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Go Back   SailNet Community > Skills and Seamanship > Learning to Sail
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  #11  
Old 09-09-2007
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Rockter will become famous soon enough
Down-wind the sail tell-tales tell of very little. Downwind, the sail is a big wind dam. On the leeward side of it, it will be a big turbulent eddy pattern.

Apparent wind can never be zero, unless the boat is stopped in no wind. You must have apparent wind or you won't have any drive.
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  #12  
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Rock...sorry to disagree with you....off course it can be zero, when the boat is moving at wind speed.

If the wind is 5 from 180 and the boat is doing 5 also apparent is zero..

I have had my boat at zero apparent wind.
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  #13  
Old 09-09-2007
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Apparent wind can be zero, as Giu has pointed out... however, you're not generating lift with the sails at that point generally—since you're usually sailing DDW when this happens.

To generate lift with the sails, you need to be going in a direction other than DDW, and the apparent wind can help the sails generate more speed through the lift caused by the apparent wind speed being higher than when you're going DDW. This works best on faster boat designs... which can make more distance made good by doing broad reaches, rather than sailing DDW, since they will actually sail at much higher speeds on a broad reach than they could going DDW—which can more than make up for the extra distance traveled.

Quote:
Originally Posted by Rockter View Post
Down-wind the sail tell-tales tell of very little. Downwind, the sail is a big wind dam. On the leeward side of it, it will be a big turbulent eddy pattern.

Apparent wind can never be zero, unless the boat is stopped in no wind. You must have apparent wind or you won't have any drive.
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You know what the first rule of sailing is? ...Love. You can learn all the math in the 'verse, but you take
a boat to the sea you don't love, she'll shake you off just as sure as the turning of the worlds. Love keeps
her going when she oughta fall down, tells you she's hurting 'fore she keens. Makes her a home.

—Cpt. Mal Reynolds, Serenity (edited)

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  #14  
Old 09-09-2007
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chris_gee is on a distinguished road
Don't think so. The boat may momentarily be at windspeed, but for a steady speed the force depends on there being a difference between the speed of the boat and the wind, both measured over the ground. If they are equal there is no force.
What the boat measures on the log is speed through the water. If there is a counter current of 2 knots, the boat speed over the ground may be 3 while through the water it is 5. The windspeed via anenometer is a measure relative to the boat. If the wind over the ground is 5, and the boat speed is 5 through the water but 3 over the ground, the wind on the anenometer will be 2, while the log reads 5. In fact the speed over ground is 5 for the wind and 3 for the boat.
If there is no current the anenometer may read 5 and the log 5, but because the anenometer reads relative to the boat that is an actual windspeed of 10.
If the wools are hanging down then either the relative wind is too light to lift them say 1 knot, or the anenometer is inaccurate, or there is a countercurrent. It is unlikely that a wool would not lift when the anenometer was showing 5.
It would be possible for the anenometer to show 5, and the log 5 only if the countercurrent was 5, assuming all gauges are accurate. This is for dead downwind.
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  #15  
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Chris—

There is no reason a boat can't be moving such that the apparent wind is zero.
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You know what the first rule of sailing is? ...Love. You can learn all the math in the 'verse, but you take
a boat to the sea you don't love, she'll shake you off just as sure as the turning of the worlds. Love keeps
her going when she oughta fall down, tells you she's hurting 'fore she keens. Makes her a home.

—Cpt. Mal Reynolds, Serenity (edited)

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  #16  
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Rockter will become famous soon enough
Not when you have been sailing, unless you have been in a 5 kt tide.

There is no way in this world, that sailing dead downwind, that any sailboat can have any drive if there is not an apparent wind.

Isaac Newton governs that one.

Downwind, the sails must SLOW the wind in order to get a drive. The wind impacts the sails with a velocity from behind, is slowed down, the slowing down of the wind (or rate of change of momentum as we like to call it) needs an action to do it, the action is provided by the sails, and the reaction is drive on the sails, hence on the boat.

If we could get drive with zero apparent wind, I would never be becalmed trying to sail downwind, and dammit I have been.

Leave tides out of the argument for now... tides are a form of propulsion.... and you will normally get an apparent wind with the motor on.
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Rockter will become famous soon enough
If I looked at your boat from above, and you are sailing downwind, and if I could see the wind, I would see that the wind ahead of your boat is moving slower than the wind behind it.
That slowing can only be done by the ship, and to slow the wind, the ship (and sails) must interact with the wind.

There can be no interaction if they are both moving at the same speed.
Zero apparent wind is not possible. You may have seen it on your gauge, but bring some feathers or wood dust with you next time and you will see that they are drawn ahead of the boat.

End of sermon.
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Its actually Bernoulli that governs it..not Newton. Bernoulli studied Pressure, flows and temperature behaviours of masses acros venturis and other sections.

Next time I go sailing in calm seas, I will do that and film it...might take some time, but its a great thing to do.

Last edited by Giulietta; 09-09-2007 at 08:23 PM.
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  #19  
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In fact take a look at this video, taken at same time as photo above.

This day we had no current, (because we need to wait for zero current to leave the place), we were doing around 4 to 5 knots boat speed over water not over ground or over wind.

The boat speed was measured at the ST60 tri data. I do remeber seeing the ST60 wind indicator (wish I had filmed it too), and apparent was 180 deg, at 0,3 or similar, almost zero.

In one frame of that video you can see the tell tale down hanging, they are made of silk, the lightest I found in the market, in the end, you can see the dinghy move and you can hear the noise of the water around.

Here is the Video of that wing on wing....note the tell tales in shrouds...we're doing wind speed.
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  #20  
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Umm... if you are sailing at five knots DDW, and the wind is at five knots... the apparent wind is going to be zero. Of course, this would only work if there were no friction...

However, the apparent wind at deck level may be zero, with the boat doing five knots DDW, and the wind speed a bit above five knots. The apparent wind at deck level isn't the same as the wind 20 feet above the deck level—which may be sufficient enough to move the boat at the speed of five knots.

Bernoulli's principle doesn't come into play unless the sails are generating lift. Running DDW, the sails aren't tgenerating lift, but acting as large wind breaks... and the force generated by the sails at that point is equal to the wind speed x surface area.

Rockter, Chris, you guys need to go back and study physics a bit more. BTW, wind speed is generally slower as you get lower in altitude due to the friction effects of the surface—applies to ground, water, or boat decks.
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You know what the first rule of sailing is? ...Love. You can learn all the math in the 'verse, but you take
a boat to the sea you don't love, she'll shake you off just as sure as the turning of the worlds. Love keeps
her going when she oughta fall down, tells you she's hurting 'fore she keens. Makes her a home.

—Cpt. Mal Reynolds, Serenity (edited)

If you're new to the Sailnet Forums... please read this
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