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 side force.
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 boat speed 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 boat speed (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.
This, incidentally, is also why conventional monohull keelboats require the fin to be behind the mast if they are to remain balanced when heeled.
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.
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.|
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
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.
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 take-off 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
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.
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.