is actually a critical value in the design of stable L/V foils. Our iterative approach, combining a range of vertical and horizontal foil designs at different angles, is proving to be a quick, economical and fun way to explore this new design space.

Check out the following video to see an unedited run on one foil in barely enough wind to be on the wire upwind. The acceleration is addictive.

Sunday, March 16, 2014


The following videos show some of our recent testing with heave-stable 'acute L' foils (L/V for short).
This experiment had one aim: To prove that a simple cheap upgrade is possible to convert existing A Class catamarans to stable foiling without major structural modifications.

The story so far

We knew from previous testing that L/V foils give stable foiling. Our conclusion was that the crossover speed was relatively large so the overall advantage of this configuration for racing would be marginal. The complexity of a cassette arrangement to satisfy the 'inserted from above' rule tipped the scales in favour of adopting our 'comma' foils for production and racing. These proved competitive giving skimming flight with neutral stability when required and minimising drag when in foil-assisted mode.

After the Worlds we revisited the crossover numbers armed with new knowledge about kinetics and the tactical options made accessible by foiling. It is now beyond any doubt that foiling will pay. L/V foils can be made to work within the rule. This requires workarounds such as hinged foils or large cassettes. Contraptions that are feasible in an engineering sense but will add complexity and cost.

While it is tempting to wonder whether there is some lateral breakthrough design somewhere within the rule space, all the evidence right now points to the fact that Rule 8 adds no value but instead imposes complexity and compromises efficiency. It creates costs that bring no benefit.

A growing majority of Class members is asking whether a simpler solution would be feasible.
Any lingering doubts revolve around ease of handling and, especially, the difficulties of converting existing boats that may otherwise become obsolete.


The foils in these videos are old Marstrom C boards with new horizontal legs bonded on.
The horizontal legs are a proprietary shape. Critical features such as the area, tip-up angle, twist and angle of incidence were determined in light of the work carried out over the course of our Paradox development programme. However the Marstrom verticals were not modified.

In the first video they are housed in their original cases installed in a Melvin A3.
In the second they are mounted through the existing foil cases of a Paradox test platform. The rotating bearings at the hull and deck were replaced with simple plastic blocks.

In both instances the foils are held down by a rope led to a deck cleat. They are retracted using pre-tensioned bungee that pulls them up when the down-line is released.

Rudders are standard Paradox with some transom reinforcement added to the older boat.

Longitudinal placement of the foils is further forward than usual because that happened to be the arrangement on these particular boats. Previous testing has already shown that, within limits, keeping the foils further aft gives a maneuverability advantage without adversely affecting foiling stability. At any rate, installing a simple foil case further forward is cheaper than putting in a cassette.


In short the transition is surprisingly gradual with drag falling away as the hulls rise. Once 'unstuck' the boat rises more rapidly until the heave control features of the L/V foils come into play.
At this point the normal instincts of a cat sailor remain applicable. However there are a few interesting differences:

The most important lesson is to do less rather than more.

You will see in the first video (my first time on this configuration) that pulling away aggressively in response to building wind strength can force a reduction in ride height, especially if it is done after the boat begins to heel in response to the gust. The foils automatically go to work to restore level flight but the momentary reduction in ride height does sap energy. A good example of this is at 0:39.

Once the coupling between steering and heel is noted (if heeled to leeward, steering into the wind will make the bow come up. Pulling away will make it go down), then one quickly learns to anticipate. So the usual response of letting the windward hull rise then bearing away is somewhat modified:

As pressure increases, the best technique seems to be to ease the sheet a tiny amount, let the boat heel to windward ever so slightly, then pull away as normal. This results in addictive exhilarating acceleration in total safety. Unlike a displacement cat where forward buoyancy gradually runs out, the foils provide more lift as pressure from the rig increases so the feeling is one of total immunity to nosediving (so far!).

It is easy enough to become accustomed to just trusting the foils and keeping steering inputs to a minimum. Obviously the boat will spin on a dime when foiling so the key is to be subtle with the tiller. Heeling to windward a tiny bit helps to increase ride height when bearing away. This really boosts VMG downwind (interestingly the same technique works upwind because luffing up to depower helps lower the bow and settle the ride height. But more on upwind foiling later).

For reasons I do not yet fully understand, Keeping the sail more open works better than strapping the sheet on. Letting the traveler down slightly (say to the hiking strap) and allowing a few degrees of twist causes the boat to fly higher. Closing the leech seems to lock the foils up so the boat settles closer to the water.

My best theory at this stage is that increasing sideforce causes the equilibrium ride height to decrease. This is based on the coupling between leeway and lift built into the L/V foil geometry.
Another factor may be that, since drag when foiling is so much smaller, it is not necessary to load up the boat with maximum sail CL. Instead the goal is to have the greatest drive force component exploiting fore-and-aft righting moment rather than resistance to heel...

You will notice that we are sailing downwind with both foils down. This is something we (Johno) stumbled upon but it has really helped to lower the crossover speed significantly.
Having both sides down effectively gives a pair of V foils. The reduction in foil area due to the inboard tips breaching the surface becomes the dominant heave-control mechanism instead of leeway-coupling. This is an advantage because stable foiling becomes possible at small sideforce values.

Raising the windward foil and relying on leeway-coupling effectively doubles righting moment whilst halving foil area. This is good when sail power is 'excessive' and speeds are very high.
In lighter winds the leeward foil would have to be bigger (if used alone) for the same takeoff speed. More importantly you would have to sail much higher to generate enough sideforce to fly a hull while trapezing.

In other words you would have to generate enough sideforce to lift the windward hull and enough speed to takeoff on one foil only. This is achievable at a much lower windspeed upwind than it is downwind. Having both foils down instead allows a very early takeoff while sailing deeper because righting moment is effectively halved and foil lift is doubled.

I suspect that top level sailors will gradually be able to bring down the critical windspeed for 'downwind windward foil raising'. But for now this option allows mere mortals to foil safely in as little as 6 knots TWS.


We have now proven objectively that it is feasible to convert an existing A Cat platform to stable foiling with minimum fuss and expense. The boat remains practical and exploitable but it offers a whole new level of performance.

On the emotional side, the feeling of foiling is just fantastic. I am sure that hundreds of Moth sailors already knew this. But it must also be said that foiling on an 18' cat while on the wire, and with no mechanical self-adjusting systems is special in a wholly unique way.

It really is simple to learn and no outstanding athletic ability is required. I have no doubt that this will be the future. Many top level sailors agree. The only question for the Class is this: Will we be permitted to do it in a way that is simple and cheap or will we have to use complex and expensive workarounds?

After the thrill of the 'magic carpet ride', touching down feels like sailing through honey. It becomes frustratingly restrictive. This really must be tried to be understood. Hopefully many A Cat sailors are about to do just that.

Saturday, March 8, 2014

A World(s) of Learning - Part 2

One more post on appendages. Then we can look at other areas of development such as aerodynamic tailoring and control systems/ergonomics.

Glenn Ashby balancing nicely on J foils and Paradox rudders.
Photo by Rhenny Fermor of
Steering system

The new kinetic techniques used to promote early flight, combined with much higher top speeds, really put our cassette and gudgeon fittings to the test.

When foiling high, immersed rudder area  is much smaller but, since speeds are higher, the equilibrium sideforce generated by the rudder is the same. Dynamic forces are bigger.
Because these forces are exerted at a greater distance from the hull (further down), the moments on the cassette assembly, gudgeons and transom are considerably greater than previously measured.

When retrofitted to the transoms of existing boats, some reinforcement may be required to create a stiff connection between gudgeons and hull. This can be done internally or externally.
Applying extra carbon reinforcement internally is ideal. But simply bonding on some carbon plate to the outer skin of the transom is an easy and quick solution.
Care must be taken that any external bonded-on reinforcement remains within the hull overall length limit. If this is exceeded then some distance must be sanded off the bows.
Some filling of the core may be needed in way of the gudgeon fastening bolts if the area around the new bolt positions is not already 'cored out' with carbon or filler.

External transom 'C-plate' reinforcement. Plus stiffening added to sideplates.
Initial observations were that our cassettes were too flexible when subjected to the higher loads. They were not in danger of failing but their flexibility impinged on the 'crispness' of the feel/feedback through the tiller extension when flying fast.
It should be noted that these cassettes evolved through our in-house testing on Paradox platforms fitted with our more stable foil setups. The inherent stability of our foiling geometries did not require the aggressive inputs needed to correct unstable configurations. Therefore we progressively reduced the gauge of the sideplates connecting the top and bottom machined 'stocks' that hold the rudders and swivel pins. The earlier versions (round holes) had thicker sideplates than the later (triangular holes) design.

Since the flex was in the sideplates only (the machined stocks are robust and fit the rudders snugly) it was easy to reinforce them by adding carbon plate or box section to the sides of the cassettes.
The production cassettes have been revised to incorporate stiffening ribs in the sideplates. Aluminium remains the better material choice for production when weight and cost are considered together.

Latest cassettes developed on stable Paradox platform had very thin gauge sideplates.
These were flexing when steering aggressively to stay 'on top of' unstable foils
The more surprising failure we had was shearing of the rod ends (off-the-shelf spherical bearings) connecting the gudgeons to the cassettes. We had been warned early on by experienced Moth sailors about the possibility of fatigue failure in these components when they are loaded cyclically in bending. Therefore we inspected them often and monitored the hours each unit had sailed so that if and when one failed, we could know the expected fatigue life.
However none had broken after the equivalent of three seasons on our various prototypes.
So it seemed the parts (8mm SS) were conservatively over specified.
Even so we replaced them before the regatta.

Brittle failure of 8mm SS rod end fitting
In our application the fittings are loaded almost exclusively sheer, with bending moment minimised: They are not used to adjust the steering geometry so they are always wound all the way into the gudgeons.
The rod ends that broke (one in practice and one during a race) exhibited a brittle failure with no sign of fatigue.
The instantaneous load simply exceeded the strength of the part.
We have since sourced a higher-spec equivalent rod end made with a forged/rolled process using a stronger steel.
These were used for the remainder of the series and held up well.

Looking failures of rudder fittings on other designs (and there were a few), especially on boats where conventional gudgeons were adapted to take some vertical lift, it is obvious that the engineering has to take into account much greater moments. The transoms and fittings must now be engineered accordingly.

As always the learning curve is absorbing and the weakest link is constantly exposed as it is chased around the system. Fascinating times!

Wednesday, February 26, 2014

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.


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 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: Using 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.
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 of 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 set up 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.
We had some failures in early testing, but 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).

Tuesday, February 4, 2014

What it is

Answer to the SA quiz about this pic:

Rapid proof of concept prototype of a system to adjust rudder winglet 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.
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.

Wednesday, January 22, 2014

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 that I thought would be 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 (read 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 moderately 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.


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 manouvrability. 
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 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 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.