Dario's Lab

Imagineering Part 2

In Part 1 we looked at straight line sailing.
We concluded that foils will always be a hindrance at very low speeds, but will give an advantage at higher speeds. More aggressive setups (read more foil area) are even worse at low speed but, since they allow earlier take-off, become superior ‘sooner’ (at a lower windspeed than more moderate foils).
The exact crossover is still being explored. It may depend on crew weight, hull characteristics, and rig choice.


Tacking and Jibing

When we introduce changes of direction the picture gets more complex.


As a general rule, asymmetrical setups are always faster in a straight line.
Water ballast, canting keels, and sail ‘stacking‘, are all examples of how making a boat asymmetrical improves performance on that tack. They offer gains additional to just moving crew weight to windward. At the extreme, all-out speed record craft have been ‘permanently’ asymmetrical for some time now.

Configurations with all the lift on one side (those that maximise righting moment) necessarily require foil settings to be swapped as the wind changes side.

We can view asymmetry as a form of specialisation.
If we must tack frequently, and our crew resources are limited, the advantage of specialisation must be weighed against the cost of transitioning from one specialised setup to another.
Two or three seconds may be lost during the tack as the sailor hauls on a foil-control line before settling in to sail the new course.
That time must be recuperated via extra straight line speed just to break even. If the distance between changes of direction is too short (because of course restrictions, shifts in the breeze, or tactical considerations), then the faster asymmetrical setup will not pay.

This trade-off is hard to quantify on the drawing board.
Obviously sailors with more practice will be able to manage such changes more readily.
Once everyone reaches the same level of proficiency, the more symmetrical solution will still free up hands and mental capacity, making manoeuvres faster.

Z foils are a compromise in a straight line, but still require management during turns. Differential rake gives a marked improvement (unloading the windward foil to minimise righting moment loss). Since foiling upwind with them only pays in rare conditions, retracting the windward one is de rigueur.

If regular raising and lowering is required, then the straighter the foil, the easier.

As an aside, configurations with an active central T-foil, such as used by Moths, tend to have the sensor wand offset to keep any wake it produces away from the vertical strut of the main foil.
Since T foils rely on windward heel to vector lift from the fully submerged foil, the wand needs to be reset on each tack.
For example, if the wand is mounted to the right, it needs to be set lower on port tack and higher on starboard tack.

Even under an open rule, a central T foil solution will most probably not be the fastest on a cat, because it would effectively halve the beam of the boat, giving up too much righting moment.
The ease-of-use advantage would probably not outweigh the loss of righting moment for this particular almost completely symmetrical configuration.
Though it would be an interesting experiment, it makes little sense to give up half the leverage available to competitors who fully exploit the inherent advantages of a catamaran platform…

In summary, some form of asymmetry, and associated extra work, will most probably be accepted as worthwhile for maximum performance around the course.
The question is how much. Where does the best compromise lie?


If we equate straight line sailing with powered flight, then tacking and jibing is like gliding.
During the turn, as sail drive force dips through zero (and below if the apparent wind goes around the front), the distance available before ‘splashdown’ is analogous to glide ratio.

There is an established and fascinating body of knowledge around unpowered flight.
One of the first facts to digest is that lift-to-drag ratio is the dominant element.
For the same configuration, a heavier glider will fly faster, but it will follow the same glide path (same sink for every unit of forward movement).
It will reach the ground sooner, but in the same place.

So, all things being equal, foils with a better lift-to-drag-ratio will keep us foiling further than less efficient ones. Regardless of crew weight.

In our case, the mechanism that controls heave will also be making the foils work harder as we sink. Surface piercing foils will be getting bigger with sink. Active foils will be lowering flap, and hence increasing lift coefficient. In both cases drag will be increasing. At some point drag will increase rapidly, taking us away from best lift-to-drag regime and shortening our glide.

One possible exception to this is L foils. Since they rely on leeway for heave control, and because mid tack/jibe there is no sideforce, they will possibly remain more efficient for longer, increasing our chances of getting through the turn without ditching the hulls.

In all cases the area of vertical shaft in the water will be getting bigger as we slow down, adding drag.
But this effect should be similar for all configurations.

One interesting take-away is that flying higher helps with maneuvers since it gives you more potential energy going into a turn.
But flying high means long foils, which are draggy at low speeds.
Ultimately an answer could be ‘jacking up’ the boat just before a turn. But realistically this is not a workable solution for a singlehanded boat…

Dynamic Effects

A fascinating observation we made when testing ‘four point’ foil configurations (Zs and active/flap foils) on A Class cats is that turning can induce significant rolling.

The cause is simple: as you turn, the foil on the outside of the track travels faster through the water, so generates more lift.
If not managed, such rolling can make the outside foil breach the surface, which then causes a splashdown of that hull.

On a boat where crew weight is so influential, this effect can be managed through good technique.
Inherent heave stability helps.

Side View Vs. Top View

Finally let’s examine getting away from a ‘parked’ position near head-to-wind.

When looking down on the boat, we want the foils to be at or just behind the centre of effort of the sail. Thus when we ease the sail, the drag from the rig tends to just lead the foils. Combined with steering input and windage on the bows, it allows us to bear away quickly and power up.
A boat with less lead will rely more on rudder sideforce to sail in a straight line. This theoretically reduces induced-drag, but takes away ‘reserve’ rudder force available to bear away.

In side view we want the lifting foils to be slightly ahead of the centre of gravity of the boat. This increases tail volume and helps with stability.

The ideal position for the main lifting surfaces is forward of that for the verticals. Moving the vertical part of the foils forward makes it more difficult to bear away.
Raking the foils bottom-forward helps (and discourages ventilation), but the longitudinal displacement is minimal.

Again a compromise is required if we want to continue using a single sail.
Again our ability to aggressively trim the boat by shifting crew weight is our friend.
Having the windward foil raised does help in this respect, scoring another point for an asymmetrical setup.

Crystal Ball

There is certainly a lot still to learn about how all these sometimes conflicting factors should be balanced for best performance around a course.
Experimentation and time will give us answers, as well as showing us new questions.
This is why we like to play in a development class.

It will be a close run thing between ‘four point’ and ‘three point’ solutions on the A Class given our unique characteristics of limited beam, modest sail area, and singlehanded crew.
Our testing indicates that, especially with more transverse span available, L foils show a lot of promise. The near future will most probably be an L or a less tortured Z.

It is doubtful that radical and/or impractical solutions, such as central foils, and/or ones that require capsized launching, will take root.
There are certain inherent advantages in a cat class that make foil retraction attractive. Just as using beam to generate righting moment is advantageous.

Everyone will weigh the pros and cons. Given the freedom to experiment, the cream will rise to the top and the best ideas will win.
After all, this approach has given us the simple, lively and enjoyable toy that is the modern A Class cat.