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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.

Tuesday, April 2, 2013

A Different Look

Some quick snaps of the last shell just out of the mould.
We are building a series of 10 Katana Marbleheads using a special hybrid cloth.
The red bits are Kevlar.
No change in structural properties, just a different and unique look.
We will keep the boats in stock so grab yours today!