NorthStar Navigation Pilot System NS3300 User Manual

Page 37

Reference Manual

7-3-2 True and apparent wind speed and direction

Apparent wind speed and direction are the values measured on the boat. True wind speed and direction are
the values after allowing for boat speed through the air.
If the boat is moving, then the apparent wind speed is different to the true wind speed and the apparent wind
direction is different to the true wind direction, as shown below.

Boat moving upwind
Apparent wind speed is greater than true wind speed
and apparent wind direction is closer to dead ahead than true wind direction

Boat is at rest

True wind speed is the same as apparent wind speed and
true wind direction is the same as apparent wind direction

True
wind
speed 20
kts

Boat moving downwind
Apparent wind speed is less than true wind speed and apparent wind
direction is closer to dead ahead than true wind direction

True
wind
speed
20 kts

Boat
speed
10 kts

True
wind
speed
20 kts

Boat
speed
10 kts

Apparent wind
speed 15 kts

Boat
speed
10 kts

Apparent wind
speed 15 kts

True wind
direction
45º

True wind
direction
135º

Apparent
wind
direction
107º

Apparent
wind
direction
30º

Apparent
wind speed
28 kts

Boat at rest