The wind perceived on a boat is affected by three separate factors. The first is the true wind experienced at that place, irrespective of boat movement. The second is the wind generated by the passage of the boat through the air, which is in an opposite direction to the boat's course and at the same speed. These two are combined to form a resultant which is usually known as the apparent wind. However, when the motion of the boat over the sea is modified by a tidal current, the apparent wind perceived aboard will be the resultant of waterflow, boat motion through the water and true wind. These three factors contributing to the apparent wind are usually represented by the vector system. Let's see first how the vector system works when there is no water movement.

Apparent wind with no water flow
If you are sailing in still water (i.e. with no water flow at all) you will perceive an apparent wind, which is the resultant of the motion wind (blowing in the opposite direction to your course and at the same speed as the boat) and the true wind.
This can be illustrated by drawing arrows to represent the wind on a vector diagram. Each arrow should be in the same direction as the wind it represents, and the length of each arrow shows the speed of the wind . These arrows can be considered as two sides of a triangle: draw in the third side and you have the resultant of the two winds - the apparent wind. You can now see that when a boat is beating the apparent wind comes forward of the true wind and increases in velocity.
To sum up: when a boat is beating, an increase in boat speed results in the wind coming forward (heading) and a decrease in boat speed results in the wind coming from further aft (freeing). An increase in boat speed results in an increase in apparent wind, and a decrease in boat speed results in the wind reducing.
If a boat is beating up to the windward mark the heading and freeing effects, as well as the change in velocities mentioned above, will affect the course the boat steers and the speed at which it travels. A, boat stopped in the water will only experience the true wind. At the other extreme a motor boat going at 10 knots in a flat calm with no tidal stream will experience a 10-knot wind directly on the bow caused by its own motion.

The effect of waterflow on boats beating
It should be stated clearly here that tide has two effects on boats racing. The first effect is on the apparent wind strength and direction. The second effect is the movement of the boat down tide, caused by the velocity of the water flowing over the sea bed. We are only concerned with the first effect for the moment - the effect on the apparent (perceived) wind.
A tidal stream has two effects on apparent wind. It changes the direction of the apparent wind, and it changes the velocity of the apparent wind.

Wind strength
- If the tidal stream is opposing the apparent wind, the apparent wind will increase in velocity .
- If the tidal stream is going with the apparent wind, the apparent wind will reduce in velocity .
- If the tidal stream is at right angles to the apparent wind, the velocity of the apparent wind will not be much affected .

Wind direction
- If the tidal stream sets towards the left of the apparent wind, boats on the starboard tac will be headed and boats on the port tack will
- If the tidal stream sets towards the right of the apparent wind, boats on the starboard tack will be freed and boats on the port tack will be headed .

The effect of waterflow on boats running and reaching
- When the tide is with the wind, the, direction of the apparent wind remains the same but its strength is lessened.
- When the tide is against the wind, the wind's direction remains the same but its strength is increased.
- When the tide is in the same direction as the boat's progress, the apparent wind comes ahead.
- When the tide is against the boat's progress, the apparent wind comes astern.

Estract taken from Tides and Currents (Sail to win) by David Arnold.
Published by Fernhurst Books
C Fernhurst Books
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