Dario's Lab

A World(s) of Learning – Part 1

One of the things I love about yacht racing is the ability to get objective feedback in testing and competition.

As with any experimental science, the feedback comes mixed with noise, and bundled with data that is correlated to, but may not be caused by, the variables being tested.

Part of the challenge is to decode this raw, dirty, but ultimately objective information and decipher the ‘meaning’ as relevant to the concepts being tested.

The channels available for collecting quantitative data are often limited by considerations of budget, time, and practicality. However qualitative feedback is plentiful.

Even in one-design sailing, the variables in settings and technique make the game fascinating. In development classes there are the same ‘fine-tune’ knobs to twiddle as well as coarser variations.

Remaining objective, being willing to set aside preconceived beliefs, and being able to adapt one’s thinking are key to making progress. Cultivating the art of seeing the signal among the noise is an absorbing pursuit.

So in this series of posts I will share some of the impressive lessons gained at our first A Class Worlds.


Given the early and immature stage of foiling development in the class, the single most critical factor this time around was mastery by some of aggressive kinetic techniques able to momentarily force a high instantaneous flow rate over the foils.

A specific sequence was evident, having been learned and perfected both in parallel and in consultation by the top players. Here is an attempt at describing it:

1) Maximise boat speed: Use a combination of energetic sail trim (pump) and steering for best speed through the water to unstick the windward hull (sail hotter if running downwind and foot off if sailing upwind).
Let the boat heel gradually through this phase, progressively transferring weight to a hiking position as the apparent wind increases causing heeling moment to grow.
The idea is to coax boat speed to rise, regardless of heading, while keeping fore-and-aft trim level, to reduce foil angle of attack (AoA).
If bow up trim is excessive in this phase, the drag caused by the foils attempting to lift the boat prematurely will make it difficult to reach takeoff speed.

2) Once up to speed, steer down to reduce sideforce, using the trapeze to roll/flick the boat upright while stepping out/back.
Hardly any rudder angle is required since rolling the boat to windward helps it to pull away.
This step adds another burst of speed due to the dynamic effect of the mast rotating to windward.
When the boat is flat both foils will have maximum projected horizontal area.
Pulling away will have reduced the sideforce, unloading the vertical part of the foils.

3) Before the speed decays, step back to increase AoA on the foils.
The kinetic energy built up in steps 1 and 2 is converted to potential energy as the foils ‘bite’ and force the boat up.

4) Once ‘popped’, work hard to co-ordinate heading, trim and sail force to remain airborne.
In this mode hydrodynamic drag is so small that remaining fast enough to stay on the foils is relatively easy.
The challenge is to react in time to the inherent instability in heave of a 4 foil system (especially J foils).
The time available to make corrections gets smaller the higher the speed (the stronger the wind).

Balancing on Unstable Foils

Since there is no automatic decrease in foil force with ride height, the two stability ‘controls’ available to the skipper are speed through the water and AoA.

Speed can be manipulated by steering toward or away from the wind.
A small blessing is that the direction of ‘salvation’ coincides with where you want to go in terms of VMG: Downwind, as your speed threatens to increase, you progressively bear away. If done correctly you take the extra energy provided by a gust in depth rather than speed, preventing an increase in foil force that would lead to a ‘launch’ followed in short order by an undignified crash.
Upwind you sail higher when you need to reduce speed, taking height back once airborne.

AoA control is related to stability in pitch.
If the boat is unstable in pitch, you have to weight-shift fore-and-aft to control trim/AoA while also controlling speed and heel. Not an easy task!

This point is worth exploring: it is important to understand the distinction between stability in pitch (rotation about a transverse axis) and stability in heave (translation up/down of the whole boat).

J foils do not have stability in heave. There is no correlation between increasing ride height and decreasing lift. Therefore manual corrections must be made to keep lift equal to weight.
Our comma foils are predicted to be neutrally stable but we have not yet tested them applying the kind of kinetic techniques described in this post. We do not yet have sufficient information to draw conclusions so I will leave them aside for this discussion.

Stability in pitch is quite independent from stability in heave. It is simply the tendency to dampen out changes in bow up/bow down attitude.
To be strictly correct there is some coupling because, as ride height varies, foil area changes. But this change is small for J foils (part of why they are unstable in heave) and is largely cancelled out by other variables such as speed and rig moment. So for the purposes of a conceptual understanding the two degrees of motion (pitch and heave) can be considered separately.

Stability in heave has been the subject of previous posts.
Most solutions to obtaining heave stability involve a drag penalty of some form.
Moths use a wand that senses the water surface and actuates a flap on the main foil, changing its camber and hence its coefficient of lift. This involves the (minor) parasitic drag from the wand ‘spoon’ and the flap hinge.

Multihulls can take advantage of asymmetric setups to obtain stability in heave.
The ‘acute L‘ foil has proven to be the most effective solution in modern racing multihulls.
Since the horizontal foil has a component of lift to leeward, its drag penalty comes in the form of added induced drag because the vertical strut must produce extra sideforce…

In previous testing we have found the drag associated with obtaining stability in heave to be prohibitive. We will revisit this conclusion given that kinetic techniques now allow for much earlier take-off than ‘steady state’ models predicted.

However stability in pitch is the ‘low hanging fruit’ and can be obtained with the right choice of rudder elevator size, section and angle.

The work we did on rudder setup gave us stability in pitch, relieving an overworked skipper from at least one variable.
With a pitch-stable but heave-unstable setup, only speed needs to be controlled to even out foil lift.

Prior to the Worlds, Glenn Ashby, Ray Davies, Peter Burling and Blair Tuke undertook an admirable testing programme in a relatively short but intense training camp.
I admire the rigour that they applied and the experimental methods they used.
One boat was always kept standard as a baseline. Rigs and skippers would be swapped on the test boats to isolate extraneous variables.
This allowed us to conclude with confidence that our rudder setup was the only one available that provided stability in pitch.
Overall drag was also lower but this is a secondary benefit since the exploitability of the boat was noticeably improved (meaning it could be sailed at a higher average percentage of available potential).

The key factor is the rate of change of elevator lift with pitch angle (AoA).
This is helped by having the elevator as far below the free surface as possible and minimising junction interference.

The design of the elevator foil was heavily influenced by my work with RC yachts.
Experience with this low Re application helped to develop a thin section with unusually straight exit runs.

Use of such a thin foil was made possible by efficient structural design and construction.
The last generation of rudders (current production spec) stood up well to incredibly punishing use.

Tough use and aggressive kinetics did lead to some very unexpected hardware failures that have informed the updated specs of our production items (more on that later).

Further Reading

For a good treatment of Kinetics, brush off High Performance Sailing by Frank Bethwaite (Chapter 23).