Dario's Lab


Some thoughts on our recent testing with heave-stable ‘acute L’ foils (L/V for short).
This experiment had one aim: To prove that a simple cheap upgrade is possible to convert existing A Class catamarans to stable foiling without major structural modifications.

The story so far

We knew from previous testing that L/V foils give stable foiling. Our conclusion was that the crossover speed was relatively high so the overall advantage of this configuration for racing would be marginal.
The complexity of ‘tacking’ the L/V foils tipped the scales in favour of adopting our ‘comma’ foils for production and racing. These proved competitive giving skimming flight with neutral stability when required and minimising drag when in foil-assisted mode.

After the Worlds we revisited the crossover numbers armed with new knowledge about kinetics and the tactical options made accessible by foiling. It is now beyond doubt that foiling will pay.
L/V foils maximise righting moment, are inherently stable and can be made to work within the rule.

While it is tempting to wonder whether there is some lateral breakthrough design somewhere within the ‘four point’ design space, all the evidence right now points to the fact that ‘three point’ foiling offers the best performance with consistent stability (and hence safety).

An objective application of existing A Class rules allows simple, cheap conversion. More tortured interpretations may require workarounds such as hinged foils or large cassettes.

Stretching Rule 8 to serve the interests of the ‘no foiling’ constituency adds no value but instead imposes complexity and compromises efficiency. It creates costs that bring no benefit.

A growing majority of Class members is asking whether a simple retrofit foiling solution would be feasible.
Any lingering doubts revolve around ease of handling and, especially, the difficulties of converting existing boats that may otherwise become obsolete.


The foils in these videos are old Marstrom C boards with new horizontal legs bonded on.
The horizontal legs are a proprietary shape. Critical features such as the area, tip-up angle, twist and angle of incidence were determined in light of the work carried out over the course of our Paradox development programme. However the Marstrom verticals were not modified.

For some runs the foils were installed in the original cases of a Melvin A3.
For other tests they were mounted in the existing foil cases of a Paradox test platform. In the latter boat the rotating bearings at the hull and deck were replaced with simple plastic blocks.

In both instances the foils are held down by a rope led to a deck cleat. They are retracted using pre-tensioned bungee that pulls them up when the down-line is released.

Rudders are standard Paradox with some transom reinforcement added to the older boat.

Longitudinal placement of the foils on the Paradox is further forward than usual because that happened to be the arrangement on this particular boat.
Previous testing has already shown that, within limits, keeping the foils further aft gives a manoeuvrability advantage without adversely affecting foiling stability. At any rate, installing a simple foil case further forward is cheaper than putting in a cassette.


In short the transition is surprisingly gradual with drag falling away as the hulls rise.
Once ‘unstuck’, the boat rises more rapidly until the heave control features of the L/V foils come into play.
At this point the normal instincts of a cat sailor remain applicable. However there are a few interesting differences:

The most important lesson is to do less rather than more.

Pulling away aggressively in response to building wind strength can force a reduction in ride height, especially if it is done after the boat begins to heel in response to the gust. The foils automatically go to work to restore level flight but the momentary reduction in ride height does sap energy.

Once the coupling between steering and heel is noted (if heeled to leeward, steering into the wind will make the bow come up. Pulling away will make it go down), then one quickly learns to anticipate. The usual response of letting the windward hull rise then bearing away is somewhat modified:

As pressure increases, the best technique seems to be to ease the sheet a tiny amount, let the boat heel to windward ever so slightly, then pull away as normal. This results in addictive, exhilarating acceleration in total safety. Unlike a displacement cat where forward buoyancy gradually runs out, the foils provide more lift as pressure from the rig increases. The feeling is one of total immunity to nosediving (so far!).

It is easy enough to become accustomed to just trusting the foils and keeping steering inputs to a minimum. Obviously the boat will spin on a dime when foiling so the key is to be subtle with the tiller.

Heeling to windward a tiny bit helps to increase ride height when bearing away. This really boosts VMG downwind. Interestingly the same technique works upwind because luffing up to depower helps lower the bow and settle the ride height. But more on upwind foiling later.

For reasons I do not yet fully understand, Keeping the sail more open works better than strapping the sheet on. Letting the traveller down slightly (say to the hiking strap) and allowing a few degrees of twist causes the boat to fly higher. Closing the leech seems to lock the foils up so the boat settles closer to the water. More investigation is needed here because as speeds rise and the apparent wind goes forward, sheeting in will become necessary.

My best theory at this stage is that increasing sideforce causes the equilibrium ride height to decrease. This is based on the coupling between leeway and lift built into the L/V foil geometry.
Another factor may be that, since drag when foiling is so much smaller, it is not necessary to load up the boat with maximum sail CL. Instead the goal is to have the greatest drive force component exploiting fore-and-aft righting moment rather than resistance to heel…

Sailing downwind with both foils down could lower the crossover speed significantly.
Having both sides down effectively gives a pair of V foils. The reduction in foil area due to the inboard tips breaching the surface becomes the dominant heave-control mechanism instead of leeway-coupling. This is an advantage because stable foiling becomes possible at small sideforce values. It is important to note that this arrangement is draggy at higher speeds and definitely unstable as soon as sideforce becomes significant.

Raising the windward foil and relying on leeway-coupling effectively doubles righting moment whilst halving foil area. This is good when sail power is ‘excessive’ and speeds are very high.
In lighter winds the leeward foil would have to be bigger (if used alone) for the same take-off speed. More importantly you would have to sail much higher to generate enough sideforce to fly a hull while trapezing.

In other words you would have to generate enough sideforce to lift the windward hull and enough speed to take-off on one foil only. This is achievable at a much lower windspeed upwind than it is downwind. Having both foils down instead allows a very early take-off while sailing deeper because righting moment is effectively halved and foil lift is doubled.

I suspect that top level sailors will gradually be able to bring down the critical windspeed for ‘downwind windward foil raising’. But for now this option allows mere mortals to foil safely in as little as 6 knots TWS.


We have now proven objectively that it is feasible to convert an existing A Cat platform to stable foiling with minimum fuss and expense. The boat remains practical and exploitable but it offers a whole new level of performance.

On the emotional side, the feeling of foiling is just fantastic. I am sure that hundreds of Moth sailors already knew this. But it must also be said that foiling on an 18′ cat while on the wire, and with no mechanical control systems is special in a wholly unique way.

It really is simple to learn and no outstanding athletic ability is required. I have no doubt that this will be the future. Many top level sailors agree. The only question for the Class is this: Will we be permitted to do it in a way that is simple and cheap or will we have to use complex and expensive workarounds?

After the thrill of the ‘magic carpet ride’, touching down feels like sailing through honey. It becomes frustratingly restrictive. This really must be tried to be understood. Hopefully many A Cat sailors are about to do just that.