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LEE BOW EFFECT

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Presuming Ed View Drop Down
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Post Options Post Options   Quote Presuming Ed Quote  Post ReplyReply Direct Link To This Post Posted: 11 Oct 11 at 5:52pm
Originally posted by Rupert

You are talking about 2 totally different things, Jim and GRF. Of course where you are in the tide has a massive effect, but the very specific effect of the tide pushing just on the lee side of your bow having a magic effect compared to it coming straight on is what the lee bow effect is, so making pointing very slightly higher worthwhile. 

And there was me thinking that the lift/drag curve for a particular foil was a fixed thing. A foil of NCAA section xxxx at an angle of attack (= leeway angle) of x will generate lift of F and drag of D. All physics. 

If you're saying that there is some magic way the keel becomes "aware" of the angle between the tidaf flow and the seabed, then could you tell me where the brain in the keel is because I haven't found one yet. Also, if there is some way we can "switch on" this "extra lift", the again, that would be really useful because I'll just switch it on all the time. 


Edited by Presuming Ed - 11 Oct 11 at 6:08pm
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Post Options Post Options   Quote Presuming Ed Quote  Post ReplyReply Direct Link To This Post Posted: 11 Oct 11 at 5:55pm
Originally posted by tgruitt

Originally posted by JimC

One of the main sources of confusion in this topic is that not everyone's talking about the same thing...
I always thought a lee bow was tacking under someone and being sucked up to windward a bit by the air rushing between the close proximity of the rigs?

IME, it's not that the boat ahead is lifted, it's that the boat behind is knocked. In the same way that the main has to be sheeted closer than the jib, because it's knocked by the jib. I've always called tacking into someone's lee bow "lee bow-ing" someone, never the "lee bow effect".

To me the "lee bow effect" is this mythical thing about magically generating extra lift out of nowhere. 


Edited by Presuming Ed - 11 Oct 11 at 6:08pm
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Presuming Ed View Drop Down
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Post Options Post Options   Quote Presuming Ed Quote  Post ReplyReply Direct Link To This Post Posted: 11 Oct 11 at 6:07pm
Originally posted by G.R.F.

Perry, that's the chap, be cool to race against him in some tidal venues..

Well, he does have a rough idea how to sail....http://www.sailing.org/biog.php?id=USADP4
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Post Options Post Options   Quote Rupert Quote  Post ReplyReply Direct Link To This Post Posted: 11 Oct 11 at 6:09pm
Originally posted by Presuming Ed

Originally posted by Rupert

You are talking about 2 totally different things, Jim and GRF. Of course where you are in the tide has a massive effect, but the very specific effect of the tide pushing just on the lee side of your bow having a magic effect compared to it coming straight on is what the lee bow effect is, so making pointing very slightly higher worthwhile. 

And there was me thinking that the lift/drag curve for a particular foil was a fixed thing. A foil of NCAA section xxxx at an angle of attack (= leeway angle) of x will generate lift of F and drag of D. All physics. 

If you're saying that there is some magic way the keel becomes "aware" of the angle between the tidaf flow and the seabed, then could you tell me where the brain in the keel is. I haven't found one yet. Also, if there is some way we can "switch on" this "extra lift", the again, that would be really useful because I'll just switch it on all the time. 
Not me saying it, just trying to define what we are discussing - pointing too high into tide sounds like a dumb thing to do. In fact, as someone who grew up sailing on rivers with current, the only way of making ground faster against current is to get where it is less, and I assume tide works just the same.
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Post Options Post Options   Quote RS400atC Quote  Post ReplyReply Direct Link To This Post Posted: 11 Oct 11 at 6:19pm
The lee bow effect debate is largely as JimC says, people talking at cross purposes.

Some people cannot add up the vectors to understand how tide affects your VMG
Some people forget the mark is moving through the tide
The deniers can't explain what they are denying because the proponents can't explain it properly either, usually because they are working on a badly worded explanation in a sailing book that was not written by a mathematician.

There is nothing magic about it, but a tide on your bow definitely has an effect.
What seems to a science graduate as the boring predictable unsurprisng addition of vectors is the same magic effect that surprises people who can't add vectors.
It's also used to explain any inconvenient/remarkable divergence of courses.

A couple of seasons sailing leadmines, well iron mines in fact, in the solent help you realise that the tide is always significant, never constant and doesn't obey the little charts they sell.
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Post Options Post Options   Quote SoggyBadger Quote  Post ReplyReply Direct Link To This Post Posted: 11 Oct 11 at 6:21pm
Originally posted by Presuming Ed

IME, it's not that the boat ahead is lifted, it's that the boat behind is knocked. In the same way that the main has to be sheeted closer than the jib, because it's knocked by the jib. I've always called tacking into someone's lee bow "lee bow-ing" someone, never the "lee bow effect".


Actually, the leeward boat is lifted slightly WRT to the apparent wind and also experiences a slight increase in wind speed.
Best wishes from deep in the woods

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Post Options Post Options   Quote JimC Quote  Post ReplyReply Direct Link To This Post Posted: 11 Oct 11 at 7:00pm
These sorts of discussions probably feel really intimidating to beginners, but they should in fact be enormously encouraging, because they so often demonstrate that a lack of understanding of the underlying science is frequently no obstruction to sailing a boat fast...

I have to admit I've never really quite understood what GRF is getting at with respect to tides and lee bows: I have no idea whether he's saying the same thing as me or something quite different.
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Post Options Post Options   Quote Presuming Ed Quote  Post ReplyReply Direct Link To This Post Posted: 11 Oct 11 at 7:01pm
I've never really quite understood what GRF is getting at with respect to pretty much anything. 

I'll get me coat. 
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Post Options Post Options   Quote Rockhopper Quote  Post ReplyReply Direct Link To This Post Posted: 11 Oct 11 at 7:49pm
Don just come over and join us when the winds from the south or south east its perfect for that lee bowing the tide
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Post Options Post Options   Quote didlydon Quote  Post ReplyReply Direct Link To This Post Posted: 11 Oct 11 at 8:36pm
Ok.... Thanks everyone for the really informative replies Thumbs Up It's just as I thought...... Bloody complicated to understand, but there is something definitely there. I can feel it when pushing into the tide (& breeze)... It's just working out how to harness it to your advantage to either make the mark without tacking again or getting that extra bit of "lift" & catching the bloke in front by surprise. So can anyone suggest a practical way of demonstrating it's effect to yourself when out practising so it's effect or otherwise is really noticeable? 


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