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    Posted: 13 Oct 11 at 10:31pm

Above I have drawn a very basic diagram.  Imagine there is a NE wind. 

The thin black, green and red lines are the starting points of three boats. They are at the same place, but on different headings.  To make it simple assume they are all going the same speed (not velocity) and there is no tide.  After 10 minutes (and 1000m) they end up in the places with the thick outlines. Not suprisingly, the red boat has sailed to windward of the others. 
 
Now suppose there is a tide.  The boats would end up after 10 minutes at the locations with the dashed lines, because all the boats are pushed back equally by the tide. 

So, after the same number of minutes, the red boat is the same amount of metres to windward.  But all the boats have moved forward 800m rather than 1000m due to the tide.  After 1000m (but, say, 12 minutes) the red boat will be further to windward than if there was no tide. 

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Posts: 641 		Post Options Post Options   Quote Presuming Ed Quote  Post ReplyReply Direct Link To This Post Posted: 13 Oct 11 at 10:42pm
But in this magic boat of yours - which can point like a badger, and lose no speed at all, why would you sail at anything but the top of the groove? ITYF that with most boat, if you foot you go faster.

Edited by Presuming Ed - 13 Oct 11 at 10:44pm
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Posts: 4028 		Post Options Post Options   Quote G.R.F. Quote  Post ReplyReply Direct Link To This Post Posted: 13 Oct 11 at 10:47pm
Originally posted by sargesail

 

GRF's post almost convinced me - but then I stepped back and remembered that the lower fluid in which those forces are working is not the tide but water.  If the water is moving east at three knots and the boat has a force applied to it by the tide which would move it east at 3 knots in the absence of other forces moving it in different directions then there is no angular force on the hull and foils.



You're not thinking about the 2nd fluid, which is air, even still air. Try to move a foil sideways and the fluid dynamics will propel it forward. When the Fluid A) The tidal water, moves your craft, Fluid B)the (even dead still )Air propels your sail forward.

So even in totally stationary air, so there is no 'upwind', the action of a sideways force of the tide against one foil, will move it at a vector of the two angles, so effectively at 90 degrees to the direction of the tidal current if the foil in fluid a) is presented to the flow at 45 degrees, doh this is hopeless trying to explain, it needs a diagram or even better a test tank.
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Posts: 1459 		Post Options Post Options   Quote sargesail Quote  Post ReplyReply Direct Link To This Post Posted: 14 Oct 11 at 7:52am
Yes - but those forces act as a totality not with changes of direction when the angle of the boat to them changes.
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Posts: 4028 		Post Options Post Options   Quote G.R.F. Quote  Post ReplyReply Direct Link To This Post Posted: 14 Oct 11 at 8:37am
Originally posted by sargesail

Yes - but those forces act as a totality not with changes of direction when the angle of the boat to them changes.
Absolutely correct, right up until the moment the 'other' foil, the sail, gets orientated against the direction of the tidal flow, even in totally still air.

I totally understand where your coming from, and it is true if a boat moves with the flow of the tide it matters not which direction it points in, it just moves along.

But once you engage the sail, you introduce another force, then if you orient the foil under th boat in opposition to that second force, you get a resultant movement of the combined forces.


There...


Edited by G.R.F. - 14 Oct 11 at 8:40am
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Posts: 1151 		Post Options Post Options   Quote 2547 Quote  Post ReplyReply Direct Link To This Post Posted: 14 Oct 11 at 10:08am
Originally posted by JimC

Originally posted by fab100

doing 1.01 knots thru the water, against a 1 knot, on the nose, adverse tide.

Ah yes, you say, but I can point 2 degrees higher than the other chap and not go any slower. In which case I say congratulations, well sailed, but that means you'd beat him just as thoroughly if there were no tide at all...


... and there JimC has made the critical point to debunk the fallacy ... lee-bow effect gibbons rely on the assumption that they can point higher with no loss of speed or that the footers don't go any faster.

When sailing we are all on a moving carpet ...

I'm waiting for GFR to tell us next how he exploited the rotation of the earth to beat Robbie Nash ...


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Posts: 4028 		Post Options Post Options   Quote G.R.F. Quote  Post ReplyReply Direct Link To This Post Posted: 14 Oct 11 at 11:34am
Do you really want to set me off on the Coriolis effect and it's application to sailing in advanced racing tactics?

Y'all can't cope with simple Lee Bow effect and I used to think we were the ignorant bozos in matters of sailing tactics..

I wouldn't expect folk who sail inland to even consider tide, why would they need to?

But at sea, on the coast, we view it as a secondary power source, simply that, it's just third dimensional thinking. If there aint enough power from the airflow then there is always power from the tidal flow, in a way it can be more relied upon than the wind, every day constant as the moons rotation around our planet they even make watches that time it.. tell me you don't have one.
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Posts: 641 		Post Options Post Options   Quote Presuming Ed Quote  Post ReplyReply Direct Link To This Post Posted: 14 Oct 11 at 11:49am
Next you'll be saying that you can sail in a 3kt easterly with a tide setting 3 kts to the west....
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Posts: 628 		Post Options Post Options   Quote I luv Wight Quote  Post ReplyReply Direct Link To This Post Posted: 14 Oct 11 at 12:02pm
But it can be faster to sail downwind in a 3 kt adverse tidal current in a 10 kt wind ( compared to hugging the shore in a 1kt current ) LOL

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Posts: 4028 		Post Options Post Options   Quote G.R.F. Quote  Post ReplyReply Direct Link To This Post Posted: 14 Oct 11 at 12:04pm
Originally posted by Presuming Ed

Next you'll be saying that you can sail in a 3kt easterly with a tide setting 3 kts to the west....

A frequent occurrence where I sail...

You need to pump a bit to get the apparent wind working, then use that.. Wink
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