Design & Technology · Foils & flight control

Sixteen feet
of lift

One curved carbon foil, a pair of T-foil rudders and a control system that adjusts them continuously. An aircraft’s logic applied to a 21-tonne yacht.

01 · The righting-moment foil

The foil that
lifts the boat

Roughly sixteen feet long and three feet wide, PB72’s curved carbon foil reaches out from the leeward hull. Its case is raked hydraulically, so its angle of attack, and the lift it makes, can be set to any value, at any time.

Below nine knots of wind it stays out of the way. From eleven to thirteen knots it begins to take the load, lifting the yacht from a point well outboard of the leeward hull, about 14 feet (4 m) from her centre of gravity, and cutting the drag of the hulls dramatically. The boat stays almost level as she flies.

The foil can carry the entire boat, but simulation showed the most stable condition is just skimming the water, with the foil carrying 80 to 90% of her weight depending on the waves.

“Unlike the full foilers seen in the America’s Cup, where yachts fly completely free of the water, we deliberately chose a configuration that retains some hull contact.”

Ferdinand van West · Seahorse, July 2025
The lime foil seen through clear water beneath the hull

02 · Two foils per side

One job
each

No other catamaran carries two foils on each side. On the original Extreme H2O, one C-foil had to do two jobs at once: it was both the daggerboard and the lifting foil. Above about 25 knots it pulled the boat to windward. For PB72, Pat insisted on separating the two forces, over objections that it would be too complicated and too heavy: a centreline daggerboard handles leeway, which frees the lifting foil to be shaped purely for lift.

As boat speed builds, the lifting foil changes rake and the daggerboard retracts. Both are infinitely adjustable, so each carries only as much area as the moment needs.

Why two foils

The lesson of
the first boat

The original Extreme H2O taught Pat the limit of asking one foil to do everything. These two drawings by Ferdinand van West show why PB72 is built differently.

Diagram: on the original C-foil catamaran, the foil’s vertical lift acts about 6 ft inboard of the centre of buoyancy, producing a heeling moment

2015 · The original Extreme H2O · C-foil

One C-foil did two jobs: daggerboard and lifting foil. Its lift acted about 6 ft (1.8 m) inboard of the boat’s centre of buoyancy, so instead of helping to hold her upright it actually added to the heel. A negative righting moment.

Diagram: on PB72, the righting-moment foil’s vertical lift acts about 14 ft outboard of the centre of gravity, producing a righting moment

PB72 · Outboard righting-moment foil

PB72 separates the jobs: a daggerboard for leeway, and a lifting foil that reaches out from the leeward hull. Its lift acts about 14 ft (4 m) outboard of her centre of gravity, so every pound of lift pushes her upright. That restoring moment is what lets her carry far more sail, and far more power.

Righting moment = foil lift × lever arm Same principle, opposite results: what matters is which side of the boat the lift acts on.

On the drawings

Where the
foils sit

From above, the lime foils sit alongside the cabin, one in each hull. From the front, the leeward foil curves out and down beneath the boat.

PB72 top view of the main deck: cabin, aft cockpit, nets and foils
Top view · main deck
PB72 front view: both hulls, the rig and the curved lime foil deployed to leeward
Front view · looking aft

Morrelli & Melvin Design & Engineering · PB72_001 Rev A · 6 August 2022 · Reproduced with permission

03 · Rudders

Pitch,
on demand

Each rudder rakes, and each carries a T-foil about six feet across, so the crew can push the bow up or down independently on each side.

The rudders turn on conical titanium bearings that let each one move fore and aft to bring the bow up or down, adjusting her attitude like an aeroplane. Together with the main foil they give PB72 two continuously adjustable forces that conventional boats simply don’t have.

“With the help of Team New Zealand engineers, they began helping us to design a flight control system.”

Dr. Pat Benz · At the design boards

04 · Flight control

Built for
fly-by-wire

Right after the last America’s Cup, engineers from Team New Zealand began helping design PB72’s flight-control system, and they remain the partner working on it. The boat is designed and fully built to support it: it will manage the angle of attack of the main foil and the rudder T-foils together, holding ride height and attitude as wind and sea change.

So far, every flight has been sailed with the foils trimmed by hand. Once the foil and wind instruments are fully calibrated during the remaining sea trials, the flight-control system comes online and the boat can be sailed to the numbers her designers predicted.

Team New Zealand
Main foil
≈ 16 × 3 ft
T-foils
≈ 6 ft
On each rudder
Draft, boards down
5.31 m
Draft, foils raised
0.65 m
Shallow anchorages

Two new forces.
Total control.

At the helm · sea trials

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