A Class Catamarans – A Look at the State of the Art Part 3

With reference to the illustrations, I will attempt to roughly cover the progression to angled and then curved foils.
Let’s start with a conventional centerboard dinghy.
On any point of sailing except dead downwind, the sail force will have a component across the boat. This is the aerodynamic sideforce.
For the boat to sail at a constant speed, all forces must cancel out as any net difference will result in some acceleration.
The forward component of sail force is balanced by hydrodynamic drag from the parts of the boat in the water (and some aerodynamic drag of the superstructure).

At rest we have sail drive force but no hydro drag. So the boat will accelerate.
As boatspeed increases, hydro drag increases. The boat will accelerate until the rising hydro drag equals the aero drive force. This will be the equilibrium speed.

Increasing the drive force and/or reducing drag will result in a higher equilibrium speed.
At the same time, sideforce is balanced by an equal and opposite hydrodynamic sideforce generated by the hull/board/rudder system, but principally by the centreboard/foil(s).
The hydro sideforce arises because the leeway component of boatspeed (arising due to sideforce) makes the water meet the foils with an angle of attack (AoA).

Since the aerodynamic force is generated above the water, and the hydrodynamic force below, there is a heeling moment that must be balanced by moving crew weight to windward.
But for now let’s concentrate on the forces as viewed from above.


Sail force and hydrodynamic reaction force can be broken down into components across the boat and parallel with the boat.
Note that a force can be thought of as the sum of any pair of components at right angles to each-other.
For example, components can be taken across and parallel with the
actual direction of motion (including leeway) or relative to the wind…

In order for the boat to be balanced in yaw (not want to luff up or bear away), the board must be positioned along the line of action of the sail force. As a boat gets wider, the board must therefore move forward.
This, incidentally, is also why conventional monohull keelboats require the fin to be behind the mast if they are to remain balanced when heeled.


Simplified concept of the line of action of sail force and hydro reaction force.
As the board moves to leeward it must move forward.
In reality contributions to sideforce from rudder and hull must also be taken into account,
usually resulting in the board being even further forward.

The earliest use of angled foils that I am aware of is on the floats of ORMA 60 trimarans.
When conventional upright foils were first placed on the floats to maintain side force when flying two hulls, they were naturally positioned forward of their counterpart on the main hull.
The move forward was due to the wide platform beam and the windward cant of the rig.
By some accounts (I heard it from Nigel Irens) it was noticed that when flying two hulls (and hence heeled significantly for a multihull), conventional upright foils tended to give a bow-up ‘assist’ that allowed the boat to be pushed harder.


Conventional upright foils give a vertical component when heeled.
If the mast does not cant, then this vertical component is canceled by the downward component of sail force that also arises with heel. If sheets are suddenly eased on a ‘conventional’ beach cat when heeled, the sail force goes away and the platform may momentarily ‘jump’ up as a result.

The next step was to angle the straight foils, but this resulted in an awkward exit angle with high interference/junction drag at the hull.
So the next solution was to curve the foils such that they exited the hull vertically and became more horizontal toward the tips.
A constant radius was the simplest solution since it allowed a snug fitting case without complex bearings, a significant factor for a large oceangoing boat.


Limiting factors are the hull exit angle and the tendency for the top of the foil to exceed any beam limit when the foil is raised. Also, the whole immersed part of the foil is displaced inboard. This reduces effective platform beam.

Note that a vertical component to the hydrodynamic reaction force produced by the foils exists even with straight boards. But it is small, and largely cancelled out by the downward component of the sail force due to heel (unless the mast cants to windward).


Once the bow up ‘help’ of angled and curved foils was noted, it became a logical progression to increase this contribution so boats could be pushed harder.
In my opinion this has been a separate evolution to ongoing attempts at hydrofoil sailing where the aim was to actually sail the boat with the hulls completely out of the water, replacing displacement with hydrodynamic lift.
That evolution had different priorities and came up with fundamentally different configurations.
Without delving too deep into this esoteric world, true foiling multihulls tended to use ‘ladder’ foils with no regard for beam limits and no hope of being competitive at sub-foiling speeds.


Experiments in ‘pure’ hydrofoil boats:
Specialist solutions included ‘ladder’ foils designed so that the total lifting surface
becomes smaller with increasing ride height and speed. Image source: www.foils.org

Hydroptere’ style angled foils bear a superficial resemblance to angled foils on displacement boats, but the physics are different.
To allow hydrofoiling, they work against each-other, the windward one providing half the sideforce plus some downward pull while the leeward one contributes the remaining half of the sideforce plus the necessary upward lift. For takeoff, the windward foil may actually be pulling up and to leeward. This is necessary when sideforce is too small to give sufficient vertical component to support all the displacement.

This configuration uses very large foils, spaced far apart, to achieve takeoff, with a large drag penalty at lower speeds.
The drawbacks would be crippling to time around a windward/leeward course.
A boat with a ‘pure foiling’ configuration of this kind would stand no chance in anything but fresh reaching conditions.
The vast majority of the time, a slender displacement hull has less drag than a true foiling configuration when averaged around a windward/leeward course.


Here the lee foil provides upward force and some side force.
The windward foil provides the rest of the sideforce and pulls down.
But for takeoff the windward foil may be configured to pull up and to leeward, with an associated induced drag penalty.
Note the extremely wide beam, surface interference, and spray.
This ‘pure hydrofoiling’ configuration evolved for specialised applications.
Image source: www.adriaan.com

Pure hydrofoil machines have tended to specialise in high speed straight line applications, falling outside established class rules.
The Moth really brought full foiling to the racecourse within a ‘box’ rule. We will be looked later at the special circumstances that made this possible.

On the other hand, conventional multihulls to the ORMA 60 rule were foil assisted, using foils to help keep the bow up in hard reaching conditions.
Angling or curving a foil that was already necessary to provide sideforce meant that very little drag penalty was incurred in more moderate conditions.
This is very important because boats that have to race within a class around a course in varied conditions cannot afford to specialise. They cannot carry around additional foils (or extra foil area) only of use in some conditions on some points of sailing.

In the next post we will look at why outright foiling does not yet pay on an A cat that has to race on upwind/downwind courses in varied conditions…

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