Multihull development in particular is really benefiting from having been ‘discovered’ by the ‘mainstream’.
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.
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.
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.
The initial concept for the foils on Paradox (one of a few we intend to test) 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 levelling 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…