Sailing Velocity Made Good (VMG) Calculator

Velocity Made Good (VMG)

VMG is the component of boat speed projected onto the wind axis (or any other target axis). It tells you how fast you’re actually closing on a windward or leeward mark, accounting for the angle you’re sailing at.

The formula: VMG = Boat Speed × cos(Angle to wind)

For upwind: angle is the true wind angle (TWA). For downwind: same formula with TWA over 90°.

Key insight:

• Pointing higher (smaller angle) reduces speed but increases the cosine factor: usually a wash
• Bearing off (larger angle) increases speed but reduces projection
• The optimal angle depends on the boat’s polar curve

Typical optimal upwind angles by boat:

Reading polar diagrams: A polar diagram shows boat speed at every wind angle and wind speed combination. The VMG sweet spot is the angle where the polar curve’s tangent is perpendicular to the wind.

Example:

• Boat speed 6 kts at 40° TWA: VMG = 6 × cos(40°) = 4.6 kts to windward
• Boat speed 7 kts at 50° TWA: VMG = 7 × cos(50°) = 4.5 kts to windward

So pointing tighter wins by 0.1 kts even though the boat is a full knot slower. Note how close it is: a tenth of a knot over a two-mile beat is about twenty seconds. Point one degree too high and you lose more than you gained, which is why the sweet spot is worth finding on your own boat rather than copying somebody else’s number.

Lay line strategy:

• Sail toward the lay line on the lifted tack when possible
• Don’t overstand the mark: every meter past the lay line is wasted
• In oscillating wind, tack on headers to stay on the lifted tack

Headers and lifts:

• Header: wind shifts forward (closer to the bow), so tack to gain
• Lift: wind shifts aft (further from bow), stay on this tack and ride it

Apparent vs True wind:

• TWA (True Wind Angle): angle from wind direction: used for VMG calculation
• AWA (Apparent Wind Angle): wind angle felt by the moving boat: what trim is set to
• For close-hauled, AWA is typically 25-35° while TWA is 35-50°

Practical use:

1. Watch boat speed and angle on instruments
2. Calculate VMG mentally: speed × cos(TWA) for upwind
3. Adjust pointing to maximize VMG, not raw speed
4. In light air, bear off slightly for boat speed; in heavy air, point higher

Modern instruments (B&G, Garmin, Raymarine) calculate VMG automatically from GPS speed, wind angle, and wind speed. Old-school sailors keep mental models for backup.