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Sunday, January 27, 2013

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...

Tuesday, January 22, 2013

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.

Tuesday, January 8, 2013

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!