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Recent Foil Design Developments

The Foiling WeekMalcesine, 08-July-2014

Martin FischerNC Raceboats

Nouméa, New Caledonia

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Outline

• Historical development

• Equilibrium of forces

• Stable flight

• Summary and Outlook

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Historical Development – the early days

• Monitor

• MayFly

• Icarus

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Monitor

• Designed by a group of engineers at the US Navy in 1957!

• The boat was far ahead of its time

• Monitor was at the leading edge of technology and cost a fortune at that time

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MayFly

• Philippe Hansford got the A-Class world speed record in 1972 with a top speed of 16.40 kts

• In 1977 after several modifications of the foils the same boat reached 23.00 kts with Ben Wyne helming

• The boat was very stable and relatively easy to sail

• MayFly is in my opinion one of the iconic foiling boats.

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Icarus – the boat that flies

• Another iconic boat: Icarus designed by James Grogono• Top speed in 1972: 21.60 kts• In 1985 after numerous

modifications top speed: 28.15 kts• Very stable• Very fast

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Summary of „the early days“

• All configurations relied on non moveable V-foils

– Heave stability (flight height) is relatively easy to achieve

– But it comes at a price – more drag

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A second approach - active foil control

• Longshot

• Rave

• Moth

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Longshot

• Designed by Dan and Greg Ketterman and Russel Long• First speed runs in 1990 with a top speed of 37.08 kts• Top speed in 1992 was 43.55 kts in A-Class configuration• L-foils with an active control of

the horizontal angle of attack• A surface sensor is used to

automatically adjust the foiling

height

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Rave

• A production foiling boat designed by Sam Bradfield• The main foil has a flap with an adjustable angle that is activeley

controled by a wand• This boat was optimised for ease of use and not for record

breaking speed• Sam Bradfield pioneered

wand controled foils

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Moth

• First race win with a foiling Moth in 2001 by Brett Burvill with a V-foil configuration but later declared illegal by the class

• Second approach: Centerline T-foil configuration with an active wand control proved to be hugely successful

• Most successful foiling class so far• The switch to T-foils was actually forced

by rule constraints

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Third approach – L-foils

• For the 34th America's Cup TNZ invented L-foils with an upward tip

• This development was entirely rule driven and finally proved very successful

• The concept has been recently adapted to production boats like the GC32, Flying Phantom and Nacra-20

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Kites and Boards

• Stabilised by the sailor• Extremely fast, especially in light conditions

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Summary - development history

• Quite different solutions for foiling boats• The driving factor in all cases is to maximise performance under

constraints (in that order):

– Get sufficient lift

– Achieve sufficient heave stability to keep the boat manageable

– Achieve sufficient pitch stability to keep the boat manageable

– Go for a solution that is sufficiently robust for the expected sailing conditions

– Cost

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Foil Design - Fly or not to fly?

• The figure below shows the ratio of Fz/Fx for a C-Class hull for different displacements as a function of boat speed – a strong dependency on boat speed can be seen

• The L/D of a foil is typically of the order 8 to 15 (horizontal green line on the plot)

• If the L/D of the foil is higher than the L/D of the hull, it is favourable to fly. In the current example the cross over between the curves is roughly at 8 m/s of boat speed

• So according to this simplified model the size of the foil should be such that the boat can take off at about 8 m/s

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Foil Design – and when to fly?

• But the real world is a bit more complicated …• The transition between displacement mode and flying is not

instantaneous. The portion of the weight carried by the foil (“lift fraction”) increases gradually from 0% at very low speed to 100% at take-off speed

• For the best possible overall performance of the boat this transition phase must be taken into account during the development / dimensioning of the foil

• This leaves us with an optimisation problem with various design and trim parameters as free variables

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Lift – fully uncoupled

• T-foil with an active flap control• Strut and tip are perpendicular, hence Fz and Fy are (almost)

fully uncoupled• Main foil with an adjustable flap • A wand system controls the flap angle as a function of ride

height

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Lift – full coupling between Fz and Fy

• As an example let's assume we have a straight foil with a dihedral of 45 degrees and a ratio of weight to side force of 2

• Leeway adjusts automatically such that the side forces are in equilibrium

• The dihedral determines entirely the vertical force• Fz is independent of speed!!• If the dihedral is not correct the boat

cannot fly on one hull

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Lift – full coupling (II)

• The foils must create at the same time vertical lift to balance the weight of the boat and lateral lift to balance the side force created by the sail

• For a weight to side force ratio of 2 we obtain an angle of the force vector with respect to the vertical of atan(2) = 63 deg

• This angle is much flatter than typical

dihedral angles• The dihedral angle must exactly match

the weight to side force ratio, otherwise

no flight possible• Unstable

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Lift – full coupling (III)

• In order to get stable fligt we need a second foil that creates additional lift and additional side force

• The total amount of vertical lift and side force is the same as for the dihedral of 63 degrees, but the total amount of lift is increased by about 26%, hence significantly more drag

• Fz and Fy get uncoupled and the situation

becomes stable• Significant reduction in

righting moment due to the

lift from the windward foil

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L-Foils

• A few geometric defnitions• Rake angle alpha and leeway angle beta• The upward tip partly uncouples Fz and Fy and makes

the foil heave stable: • If the boat flies higher leeway increases and this

reduces the effective angle of attack of the tip

hence Fz decreases• The reduction of the angle of attack with beta is

approximately given by: beta * cos(90-tip_angle)• Thus a larger tip angle leads to more

heave stability

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L-Foils (II)

• Effective angle reduction as a function of tip angle

0 10 20 30 40

0.0

0.2

0.4

0.6

tip.angle

ff

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L-Foils (III)

• Alpha = 2, beta = 0 (left) and beta = -3 (right)

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L-Foils (IV)

• Upwind foiling situation, rake = 3, leeway = 0 (upper panel) and leeway = -1 (lower panel

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L-Foils (V)

• Stability diagrams: change of Fz with foiling height for a given side force

• Stable (left), unstable (centre), initially unstable then stable (right)

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Pitch stability

• Pitch stability is the capacity of the boat to get automatically back to a stable pitch angle after a perturbation

• The rudder elevator provides pitch stability• A larger rudder elevator provides more stability but creates also

more drag

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Pitch Stability

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Some conclusions

• For the L-foil with an upward tip the tip angle is the driving parameter for heave stability

• Obviously a large tip angle creates also more drag, hence the right trade off between stability and performance has to be found

• A larger rudder elevator provides better pitch stability but creates also more drag

• It might be faster to go for a potentially slower confguration that

is easier to sail and hence that can be raced at a higher percentage of its theoretical performance

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C-Class Catamaran

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Outlook

• At least with multihulls we are just at the beginning of this new development

• In the future the foil solutions will become smarter and the boats will be easier to sail and faster

• First race classes start to appear and I think in a few years we'll have active racing as we see it nowadays with non-foiling catamaran classes

• I think foiling will very quickly become well established in multihull racing, but even for fast cruising multihulls it should be feasible