Sailboat Polar Calculator
Estimate sailboat speed at a given true wind angle by entering polar performance data. The calculator interpolates between your data points and calculates VMG to windward or downwind.
Polar Calculator Results
VMG = Boat Speed × |cos(TWA)|
The boat speed at the requested angle is interpolated between the nearest entered polar data points.
What is a sailboat polar?
A sailboat polar is a representation of the boat's expected speed at different true wind speeds and true wind angles. It shows how fast a boat can sail in different conditions and is commonly used for route planning, performance analysis, and racing.
The polar normally has true wind angle on one axis and boat speed on the other. A separate polar curve is usually provided for each true wind speed.
How does this sailboat polar calculator work?
Enter several points from your boat's polar table. For example, you might enter boat speeds at 30°, 45°, 60°, 90°, 120°, 150° and 180° true wind angles.
The calculator finds the two polar points surrounding your requested true wind angle and performs linear interpolation between them.
$$V = V_1 + \frac{(TWA - TWA_1)} {(TWA_2 - TWA_1)} (V_2 - V_1)$$
- V: Estimated boat speed at the requested angle.
- V₁: Boat speed at the lower polar angle.
- V₂: Boat speed at the higher polar angle.
- TWA: Requested true wind angle.
- TWA₁ and TWA₂: The surrounding polar angles.
What is VMG?
Velocity Made Good, or VMG, describes the component of boat speed that moves the boat directly toward or away from the wind. It is particularly useful for determining efficient upwind and downwind sailing angles.
$$VMG = V_{boat} \times |\cos(TWA)|$$
When sailing close to the wind, a boat may travel faster by bearing away slightly, even though its heading is less direct. The best VMG angle is therefore not necessarily the angle at which the boat points closest to the wind.
What true wind angles are used in a polar?
Typical polar data covers approximately 0° to 180°. Angles near 0° represent sailing directly into the wind, while 90° represents a beam reach and 180° represents sailing directly downwind.
Why does wind speed matter?
A sailboat's performance changes substantially with true wind speed. A boat might reach 7 knots in 10 knots of wind but considerably more in 20 knots of wind. For accurate routing, polar data is therefore normally stored as several curves, one for each true wind speed.
What is the best sailing angle?
The best angle depends on the objective. The optimum VMG angle is useful when trying to move directly toward or away from the wind. The fastest boat speed may occur at a different angle, particularly on a reach. Sailors and routing software often use the polar to choose the appropriate compromise between boat speed and direction.
How accurate is a polar calculator?
The calculator is only as accurate as the polar data supplied. Real boat speed can differ because of sea state, sail trim, hull condition, crew weight, current, waves, wind shifts, and other factors. Interpolation between polar points also provides an estimate rather than a measured value.