A Sailchart answers one question, for every pair of true wind angle (TWA) and true wind speed (TWS): which sail are we carrying? It is the digital form of the table a sailmaker hands over with a set of sails, and which racing software also calls a crossover.
What it changes, and what it does not
The key point: a Sailchart never changes your boat's speed. That speed comes from your polar, and from nothing else. The Sailchart adds identity: it names the sail carried on each leg.
Once a Sailchart is in place:
- your routing legs can be coloured by sail;
- the routing table names the sail for each step;
- the statistics tell you how long and how far you sail under each one;
- the router can charge a sail-change penalty, just as it charges a tack or a gybe;
- under way, an instrument tells you which sail to carry now.
Without a Sailchart everything keeps working exactly as before: your routing is identical, simply without sail names.
How it is built
The Sailchart is a grid: rows are wind angles, columns are wind speeds, and each cell holds a sail name.
A cell stands for a band, not a point: the cell placed at 12 knots holds from 12 knots up to the next cell. You therefore do not need one cell per knot — a few well-placed bands are enough, exactly as on a sailmaker's paper table.
Overlaps
In real life two sails are often equally good across a transition band. A cell can therefore carry a primary sail and up to two alternates, written J2|C0.
This matters for routing: if the boat already carries one of the sails the cell allows, it keeps it and pays no change penalty. That is what stops a routing sending you up the mast to cross a two-knot band.
A Sailchart belongs to one polar and follows it everywhere, including when syncing between your devices.
To create one: How do I create my boat's Sailchart?