Timeline measures (STW, SOG, TWS, TWA, VMG, polar %…)
Meaning of each curve of the Avelsta timeline: speed through water and over ground, true wind, wind angle and direction, VMG, polar percentage, heel, pitch and current.
The essentials
The timeline shows one row per measure, all on the same time axis. By default, the Essentials preset shows the boat speed, the wind speed and the wind angle.
- Click All to show every measure available in your logs.
- Open Measures to tick or untick each row.
- Hover over a row: its value is shown, and the Live tab gives all the values at that moment.
Going further
The measures
| Measure | Abbreviation | Definition |
|---|---|---|
| Speed through water | STW | Boat speed relative to the water, measured by the log (paddlewheel). The performance reference. |
| Speed over ground | SOG | Speed relative to the seabed, measured by the GPS. Includes the effect of the current. |
| True wind speed | TWS | True wind in knots, computed by the instruments from the apparent wind and the boat speed. |
| True wind angle | TWA | Angle between the boat’s axis and the true wind. Green = starboard, red = port. |
| True wind direction | TWD | Where the wind comes from, in degrees true (NMEA MWD sentence). |
| VMG | VMG | Speed projected on the true wind axis: positive upwind, negative downwind. Computed by Avelsta. |
| Polar percentage | polar %, POL | Boat speed divided by the polar’s target speed at the same TWA and TWS. 100% = target reached. |
| Heel | HEEL (ROLL) | Sideways tilt in degrees (attitude sensor, XDR ROLL). |
| Pitch, trim | PTCH | Fore-and-aft tilt in degrees (XDR PTCH). |
| Current speed | CUR | In knots, computed by the instruments (NMEA CUR sentence). |
| Current direction | CDIR | Direction the current flows towards, in degrees true. |
A measure missing from your logs does not appear in the list.
Target speed (dashes)
On the boat speed row, the dashes give the target speed of the compared polar(s), interpolated at the TWA and TWS of each moment. The gap between the solid curve and the dashes reads directly as a percentage on the “polar %” row. The timeline’s Polars menu chooses the polars compared.
STW or SOG
The session’s “⋯” menu chooses the reference speed. It is used by the boat speed row, the VMG and the polar percentage. Prefer STW: with SOG, a favourable current artificially inflates the performance, a foul current lowers it.
Signed angles
Avelsta keeps the sign of the TWA: positive to starboard, negative to port. The polar, however, is symmetrical: the comparison uses the absolute value of the angle.
Frequently asked questions
What is the difference between TWA and AWA?
AWA (apparent wind angle) is measured by the wind vane at the masthead; it combines the true wind and the wind created by the boat’s speed. TWA (true wind angle) is derived from it by the instruments. Performance and polars are read in TWA and TWS.
Are the box VMG and the Avelsta VMG the same?
Avelsta recomputes the VMG from the chosen reference speed (STW or SOG) and the TWA. The VMG recorded by some NKE boxes is kept in the data but is not the one shown.