Lee bowing

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You are all correct of course about a Channel crossing, but there is another type of occasion when lee-bowing can be significant, and that is when close-hauled and e.g. trying to clear a bend in a river. It is possible for an experienced sailor to pinch up a bit and gain more ground from the tide/current while another sailor allows himself to be set towards the bank and forced to tack.

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I think that'a a different effect. That's caused by the boat trapping a wedge of water between the hull and the river bank, and an experienced helmsman can "bounce" off it and momentarily claw to windward. This can be enough to clear the bend. If it fails one is left ignominiously stuck in irons up against the bank.
 
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so where are you saying you are going to find the magic component that will lift you to windward

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I didn't think that I said that there was one.
Re Clifford Pope; generations of pre-auxiliary Norfolk Broads sailors know this trick well !
 
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False - The aerofoil shape of the keel works in the same way as a glider wing - we all know that gliders gain LIFT from the air passing over the wing - without this they would just fall out of the sky.



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That too is a fallacy. It is not necessary for a wing to have an aerofoil shape for it to lift. A flat board will lift too, if driven through the air. There is an old salmon-poacher's tool consisting of a floating weighted vertical board, controlled by two lines. By angling it across the current it can be made to go closer or further from the bank. (How you catch the fish I've no idea, I just remember being shown one by a chap in a museum)
An aerofoil may be more efficient, but a flat board achieves the same result - the pressure difference between top and bottom causes the lift. That is why you can steer with a perfectly flat rudder.
 
The simple explanation of the reported racing advantage is surely that sometimes pinching into the wind pays off and sometimes it doesn't. Sometimes gaining a bit of time takes advantage of the tide, sometimes it doesn't.
In an extreme case, you might just heave to and let the tide take you where you want to go, while all the others feel obliged to go through the motions of pretending to tack out across it and back again.
 
Re: Lee bowing the tide

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That too is a fallacy. It is not necessary for a wing to have an aerofoil shape for it to lift. A flat board will lift too, if driven through the air.

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Please try this by replacing your sails with some plywood ... see just how far you get to windward ... you'll make a little progress, but not huge amounts - if it didn't matter at all, why can't square riggers point very well?

the shape is important - keeping the wind "attached" to the sails is very important - which is why we have telltails on them so we can see if we are over/under sheeted.

The same applies to the keel - an increase of flow over the aerofoiled keel in the right direction will generate more "lift" - this can be achieved by using the tidal flow of water too.

Anyone with a degree in Hydrodynamics care to counter?
 
Re: Lee bowing the tide

A force is created if you change the momentum of a fluid. Keels work by adding leeward momentum to water. They shove it mainly sideways relative to the boat (nice), but also forwards a bit (nasty). The ratio between nice and nasty is efficiency.

Flat plates (many centre boards) work well. So do aerofoils. For similar aspect ratios, there's very little difference between them.

Aspect ratio is far more important. Deep and narrow is good - the vortex at the bottom end is very small. Shallow and long is bad - enormous vortices. Adding spin to water, as in government, doesn't achieve the objective - a sideways force. It's a bit of a drag, actually.

Doesn't need a hydrodynamics expert.

Back to the subject. Now, how about the situation where there's no wind relative to the land, but a nice 4kt tide first from one side, then from the other . . . yes, indeed, it's possible to cross the channel when there's no geographic wind. Now, what's that called? Lee-bowing?
 
Re: Lee bowing the tide

You can't use the tidal flow of water over a keel to generate "lift", otherwise it would be possible to tack up-tide using just the keel. There is no boating equivalent of "gliding", because gliders generate speed using gravity, which gives them sufficient lift to keep them in the air while they search for the next up-current. In a constant laminar wind they would indeed fall out of the sky.

Square riggers were not so good to windward because of the difficulty of rotating a square sail sufficiently, not because the sails were flat, like plywood. They weren't, and designs that can turn the yards sufficiently can sail to windward.

I said doubtless an aerofoil shape is more efficient, but it is not essential for the principle to work. A paper dart with flat wings will lift, if angled upwards, as long as it has forward momentum.

Has anyone ever tried a boat with plywood sails, as a matter of interest? I bet it would sail surprisingly well, although obviously not as well as ones with curvature. Who believes it wouldn't work at all?
 
Re: Lee bowing the tide

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Note the question, how can an aeroplane fly upside down, if the aerofoil shape is the only cause of the lift?
Answer; it is the angle of attack that matters.

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Acrobatic aircraft have symetrical wings, whilst those not intended for such antics have symetrical wings - why do you think this is?
 
Re: Lee bowing the tide

My last reply to no one in particular then I am out of here.

Many things have now been mentioned including bouncing off wedges of water on river bends and now fredrift onto how sails and foils work - but getting back to the lee bow effect if someone cares to reread my earlier attempts to explain it. The first part in the context of a channel crossing or crossing any tide at right angles with the wind then on the nose by lee bowing I agree that you are effectively making good use of the tide to go in the direction that you want to go.

The second part though regarding true and apparent wind is the nub of the matter. To understand this you have to exagerate the speeds then if you agree there is a benefit to lee bowing there is a small benefit at realistic speeds.

Lets try the concept of true and apparent wind. If stationary on a motorcycle and the wind is comming from right angles to you at 50mph when the motorcycle goes forward at 10mph the wind shifts slightly ahead as per the vectors. When the motorcycle is doing the same speed as the wind it is at 45deg in front of you. Now if the tide was taking you sideways at 50mph and the wind was on the nose at 50mph it is the same effect and the wind is at 45 deg to you.

Now in the context of a boat with say 20 kts of true wind on the nose and crossing a current the movement of the boat in that current shifts the true wind. When lee bowing it slightly increases the wind and gives you a small lift and if on the other tack it still increases the wind but gives you a small header. Do the vectors.

Whichever way you look at it either 50% of the sailors that have posted understand it or just believe it and will use it and the other 50% believe that it does not exist and there is no benefit! Its therefore a no brainer go for it - if 50% are correct you gain something and if the other 50% are correct you lose nothing!!
 
Re: Lee bowing the tide

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You can't use the tidal flow of water over a keel to generate "lift", otherwise it would be possible to tack up-tide using just the keel. There is no boating equivalent of "gliding", because gliders generate speed using gravity, which gives them sufficient lift to keep them in the air while they search for the next up-current. In a constant laminar wind they would indeed fall out of the sky.

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So - a glider going through the air heading downwards generates lift that pushes it up? .... so not all to do with angle of attack then - because otherwise they could just put a couple of plywood planks on instead of wings and fly with them instead .... except that to get the drag off the plywood wings it would generate so much drag that they wouldn't be able to generate the lift whilst keeping the speed up ...

Jeeze - do you guys actually study this subject or are you just floundering around in the dark .... go and get your O-Level physics books out and re-read them please!
 
Re: Lee bowing the tide

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Whichever way you look at it either 50% of the sailors that have posted understand it or just believe it and will use it and the other 50% believe that it does not exist and there is no benefit! Its therefore a no brainer go for it - if 50% are correct you gain something and if the other 50% are correct you lose nothing!!

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That must be why half the fleet are always at the back!! /forums/images/graemlins/wink.gif
 
Re: Lee bowing the tide

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The second part though regarding true and apparent wind is the nub of the matter. To understand this you have to exagerate the speeds then if you agree there is a benefit to lee bowing there is a small benefit at realistic speeds.


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Try doing the vectors for a 60NM channel crossing at the top end of a F3 (10 kn TWS) assuming a tide of 3 kts - those are very realistic figures, although the 3kt is not for the whole tide of course. Assume the boat can make 5kts at a AWA of 45 deg. Then work out the ETA assuming (a) tide on lee bow and (b) tide on weather bow.
 
Re: Lee bowing the tide

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Note the question, how can an aeroplane fly upside down, if the aerofoil shape is the only cause of the lift?
Answer; it is the angle of attack that matters.

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Acrobatic aircraft have symetrical wings, whilst those not intended for such antics have symetrical wings - why do you think this is?

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Because it is more efficient. But not impossible with ordinary wings - ask a dare-devil ex-spitfire pilot how he managed to do a victory roll, or watch a clip of one of those WW1 biplanes flying upside down with people doing silly stunts.
 
Re: Lee bowing the tide

As I understand the theory of gliding, the skill comes from searching around for upward thermals. They are the only way of gaining height. But having gained height it is possible to minimise the tendency towards falling by gaining speed by a short bit of descent, and then in effect "bouncing" off the air while casting around for another thermal.
It is rather like bouncing a boat off trapped water on a river bend - you can trade in speed in order to gain a bit of distance, which might get you to a more favourable position - ie the river bends to leeward, or you find an up-current of air.
 
Re: Lee bowing the tide

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up-current of air

[/ QUOTE ] Which just happens to be the equivalent of the flow of water on the lee bow .... ta da!

Where ppl may get confused is the Lift generated by the keel doesn't over-counter the leeway induced by sailing, but does reduce it (by varying degrees) ...
The Lee Bow current increases the flow over the keel and therefore reduces your leeway still further - if the current is strong enough it will carry you INTO the wind - thus increasing your apparent wind (and direction of), and as well all know, stronger winds = faster sailing (to a point) ....

As I sail to get somewhere (either round the cans or to a destination) I am interested in this effect as it can allow me to get to my destination sooner (vital when racing!!)
 
Re: Lee bowing the tide

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up-current of air

[/ QUOTE ] Which just happens to be the equivalent of the flow of water on the lee bow .... ta da!



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No it isn't. It would be the equivalent of a fortuitous swirl in the current, eg behind a headland or on the inside of a river bend, which allowed you a free passage against the general drift, if you were smart enough to spot it and take advantage of it.

BTW
For a discussion of the "scientific" v, the "ordinary" explanations of aerofoils, see:

http://www.turnertoys.com/G1/aeroScience/default.htm
 
Re: Lee bowing the tide

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The physics explanation, PRO-NEWTON or ATTACK ANGLE: wings are forced upwards because they are tilted and they deflect air downwards.

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I assume your keel doesn't move sideways through the water then .... that's rather interesting - can you tell us how you manage that?
 
Re: Lee bowing the tide

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Lift generated by the keel doesn't over-counter the leeway induced by sailing, but does reduce it (by varying degrees) ... The Lee Bow current increases the flow over the keel and therefore reduces your leeway still further -

[/ QUOTE ] Dear Oh Dear, what a lot of confusion.

Some simple facts:

1. The propulsion energy for a sail boat comes from the local difference between water speed and air speed.

2. Two shapes are used to create forces which (hopefully) make the boat move through the water; shapes above the water, and shapes below the water.

3. From the point of view of creating motion through the water, tidal/current direction is irrelevant.

4. From the point of view of navigating, tides matter. The task is to make good the best speed towards your destination. This involves pointing into improbable directions, caused by tidal predictions, or predicted windshifts.

One element of predicted windshift is the possibility that the wind remains constant in direction relative to the land, while the tidal currents change. This will create a change in wind direction relative to the water.

When crossing the channel in zero wind (relative to the land) it is possible to move southwards harnessing the difference between the alternating tidal streams and the (geographically) static air.

This could be called 'lee bowing'. This additive effect is very small, and will apply in any direct headwind situation. However, since headwinds are not usually direct, and change direction unpredictably, it is always more sensible to choose the tack which gives you best speed towards your destination (allowing for future currents).

Leeway has nothing to do with this. Leeway is just a consequence of the two shapes having a bit of a struggle. The shapes don't have much to do with this either. They're either more or less efficient at getting the best out of the local speed difference between wind and water.
 
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