Explain what it is and what it means in windsurfing terms.
[5 marks]

It's the wind just as you launch that was apparently there rigging up but is now all but gone.

^^^^yep

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sausage said...
It's the wind just as you launch that was apparently there rigging up but is now all but gone.

That's 10 points!

Shane, you can't ask questions that are too sensible

en.wikipedia.org/wiki/Apparent_wind

while Sausage has clearly taken top podium position...

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FlickySpinny said..

en.wikipedia.org/wiki/Apparent_wind

This article is what made me post in the first place. So apparent wind is the true wind speed minus the board speed (assuming you were traveling perfectly downwind, which you never do)? Is apparent wind just what the observer 'feels' the wind to be, proportional to how fast they're traveling in the direction of the wind? Is the sail not always responding to true wind speed, not apparent speed? So many questions...

It is simply where the wind appears to be coming from.
So combination of board speed/direction and wind speed/direction.

EG if sailing on a reach and wind drops to nil, for the brief moment you remain planing your apparent wind is totally head on
EG2 - 30kn wind and you are sailing on a reach then suddenly stop dead, the apparent wind moves from say 45 deg to 90deg as you have zero board speed.

rather simple I thought...?

I think the easy answer is the it's the wind you feel on your face when moving faster than actual wind speed. But there have been a few hazy nights/years since sailing school so could be completely wrong and need correcting .

^^^ you don't need to be moving faster than the wind. On a reach as soon as you move forwards at 5kn or 1kn, the wind that was coming from behind you swings to the front (apparently). Faster you go, the more it appears to be coming from the front.

That's all you need to know.

Sorry. I thought that Wikipedia article was pretty clear.

Scenario 1:
I am stood stationary on the beach.
The wind is blowing 20 knots from the north.
The true wind speed is 20 knots.
The apparent wind is 20 knots from the north.

Scenario 2:
I am travelling at 20 knots EAST (i.e. at 90 degrees to the wind).
The wind is blowing 20 knots from the north.
The true wind speed is 20 knots.
The apparent wind is 28 knots from the NORTH EAST.

This means that the wind APPEARS to be blowing from 45 degrees to my direction of travel, instead of 90 degrees to my direction of travel.

This means that as you go faster and faster, you have to bear off more and more from the wind in order to keep the wind in your sail...

Wanna play further?

http://www.sailingcourse.com/keelboat/true_wind_calculator.htm

I'm not entirely happy with the word "appears". I know that's what it's called, but relative to the moving object, it's very real.
It is the actual speed and direction of the wind relative to the moving object, and if your using wind power, it's what you have to play with, not the wind relative to something stationery.

^^ I agree "appears" is not ideal - but it is easier than using vectors

I gave up when they started to ad the alphabet together.

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stone said..

I gave up when they started to ad the alphabet together.

Ignore that stuff unless you love the techy bits - they're not necessary for understanding the concept.

To make it easier to visualise.

You're stood in the back of a stationary ute.
The wind is blowing 20 km/h from the north.
You experience the wind as being 20 km/h from the north.

The ute drives east at 20 km/h.
The wind is still blowing 20 km/h from the north.
You experience the wind as being 28 km/h from the north-east.

Why 28 km/h? Because of maths with the alphabet.

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FlickySpinny said...

stone said..

I gave up when they started to ad the alphabet together.

Ignore that stuff unless you love the techy bits - they're not necessary for understanding the concept.

To make it easier to visualise.

You're stood in the back of a stationary ute.
The wind is blowing 20 km/h from the north.
You experience the wind as being 20 km/h from the north.

The ute drives east at 20 km/h.
The wind is still blowing 20 km/h from the north.
You experience the wind as being 28 km/h from the north-east.

Why 28 km/h? Because of maths with the alphabet.

Haha. I got it Flicky.

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sausage said..

It's the wind just as you launch that was apparently there rigging up but is now all but gone.

Ok, I'm glad it's not just me and my inexperience that finds this happening a lot. Find yourself often coming back to shore to switch sails due to changed wind conditions too, only to have the wind change again by the time you're back on the water? I should just have multiple of everything to save the time de-rigging and rigging when I want to switch!

Last year was my final year of school and for one of my subjects I did a study of the physics of lift in relation to planes and sailing. Thought I might as well share the page about thrust and apparent wind seeing as it is relevant.

^It's been a long time since I did vectors and physics at school...in fact a long long long time ago but if I'm travelling north at 100km an hour with a 20km tail wind (southerly) isn't the apparent wind 80km from the north, not 120km as per the above school text book????? Don't see many planes taking off with a tail wind!

Maybe they mean the wind is coming directly from the direction you're travelling in - not well worded

Oh wow, glad the assessors didn't notice that; you are indeed correct. In the brackets it should be "(assuming you are travelling against the wind)" or something to that effect. And I agree, the wording is a bit awkward. Cheers :)

I find the vector picture the easiest to understand. Note that if your speed and the wind speed are drawn in proportion, you get the exact apparent wind by measuring that side. Try with different wind angles, board speed etc, you will find why it's so important to bear off a lot to increase your speed

Makes sense. You are travelling perpendicular to the wind. Wind is 20kts. You're travelling at 20kts. Apparent wind is 28kts. You should go exponentially faster though right? Guess not, because its only "apparent" wind (well 8kts of it), not real wind.

Now I'm confused.

It is real - it is the collision speed of the air molecules you are hitting.

(Leaving the term "exponentially" out of this...)

As such, the faster you go, the faster you can go (to a certain extent limited by a massive heap of other physics like drag, and the fact that you constantly need to bear away as you get faster which has limits).

Sounds crazy right?


This is why the Sandy Point speed run works so well.


You start off running at approx 90 degrees to the wind. As you get faster the apparent wind moves forward and increases. This means that you can go faster, but not pointing the same direction - you need to bear away.

If you look at the end of the speed run on Google Maps you're no longer travelling south-east... instead you're travelling east but still sheeted right in.

Farting while sailing.

Out of interest the Americas Cup 72'ers were so efficient dead down wind that they effectively created a head wind ......apparently

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Sputnik11 said..
Makes sense. You are travelling perpendicular to the wind. Wind is 20kts. You're travelling at 20kts. Apparent wind is 28kts. You should go exponentially faster though right? Guess not, because its only "apparent" wind (well 8kts of it), not real wind.

Now I'm confused.

No because drag increases exponentially.

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sausage said..

Out of interest the Americas Cup 72'ers were so efficient dead down wind that they effectively created a head wind ......apparently

Not quite.

You can't sail dead down wind and expect to move faster than the wind.

AC 72's, Windsurfers, Sports yachts (etc) operate on the principle of sailing at an angle down wind (or broad reach) to the wind in order to plane or foil and achieve maximum speed and VMG.

To sail at maximum VMG (velocity made good) requires you to zig-zag down wind to reach the bottom mark.

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FlickySpinny said..

It is real

As such, the faster you go, the faster you can go (to a certain extent limited by a massive heap of other physics like drag, and the fact that you constantly need to bear away as you get faster which has limits).

There are 2 limiting factors:
- firstly drag from the board and fin and
- secondly, the faster you go, the more the apparent wind rotates round to head you, until eventually you appear to be pointing into the wind, Hence you are limited by how high the sail can point into the wind

Normally drag kicks in first, but in the case of land and ice boats the drag is so small, then the angle to windward is the limiting factor.

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sausage said..

Out of interest the Americas Cup 72'ers were so efficient dead down wind that they effectively created a head wind ......apparently

No they don't go "dead down wind" they do downwind tacking. But the downwind VMG is faster than the wind.

The only device that can go "dead down wind" faster than the wind is this thing


But the blades of the propellors don't go dead downwind, they go downwind in a spiral at , I'm guessing, about the same angle as the sail on an AC 72. It's no big deal if you think about it long enough. It's the same fundamental thing. Why the folks on the video had to build such a large machine to prove the obvious I don't know? If you stuck a water skier on a long rope behind a quick downwind tacking AC 72 he could probably swing left and right off the towline and cancel out the zig zag of the AC 72. He'd then be negotiating a course dead downwind faster than the wind. Can we consider the skier to be sailing? Of course he is.

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shadow said...

sausage said..

Out of interest the Americas Cup 72'ers were so efficient dead down wind that they effectively created a head wind ......apparently

Not quite.

You can't sail dead down wind and expect to move faster than the wind.

AC 72's, Windsurfers, Sports yachts (etc) operate on the principle of sailing at an angle down wind (or broad reach) to the wind in order to plane or foil and achieve maximum speed and VMG.

To sail at maximum VMG (velocity made good) requires you to zig-zag down wind to reach the bottom mark.

Looks like I could get myself a job writing school text books . Yes there's no excuse for my tardy explanation even though I watched every single AC race and understood the dynamics of sailing the downwind leg.

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Trousers said..
Explain what it is and what it means in windsurfing terms.[5 marks]

I drew myself a rough table at 135 degrees off the wind and it seems to show that if I was capable of getting to 40 knots
in 40 knots ambient wind then apparent wind should be about 30 knots at 68 degrees sheeting angle
in 30 knots ambient wind then apparent wind should be about 28 knots at 48 degrees sheeting angle and
in 20 knots ambient wind then apparent wind should be about 30 knots at 29 degrees sheeting angle.