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Wednesday, June 17, 2015


Some thoughts following some recent testing with a focus on handling.
Specifically, we worked to gain data on how different foil configurations affect dynamic behaviour during turns in strong wind.

The results confirmed observations we hear regularly from experienced Moth sailors, as well as those who race foiling multihulls such as the NACRA F20 Carbon FCS:
Being foilborne on the upwind leg makes the bearaway a lot safer.

Intuitively it is easy to understand that a foil has the ability to 'push back' with increasing force as the bow-down moment from the rig increases. But this is only part of the picture.

In displacement mode, an increase in bow-down trimming moment must result in some bow-down trim in order to move the centre of buoyancy forward.
More volume has to be displaced closer to the bow so that a restoring bow-up moment can exist to counter the increasing bow-down moment from the rig (which in turn is a result of sail force increasing and rotating to point more forward during the bearaway).
A secondary effect of the bow-down trim is that the rig increasingly pushes down, effectively increasing displacement.
All the while drag is increasing, speed is diminishing (or increasing at a reducing rate), and available volume (forward buyancy) is running out.

Staying on the foils instead allows the foil/elevator system to dynamically counter the changing sail vector.
Less obvious, and possibly more important, is the fact that, with less drag and smoother acceleration, the apparent wind stays forward so bow-down trimming moment is much smaller.

To reap the benefit, foiling upwind has to be competitive.
This can only be the case when maximum righting moment is available.
To maximise righting moment the leeward foil must be able to carry the boat at moderate (upwind) speeds and give some heave stability unaided.

Safety is perhaps the most compelling argument for ending the absurd restrictions imposed on the A Class by a shrinking minority.

Images of customers who have retrofitted our L foils to existing (mostly older) A Class cats.
A low-cost upgrade that increases performance and improves handling.
Incidentally it makes beach launching easier, as the foils support the hulls, giving a small point of contact rather than potentially scratching a larger area. Obviously it is better to always use a set of 'beach wheels'... But where laziness or circumstances do not allow it, the foils make for less widespread damage.

Friday, June 5, 2015

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 takeoff, 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 tradeoff 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 maneuvers 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 sulutions, 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.
Afterall, this approach has given us the simple, lively and enjoyable toy that is the modern A Class cat.

Wednesday, June 3, 2015


Congratulations to Sergio Vela, who placed third at the European Spring Championship, using retrofitted Paradox 2014 steering system. Full results here.

Image source: Circolo Vela Arco

Monday, June 1, 2015

Imagineering Part 1

In response to questions about where catamaran foil design may go in the future, especially if rule constraints are relaxed, here are some thoughts on the incentives driving design choices.

If our goal is fastest time around a windward/leeward course, then the considerations are:
- VMG upwind.
- VMG downwind.
- Control at low speed, specifically to bear away and accelerate off the start line.
- Speed profile through tacks.
- Speed profile through jibes.
- Sensitivity to setup.

VMG Upwind

To get to the windward mark first, we want the right combination of speed through the water and heading angle to the wind.
We can sail faster through the water by footing. But sailing at a bigger angle away from the wind direction means covering more distance for the same ground gained toward the mark. So our extra speed must be enough to make up for the longer course sailed.

Any form of foil assistance will involve an initial drag penalty.
At very low speeds the foils do nothing but add drag.
As we go faster and the foils begin to produce lift, they contribute even more resistance. This is added to the drag of the hull(s) that are still in the water.

The 'foiler' will remain at a disadvantage until enough weight is transferred to her foils to at least reduce hull displacement-to-length ratio enough so hull drag reduces enough to make total drag of foils+hull lower than just hull drag would have been with no foils...

At any speed above such crossover, all other things being equal, the foiler will have less total drag than her displacement counterpart. Less drag for a given speed means that speed can be maintained with less sail force.
The advantage will get bigger as speed increases.
At some arbitrary point the hulls will be completely free of the water, hull drag will go to zero, and the only contribution to hydrodynamic drag will come from foils and rudders.

Indicative graph showing how drag rises with speed. Simple displacement hull has the least drag at lower speeds because the drag of foil-assisted and foiling boats at those same speeds is hull drag + foil drag. Foil assisted shadows displacement but has less drag at high speed because the effective displacement of the hull is reduced. Aggressive foils pay an initial drag penalty for early takeoff. Note that takeoff constitutes a quasi-discontinuity in the drag curve. Foils are shown 'loaded'. Reducing their angle of incidence at low speed would reduce their drag, but the assumption here is that takeoff is being attempted. 
From the above paragraph we can conclude that a foiler needs to be moving fast through the water to get an advantage. After all, foils don't work unless they have water flowing over them.
So we can conclude that foils will only give us a winning edge upwind if they are efficient enough to support enough weight to significantly reduce hull drag (or better yet eliminate it completely) at speeds that are achievable without having to give up too much pointing angle.

Looking at foil drag alone, this explains why angled and C foils are formidable upwind:
They have negligible extra area compared to a straight foil that just contributes sideforce.
But through curvature or cant angle, they vector some lift upwards, and reduce hull displacement with almost no foil drag penalty.

In contrast, a foil with a dedicated lifting surface (such as an L) must have the same area in its vertical shaft as a conventional foil in order to provide sideforce. The horizontal leg is extra. It just adds drag until speed is high enough for the lift to start making a difference.

Angled and C foils, for foil-assisted sailing, have little more area than upright ones.
Dedicated lifting foils such as Z (centre) and L/V (right) have considerably more.
So now the question is:
Can I go fast enough, enough of the time, to get my dedicated lifting foil working, without reaching away from the top mark too much?

To answer that, we have to look at other speed-producing factors.
Upwind the dominant one is righting moment.
Assuming enough wind to be fully 'powered up', more righting moment means we can keep increasing sail force without capsizing.

If boat mass and crew weight are the same, the only way to increase righting moment is to get more leverage. Meaning lengthen the distance across the boat between the centre of gravity (CoG) and the point where the mass of the boat is being supported (let's leave out downforce from the windward foil for simplicity).

For a displacement cat, flying a hull moves the centre of buoyancy (CoB) to the leeward side, maximising the lever arm.
For a foiler you can see that ideally we would want all our lift to be centred right under the leeward hull.
Any movement toward the centreline carries a penalty in righting moment.
However some compromise may be optimum for an L foil because increasing the span of the horizontal leg (making the aspect ratio higher) improves foil efficiency.

Single foil to leeward (blue), with straight vertical and short horizontal, gives maximum leverage.
Curved vertical and long horizontal (red) gives less lift-induced drag.
Some compromise (orange) is usually the fastest solution.
Z foils may add less drag at lower speeds but will also reduce righting moment.
Given no other constraint, getting all our lift from the leeward foil would be ideal.
If the rules limit the available span on each side, then one foil may not be enough to get our desired 'breakeven' speed to make foiling work.
Then we can go to 'strange' solutions, like having two or more foils on the same hull...
Or use the windward foil to help by contributing some lift.

Since the windward foil is adding to heeling moment, lift generated under the windward hull is very 'expensive'. It has a direct cost in sail carrying power.
That is why Z foil A cats feel very 'tippy' when set up for maximum lift.
It is also why boats like Hydroptere go for extreme beam.

Once fully foiling, stability will come into play.
There must be a way to keep lift constant as speed and ride height change.
This can be done by:
- Varying immersed foil area (surface-piercing foils).
- Coupling lift with leeway (L/V or 'acute L' foils).
- Active control systems (wands/flaps as on a Moth).

Heave stability is not so critical upwind because boatspeed can be controlled relatively easily by coming up into the wind in the gusts, and bearing away in the lulls. Effectively sail force can be kept constant as apparent wind varies.
Pitch stability is surprisingly important upwind because drag from the top of the rig tends to make the sterns squat down, so rudder winglet (elevator) lift is critical.

VMG Downwind

Initially it would seem that downwind the tradeoffs are simpler: Righting moment is less vital because the sail force vector can be more in line with where the bows are pointing.
This is true in strong wind, at low to moderate boatspeeds. In such circumstances, you can sail 'deep', with the apparent wind over your shoulder, and sails eased. Crossover boatspeed can be reached easily and foiling pays thereafter.

However, as boatspeed increases, and the apparent wind goes forward, righting moment again becomes king. Even downwind.

Getting to takeoff speed is also just as critical downwind in lighter conditions.
If there is not enough wind for the sail to basically 'push' the boat to takeoff speed, then we need to luff up to a reaching angle to get sufficient boatspeed.
This is taking us away from the bottom mark. So the speed gain when foiling has to be enough to repay the extra distance traveled in our attempt to 'unstick'.

Again, the tradeoff becomes about how efficient the foils are. We want to be able to accelerate to takeoff speed, despite foil drag, at the 'deepest' possible heading angle, to keep our VMG up.
Once foiling, since overall drag is less, we can improve VMG by sailing broader than a displacement boat, because the necessary drive force is less (drive must oppose drag to sail at a constant speed).


At low speed foils are a handicap.
To lower takeoff speed we want big dedicated foils, but we want them to be efficient so they don't hold us back too much in sub-foiling conditions.

Righting moment is vital, first to reach takeoff speed, then to continue foiling at speed when the apparent wind moves forward. Ideally we want all our lift on the leeward side for best performance in a straight line.

It is not at all clear that the most aggressive foiling setup will be fastest across a broad range of conditions. Experimentation and competition will tell us where the balance lies.

So much for straight line sailing. In the next post we will look at control at low speeds, and speed profile through manouvres.