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Extraordinary boats: TF35 semi-automated foiler

The TF35 is a new one-design catamaran with a unique computer-controlled automatic foil system, that will race in the TF35 Trophy. We get all the info on these incredible foiling catamarans

In recent America’s Cups we have seen a small army of grinders pumping away to bring pressure to their flying yacht’s hydraulic lifeblood, while flight controllers attempt to keep up with their speeding craft’s accelerations in three dimensions. But while these athletic and technical skills are impressive, these days there are simply better alternatives to humans carrying out these roles. Enter the TF35.

The TF35 is a new one design foiling catamaran created to take foil-borne sailing to the next level. It has no hydraulics to keep primed. Conventional ropes and winches handle the sails and raising and lowering of foils, but battery powered electric actuators drive the rest, including split-second adjustments of the foils and rudder elevators. What’s more, these operations are automated, leaving crew free to focus on skills such as helming and tactics.

The shape, size and efficiency of the TF35’s foils means it requires just seven knots of wind to take off upwind and nine downwind (when boat speed reaches 2x and 3x wind speed respectively). It also passes the acid test of foiling upwind and through tacks.

The racks increase righting moment but also protect the foils when alongside pontoons and improve crew safety. Photo: Loris von Siebenthal

A small penalty for having such light wind take-off is ultimate top speed, which is 18-19 knots upwind and 34-35 knots downwind, but still plenty fast enough.

After the prototype launched at the end of 2019, the new TF35 is enjoying its first full season of competition this year. The class follows on from the D35, the one design catamaran that provided sterling service on Lake Geneva for 16 years and countless Bol d’Or Mirabaud victories for Ernesto Bertarelli’s Alinghi, his sister Dona’s Ladycat-Spindrift and many others.

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Aside from becoming fully foiling, a significant difference between the two boats is that the TF35 is designed to race not just on lakes but also on open water, and this year the circuit will include events in Scarlino, Italy, in September.

Behind the TF35 is an America’s Cup-level design team led by Gonzalo Redondo of d3 Applied Technologies, who most recently headed the CFD team for American Magic. On structures is Cup veteran Dirk Kramers with Swiss genius Luc du Bois on the design and flight control system, plus Britain’s own Adam May, Marc Menec and Jean-Marie Fragniere.

The majority of the TF35’s components were created in Brittany with Multiplast building the hulls, beams by Airbus, and Heol Composites fabricating the intricate foils. Spars are from Lorima, with some of the few non-French parts including carbon diamonds and PBO shrouds from Future Fibres.

The mainsail foot touches the central hull to create an endplate effect, but requires an unusual half-wishbone boom. Photo: Loris von Siebenthal

Surprisingly, the TF35’s platform was one of its least important parts. The aim was to remove every gram of weight while maintaining a structure stiff enough to prevent deformation due to high foil and rig loads.

The D35’s flying central hull now terminates beneath the mast with a long, heavily-rigged bowsprit protruding more than 4.5m forward of the bows. The rig is relatively conventional for a modern day multihull, with a wing section rotating spar and sails that, thanks to the boat’s near permanent upwind apparent wind angle, are never eased far.

To reduce weight the mast has a diamond and a two-part construction, allowing it to fit into a 40ft container along with the boat.

The mainsail is a ‘deck sweeper’, with part of the foot touching the flat deck of the centre hull, creating an end plate to increase the sail’s efficiency. However, this requires an unusual boom – it is curved, like half of a windsurfer’s wishbone boom.

The foil sits inside its own case enabling it to be raised/lowered in manoeuvres. The case can be inclined using a worm drive to alter overall rake

Understandably, much of the development has been focussed on the TF35’s foils, their operation and software control.

Most other flying cats have J-shaped lifting foils, with some ‘V’ (dihedral). This shape provides some inherent ride height stability (the more immersed the foil becomes, the more surface area there is to provide lift) without requiring constant trimming.

However, due to the foils on the TF35 being computer controlled this is less necessary, so they can have a more efficient, flatter and less draggy shape, albeit with a massive 2.7m span.

TF35 auto adjustment

On most flying catamarans the amount of lift a foil produces is determined by its rake. On the TF35 rake of the entire foil is controlled by a rope-driven worm-drive. This is a broad brush setting, determined by conditions, point of sail, wave state, etc.

The T-foils span 2.7m; substantial but necessary to achieve light wind lift off. Photo:

Split second adjustment of the amount of lift comes from an aircraft wing-style flap on its trailing edge which is able to articulate by up to 5°. This demanded some intricate construction but its operation requires substantially less power than adjusting the rake of the entire foil.

Compared to the main foils, the TF35’s rudders are a simpler inverted ‘T’ configuration with the shaft on each fitting into a cassette attached to each transom via gudgeons and pintles.

Like the main foil, lift is also adjusted by computer/electric actuator via a flap on the elevator’s trailing edge. The amount of rudder lift determines the overall pitch of the boat. This, in turn, affects the rake of the main foils, thus trimming them in conjunction is vital – another job that’s vastly improved by automation.

A key feature of the AC50s was rudder differential – ie negative lift in the windward elevator, or a downward force that dramatically increases righting moment.

The TF35 also offers rudder differential, but to a much more limited degree. The automation means crew only have the option of switching rudder differential on or off, similarly they have just four settings for ride height and overall boat pitch.

This leaves the software to ensure that foils are trimmed to the optimum and that rudder differential (where loads can quickly spiral out of control) never causes structural limits to be exceeded.

The main lifting foil hangs off the end of a titanium bulb, with its trailing edge flaps all but invisible

The computer is stored alongside a lithium-ion battery within the central flying hull, controlling four electric motors operating trim flaps – a relatively straightforward system. However, the software behind it, monitoring parameters such as ride height, 3D attitude and accelerations with split second accuracy, is monumentally complex. It requires a multitude of sensors from state of the art GPS to ultrasonic altitude sensors and rate gyros.

While maxi racing yachts with complex hydraulic systems typically use a programmable logic controller (PLC), so the TF35 has a similarly programmable brain, the difference being that it is electronic, controlled via software.

This is especially tested during manoeuvres, when the software must recognise when two foils are in the water and the boat is developing twice its usual lift. So vital is this software that it’s one of the TF35’s strictest one design elements.

As a piece of ground-breaking technology, the TF35 is perhaps the first one-design class to have its own operating system.

TF35 specifications

Hull length: 10.85m / 35ft 7in
Beam: 6.30m / 20ft 7in
Draught: 2.45m / 8ft 10in
Mast height: 21m / 68ft 11in
Weight: 1,300kg / 2,866lb
Sail area Mainsail: 79m² / 850ft2
Sail area Jib: 34-41m² / 365-440ft2
Sail area Gennaker: 120-140m² / 1,290-1,506ft2


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