Insight

Some thoughts following some recent testing with a focus on handling.
Specifically, we worked to gain data on how different foil configurations affect dynamic behaviour during turns in strong wind.

The results confirmed observations we hear regularly from experienced Moth sailors, as well as those who race foiling multihulls such as the NACRA F20 Carbon FCS:
Being foilborne on the upwind leg makes the bearaway a lot safer.

Intuitively it is easy to understand that a foil has the ability to 'push back' with increasing force as the bow-down moment from the rig increases. But this is only part of the picture.

In displacement mode, an increase in bow-down trimming moment must result in some bow-down trim in order to move the centre of buoyancy forward.
More volume has to be displaced closer to the bow so that a restoring bow-up moment can exist to counter the increasing bow-down moment from the rig (which in turn is a result of sail force increasing and rotating to point more forward during the bearaway).
A secondary effect of the bow-down trim is that the rig increasingly pushes down, effectively increasing displacement.
All the while drag is increasing, speed is diminishing (or increasing at a reducing rate), and available volume (forward buyancy) is running out.

Staying on the foils instead allows the foil/elevator system to dynamically counter the changing sail vector.
Less obvious, and possibly more important, is the fact that, with less drag and smoother acceleration, the apparent wind stays forward so bow-down trimming moment is much smaller.

To reap the benefit, foiling upwind has to be competitive.
This can only be the case when maximum righting moment is available.
To maximise righting moment the leeward foil must be able to carry the boat at moderate (upwind) speeds and give some heave stability unaided.

Safety is perhaps the most compelling argument for ending the absurd restrictions imposed on the A Class by a shrinking minority.

Images of customers who have retrofitted our L foils to existing (mostly older) A Class cats.
A low-cost upgrade that increases performance and improves handling.
Incidentally it makes beach launching easier, as the foils support the hulls, giving a small point of contact rather than potentially scratching a larger area. Obviously it is better to always use a set of 'beach wheels'... But where laziness or circumstances do not allow it, the foils make for less widespread damage.

Imagineering Part 2

In Part 1 we looked at straight line sailing.
We concluded that foils will always be a hindrance at very low speeds, but will give an advantage at higher speeds. More aggressive setups (read more foil area) are even worse at low speed but, since they allow earlier takeoff, become superior 'sooner' (at a lower windspeed than more moderate foils).
The exact crossover is still being explored. It may depend on crew weight, hull characteristics, and rig choice.

Tacking and Jibing

When we introduce changes of direction the picture gets more complex.

Re-Configuring 

As a general rule, asymmetrical setups are always faster in a straight line.
Water ballast, canting keels, and sail 'stacking', are all examples of how making a boat asymmetrical improves performance on that tack. They offer gains additional to just moving crew weight to windward. At the extreme, all-out speed record craft have been 'permanently' asymmetrical for some time now.

Configurations with all the lift on one side (those that maximise righting moment) necessarily require foil settings to be swapped as the wind changes side.

We can view asymmetry as a form of specialisation.
If we must tack frequently, and our crew resources are limited, the advantage of specialisation must be weighed against the cost of transitioning from one specialised setup to another.
Two or three seconds may be lost during the tack as the sailor hauls on a foil-control line before settling in to sail the new course.
That time must be recuperated via extra straight line speed just to break even. If the distance between changes of direction is too short (because of course restrictions, shifts in the breeze, or tactical considerations), then the faster asymmetrical setup will not pay.

This tradeoff is hard to quantify on the drawing board.
Obviously sailors with more practice will be able to manage such changes more readily.
Once everyone reaches the same level of proficiency, the more symmetrical solution will still free up hands and mental capacity, making maneuvers faster.

Z foils are a compromise in a straight line, but still require management during turns. Differential rake gives a marked improvement (unloading the windward foil to minimise righting moment loss). Since foiling upwind with them only pays in rare conditions, retracting the windward one is de rigueur.

If regular raising and lowering is required, then the straighter the foil, the easier.

As an aside, configurations with an active central T-foil, such as used by Moths, tend to have the sensor wand offset to keep any wake it produces away from the vertical strut of the main foil.
Since T foils rely on windward heel to vector lift from the fully submerged foil, the wand needs to be reset on each tack.
For example, if the wand is mounted to the right, it needs to be set lower on port tack and higher on starboard tack.

Even under an open rule, a central T foil solution will most probably not be the fastest on a cat, because it would effectively halve the beam of the boat, giving up too much righting moment.
The ease-of-use advantage would probably not outweigh the loss of righting moment for this particular almost completely symmetrical configuration.
Though it would be an interesting experiment, it makes little sense to give up half the leverage available to competitors who fully exploit the inherent advantages of a catamaran platform...

In summary, some form of asymmetry, and associated extra work, will most probably be accepted as worthwhile for maximum performance around the course.
The question is how much. Where does the best compromise lie?

Gliding

If we equate straight line sailing with powered flight, then tacking and jibing is like gliding.
During the turn, as sail drive force dips through zero (and below if the apparent wind goes around the front), the distance available before 'splashdown' is analogous to glide ratio.

There is an established and fascinating body of knowledge around unpowered flight.
One of the first facts to digest is that lift-to-drag ratio is the dominant element.
For the same configuration, a heavier glider will fly faster, but it will follow the same glide path (same sink for every unit of forward movement).
It will reach the ground sooner, but in the same place.

So, all things being equal, foils with a better lift-to-drag-ratio will keep us foiling further than less efficient ones. Regardless of crew weight.

In our case, the mechanism that controls heave will also be making the foils work harder as we sink. Surface piercing foils will be getting bigger with sink. Active foils will be lowering flap, and hence increasing lift coefficient. In both cases drag will be increasing. At some point drag will increase rapidly, taking us away from best lift-to-drag regime and shortening our glide.

One possible exception to this is L foils. Since they rely on leeway for heave control, and because mid tack/jibe there is no sideforce, they will possibly remain more efficient for longer, increasing our chances of getting through the turn without ditching the hulls.

In all cases the area of vertical shaft in the water will be getting bigger as we slow down, adding drag.
But this effect should be similar for all configurations.

One interesting take-away is that flying higher helps with maneuvers since it gives you more potential energy going into a turn.
But flying high means long foils, which are draggy at low speeds.
Ultimately an answer could be 'jacking up' the boat just before a turn. But realistically this is not a workable solution for a singlehanded boat...

Dynamic Effects

A fascinating observation we made when testing 'four point' foil configurations (Zs and active/flap foils) on A Class cats is that turning can induce significant rolling.

The cause is simple: as you turn, the foil on the outside of the track travels faster through the water, so generates more lift.
If not managed, such rolling can make the outside foil breach the surface, which then causes a splashdown of that hull.

On a boat where crew weight is so influential, this effect can be managed through good technique.
Inherent heave stability helps.

Side View Vs. Top View

Finally let's examine getting away from a 'parked' position near head-to-wind.

When looking down on the boat, we want the foils to be at or just behind the centre of effort of the sail. Thus when we ease the sail, the drag from the rig tends to just lead the foils. Combined with steering input and windage on the bows, it allows us to bear away quickly and power up.
A boat with less lead will rely more on rudder sideforce to sail in a straight line. This theoretically reduces induced-drag, but takes away 'reserve' rudder force available to bear away.

In side view we want the lifting foils to be slightly ahead of the centre of gravity of the boat. This increases tail volume and helps with stability.

The ideal position for the main lifting surfaces is forward of that for the verticals. Moving the vertical part of the foils forward makes it more difficult to bear away.
Raking the foils bottom-forward helps (and discourages ventilation), but the longitudinal displacement is minimal.

Again a compromise is required if we want to continue using a single sail.
Again our ability to aggressively trim the boat by shifting crew weight is our friend.
Having the windward foil raised does help in this respect, scoring another point for an asymmetrical setup.

Crystal Ball

There is certainly a lot still to learn about how all these sometimes conflicting factors should be balanced for best performance around a course.
Experimentation and time will give us answers, as well as showing us new questions.
This is why we like to play in a development class.

It will be a close run thing between 'four point' and 'three point' solutions on the A Class given our unique characteristics of limited beam, modest sail area, and singlehanded crew.
Our testing indicates that, especially with more transverse span available, L foils show a lot of promise. The near future will most probably be an L or a less tortured Z.

It is doubtful that radical and/or impractical sulutions, such as central foils, and/or ones that require capsized launching, will take root.
There are certain inherent advantages in a cat class that make foil retraction attractive. Just as using beam to generate righting moment is advantageous.

Everyone will weigh the pros and cons. Given the freedom to experiment, the cream will rise to the top and the best ideas will win.
Afterall, this approach has given us the simple, lively and enjoyable toy that is the modern A Class cat.

Bravo

Congratulations to Sergio Vela, who placed third at the European Spring Championship, using retrofitted Paradox 2014 steering system. Full results here.

Image source: Circolo Vela Arco

Imagineering Part 1

In response to questions about where catamaran foil design may go in the future, especially if rule constraints are relaxed, here are some thoughts on the incentives driving design choices.

If our goal is fastest time around a windward/leeward course, then the considerations are:
- VMG upwind.
- VMG downwind.
- Control at low speed, specifically to bear away and accelerate off the start line.
- Speed profile through tacks.
- Speed profile through jibes.
- Sensitivity to setup.

VMG Upwind

To get to the windward mark first, we want the right combination of speed through the water and heading angle to the wind.
We can sail faster through the water by footing. But sailing at a bigger angle away from the wind direction means covering more distance for the same ground gained toward the mark. So our extra speed must be enough to make up for the longer course sailed.

Any form of foil assistance will involve an initial drag penalty.
At very low speeds the foils do nothing but add drag.
As we go faster and the foils begin to produce lift, they contribute even more resistance. This is added to the drag of the hull(s) that are still in the water.

The 'foiler' will remain at a disadvantage until enough weight is transferred to her foils to at least reduce hull displacement-to-length ratio enough so hull drag reduces enough to make total drag of foils+hull lower than just hull drag would have been with no foils...

At any speed above such crossover, all other things being equal, the foiler will have less total drag than her displacement counterpart. Less drag for a given speed means that speed can be maintained with less sail force.
The advantage will get bigger as speed increases.
At some arbitrary point the hulls will be completely free of the water, hull drag will go to zero, and the only contribution to hydrodynamic drag will come from foils and rudders.

Indicative graph showing how drag rises with speed. Simple displacement hull has the least drag at lower speeds because the drag of foil-assisted and foiling boats at those same speeds is hull drag + foil drag. Foil assisted shadows displacement but has less drag at high speed because the effective displacement of the hull is reduced. Aggressive foils pay an initial drag penalty for early takeoff. Note that takeoff constitutes a quasi-discontinuity in the drag curve. Foils are shown 'loaded'. Reducing their angle of incidence at low speed would reduce their drag, but the assumption here is that takeoff is being attempted. 

From the above paragraph we can conclude that a foiler needs to be moving fast through the water to get an advantage. After all, foils don't work unless they have water flowing over them.
So we can conclude that foils will only give us a winning edge upwind if they are efficient enough to support enough weight to significantly reduce hull drag (or better yet eliminate it completely) at speeds that are achievable without having to give up too much pointing angle.

Looking at foil drag alone, this explains why angled and C foils are formidable upwind:
They have negligible extra area compared to a straight foil that just contributes sideforce.
But through curvature or cant angle, they vector some lift upwards, and reduce hull displacement with almost no foil drag penalty.

In contrast, a foil with a dedicated lifting surface (such as an L) must have the same area in its vertical shaft as a conventional foil in order to provide sideforce. The horizontal leg is extra. It just adds drag until speed is high enough for the lift to start making a difference.

Angled and C foils, for foil-assisted sailing, have little more area than upright ones.
Dedicated lifting foils such as Z (centre) and L/V (right) have considerably more.

So now the question is:
Can I go fast enough, enough of the time, to get my dedicated lifting foil working, without reaching away from the top mark too much?

To answer that, we have to look at other speed-producing factors.
Upwind the dominant one is righting moment.
Assuming enough wind to be fully 'powered up', more righting moment means we can keep increasing sail force without capsizing.

If boat mass and crew weight are the same, the only way to increase righting moment is to get more leverage. Meaning lengthen the distance across the boat between the centre of gravity (CoG) and the point where the mass of the boat is being supported (let's leave out downforce from the windward foil for simplicity).

For a displacement cat, flying a hull moves the centre of buoyancy (CoB) to the leeward side, maximising the lever arm.
For a foiler you can see that ideally we would want all our lift to be centred right under the leeward hull.
Any movement toward the centreline carries a penalty in righting moment.
However some compromise may be optimum for an L foil because increasing the span of the horizontal leg (making the aspect ratio higher) improves foil efficiency.

Single foil to leeward (blue), with straight vertical and short horizontal, gives maximum leverage.
Curved vertical and long horizontal (red) gives less lift-induced drag.
Some compromise (orange) is usually the fastest solution.
Z foils may add less drag at lower speeds but will also reduce righting moment.

Given no other constraint, getting all our lift from the leeward foil would be ideal.
If the rules limit the available span on each side, then one foil may not be enough to get our desired 'breakeven' speed to make foiling work.
Then we can go to 'strange' solutions, like having two or more foils on the same hull...
Or use the windward foil to help by contributing some lift.

Since the windward foil is adding to heeling moment, lift generated under the windward hull is very 'expensive'. It has a direct cost in sail carrying power.
That is why Z foil A cats feel very 'tippy' when set up for maximum lift.
It is also why boats like Hydroptere go for extreme beam.

Once fully foiling, stability will come into play.
There must be a way to keep lift constant as speed and ride height change.
This can be done by:
- Varying immersed foil area (surface-piercing foils).
- Coupling lift with leeway (L/V or 'acute L' foils).
- Active control systems (wands/flaps as on a Moth).

Heave stability is not so critical upwind because boatspeed can be controlled relatively easily by coming up into the wind in the gusts, and bearing away in the lulls. Effectively sail force can be kept constant as apparent wind varies.
Pitch stability is surprisingly important upwind because drag from the top of the rig tends to make the sterns squat down, so rudder winglet (elevator) lift is critical.

VMG Downwind

Initially it would seem that downwind the tradeoffs are simpler: Righting moment is less vital because the sail force vector can be more in line with where the bows are pointing.
This is true in strong wind, at low to moderate boatspeeds. In such circumstances, you can sail 'deep', with the apparent wind over your shoulder, and sails eased. Crossover boatspeed can be reached easily and foiling pays thereafter.

However, as boatspeed increases, and the apparent wind goes forward, righting moment again becomes king. Even downwind.

Getting to takeoff speed is also just as critical downwind in lighter conditions.
If there is not enough wind for the sail to basically 'push' the boat to takeoff speed, then we need to luff up to a reaching angle to get sufficient boatspeed.
This is taking us away from the bottom mark. So the speed gain when foiling has to be enough to repay the extra distance traveled in our attempt to 'unstick'.

Again, the tradeoff becomes about how efficient the foils are. We want to be able to accelerate to takeoff speed, despite foil drag, at the 'deepest' possible heading angle, to keep our VMG up.
Once foiling, since overall drag is less, we can improve VMG by sailing broader than a displacement boat, because the necessary drive force is less (drive must oppose drag to sail at a constant speed).

Summary

At low speed foils are a handicap.
To lower takeoff speed we want big dedicated foils, but we want them to be efficient so they don't hold us back too much in sub-foiling conditions.

Righting moment is vital, first to reach takeoff speed, then to continue foiling at speed when the apparent wind moves forward. Ideally we want all our lift on the leeward side for best performance in a straight line.

It is not at all clear that the most aggressive foiling setup will be fastest across a broad range of conditions. Experimentation and competition will tell us where the balance lies.

So much for straight line sailing. In the next post we will look at control at low speeds, and speed profile through manouvres.

Paradox 2015

First Public Introduction of our All New A Class Catamaran

Concept

-          Speed, stability, easy to tune for different conditions, value, elegant engineering.

-          Benefiting from three years of structured testing, data collection and validation.

-          Developed in close collaboration with Glenn Ashby.

-          Built in Australia to aerospace standards.

Hull Shape

-          High volume combined with narrow waterline beam through U shaped sections.

-          Flat bottoms for maximum planing lift and minimum dynamic wetted area.

-          Rocker shaped for responsive trimming: Easy transition from bow-down (lowriding) to bow-up (step back/takeoff).

-          Bows have generous volume underneath and peaked low-freeboard tops for wave piercing and water shedding.

Platform

-          Low windage and high stiffness.

-          High modulus beams, Nomex cored hulls.

-          Integrated construction, sealed low-stretch trampoline, streamlined rear beam.

-          Future-proof foil case design, able to take any shape foil. Go from Z to L with no mods.

Foils

-          Optimised Z foils with variable section (camber changes along the span).

-          Full use of permitted lifting span.

-          Precise rake adjustment through worm-gear, as used by proven foiling classes.

-          Good foil support (no slop, no jamming) with precise toe-in through rotating bearings.
-          Optional L/V foils, plug and play. 

Steering

-          All new: A leap forward from the existing dagger/cassette concept that we pioneered and has since been widely adopted.

-          The new system allows much better refinement of rudder planform as well as easy rake adjustment on the water and greater safety.

-          Superior grip at low speed, low drag and precise control when foiling.

Availability

-          Customer deliveries expected to start in August 2015.

-          Ongoing deliveries after September including containers to the Americas and Europe.

-          Contact us now to lock in a hull number with a conditional holding deposit.

Contact

Email: info@carbonicboats.com

Phone: +61 412 127 388

Farewell 2014

Some images looking back on a year of regrouping, transition and growth:

Production L rudders, proven at the A Class Catamaran Worlds 

Mould for experimental T rudder elevator with junction bulb.

 Part of extensive R&D work on appendages

Rudder gudgeon assembly with 'between races' rake adjustment

Experimental gudgeons with 'on the fly' rake adjustment
via tiller extension twist-grip

Billet rudder cassette. Our concept of 'dagger' rudders with offset axis has been widely adopted

First A Class 'V' foil concept. 'Inspiration' for current Z foils

Retrofit foil case kit with rotating bearings

Moth bow swivel fitting developed with Scott Babbage. Production version available here: http://www.sailingbits.com/class-specific/moth-bow-mechanism/

Moth bellcrank developed with Scott Babbage. 
Production version available here: http://www.sailingbits.com/class-specific/moth/moth-adjustable-bellcrank/

Bolts with streamlined heads. Used on UAVs and various sailboat classes

Experimental Finn mast chocks for NB Sailsports

18' Skiff rig spanners for Allmarine. Available here:
http://www.allmarine.com.au/shop/boat-specific-products/18-foot-skiffs/all-marine-rig-spanner/

Tasar fittings for NB Sailsports.
Available here: http://www.nbsailsports.com.au/store/product-info.php?pid1365.html

Fairleads

Trophies for A Class Catamaran Nationals

Choices

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.

Active

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.
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).

One concept we tested, designed by Dave Lister, showed promise by combining active heave control and lift vectoring via angled fully submerged lifting spans for 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)

Passive

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.

Tradeoffs

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.

Graph 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

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.

The interview covers our V3 Paradox A Class design (nearing production) as well as our ongoing foil R&D work.

The final questions are about how some of our recent experiments with control system foils fit with the A Class rule. We reiterate that our design decisions for production are informed by proactive consultation with the Technical Committee to make sure we are always within current rule interpretations when introducing innovations to the market.
Experimental work goes on in parallel. It is aimed at demonstrating what is possible and, increasingly, at satisfying market demand for 'pure' full foiling solutions.
Personally my hope is that fair, objective, literal and consistent rule interpretations will allow further development within a knowable and predictable design space.  

http://www.catsailingnews.com/2014/11/a-class-development-dario-valenza-will.html
http://www.a-cat.org/?q=node/474

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.

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 system of hull and deck bearings.

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.

Bel Paese

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

Think with a Twist

In response to an avalanche of questions about how our experimental foils with the lifting surface mounted at the forward end of a 'fuselage' tube can be extracted from above... Here are some illustrations:

First the foil is raked top-forward so the tube sits vertically through-hull slot.
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 legal 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 35mm.

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...

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 2015 season, resolving the details for all new tooling to be created in-house at a new facility. 

Now that our new 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:

Rudders

Two candidate revised rudder designs were tested. More on the selection of rudder design in future posts. 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 number of variables. 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 extremely 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 'bent' foils (as opposed to curved) that exit the hull vertically then transition quickly to a span with pronounced dihedral, have 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 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. 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. Getting the foil to locate properly when partially retracted requires engineered bearings rather than a simple slot. Our bearing technology remains unsurpassed. The effectiveness of our self-aligning bearing design is such that our ‘bent’ foils ‘autotack’.

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 with 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. Mischa Heemskirk using sections designed by Gonzalo Redondo of D3 seems to have nailed that aspect of foil setup.

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 steps are already in testing. 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.