Prototype Under Lights

par·a·dox  (par-a-dks)
noun
1. A tenet contrary to received opinion.
2. A statement that is seemingly contradictory or opposed to common sense but is nonetheless true.
3. An assertion that is essentially self-contradictory, though based on a valid deduction from acceptable premises.

Sweet Spot Part 1

Optimising a system such as a sailboat is a fascinating process. Once the basics are right, picking the best settings for different conditions is an art in its own right. Varying degrees of scientific thinking and 'gut feel' based on experience seem to be called for and an open mind is definitely an advantage.

Even with very sophisticated tools, the core of the problem is the large number of interconnected variables. As with any endeavour involving the scientific method, all bias toward trying to prove a concept that is arbitrarily appealing must be 'checked at the door' if the results are to be objective. Then data gathered on the water has to be sorted, filtering out extraneous noise, so it can be used to validate the initial predictions. 

If done correctly, the discrepancy between observations and predictions will illuminate the designer about where predictions were mistaken. 

If the source of the discrepancy can be identified correctly, then the next lot of predictions should be closer to reality.

Those with an interest in such things will know that arriving at quantitative predictions requires a discipline and constraint quite at odds with the creative thinking process that generates the concepts in the first place.
To get useful numbers, many assumptions must be made so that each calculation discriminates between only a small subset of variables. Getting the assumptions right is vital and that is where real observations are of great value.

As the process is repeated, the calculations will get better and more predictive of actual behaviour.

On a macroscopic scale this process has been going on in design over generations.

Computational fluid dynamics and tank testing are a great example of how one tool has been vital in honing another to the point where the older one is almost redundant (almost!).

On a smaller scale, each individual project goes through this cycle. Each boat, once the hull shape, rig design and foil package have been locked in, goes through a process of discovery on the road to delivering its full performance potential.

Every sailor will know an example of a boat that just seemed to respond to a certain setup that was not the obvious first candidate. A bit more rake, a bit more jib twist, traveler down a few inches... The winning teams are the ones who accept the observations and figure out how they fit in the overall model.
Complaining that 'it isn't supposed to be faster like that', won't change the reality.
Just replicating the fast setting without understanding why it works is dangerously limiting since its effect may vary with different conditions so the advantage it confers cannot be relied upon. 

Part of what makes our game so engaging is that there is a very stimulating interplay of different forces, at the interface of two fluids, acting on different pieces of equipment that all have multiple dimensions of adjustment.  

This post was prompted by the work we are doing now to make sense of the observations we collected when Paradox last competed.

As explained in the post following the Gosford regatta, Paradox first raced with very conservative foil settings. 

For the recent regatta we dialed in maximum toe-in angle, giving the most aggressive lift profile in the fleet. 

Now that we have tested the two extremes in the range of this important setting, we can plot how lift and drag interact, giving us an informed guess at where the optimum might be found.

During the regatta we also tested two different sails from two well known sailmakers. Since Paradox is much more forgiving than other A Cats, it does not require as much mast rake. With a more upright mast the newer high-clew sails open up an unnecessarily large gap under the foot of the sail and we suspect this leads to significant aerodynamic losses. Once we settled on the lower-clew sail (last day), we replicated known fast settings for diamond tension and spreader rake and obtained a sail shape very similar to other leading boats. 

Having eliminated the rig as a variable (with the exception of rake angle which remains an area of investigation) we could be reasonably sure that variations in performance came down to foil settings. 

Details of the degrees of adjustment of our foils and their effects on performance will be explored in Part 2.

This great shot by Karen Parker

Learning Curve

All packed up after the Victorian State Championships, ready for the long drive back to Sydney.
The event was run with great efficiency so that we were able to have seven races over three days leaving plenty of time to enjoy the great atmosphere ashore.
It is interesting to reflect on the places that our sport leads us to visit and the diverse people it gives us the pleasure of meeting.

Jason Waterhouse sailed Paradox faultlessly to a standard well beyond what would be expected of anyone new to the class.

It was apparent however that we simply did not have the speed to match the top boats in this very competitive fleet.

Our task now is to analyse the wealth of data we collected over the regatta and draw the conclusions that will allow us to make the necessary changes.
Initially we will check that our predictions match our measurements for the settings that had been identified as optimum. Depending on how that goes we will evaluate the possibility of making changes to some aspects of the overall geometry.
As always, the process is one of elimination given the number of variables involved.
The boat behaved well, beginning to transition onto the foils in the brief periods where the wind exceeded the predicted takeoff speed (the regatta was a light wind one overall).
However our speed was not what it should have been so we must acknowledge that and proceed to address it.

As always we will share our findings as development continues.
This is the nature of the game and such lessons are steps on the path to our goal of creating a competitive, high quality production A Cat that is a joy to own at a great value price.

Interesting Times

Great interview with Martin Fischer on Catamaran Racing News and Design.
Discusses Paradox and other foiling multihull projects...

Busy Season

The end of our Southern summer season is approaching so we have been making best use of each long warm day whenever the breeze has been good.

Lots of time on the water for Paradox, putting the theories to the test and giving many interested parties the opportunity to experience A Cat sailing on stable foils.

In just a few short weeks we will be back in the office so the technical content many of you tune in for should start flowing again.

In the factory we have selected the final contractors who are setting up for building a Version 2 production prototype that, after testing against Version 1, will be followed by customer boats.
Sooner than we had expected thanks to a few prospective early adopters who have believed in us and encouraged us, sharing the passion that drives us to do what we do.

Build slot number four is the next available. A few slots may still be available for delivery at the AUS Nationals. Then the lead-up to the Worlds will begin.

Now that the basics are in place, the slower process of tuning for competition begins. As with any new boat, there is a learning process discovering the quirks, optimising the systems and sniffing out the small hidden weaknesses that only pop up during the heat of battle.
Everything we are learning is informing the user manual that will come with every boat.

Interesting times indeed!

S Foils FAQ: Why the Top Bend?

The bottom inflection on the foils for Paradox is there to give stability in heave (controlling ride height). This solution is unique to our A Cat and other Martin Fischer designs such as the GC32 and Flying Phantom.

The upper inflection is a way to adjust overall dihedral angle. It has also been seen on the ETNZ AC72 and is being explored by at least one other A Cat builder in an upcoming model release.

Adjusting dihedral is simply a way to eliminate unwanted vertical lift when it is not required.
On Paradox we want to reduce lift when sailing at lower speeds such as in sub-foiling wind speeds and upwind in light to moderate conditions.
Other designs need to reduce lift at high speeds to avoid transitioning from foil assisted to fully foiling since stable foiling was not a design goal.

Dihedral adjustment could be achieved by moving the top foil bearing inboard/outboard.
This is a valid solution which would not require the top S bend and would give 'infinite' adjustment within the available deck width.
It was a solution we considered, however, when carefully analysed, it had several drawbacks that made the S foil solution more attractive:

1) Low speed sailing, when we want less dihedral angle, is also the condition when we want more foil span. This is because deeper foils increase heeling moment (by moving the centre of lateral resistance vertically down away from the sail centre of effort), helping to load up the leeward hull and 'unstick' the windward one sooner.
Also, increasing foil area in these conditions allows a lower loading per unit area and a higher aspect ratio, reducing induced drag.
If we used an inboard/outboard adjustment, the sailor would have to make two discreet changes each time: lower the foil, then pull the top bearing inboard to cant the bottom end outboard making the foil more upright.
With the top S bend, the foil only needs to be pushed down and the dihedral angle decreases automatically to the precise value required.

2) Mechanically adjusting the top bearing would require an arrangement strong enough to take all the sideforce (meaning maximum sideforce when sailing very fast on the foils) while maintaining manageable levels of friction when being adjusted.
This would mean making a very stiff sliding plate at the top that is supported such that it cannot skew on the two transverse 'rails' which are necessarily separated by the chord length of the foil plus the necessary fore/aft adjustment distance needed to control the angle of attack of the part of the foil that provides vertical lift...
This can be done but results in a heavy and complex piece of engineering that is not ideal for a production boat.
Having a top bearing that only moves fore-and-aft is a much lighter solution.
Pulling the foil up and down in this laterally fixed bearing requires much smaller forces on the control systems, resulting in a lighter arrangement overall.

3) A 'V' shaped foil case with a wide opening at the top would hold much more water than a fitted case just big enough to accommodate an S foil.

4) The legality of an inboard/outboard adjustable system is questionable since when the foil is partially retracted the top of the foil would breach the beam restriction if the top bearing plate were adjusted to the outboard position.

For our requirements the S foil seemed to offer more advantages than disadvantages.
Other applications may lead to different conclusions.
As always our approach is to share the process and our thinking to convey to fellow enthusiasts our reasons for doing things a certain way.
This is an expression of our philosophy that a clear brief is vital to good, elegant design and the best design response is the one which on balance satisfies the brief with the smallest possible drawbacks.

Smooth Sailing

What a great time to be in this game: So many factors are converging to make fast sailing accessible and affordable.
Multihull development in particular is really benefiting from having been 'discovered' by the 'mainstream'.
I speak as a converted monohull sailor but also as a member of the industry with some understanding of economics. Competition is the key to development and the forces it brings to bear are the driver of innovation.
The tools now available thanks to Moore's law and relatively commonplace access to CAM technology, have opened up a vast new space. Freeing up the designer to draw, analyse and specify complex shapes such as curved foils.
However this technology also puts a premium on properly understanding the new powerful tools available. It has never been more true that 'garbage in equals garbage out'.
Getting an accurate sounding result to ten decimal places may feel impressive but it can still be as wrong as an uneducated guess if the assumptions in the model do not reflect reality and the brief.
So success comes back to having the right team, a well formulated project brief and a selective approach.

One feature that is going to become more commonplace as a result of the new tools mentioned above is curved foils with non-constant radius, changing sections and even twist.

As always it is important to understand that the thinking behind different curved foils is not always the same.

As with 'wavepiercing bows', foil curvature is a feature that can be adopted to achieve any number of different things.
I will attempt a more detailed analysis in future posts but here is a brief guide:

J boards are a variation on C boards. They have progressively more horizontal component toward the bottom tip. They get all their vertical lift from the bottom half of the foil.
J boards with an outward inflection at the top are a way to reduce dihedral with increasing span (as the board is pushed down the bottom tip moves outboard making the average angle more vertical).
J boards with a tip inflection are a way to optimise tip shape to reduce induced drag (similar to winglets on commercial aircraft). The inflected tip is there to address local flow conditions and does not influence overall behaviour.
What the above solutions have in common is that they provide lift efficiently for foil assisted sailing without considerations of stability.

Martin's concept for the foils on Paradox is a bit different and I would like to clarify this point.

J boards have progressively more lift toward the lower tips since the foils become more horizontal at the bottom.
This means that the lift happens at the most deeply submerged part of the foil.
Therefore, if the boat rises too high on the foils, the lift will continue, pushing the boat up even further.

His concept calls for foils that have the greatest lifting surface right under the hull.
As soon as the hull rises, the lift starts decreasing, encouraging ride height to settle.
The rate at which this happens, controlled by foil curvature and twist, is the key to the concept and is vital for heave (ride height) stability.

Paradox is designed to be pushed hard without needing to continuously correct ride height manually.
This means the sailor can concentrate on the race instead of trying to prevent the boat from jumping up or burying.

Our foils do have an outward inflection at the top to make the foils more upright as they are pushed fully down (reducing dihedral). This reduces lift for upwind (lower speed) sailing.
Our geometry allows this effect to be more pronounced as there is room for a greater change in angle before approaching the maximum beam limit.

Our L rudders are designed to provide a sufficiently strong negative feedback on the trim angle for good pitch stability. They do not only provide pitch damping but a real negative feedback leveling the attitude of the boat. The lower part of the rudder is not horizontal. It is angled to complement the foils in three dimensions.
By having a single 'bent' surface instead of two intersecting surfaces (as with T or + rudders) the junction drag is significantly less. Also we can place the rudder blades further outboard which is beneficial. Finally, being able to partially retract the rudders makes them much more practical than 'kick-up' T or + rudders.

Watching Paradox sailing in moderate conditions, with foils set at an intermediate level (not maximum lift but not neutral), you can see that the boat has a very steady motion with noticeably reduced pitching. The crew stands a little further forward than is common on other A Cat designs and the foils do all the work...

Get Your Copy

Six page piece on Paradox in the latest Australian Sailing + Yachting magazine.
Great to see the A Class Catamaran get such visibility in the mainstream sailing press!
This exciting class has certainly moved on from being 'the best kept secret in sailing' to being recognised as the fun, technically advanced and elegant boat it is, with a growing competitive scene of enthusiastic participants.
If you have not discovered it yet, there is still time to do so...

Balancing Act

After testing four different prototypes, here is the latest version of the rudder cassettes that we will supply with your Paradox A Class Catamaran.

Once again, the optimum choice strikes a balance between different considerations. 

In fact, the design of this component is a great illustration of how the best solutions use the most appropriate materials and techniques for a given application, regardless of trends.

The cassette assembly requires very high dimensional accuracy since it sets the steering geometry.

At the same time it needs to be light, stiff, easily manufactured to reliably tight tolerances, and economical.

This assembly determines the position of the pivot axis relative to the rudder blade (fore-and-aft) and the angle of the blade relative to the tiller (in the horizontal plane).

The position of the pivot axis is critical to the balance of the rudder: The further back along the blade (more foil area ahead of the pivot axis) the more ‘compensation’. Meaning more area helping to turn the rudder relative to the area behind the axis pushing to straighten the rudder. 

On a swing-up rudder compensation can be adjusted by ‘kicking’ the rudder forward past vertical so that the bottom tip is ahead of the pivot axis. 
This solution is fine when the rudder is fully submerged, but it has the drawback that compensation will increase as the upper part of the blade exits the water. When only the tip is left in the water, the rudder will most probably be over-balanced, resulting in a light and ‘skittish’ feel through the tiller extension. At high speeds, especially when foiling, this can be dangerous. 

A Class cats so far have not had to confront this problem because stable foiling has been impossible. But the effect can still be felt when in displacement mode as immersion of the windward rudder varies.

Our solution maintains the correct compensation regardless of ride height and heel angle. 
Actually the rudders are designed to provide a more positive feel as they come out of the water. 
This is achieved by tapering the leading edge aft toward the rudder tips… 

Another advantage over kick-up rudders is that compensation does not change as the angle of attack of the horizontal foil on the rudder is altered (rake). 'T' or ‘+’ foils on the rudders of other boats cannot be tuned for horizontal angle without altering rudder compensation. 

Coming into the beach, kick-up rudders with winglets also cannot provide any control because they can only be fully up clear of the water or fully down. Our rudders can be partially retracted to still provide steerage in shallow water.

To achieve the goal of positioning the rudders accurately, there are important material and process considerations that are not immediately obvious. 

The bearing surface needs to be precise, have a low coefficient of friction, be dimensionally stable and able to hold the rudder without damaging its surface and trailing edge. 

The connection with the pivot axis has very high point loads that need to be reacted out into the cassette. 

The tiller needs to be supported at the correct angle both inboard and upward and be tough enough to withstand the occasional abuse such as a rough tack or jibe.

Satisfying these needs with carbon mouldings requires the build-up of considerable thickness in high load areas. 
The processes are necessarily complex because the shapes involved cannot be moulded in one piece. Dimensional accuracy is not ideal because of the nature of the process and materials. 

The relationship between any weight savings and the additional cost is so disproportionate that it raises doubts about the appropriateness of the material.

A carefully optimised machined aluminium fabrication results in a very efficient structure that is competitive in terms of weight and can be made reliably at a fraction of the cost. 

As a bonus there is a certain ‘aeronautical’ aesthetic that is unique and very pleasing.

Rake adjustment is through shims between the gudgeons and the transom.
Tapping plates inside the boat mean you can replace the shims without having to access the interior.

Walking Before Running

Back in the office after a great A Cat regatta. Efficient race management allowed competitors to enjoy nine races in an ideal venue over three days of sunshine and consistent wind between 12 and 18 knots.

A satisfying debut for Paradox in the hands of young Dave Parker who, despite being new to the boat and the venue, was giving the top guys a run for their money.

The main aim for us was to test the structure and systems of Paradox under race conditions. 

We were closely monitoring the boat and foils to ensure all the loads are within design values and deflections are within predicted limits. 

An A Cat is a small but relatively complex structure and ours is all new, so gathering this kind of data is very important to ensure future reliability.

We are very pleased to report that it all went well with no major issues flagged and all races completed with the boat in good health. 

Measurements confirm that our target to achieve the stiffest platform on the market has been achieved. 
This confirms the feedback from every sailor who has been on board saying Paradox feels like the stiffest A Cat they have tried.

For this first event the foils were set with conservative 'toe-in', effectively reducing their contribution so that we could evaluate the hull shape and handling before going to more aggressive lift settings.

Paradox can be set up with different degrees of foil toe-in to effectively control takeoff speed. This is an ‘on the beach’ adjustment and is selected for the day rather than altered on the water. 

Self-aligning foil bearings sit in the deck (left) and hull (right).
It is simple to replace the central element in each bearing assembly for one with a slot cut at a different toe-in angle.
Or even cut for a different foil section...

More toe-in (meaning port and starboard leading edges closer together) increases the AoA of the leeward foil for a given leeway angle. 
This makes the leeward foil work harder than the windward one. 
Since the leeward foil is under the hull and away from the free surface, it is more efficient. 
Therefore making it work harder than the windward one can reduce overall drag.

At very small leeway angles (such as when sailing downwind), large toe-in angles allow the windward foil to go beyond neutral and develop some lift to leeward. 

Remember that vertical lift is proportional to sideforce: it is the vertical component of the total foil force - the horizontal component being a reaction to sideforce generated by the sail.
Since sideforce is 'pegged' by sail force, then its vertical component is limited indirectly by sail force.

With Martin Fischer S foils, vertical lift from the leeward foil is therefore limited when sideforce is small.
With large toe-in angles the windward foil can contribute vertical lift by generating a force pulling up and to leeward. 

The component to leeward adds to the sideforce made by the sail, and cancels some of the reaction sideforce (to windward) provided by the leeward foil.

The vertical components from the two foils add up while the two sideforces cancel. 

Thus vertical lift can be increased sooner (at lower sail force values) than would be possible otherwise. 

There is a penalty in the form of lift-induced drag because both foils are generating some lift against one-another (think of a kind of snow-plow effect between the two foils), but the trick is sizing the foils so that this penalty is smaller than the saving in hull drag afforded by the extra vertical lift available. 
Discovering whether the drag penalty is worth the saving in hull drag will be part of the next lot of tests, as we crank up tor-in and hence vertical lift.

If reducing the displacement to length ratio of the hull gives a saving greater than the cost in induced drag, then the boat will be faster.

In fact Martin’s optimisation algorithms suggest that greater toe-in (up to an optimum value) can be beneficial pretty much all the time. 

For this first regatta the foils were set close to parallel for the reasons already mentioned: we wanted to keep vertical lift down to values comparable with other boats as a way to evaluate hull shape. 

The next step will be to increase toe-in angle to get the most out of our unique foil configuration.

Despite the conservative settings, Dave was regularly in the top pack, often in the top three at the windward mark, with decent pace upwind. 

As expected, downwind performance was comparable to the other boats, but we hope much untapped potential lies beneath the surface.

Not bad for a debut!

A Different Slant

We have received several questions regarding the unusually flared mid sections on Paradox compared to other A Cat designs.

The simple thinking is to push the outer gunwale (hull to deck joint) between the beams as far outboard as possible.
If each hull had vertical topsides (slab sided), the outer gunwales would move inboard as the the hulls are canted.
Since maximum beam is measured as the widest point overall, the gunwales would end up inboard of the turn of the bilge (or more precisely the point where the tangent to the surface is vertical, so somewhere on the radius where the bottom of the hull turns up to meet the topsides).

On Paradox the outboard edge of the deck is at maximum beam with the hulls canted.
This means you are trapezing as far outboard as possible, giving you more leverage and maximising righting moment.

A byproduct of the wider deck is that overall hull stiffness is greater.
Since there is a bit more deck area, deck weight would be slightly greater for a given laminate spec. However the inherent stiffness of the flared shape means that less reinforcement is needed so there is no net weight gain.
In addition, since the inner gunwales are closer together, the trampoline is marginally narrower and hence a bit lighter, offsetting any weight gains in the deck.

Moving aft, topside flare is maintained to dampen pitching: as the stern submerges, the waterplane gets progressively wider, rapidly increasing waterplane area, and offering more support to restore a level attitude.

Keep the questions coming and we will attempt to post on the more frequent ones whenever time permits.

Portraits

Some close up shots of the new Paradox A Class Cat on the water, snapped by Andrea Francolini.
Detailed reports on the first weeks of testing sessions coming up soon.
This project is definitely 'centre stage' for Carbonicboats now that the summer is here, even though other developments such as RC yachts and UAV airframe work are also coming along nicely.

Exploring the Envelope

Not much time behind the desk recently, and for the coming weeks.
After the planning and design phase and a smooth build period, we are now exploring the performance profile of the new A Cat.

Here is our first in-depth discussion of the project outside this blog:
http://www.catsailingnews.com/2012/12/new-paradox-cat-in-detail-dario-valenza.html

There is a bewildering array of variables to grapple with.
And this is just one of the configurations we plan to test.

Our approach is to quantify as many as is practical and gradually develop a tuning manual to allow customers to dial in fast settings out of the box as a starting point for individualised tuning.

We will post more detail but the short summary is as follows:
The skipper controls trim through fore/aft weight placement.
Foil AoA is controlled by a single line system and rudder foil angle is set 'on the beach'.
The interaction of these three variables is fertile ground for getting the best performance out of the boat.

In addition there are the usual tuning variables such as mast rake, rudder toe-in angle as well as sail trim settings.
Simple enough to explore with a good plan!

More Machining

Another nice piece of CNC milling for production tooling...