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Sunday, March 31, 2013

Illustrated

A few images to further explain some characteristics of our Version 1 S foil configuration. Posted in response to questions sent in by some of you.

Firstly the effect of foil rake on the angle of attack of the horizontal part of the foil, the part that provides lift in the up/down direction:
In the first picture the foils are in the fully aft position in the top bearing.
This is the 'max lift' position, resulting in the lowest takeoff speed for transitioning to full foiling.
With the new foil bearings this will also correspond to max toe-in, further augmenting lift.
Full scale testing will confirm the optimum angle (both rake and toe-in) for lowest overall drag.

Yellow lines show foil leading edge angle to vertical and the Angle of Attack (AoA) of the lifting portion when set as pictured.
Orange lines show position for windward foil when sailing upwind.
In this position the foil will pull down, adding 'weight' to the windward hull
The trade-off is that more lift increases the drag of the foils. This additional foil drag buys a reduction in hull drag since less of the hull is in the water thanks to the lift from the foils.
If the reduction in hull drag is greater than the additional foil drag, then there will be a net gain.
Since hull drag and foil drag rise at different rates with speed, the exact 'crossover' (think of it as the optimum takeoff speed) remains to be seen. On an A, with current foil technology, it is somewhere in the teens when measured in knots. At lower speeds the efficiency of the slender leeward hull is formidable. Since sail area is limited, the option of increasing power is precluded so drag reduction becomes imperative.

Tuning the foil angles is key to assessing the true potential of this configuration. It is easy to do by simply swapping out the centre element of the top foil bearings. The difference in observed performance with different angles is significant. Getting these angles wrong means the true performance available is not realised.

Dihedral angle (and hence vertical lift fraction) is set by foil depth. It is zero with the foils fully down and maximum at the 'reaching position' as seen in the image below. This is the position always used except in consistently light ('sub-trapezing') conditions.
When underway, the effective dihedral angle is self regulating through the variation in curvature of the S foils. As ride height increases, the part of each foil that remains in the water becomes progressively more vertical, reducing dihedral.


The role of the rudder 'L's is to control pitch angle. Our new rudders will maintain stability in pitch with a considerable reduction in drag.
Remember that stability in pitch only comes into play when fully foiling. If full foiling proves to only pay in extreme conditions, then the Ls can be made even smaller (it would not make sense to carry the associated drag penalty, however small, at speeds where stability is not critical if such speeds are relevant most of the time). Since the effective split of vertical lift between foils and rudders changes with sail force, it is relatively straightforward to hone in on a winglet area that begins providing negative feedback only when the hulls leave the water completely.

Positioning the foils forward (ahead of the sidestays) has the principal advantage of reducing induced drag by letting the rudders share sideforce with the main foils.
A secondary advantage is that the Ls on the rudders have more leverage to control pitch attitude.
One question to be tested is whether the forward position of the foils adversely affects maneuverability.


Our policy is to share findings as we learn new things and develop new products.
The reason is simply that all the work we do is motivated by passion for the sport. Sharing this passion is profoundly rewarding.
Our customers realise why we do what we do. That is why they decide to become our customers.

In light of this, no sweeping claims have been or will be made. Martin Fischer's stable foiling configuration makes sense and we are working to unlock its potential. We will only adopt it for production if it proves superior.
At the same time we are prototyping a less extreme arrangement that will be simpler to use and have a less critical performance profile. This is being designed by Dario Valenza and will be mated to a revised hull shape made to match the new concept.
This alternative arrangement will be unveiled very soon. Buyers will be eventually offered the configuration that emerges as superior after testing, modifications and more testing.

Much of the thinking behind the S foil arrangement will still be applicable and the eventual, hopefully simpler, production version/alternative will benefit from our extensive R&D process, carried out using the very best tools and methods.

As the simpler alternative is going through the final analysis stage, it appears that it will have a slight edge in light and moderate conditions. This makes sense as it will have less wetted area, simply because the emphasis is on drag reduction at moderate speeds before dynamic stability becomes dominant.
In the top end of the wind range, performance should be comparable but the sailing style will be more conventional. Safety margins will still be higher than existing foil assisted designs and full foiling will still be possible, albeit with higher takeoff speeds.
This may yet prove the fastest way around a course most of the time in a class where the displacement to length ratio is exceptionally low and sail area is limited.

The reason to buy a Paradox remains the same: carefully researched design, build quality that goes beyond mere performance into aesthetic excellence, personal customer support and, most importantly the driving passion we want to share with our customers.

Thursday, March 28, 2013

Versatility

This week we started production of tooling for an improved rudder design for Paradox.
Design work from here on in is entirely in-house.
Considerable drag reductions should be possible through increasing the efficiency of the ‘L’ surface. 
This is achieved by improving the aspect ratio of the horizontal foil. 
Increasing the aspect ratio dramatically reduces induced drag. The new aspect ratio is in the realm of competition sailplane wings
Careful structural optimisation has allowed us to exploit this solution with no cost or weight penalties.

Mk1 rudder shown yellow, new version in red
At the same time we have confirmed that the actual force the winglets have to generate is considerably less than initially predicted. We will test progressively smaller winglets to validate the new calculations. 

When you add together the increased efficiency and the smaller force required, the size of the winglets reduces dramatically (close to 50%), giving a significant drag reduction.

Mk1 rudder shown yellow, new version in red
The new rudders are conceived to be versatile in two ways:

1) The winglets have an untapered portion near the tip so they can be trimmed off at any length and still maintain an efficient planform shape. 
Cutting them off 180mm from the root gives equivalent area to the “+” winglets being offered by other manufacturers (typically a pair with 100mm span each, giving 200mm total span). 
However the L solution is free of the interference drag created by the intersection of three separate foils. 
Any intermediate area can be chosen to suit the preferences of the user.


2) The top of the rudder is tapered such that the whole blade can be reversed. This gives the option of eliminating the winglets all together in light conditions while having them 'on standby' ready to deploy if the weather changes.


The compensation has also been refined (increased and redistributed vertically) to give a lighter feel on the tiller.

The new rudder will come standard with every Paradox.

The versatility of the new rudders allows us to confidently offer them for sale separately to customers who want to retrofit them to existing boats, with or without our cassette and gudgeon system.

Monday, March 25, 2013

Sweet Spot Part 2

Rerunning the Numbers

In light of observations over the season and the 'below expectations' performance results, Martin has decided to check and optimise the foil setup on Paradox.
Using updated software he plans to compare optimised setups for S foils and for hypothetical C foils similar to ones used by the current generation of A Cats. 
The calculations will be set up to arrive at the lowest drag combination for a given sideforce.
Interestingly, vertical lift as a percentage of boat mass (lift fraction) will be left as a variable. 
The calculations will be repeated for several boatspeeds. The result for each boatspeed being the lowest drag combination of foil immersion, toe-in angle and lift fraction. 
Computing a series comprising different boatspeeds (sideforce values) for each foil type will give a level comparison between the two configurations over a range of conditions.

Since lift fraction will be a free variable, the calculations will effectively compare total drag for foil assisted sailing against full foiling. Foil assisted will naturally have a bit more hull drag as the lift fraction will be smaller. However the number that interests us is total drag - being made up of both hull drag and foil drag.
For the same speed, the lowest drag combination for C foils might involve carrying, say, 50% of displacement on the foils while, at the same speed, an S foil boat would be happier carrying 60% of total weight on the foils.
If foil drag is higher, the gain to be had by lifting the hulls out of the water will be more expensive so carrying a lower lift fraction on the foils will give a lower total drag value.
What interests us is how that optimised lower total drag figure compares to the analogous lowest number for the alternative foils at the same sideforce value, even if the lift fraction might be different.

As described in part 1, the second round of calculations will be closer to representing reality as it will be informed by observations on the water. 

Leaving aside stability when foiling, it is thought that the span-wise lift distribution on S foils is inherently less biased toward the tip so their induced drag should be less (we will soon find out).
This incidentally would explain why it does not pay to set up C foils with aggressive rake (large angle of attack at the tip). Something many sailors have observed on existing boats.

Toe-In Angle

The biggest question mark right now is toe-in angle.
As explained previously, toe-in is an important variable as it has two effects:

1) Upwind it ‘feathers’ the windward foil which is surface-piercing so less efficient. At the same time it loads up the leeward foil which is working under the hull so effectively has a rigid ‘end plate’ above it. To understand this effect, think of the angle of each foil with respect to leeway. Toe-in aligns the windward foil with the true course through the water and gives the leeward one more 'bite'...

2) Downwind, toe-in causes the windward foil to pull up and to leeward, contributing additional vertical lift and, crucially, adding sideforce to allow the leeward foil to make even more lift.

More toe-in effectively decreases the ‘takeoff speed’. The question is whether taking off ‘early' gives a net gain. Meaning, is foil drag in fact less than the hull drag reduction may buy us... 

We found that aggressive toe-in is definitely slow upwind (slower than initially predicted) and not helpful downwind, especially so below about 15 knots of boatspeed. Yet it is necessary to foil but that is now thought to only pay at very high speeds. 
So it may be that toe-in should change at the end of every leg! In fact it should increase with increasing speed downwind.

Rather than adding another control, we plan to solve this conflict by machining new foil bearings with an angled slot: When the foil is pulled back the slot in the top bearing will guide the trailing edge outboard. When eased forward, the leading edge will rotate outboard. The range is from 0.5 degrees when fully forward to just more than 2 degrees (exact value pending) when fully back.

This solution in theory should give us the best of both worlds.
More importantly, it will not require an additional control.

Exploitability

The only extra control on Paradox (compared to other As) for now is the foil fwd/aft line
You pull it and cleat it to move the top of the relevant foil aft, or release it to allow the top of the foil to go forward (it should go forward automatically, aided by a bungee and by the drag of the foil in the water).

When the top of the foil is all the way back, the horizontal part below the hull is at the greatest angle of attack so lift is maximised. 
When the top of the foil is forward, the lift is actually pulling down (this can help upwind by sucking the windward hull down, giving a bit more power). 
Half way along and the foil is neutral.

Going upwind you tug on the windward foil control line as you come in off the wire, then release the new windward one as you complete the tack.

Downwind, both foils are set with the top fully aft to give maximum lift (and soon max toe-in) so you don’t need to do anything with them between top mark and bottom mark.

In the design brief we imposed the constraint that foil depth be set for the conditions: In anything over 8 knots TWS they are cleated in the ‘reaching position’. This is when the top part sticks out about 150 mm (as you can see in most pictures of the boat underway). In this position dihedral angle is maximised. 
In light winds there are two options: Pull both foils half way up so that they become upright (but each with halved immersed area) or always have the windward one up completely and the leeward one down completely such that it is vertical (no dihedral so no upward lift). 
Having the leeward one fully down and the windward one up gives an advantage in theory but requires an additional adjustment at every tack.

Handling

Our last observation is that having the foils so far forward, which must necessarily load up the rudders to keep the boat balanced, does require some getting used to. 
The reason for having the rudders hydrodynamically loaded is so they share with the foils the task of generating sideforce. 
This is a principle used on big boats such as CBTF maxis and is at the root of the tandem keel solution seen on several IACC boats. It is also why conventional IACC boats had such deep rudders.
If the rudder were completely neutral it would be ‘coming along for the ride’, contributing only drag. If the rudder is working to help resist leeway, it reduces the lift-induced drag being generated by the foils. 
This hydrodynamic load is felt through the tiller as a bit of weather helm. It is something that does reduce the margin for error, especially when tacking.

Conclusion

At the last regatta our setup may have been too aggressive, taking us past the optimum lift fraction. 
With excessive toe-in the foils were working too hard, giving a lot of lift but adding more foil drag than what is being saved in hull drag.

As with any new concept, there is a process of learning, to optimise both setup and sailing technique to exploit the advantages on offer. Martin points out that any comparable innovation (such as the foiling Moth) has an associated learning curve. Perseverance could be the key to getting it right and several design improvements may be required along the way. Our plan is to see this process through but also test different alternatives in pursuit of the best all-round solution.

In summary, the S foil and L rudder solution is still thought to have potential. 
It is arguably more demanding to exploit fully than a conventional setup but it might be more correct to say that it requires an adaptation in technique and a deeper understanding of foil dynamics than is sufficient on boats where the foils play a less crucial role. 

It is vital to understand where the gains lie and work through the solutions without ‘throwing the baby out with the bathwater’. An objective analysis is the only way to draw meaningful conclusions and make progress.

Going Forward

We will continue to tune and test the system on our existing prototype and work to fully understand what makes it tick. We will compare the best version of this concept with other ideas worth testing, then decide what to put into production.

Wednesday, March 20, 2013

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.











Tuesday, March 19, 2013

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

Tuesday, March 12, 2013

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.




Monday, March 11, 2013

Interesting Times

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

Monday, March 4, 2013

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!