Apparent

Physics

From published sources, not from feel.

The engine is a multibody simulation written in Rust: rigid bodies on joints, every force acting where it really acts. It runs at a fixed step in double precision, so a session replays bit for bit, and it compiles to WebAssembly to run in the browser.

• Foils and sails are strips, each seeing its own flow, with Prandtl's lifting line for induced drag and Wagner's lag on sudden changes.
• Foils ventilate and cavitate, and lose lift near the surface (ventilation inception after Damley-Strnad et al., 2019).
• The foils bend and twist under load, by the arms' stiffness: the flaps lose a quarter of their effect at 36 knots.
• The hull floats on its sections, with ITTC-57 friction, Delft series residuary resistance, Savitsky's planing drag and Zarnick's strip theory.

The boat

An AC75 to its class rule.

The boat is built from the AC75 Class Rule V3.01, the rule of the 38th America's Cup, and from Tannenberg et al. (2023): the hull's lines, the foil arms on the rule's cant axes with their T-wings and flaps, the rudder and its elevator, the double-skin main on a wing mast, and the crew where the rule seats them.

• What the crew sees is what the rule allows: the live displays, the instruments two seconds late, TRACI's race data, the alarms.
• No flow sensors, as the rule forbids them: the flight display estimates each foil's margin to stall from strain gauges, the IMU and ride height.
• Power is limited: the rig's power system limits how fast the sails come in; the cant rams and flaps run on batteries.

San Francisco Bay

The bay, from public survey data.

The land, the bay floor and the aerial photographs come from NOAA and USGS; the city's 15,215 buildings from OpenStreetMap, each with its own colours and windows. Seven landmarks are modelled on their footprints, and both bridges are built to their published dimensions.

• The bottom is in the physics. Every hull, foil and rudder can touch it: sail into a seawall and the boat stops; cut a shoal and the rudder hits.
• Round the course the ground is on a 5 m grid and the photographs at 1 m; out to the skyline, 82 km across.

Wind and tide

The land shapes the wind, and the tide moves the water.

The summer westerly pours through the Gate under the marine layer, flows round and over the hills and the city, slows over rough ground and leaves lees behind steep slopes, all computed per direction from the terrain and the buildings. Gusts, oscillations and turbulence ride on top.

• Through the Gate the wind runs 1.1 to 1.2 times the free stream; in the lees of Sausalito and Tiburon, about half. The water shows it: rougher in the jet, glassy in the lees.
• The tide is a day of NOAA's San Francisco Bay forecast model, hourly. Its current moves the water under every foil, and against NOAA's predictions at four stations it correlates at 0.99.

Against the bay's weather stations the wind is close on the course and wrong in the east and north bay, where the model has no inland pull.

In the boat

VR or a desktop, alone or with friends.

On a desktop each station has its keys, its console and its display. In a headset you sit in the station's pod: grip the wheel's rim to turn it, press the console's buttons by touch or by pointing, and feel the wheel's load as vibration.

• Several people, one boat. A crew server runs the AC75 on one PC and each browser or headset on the network takes a free station. Whoever leaves hands the station back to the emulated crew.
• Every crew session is logged and replays bit for bit, faster than real time.

The VR crew has been checked in an emulated headset in a desktop browser, not yet on a headset. Whether the wheel, the panels and the vibration feel right is for the first sessions on a Quest to say.

The race

From the warning signal to the finish.

The race committee measures the wind, lays the course and runs the start sequence. Then the legs, through the gates, to the finish at the leeward gate, with the boundary, the laylines and each mark's zone on the water.

• Umpires call an early start, the entry, the boundary and late penalties. A penalty is done once the boat has lost 75 m on a reference boat sailing the target speed.
• TRACI, the rule's race data, gives the time to the line, the laylines and the boundary on every station's display.
• Results: the start, each leg's time and speed, and the time gained or lost on the reference boat. The best time in the same conditions is kept.

The bench

A design is a few lines of what it changes.

Hull, foils, sails and systems are inputs, never constants in the code. A design names the boat it starts from and lists its changes; on loading it is checked against the class rule and against the range each model is valid in.

• Compare two designs on the same scenarios, wave seeds and wind: the steady-trim speed polar, the flight modes and their stability margins, the take-off.
• Sail any design in the game too, at the helm, in real time.
• The models say what they rest on: physics from geometry carries over to a new design; a regression carries over within its range; a calibration to one boat is flagged.

# designs/ac75/span-4.0.toml: a variant lists only what it changes
base  = "default.toml"
about = "wing span 4.0 m against the box's 4.5 m, same chords"
[wing]
span  = 4.0  # m

$ sail-lab ac75 compare default span-4.0
metric                  default  span-4.0   B − A    unc  res
take-off  8 kn, kn        18.39     19.30   +0.91   0.39  yes
take-off 12 kn, s         21.00     23.51   +2.51   0.11  yes
upwind    8 kn, VMG kn    17.45     17.40   −0.05   0.09  no
upwind   11 kn, VMG kn    20.73     21.00   +0.26   0.10  yes
downwind 11 kn, kn        35.21     35.66   +0.45   0.04  yes