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r2d2 View Drop Down
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    Posted: 13 Oct 11 at 11:16am
Originally posted by G.R.F.

Originally posted by sargesail




But can we agree that that push is the same no matter where the boat is pointing?

No, we can't agree on that either.

If it were a boat with no means of propulsion then yes we could agree with your 'theory'

But it isn't, it's a sailboat with two sets of foils set in a plane at the conjunction of two 'fluids' moving in different directions.

If the fluids were both moving in the same direction, then yes the push would be the same no matter where the boat was pointing.

But the moment you introduce a means of resistance against either fluid force, then that produces an equal and opposite response <insert all sorts of maths and physics bollox>

Now come on, you must see the sense of this...


In all seriousness here for a second and joking and banter aside, have there been no books or lectures or videos about sailing in tide, tidal currents, waves fast and slow, how to spot them all this stuff we're bantering about here? I can't believe over the years somebody hasn't already detailed it, somebody y'all might actually believe that is.. I bet if my name was Bethwaite y'all would be going ooh ahh it must be true then...
 
GRF you'll never get that v-twin thing finished if you stop to write a book for us!
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To have a mag article you'd have to agree on what "this effect" is first... A sailing in tide series wouldn't be a bad idea for the mag, hink its been a while.

GRF seems to be talking about tactical considerations affecting which tack you should take first. I hope no-one is going to disagree that the tidal stream profoundly affects tactical considerations, most especially since by and large tidal sailing is done in places where the tidal stream and direction is anything but constant.

The direction and strength of the tide also affects the wind direction felt on the boat: if you've got 5 knots of tide from East to West and 5 knots of wind from Nth to Sth then the boat is effectively sailing in about 7 knots of wind from the North West...

If the boat can do 5 knots though the water on a beat then on one tack it will be making 10 knots across the ground heading west, and on the other tack about 7 knots across the ground heading north west, but on both tacks it will be making 5 knots through the water...

Why this gets complicated is that the marks are fixed relative to the ground, not relative to the water, so in this example, compared to the water the boat is sailing on the windward mark is travelling east at 5 knots.

What GRF seems to me to be saying is that when in doubt always sail the tack that is getting you nearer to the mark quickest, because if the tide stream isn't constant you win, and if it is constant then you don't lose. Seems uncontroversial to me...
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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 11:38am
Originally posted by Ian29937

A wonderfull rambling thread as usual. 
 
The question is not whether the tide has an impact on the apparent wind as clearly it will.  The question is whether or not pinching to shift from "tide on the windward bow" to "tide on the leeward bow" mode works or not. 
 
The theorists say no, some people say experience indicates yes.
 
I've certainly been overtaken whilst trying to pinch up and gain a leebow effect by people sailing normally to the true apparent wind so my experience dictates that the academics are correct.  Smile
 
Ian

No, that wasn't the question, the question was 'what is lee bow effect'. and what does it do?

The answer to which I've already given.

What then developed was itty nitty picking, the answer to your bit was no not always, pinching in tide is never a good idea, unlikely that you could even pinch to that large a degree that the tide would shift from the weather to the lee bow, it might shift from the nose to the lee bow and tide on the nose is better than tide on the weather side of the foil for all those fluid dynamics equations that I failed to list above..
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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 11:50am
I've had a thought, trying to get folks head round wind bends and when and why you sail into a header, imagine trying to explain where you go on a tide bend into a wind bend (we have one at our club).. At certain states of the tide, you do all the text book stuff get the tide on the lee bow, but as you near the mark it knocks, you get fooled into thinking its a wind bend when it isn't so you persist, (because the curved coastline is classic wind bend geography and does produce it at times) you sail what is now a tidal knock (although you still think the tide is on the lee or at the very worse the nose, until you're on what should be the lay line, flack over and bosh the tides now on that weather bow as well...

Nightmare.


Edited by G.R.F. - 13 Oct 11 at 11:54am
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Posts: 241 		Post Options Post Options   Quote fudheid Quote  Post ReplyReply Direct Link To This Post Posted: 13 Oct 11 at 1:41pm
Undecided on who to believe on this one. Certainly sailing in alot of tide on the east coast the 'lee bow' theory certainly helps your COG whether it makes the boat faster - No i don't think so.
If the theory is wrong why do gybing boards work surely its the same theory the angle of attack of the blade on the water. Its all changing the angle of upwash on the foils????
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lee bowing the tide is really just cross track error.
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i think i'll go with that Clap
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Posts: 409 		Post Options Post Options   Quote Ian29937 Quote  Post ReplyReply Direct Link To This Post Posted: 13 Oct 11 at 2:24pm
Originally posted by G.R.F.


No, that wasn't the question, the question was 'what is lee bow effect'. and what does it do?

OK. to rephrase
 
What is the lee bow effect - the lee bow effect is non existant, there is only a tidal effect on your apparent wind which effects you consistently whether you are sailing high or low, port or starboard
 
What does it do - nothing, it doesn't exist!  However for those who believe, it is the fictional notion that there is an advantage gained (the lee bow effect) by pinching higher to get the tide pushing on the leeward bow.
 
There I've said it!
 
Ian
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Posts: 409 		Post Options Post Options   Quote Ian29937 Quote  Post ReplyReply Direct Link To This Post Posted: 13 Oct 11 at 2:28pm
Originally posted by fudheid

Undecided on who to believe on this one. Certainly sailing in alot of tide on the east coast the 'lee bow' theory certainly helps your COG whether it makes the boat faster - No i don't think so.
If the theory is wrong why do gybing boards work surely its the same theory the angle of attack of the blade on the water. Its all changing the angle of upwash on the foils????
Do gybing boards work?  I used to sail fireballs which allowed them at the time.  They were only used by a few people and then only in some very specific circumstances  (e.g. flat water, medium breeze when the height gain exceded the extra drag and speed loss)
 
This sounds like a whole new topic
 
Ian
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505's use gybing boards to great effect.
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