Upwind Tactics Simulator

Upwind polar

One row per wind speed: the optimal upwind tacking angle to the true wind and the boat speed through the water. Between rows the simulation interpolates linearly; outside the range it clamps to the nearest row. One row is enough for a constant-performance boat.

TWS (kn)	Upwind TWA (°)	Boat speed (kn)

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1 · Motion: vector addition every 2 seconds

The boat is a point mass integrated with a simple time step (dt = 2 s). Its velocity over the ground is the sum of two vectors: the velocity through the water (pointed along the heading = wind direction ± tacking angle) and the local current. The tacking angle and boat speed come from the selected boat's upwind polar. With Tidal wind on (the default) they are re-evaluated at every step against the wind over the water — the true wind vector minus the local current; with it off they are interpolated once at the entered wind speed and stay constant for the whole run.

V_ground = V_boat(heading, through water) + V_current(x, y) position(t+dt) = position(t) + V_ground · dt

2 · The current field: interpolation between your 4 measurements

The measured currents (start, mark, left and right layline) are blended into a continuous field by inverse-distance weighting: at any position, nearby measurements dominate, distant ones fade out with the square of the distance. Points you leave blank are treated as not measured and dropped from the sum — so with only two points filled, the whole field is interpolated from those two, never from a false zero. With no points measured at all, the field is simply zero everywhere.

V_current(P) = Σ wᵢ·Vᵢ / Σ wᵢ   where   wᵢ = 1 / dᵢ²

3 · Tidal wind: the sails feel the wind over the water

The rig does not care how the air moves over the ground — it feels the air moving relative to the water the boat floats in. With the Tidal wind switch on (the default), at every step the local current vector is subtracted from the true wind vector, and the tacking angle and boat speed are re-read from the polar at this effective wind:

W_water = W_true − V_current(x, y)

A current running up-wind adds wind speed, a down-wind current takes it away, and a cross current rotates the effective wind by a few degrees — which shifts the no-go zone, the laylines and the tack points. Because the current field varies over the course, the two sides can literally sail in slightly different winds. Switching it off reverts to the simpler model where the boat holds its polar values against the true (over-ground) wind.

4 · The tack decision: current-corrected laylines

At every step the model asks: if I tacked right now, would my course over ground on the other tack point at the mark? It computes the opposite tack's ground vector (boat vector on the other heading + local current) and compares its direction with the bearing to the mark. When the two align — the sign of the angle between them flips — the boat tacks.

tack when: angle( V_ground_opposite , bearing to mark ) crosses 0

One tack is not guaranteed. The test is re-run at every step against the local current — and, with tidal wind on, the local effective wind — so the corrected layline is a curve, not a straight line fixed in advance. In a varying field the condition can be satisfied, then lost again as the boat sails into different water (the opposite tack's ground vector drifts off the mark), so a run may legitimately contain two or more tacks before the final approach. A 20 s lock between tacks (section 5) keeps this from degenerating into rail-to-rail oscillation on the layline.

Direct sailing (fetch mode). In parallel, the model checks whether the mark is reachable without beating: it solves for the current-compensated heading whose ground velocity points straight at the mark, and if that heading lies outside the no-go zone (≥ TWA off the wind), the boat steers it directly. This mode takes over exactly where the direct heading exits the no-go zone — i.e. on the current-corrected layline — so the final leg is a continuously corrected curve that always converges on the mark, recovering any drift-induced overstand along the way. A run where even this cannot make progress (current stronger than the boat) is reported as unreachable.

5 · The six runs

Each scenario–side pair is simulated independently: 3 wind directions (left phase, mean, right phase — each held constant for the whole run) × 2 strategies (left side = starboard tack first, right side = port tack first). Each strategy starts from its own end of the line: the left strategy at the pin, the right strategy at the committee boat. A run ends when the boat is within 5 m of the mark (the final metres are sailed in fetch mode, homing on the mark); the elapsed times feed the results table.

Two guards keep the integration honest: after a tack the boat must sail at least 20 s before tacking again (prevents oscillation on the layline), and a run is abandoned after 4 simulated hours (reported as unreachable — typically when the current overpowers the boat). The chart frame and the four current measurement points are fixed to the start→mark course axis, so changing the wind direction moves the trajectories but never the course, the frame, or the current field.

Accounted for

• Current as a vector, everywhere on the course. Ground velocity = boat velocity through the water + local current. Head current slows progress towards the mark, favourable current speeds it up, and cross current pushes the boat sideways.
• Spatially varying current. The field is interpolated between your 4 measurement points (start, mark, left/right layline) by inverse-distance weighting, so each side of the course can have genuinely different water.
• Current-corrected laylines. The boat tacks when the course over ground of the opposite tack fetches the mark — not the geometric layline. The chart draws these corrected laylines as thin blue lines (for the mean wind); the dashed grey lines are shown only as a no-current reference for comparison.
• Wind direction scenarios. Each run uses a fixed wind direction: left phase, mean, or right phase. Comparing the three shows how sensitive the side choice is to where the wind sits in its oscillation.
• Boat performance from an upwind polar. The tacking angle and boat speed are read from the selected boat's polar table, linearly interpolated at the wind speed the boat actually feels.
• Current wind (tidal wind). With the Tidal wind switch on (default), the boat sails to the wind over the water: at every step the local current vector is subtracted from the true wind vector, and the polar angle/speed are re-read at that effective wind. A current running up-wind adds wind speed, a down-wind current removes it, and a cross current rotates the effective direction by a few degrees — enough to shift laylines and tack points. Switch it off to reproduce the simpler "boat holds its angle to the true wind" model.
• Fetch/reach sailing on the final approach. When the mark no longer requires beating, the boat steers a current-compensated heading straight at it — so it always closes on the mark, recovers small overstands, and effectively tacks exactly on the current-corrected layline.
• Start line geometry. Line length, plus an adjustable line skew: at 0° the line is square to the start→mark axis, and a non-zero skew swings the pin end around the committee boat (positive = pin up-course). The left strategy starts at the pin end, the right strategy at the committee-boat end, so line bias feeds directly into the side comparison.

Not accounted for

• Wind oscillation during a run. The wind direction is constant within each run — the boat does not gain from tacking on shifts. Phases are compared as separate static scenarios, not as a time series.
• Wind speed variation and sailing modes. Wind speed sets the polar point but is constant in time and uniform across the course; there is no pinching/footing trade-off, no wave-state effect, and no gust response.
• Tack losses. Tacks are instantaneous and free — no speed loss or turning time.
• Wind speed gradient / pressure differences. More breeze on one side of the course is not modelled.
• Other boats. No dirty air, no right-of-way situations, no start-line traffic.
• Waves, leeway, sea state. Leeway is implicitly folded into the tacking angle you enter; waves are ignored.
• Start position and timing. Every run starts at t = 0 from its own end of the line (pin / committee boat) — the start spot is fixed, not optimised, so line bias is not exploited beyond what the fixed end positions imply.
• Time-varying current. The current field is frozen in time (no tidal cycle within a beat).

Practical reading: the tool answers "given this water and these three wind directions, which side is faster on pure geometry + current?" Treat the second list as the tactical layer you add on top of it.