A Brief Guide to Sailing Foils
By Dario Valenza
CarbonicBoats
Enquiries by email: [email protected]
Phone: +61 412 127 388
Post: PO BOX 2123 Rose Bay North, NSW, Australia 2030
This (photo supplemented) article is reproduced from - SailingAnarchy.com
alphabet soup
With lifting foils getting more and more important and complex at
the top end of multihull racing, our pal Dario from Carbonic Boats
wrote a relatively simple little guide to the different solutions and
how they work.
1) Angled Board
The simplest way to obtain a vertical component by just canting the foil
lift vector. This solution is extremely constrained in angle and span if a
beam limit is to be respected when the foil is retracted. The same
constraint also forces the foil exit point in the hull inboard toward the
middle of the boat, moving the hull/foil junction closer to the free
surface and reducing righting moment (because the centre of vertical lift
moves inboard).
Every part of the foil span contributes evenly to vertical lift so, assuming
enough foil angle is possible to lift the boat clear of the water, there is
no stability in heave (ride height).
Using two such foils together on a very wide platform such
as Hydroptere (photo above and diagram below) can give heave stability
by simply reducing immersed foil area with altitude. But this
arrangement is not practical in most classes racing ‘around the cans’.
2) C, J, and L Foils:
C Foil: Side-force (to windward) is unevenly vectored to generate upward
lift. Vertical component is greatest near the bottom. By tightening the
radius, more extreme lift characteristics can be obtained regardless of
beam restrictions. On the practical side, C foils are easy to install
because they fit in a constant-radius foil case.
C foils are unstable in heave: as ride height increases, vertical force does
not decrease. Given constant thrust, if lift is greater than total weight,
the boat will rise until the foil stalls, causing a crash. C foils are helpful
in foil-assisted sailing as long as they lift less than 100% of the weight of
the boat.
J Foil: Similar to C foils but maximum lift remains available when a J foil
is partially retracted (shown orange). The lower part of a J foil stays
‘canted’ until the junction radius reaches the hull. Unlike a C foil that
becomes more upright as you pull it up. J foils are also unstable in heave
so are suited to foil-assisted sailing rather than full foiling. They
potentially have less drag when sailing downwind because their draught
(and hence frontal area) can be reduced when vertical lift is still
beneficial but less side-force is required.
Both C and J foils can have high induced drag when set for max lift (raked
– see last diagram below) because the lift distribution along the span
becomes biased toward the tip.
End devices such as winglets or washout at the tip help alleviate this but
cause parasitic drag at other times and add complexity to the foil case
design if the foil is to be fully retractable. Note that tightening the
transition radius on a J foil progressively gives a ‘traditional’ 90 degree L
foil that is also unstable in heave.
‘Acute L’ Foil: A very elegant way to automatically regulate heave for
full foiling on only one (leeward) foil. First “stumbled upon” by the ETNZ
design team, this idea is a great example of how rule constraints can
push innovation by forcing competitors to think laterally.
As ride height goes up, the immersed area of vertical ‘strut’ decreases
(lateral area is lost). This makes leeway increase, in turn reducing the
Angle of Attack (AoA) on the ‘horizontal’ foil. To get your head around
this, imagine what would happen if you made leeway extremely large
(like 90 degrees): The horizontal foil would actually start pulling
down! Under normal conditions the change in leeway is small (say 5
degrees) but the component across the boat works to reduce the AoA on
the horizontal foil, moderating lift to stop a runaway leap into the air.
So: boat goes up > lateral area gets smaller > boat starts slipping
sideways a bit more > horizontal foil moves toward its own low pressure
field > lift decreases > boat settles > lateral area increases > leeway
decreases > vertical lift grows again… And so on until an equilibrium is
reached.
The higher the inboard tip relative to the outboard root/junction, the
closer the coupling between ride height (through sideforce) and vertical
lift.
At extreme ride heights, the acute L foil begins to work as a
conventional (powerboat) V hydrofoil: When the inboard tip of the
horizontal foil breaches the surface, immersed foil area is gradually
reduced regardless of sideforce. This is vital to avoid a crash when pulling
away to a near square run in reaction to a gust. It is a good ‘safety valve’
in situations where speed (and lift) may be high but sideforce is small.
3. Combos
With the basic components described above, designers have a kit of parts
that can be mixed and matched to suit the particular application at
hand. The principal groups that can be seen when observing recent AC72
testing are described below in the order pictured above.
L Foil with Polyhedral: The bent inboard tip provides stability in the
same way as an acute L foil. Kinking the horizontal foil reduces junction
angle between vertical strut and horizontal foil to 90 degrees. In a way
similar to introducing a bulb or a radius, this decreases drag where
interference effects are most prevalent.
The root of the horizontal is heavily influenced by the low pressure area
inboard of the vertical strut so is less affected by leeway than the tip. It
makes sense therefore to use the root to generate the bulk of vertical lift
and exploit the tip for heave control. The penalty is a bit more parasitic
drag as there is more foil area for a given effective span. The bent
horizontal foil can also hug the hull more snugly when the foil is
retracted, reducing drag when the windward hull is near the water.
Acute L with Kinked Strut: Bending the vertical strut enables some
adjustment of the angle of the horizontal foil so that stability in heave
can be fine-tuned. A bend may also be necessary to stay inside the beam
restriction if designers want to cant the strut inboard to get an effect
similar to a C-L foil.
C-L Foil: Combines the heave stability of an acute L with some lift
vectoring of the strut for lower overall drag. The cost is a shift inboard of
the centre of lift which reduces righting moment.
S-L Foil: Similar objective to a C-L: more even lift sharing for lower
overall drag. But the inflection at the top moves the bottom outboard
again, recovering full righting moment.
The S also fine-tunes the angle of
the horizontal foil to adjust ride height and heave stability. The
downsides are mechanical complexity at the bearings, a foil case that
holds more water, and more friction when raising and lowering. Bending
the foil at the highly loaded area between hull and deck bearings is also
structurally more demanding, especially on bigger boats.
And finally, a diagram (left) showing how foil rake affects vertical lift.
Remember that heave stability is the tendency for lift to vary inversely
with ride height. For effective foiling it must be combined with pitch
stability which is a bit simpler to obtain using properly sized T, + or L
rudder foils.
On small boats such as the A Class, it may be possible to ‘stay on top of’
an unstable platform by actively managing weight placement and sideforce, countering in real time the continuous tendency to depart stable
flight. Like riding a unicycle this is difficult but humanly possible.
Until
now this solution, though far from optimum, seems to be the best real
world choice for racing around the course in the A Class, mainly due to
rule constraints on foils. The challenge for the future is getting stability
with an acceptable drag penalty within the rule. Bigger boats do not
have the option of quickly shifting weight and aggressively trimming the
sails so true stability is important for safety and speed.
I hope this post has been informative for keen observers of the
spectacular innovations on show in today’s multihull scene. Remember to
look critically and skeptically at the physics when assessing how effective
and stable various solutions might be. Interesting times indeed.
Post Script
Hydroptere reached peak speeds of around 61 knots – and in 2009 was the
world sailing speed record holder at 51.36 knots. Hydroptere and Alain
Thebault and team aim to set oceanic sailing speed records.
Paul Larsen and another group of pioneers of speed sailing also used foils
in a different configuration – one of which is “hooked to windward” and
designed to “hold” the craft in the water and prevent it from flipping
(and slipping) to leeward.
Vestas Sail Rocket 2 – at and average speed of 65.45 knots – peak 68.01
knots.