Dario's Lab

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.