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Apparent wind

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Wave Rider View Drop Down
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    Posted: 15 Mar 05 at 6:02pm

Hi,

I realise in a Topper and Fireball i don't really have to worry about apprent wind but whenever i ask anyone what Apparent wind is i always get a very vague answer involving wind created when travelling at high speeds.

 

So my awful understanding of it is that when you travel at high speeds you create wind which you can sail on and then i think something happens about the centre of effort moving further back in the sail therefore you can move further foward???

 

Please can i have a definition which makes sense

Thanks

           -[Franko]-
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Brian View Drop Down
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Post Options Post Options   Quote Brian Quote  Post ReplyReply Direct Link To This Post Posted: 15 Mar 05 at 6:04pm
yeah thats what iv always understood aswell, its like when ur running u
can feel "wind" just coz ur moving. ithink
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Brian View Drop Down
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Post Options Post Options   Quote Brian Quote  Post ReplyReply Direct Link To This Post Posted: 15 Mar 05 at 6:06pm
http://www.sailnet.com/collections/learningtosail/theory/ind ex.cfm

click>understanding apparent wind

Edited by Brian
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Jon Emmett View Drop Down
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Post Options Post Options   Quote Jon Emmett Quote  Post ReplyReply Direct Link To This Post Posted: 15 Mar 05 at 6:41pm
I actually wrote a piece for Y&Y on apparant wind, hopefully it will come out in the near future...
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ssailor View Drop Down
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Post Options Post Options   Quote ssailor Quote  Post ReplyReply Direct Link To This Post Posted: 15 Mar 05 at 7:20pm
A good way of thinking of it is: If you were to put a burgee on a car, driving along it would point forward due to the 'apparent wind' or wind being created by  YOU moving through the air not the air moving past you. On a fast boat when heading downind as you pick up speed the wind moves further forward allowing you to bear away and thus you travel faster as the kite works more efficiently and at a deeper angle due to the apparent wind. - if you were to sail the same angle without gaining speed by heading up initially then the boat would simply wallow along not even planing!
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redback View Drop Down
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Post Options Post Options   Quote redback Quote  Post ReplyReply Direct Link To This Post Posted: 15 Mar 05 at 11:05pm

In a Laser, or similar speed of boat, the affects of apparent wind is limited to the way the wind appears to come from more ahead due to your movement through the air.  So when you are beating at about 45 deg to the wind you need to trim your sail as if the wind was coming from about 30 deg ahead.

"Apparent Wind Sailing" occurs in boats which are fast enough downwind to make the wind appear to come from some angle in front.  In that situation the wind gets stronger the faster you go and so you can find yourself sailng maybe only 30 deg off of dead downwind but the wind appearing to come from some angle in front.  Boats with a PY of somewhere around 950 or lower encounter this quite a bit and it enables them to get downwind faster by zig zaging downwind rather than running dead.  It can be provoked by luffing a bit after hoisting, building up speed, and bearing away as the pressure increases.  It turns the runs into the sort of challenge of the beat where you have to choose the right compromise between speed and pointing closer to the mark.

My Laser4000 is often sailed best downwind with the main sheeted at much the same angle as when I'm sailing upwind.  In a medium breeze, if I didn't luff a bit after hoisting I would go downwind at about the same speed and angle as a Laser2000.  If I luff I accelerate and the apparent wind pressure is then enough to require me to get the crew on the wire.  The pressure is such that I have to bear away to keep the boat upright and I'm then sailing the same sort of angle as the Laser2000 - but at about twice the speed. Apparent Wind Sailing.

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Stefan Lloyd View Drop Down
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Post Options Post Options   Quote Stefan Lloyd Quote  Post ReplyReply Direct Link To This Post Posted: 16 Mar 05 at 7:05am

Apparent wind is the breeze you trim your sails to. It is a combination of the wind blowing over the water and the wind that arises because the boat is moving forwards. Therefore it is always further towards the bow than the breeze blowing over the water.

It certainly can be important in a boat like a Fireball. Here is an example. You are two sail reaching, look up at the burgee and think from the angle that you'd just hold the kite. Should you set? Well possibly not. With a kite up, you will go faster (good). But the increased speed will bring the apparent wind forward. So maybe you won't hold the kite after all.

Another example, true for any boat, is ghosting conditions. There is almost no true wind. If you can get some motion on the boat, the apparent wind will be much stronger. Your sails only know about apparent wind and will develop thrust to keep you moving. That is why in ghosting conditions, the key is to keep the boat moving, almost regardless of the direction you are going in.

 

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