⛵Sailing Physics - Apparent Wind, Sail Lift, Polar Diagram & VMG, Hull Speed

How to use Sailing Physics: Apparent Wind, Sail Forces & Polars

1. 1Choose a boat and the true wind speed
2. 2In Apparent wind & sail forces, drag the boat on the canvas to change heading and watch the apparent wind, point of sail and forces
3. 3In Polar & beating upwind, read the polar and the best tacking angle, set the mark distance and number of tacks, and press Play
4. 4In Heeling & righting, change apparent wind and crew hiking to see the equilibrium heel
5. 5In Hull speed & faster than wind, enter a waterline length and compare speed limits for different drag angles

FAQ

How can a sailing boat sail against the wind?

A sail is an upright wing: the apparent wind flowing past it creates lift at right angles to the wind. As long as the bow is at a large enough angle to the wind, that lift has a forward component that drives the boat, while the large sideways component is cancelled by the keel acting as an underwater wing. So a boat cannot point straight into the wind, but it can sail at 40–45° to it and zigzag from tack to tack to get upwind.

What is apparent wind, and why does it move forward as the boat speeds up?

Apparent wind is the wind you feel on board: the true wind minus the boat’s own velocity. Once the boat moves it creates a headwind, and combined with the true wind this swings the apparent wind toward the bow and usually makes it stronger. The faster the boat on a reach, the further forward the apparent wind and the tighter the sails must be trimmed. Iceboats and foilers go so fast that even on a reach or a broad reach the apparent wind comes from almost dead ahead.

Why is a beam reach faster than running downwind?

Dead downwind the sail is just a barrier pushed by drag, and the faster the boat goes the weaker the wind across the sail, so the boat must stay slower than the wind. On a reach the sail works at a good angle of attack and its lift points almost straight ahead; lift coefficients are much larger than drag coefficients, so the drive is greatest, and the apparent wind grows with boat speed instead of falling. The fastest heading on a polar is usually at 90–120° true wind angle.

How is the best tacking angle found?

Sailing closer to the wind makes the boat slower; bearing away makes it faster but gains less ground to windward. What decides the time to a windward mark is VMG = boat speed × cos(TWA). The tool evaluates VMG at every angle on the polar and takes the maximum, typically 40–45° for keelboats, so the two tacks are about 85–90° apart. The table also lists the time to the mark at different angles.

What does hull speed 1.34√LWL mean, and can it be exceeded?

A displacement hull makes transverse waves whose speed depends on their wavelength. When the bow wave’s wavelength equals the waterline length (about 1.34√LWL knots with LWL in feet, a Froude number near 0.4), the boat sits in its own wave trough and wave resistance rises steeply if it tries to go faster: that is hull speed. Light, flat dinghies and multihulls can plane on hydrodynamic lift, and iceboats and foilers escape most water drag altogether, which lets them sail faster than the wind.