Sailing start line bias calculator.
Is the line square to the wind? Enter the wind and the line, and see the bias in degrees, the favoured end and how far upwind it is — on a diagram.
Free to use in your browser. No account, no app.
Run it for the whole fleet — everyone’s phone shows the same countdown with VesselSOS Racing (pilot).
Run your next raceHow it is calculated
bias = W + 90° − L, normalised to ±180° W is the true wind direction (where the wind comes from); L is the line bearing from the port end to the starboard end, as seen looking upwind. A positive bias favours the starboard end, a negative bias the port end; under 3° we call the line square. The favoured end lies line length × sin(bias) further upwind. With two GPS points the bearing and length are great-circle values.
Questions
What is start line bias?
The angle between the start line and a line square (90°) to the wind. On a biased line one end is further upwind, and a boat starting there is ahead of a boat at the other end on the same tack.
Which end is favoured?
The end that is further upwind. If the wind veers (turns clockwise) on a square line, the starboard end (usually the committee boat) becomes favoured; if it backs, the port end (usually the pin).
How do I measure the line bearing?
Sight along the line with a hand-bearing compass, or record both ends with GPS. Use true or magnetic bearings for both wind and line — just do not mix them. With GPS points the calculator works in true degrees, so give the wind in true degrees too.
How much is the gain worth?
The favoured end is line length × sin(bias) further upwind: on a 300 m line with 10° of bias that is about 52 m. Shifts, current, traffic and the first beat matter as well — the number is geometry, not a tactic.
How long should a start line be?
A common rule of thumb is at least 1.5 times the total length of the boats starting (number of boats × boat length). Fill in both to get a warning when the line is shorter.
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