Bump's World

APPARENT WIND

There are 2 types of wind. True wind and apparent wind. True wind is the actual speed and direction of wind over the land or sea.

Apparent wind is the speed and direction of the wind over the vehicle that is moving over the land or sea.

This can be a car, airplane, boat or even a person moving ie walking or running. Anything that is moving. The speed and direction the object is moving will affect the apparent wind.

In the case of a sailboat the only wind that matters is the apparent wind. The true wind will only give you an idea of what the apparent wind is going to be. The flow of the wind over the sails is what moves the boat forward, therefore the speed and the direction is what you need to know to make the boat perform the best it can under current conditions.

If you are driving north on route 95 at 60 miles per hour and the true wind is 15 miles per hour out of the northeast which way does your hair blow when you stick your head out the window?

Your hair blows straight back. The movement of the car creates an apparent wind, 60 miles per hour which way over whelms the true wind speed and direction. When you are on your mooring about to go sailing, your wind instruments will show the true wind and apparent to be the same since you are not moving.

Lets say the wind is 10 knots. Out of the south 180 degrees magnetic.

At 10 knots your boat goes up wind at 6 knots. Since you cannot sail directly into the wind, usually about 40 degrees off directly into the true wind which is 180 degrees. This means you can sail at 140 degrees on starboard tack and 220 degrees on port tack. Since you can not sail directly into the wind the apparent angle will be reduced. You are sailing at 140 degrees at 6 knots of boat speed and the true wind is 10 the apparent wind, actual flow of wind over the sails, is 14.5 knots due to the boat moving thru the water, into the wind. The sails see 14.5 knots of wind over the sails or about a 35% increase in true wind. It feels like a pretty nice breeze. Everyone is on rail and sails are trimmed for 14.5 knots of breeze. The apparent direction has also changed. Because you are moving with a true wind angle of 40 the apparent angle, which again is what the sails see is pulled forward to about 33 degrees. So the boat thinks the wind is blowing 14.5 and the angle of the wind to the boat is 33 degrees, when in fact the true wind is 10 knots and the true angle is 40 degrees.

As we approach the windward mark, I look around and say, 'looks like the wind is going to die down'. Everyone looks at the clouds and the sea and there is no sign the wind is going to die down.

We go around the windward mark and put up a huge spinnaker. We are now sailing at 25 degrees on the compass, opposite direction. We need to go 0 degrees to get to the next mark but that reduces the flow over the sail so much that we go really slow. At 25 degrees magnetic we have a flow over the sails. We are on starboard tack and the wind is still at 10 knots true. We are now going away from the 180 true wind direction so the apparent wind is reduced not increased. There is less flow over the sails which is why we can put up huge sails. The boat speed is probably still about 6 but the apparent wind speed has dropped to 6.5. The 14.5 apparent we had up wind is now 6.5 downwind. Huge difference about 70% less. The crew is amazed that I have predicted the reduction in wind speed. It's a reduction in apparent wind not true wind. If we go upwind again the crew will be amazed at how the wind picked up again.

Downwind the true wind angle will be 155 degrees but the apparent will be pulled forward by our speed to about 140 degrees.

Now we have a reaching leg. The true wind direction is still 180 degrees (south) and wind speed is still 10 knots.

The course is now 90 degrees which is also 90 degrees to the true wind angle of 180. The apparent wind angle will be pulled forward due to the speed of the boat. True angle 90 apparent angle 75.

Apparent is forward of the beam of the boat so the apparent wind speed will go up. The boat thinks its going into the wind when in fact its going side ways to the wind. But again what counts it what the boat thinks not what is true.

With an increase in apparent wind, going from 10 to 13, the boat speed increases to 7. Again at 25% increase in apparent speed.

On mono hull displacement sailboats the speed is reduced by the drag of the hull. Eliminate the drag (like an iceboat) and the apparent goes way up. 10 knots of wind could result in 40 knots of boat speed and the apparent angle is way forward due to the speed of the ice boat. Remember the car example. Ice boats go so fast the sails are trimmed in up and down wind due to high apparent wind speeds causing tight apparent wind angles. As the ice boat goes faster the apparent wind speed keeps going up and the ice boat keeps going faster.

The good part of sailing is the wind constantly changes wind speed and angles so you need to constantly recalculate your sail trim etc. Just when you think you got it right, you don't.