5. Technical Background
The plugin is capable to calculate the magnetic Variation for you current position. It is using World Magnetic Model (WMM). The WMM2020 Coefficient file (wmm2020.cof) valid for 2020.0 - 2025.0 is part of the software package. The calculation is done with the github geomag-library provided by Christopher Weiss [email protected].
You have to provide polar data for your boat in avnav/user/viewer/polar.json for calculating the laylines. If there is no such file, the plugin will copy one to this location, and you can use it as a template for your own polar data.
If you do not have any polar data, you can enter tack and gybe angle in the plugin configuration and use these fixed values instead.
A source for polar data can be ORC sailboat data or Seapilot.com.
Leeway is estimated from heel and STW as
LEE = LEF * HEL / STW^2
With LEF being a boat specific factor from within (0,20). Heel could be a measured value (either from signalk.navigation.attitude.roll or a specific transducer in gps.transducers.ROLL). If data is not available it is interpolated from the heel polar in avnav/user/viewer/heel.json. As the avnav/user/viewer/polar.json it contains an interpolation table to map TWA/TWS to heel angle HEL.
To understand the technical background of the laylines one has first to have an understanding of the terms VMG and VMC.
-
VMG - Velocity Made Good against
wind is defined as
VMG = boatspeed * cos(TWA)boatspeed vector projected onto true wind direction -
VMC - Velocity Made good on
Course is defined as
VMC = boatspeed * cos(BRG-HDG)boatspeed vector projected onto direction to waypoint
Unfortunately there is a lot of confusion on these two terms and also most of the commercial products are mixing the two items and indicate VMG but actually showing VMC (and so does AvNav).
The laylines are computed from the beat_angle and run_angle vectors in the polar file, which contain a mapping of TWS to TWA for maximum VMG. As a result the laylines show the optimal TWA to travel upwind in general, but not the optimal TWA to get towards the waypoint. Optionally it is possible to calculate the laylines from the STW matrix.
From the STW matrix in the polar data, which is a mapping of TWS and TWA to STW, one can calculate the optimal TWA such that VMC is maximised,