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Sunday, February 17, 2013

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.

Sunday, February 10, 2013

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...