Dario's Lab

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!