1 Finding the manoeuvre
A manoeuvre is found by the same detector the guided calibration uses. It watches for the boat leaving one settled sailing state and arriving in another: close-hauled on port to close-hauled on starboard is a tack, broad reaching on one board to the other is a gybe.
| Rule | Value | Why |
|---|---|---|
| Upwind band | 25–65° true wind angle | Close-hauled sailing, either board. |
| Downwind band | 120–175° | Broad reaching and running. |
| Boat-speed floor | 2 knots | Below it the wind instruments are not measuring sailing. |
| Longest transition | 60 seconds | A change that takes longer than this is not a manoeuvre. It is a boat that stopped sailing and started again. |
| Shortest settled run | 10 seconds | Either side of the transition, so a wobble is not counted as two manoeuvres. |
The detector records the manoeuvre at the midpoint of that transition. That is as precise as a state-change detector can be: it knows when the boat left one state and when it arrived in the next, and the middle is its best single answer.
2 Where t = 0 is, and why it is not the midpoint
Comparing three tacks is only meaningful about a moment all three share. The midpoint is not that moment: it depends on how long each transition took, so aligning on it lines up the middles of three turns of different lengths. The moment three tacks genuinely share is the bow passing through the wind.
Read as a sentence: t = 0 is head to wind on a tack and dead downwind on a gybe, found by searching the transition for the sample closest to it.
The stored midpoint is never relabelled as this instant. They are two different times about the same manoeuvre, and every figure that aligns curves says on its face which one it used.
3 What a loss is
Loss is metres of progress to windward given away through the manoeuvre. It compares what the boat actually made good against what it would have made good had it kept sailing the way it was sailing when it started the turn.
Read as a sentence: add up, second by second over the minute around the turn, how much less progress the boat was making than it had been making before it started.
The window is fifteen seconds before t = 0 and forty-five seconds after it. Fifteen seconds before, because a tack begins with the trim and the steering that precede it. Forty-five after, because a boat is not finished tacking when it is back on the new angle — it is finished when it is back at speed.
The reference speed is the median of the settled run the boat came from, not its last second. A single second before the helm goes down is as likely to be a puff as a trend.
A loss is not a penalty and it is not a mistake. Every tack costs something; the question this measures is how much, and whether it was more than usual.
4 Distance through the manoeuvre
Distance is measured from recorded positions over the same window: the length of the path the boat sailed over the ground between the first and last second of the manoeuvre. It is a different number from the loss and is reported beside it, never instead of it.
| Number | What it is | What moves it |
|---|---|---|
| Loss | Metres of windward progress given away | How slow the turn was, and how long the recovery took. |
| Distance | Metres of path sailed over the ground | How wide the turn was. A tack that carries too much speed through the head-to-wind travels further and can still lose less. |
Where positions were not recorded, the distance column is empty. It is not estimated from boat speed and heading: that would be a second measurement of a different thing wearing the same name.
5 The reference is your own best
A loss in metres means little until it is compared with something. The comparison is this boat’s own best manoeuvres in the same wind speed: the twentieth percentile of its recorded history of that manoeuvre kind, as a curve against true wind speed.
Read as a sentence: one tack in five of the ones this boat has already sailed in this breeze was at least this good.
A reference the boat has itself achieved is achievable by construction, which no model can promise. It also moves as the crew improves, which is the point: a target that stays where it was in April is a target nobody is still measuring against by August.
| Gate | Value | Why |
|---|---|---|
| Manoeuvres of this kind | 12 | A twentieth percentile of four tacks is one tack, chosen by rounding. |
| Wind speed bins with data | 3 | A curve against wind speed needs more than one wind speed to be a curve. |
Below either gate the target column and the reference curve are not drawn, and the report says so where they would have been. A target fitted to four tacks is a target fitted to one day of steering.
6 What this does not compute
The platform does not compute the tablet’s burn. It never has, and this page does not change that.
Race Control on the boat carries its own model of how much time and distance a manoeuvre costs, and uses it to answer the question a crew asks before a start: can we still make the line. That model lives on the tablet, is calibrated with tables loaded onto the tablet, and is the single source of those numbers. The platform stores those tables and serves them back unchanged; it computes nothing from them.
So the reference on this page is not the tablet’s figure and is not derived from it. The two answer different questions:
| Question | Answered by | From |
|---|---|---|
| What will this tack cost me, in the next ninety seconds? | Race Control, on the boat | Its own tables, before the manoeuvre. |
| What did that tack cost, and was it worse than usual? | The manoeuvres report | Recorded telemetry, afterwards. |
7 What can make a number wrong
Every quantity here rests on the wind instruments, and a manoeuvre is the hardest minute for them. The report states what it can see and what it cannot:
- The wind angle through the turn. Masthead instruments swing as the rig comes upright and over. The detector uses the sailed angle either side of the turn to classify the manoeuvre, not the readings from the middle of it.
- A missing second. Curves are drawn with gaps where a channel had no value. A gap is never bridged: a straight line through the seconds the reader came to look at would be an invention.
- Wind that changed. A tack in a shift is compared with the reference for the wind speed it was sailed in, not the wind direction. A large shift during a manoeuvre makes the loss look different, and no report can separate the two from one boat’s data.
- Current. Distance is measured over the ground. Progress to windward is computed from boat speed through the water and the sailed angle, so a manoeuvre in current has a loss and a distance that describe different frames.
8 Reading the report
The performance reports guide walks through opening a manoeuvres report, what each figure shows and what a missing figure says. The calibration these numbers pass through before anything is computed is described in the calibration lens, and the wider reduction of a sailing day to trends in performance trends.