Let's Get to the Bottom of the Lee-Bow Effect Once and For All

[Dockhead]

Originally Posted by Paul Elliott

Not by any definition of Apparent Wind that I am familiar with.

Apparent Wind is the wind seen by your boat as a result of its motion through the water. In a constant current, the Apparent (water-referenced) Wind will be the same on either tack. Your progress towards a fixed (land-referenced) mark will be affected by the current.

I think we could all agree on this if we could first all agree on the terminology and reference planes.

It is a mark of complicated this subject is.

Actually, current does increase the TRUE wind. True wind will be same no matter what direction you are sailing in.

I'm using the instrument makers' definitions, to wit:

Ground wind -- wind in relation to the ground, derived from COG and SOG
True wind -- wind in relation to the surface of the water, derived from Heading and STW.
Apparent wind -- directly measured wind speed and direction, in relation to the moving boat.


The fact that the True Wind will be stronger in one tide than the other can play a role in tactics. It can make a huge difference in the English Channel, for example -- 6 or even 8 knots at springs (3 or 4 knot current one way, then in the opposite direction), if the wind is aligned with the direction of the tide (not our tacking case, however).

So from this point of view, you might prefer to lee-bow (increases the overall true wind speed) or to weather-bow (reduces the apparent wind speed -- which you might want if the higher speed from lee-bowing would force you to reef). Only problem is that this effect goes away as the wind direction deviates from the direction of the tide, so tacking directly upwind across the currents, there would be such no effect at all.

[Dockhead]

Originally Posted by Seaworthy Lass

No. Think of the current as a tablecloth you can slide around. The boat in current is just like an object on the tablecloth. The object on the table cloth does not know if the tablecloth is moving, except for the apparent wind generated.

Correct. Common misunderstanding. The current can't "push" the bow, because there is nothing to push against. The boat is floating in the water, and doesn't feel the current.

[Dockhead]

Originally Posted by Seaworthy Lass

Start with the situation where there is zero wind and the current is partly with you. Make it a say a five knot current (not impossible), generating a five knot headwind that you need to tack into. Yes, as you get moving, your boat motion is will alter the angle of this "wind", but there will still be a some angle you can sail at. You will progress forward (as well as the progress with the current that will of course cancel out once the current turns).

Over to you .

This would be a lot less confusing if we use proper terminology.

Your boat doesn't care whether wind comes from current or moving air masses -- it's all the same to your boat and her sails. Your boat sails in the interface between water and air, and never touches the ground (unless you are sitting in a dock, lying at anchor, or run aground ). Your boat never feels Ground Wind, only True.

So a 5 knot or 50 knot current in the absence of a Ground Wind will produce True Wind in the opposite direction of the current.

So your boat sails just the same as she would on a still body of water with 5 or 50 knots of wind.

It just that in relation to ground, she will be moving with the current, in addition to her own motion through the water.

[Seaworthy Lass]

Originally Posted by Dockhead

Sure, it's pretty obvious that you drift with the current, right? I wasn't sure what the point of that Gedankenexperiment was.

You stay in one place relative to the water. Right? Or what was the point?

If you have zero ground wind and current, you have true wind. Why can't you sail to the true wind? Am I missing something major here?

[Dockhead]

Originally Posted by Seaworthy Lass

If you have zero ground wind and current, you have true wind. Why can't you sail to the true wind? Am I missing something major here?

Of course you do. Exactly.

As I wrote above -- Ground Wind is irrelevant to sailors except to predict how the wind will change with a change of tide.

We sail in True Wind -- in the interface between air and water.

It would actually be conceptually and probably mathematically much easier, if we envision the water as stationary, and the current moving the land and racing marks around.

[Paul Elliott]

Originally Posted by Seaworthy Lass

If you have zero ground wind and current, you have true wind. Why can't you sail to the true wind? Am I missing something major here?

If (above) you there is some current, then of course you can. I think we are all agreeing on this point.

[Seaworthy Lass]

Originally Posted by Dockhead

Of course you do. Exactly.

As I wrote above -- Ground Wind is irrelevant to sailors except to predict how the wind will change with a change of tide.

We sail in True Wind -- in the interface between air and water.

Originally Posted by Paul Elliott

If (above) you there is some current, then of course you can. I think we are all agreeing on this point.

OK, we agree on this

So lets take this one step at a time and go further.

If the current is relatively strong and only just off your bow and you are close hauled, your ability to point will depend on whether the current is on your windward or leeward side. Any agreement with this?

[Dockhead]

Originally Posted by Seaworthy Lass

OK, we agree on this

So lets take this one step at a time and go further.

If the current is relatively strong and only just off your bow and you are close hauled, your ability to point will depend on whether the current is on your windward or leeward side. Any agreement with this?

I don't think the ground belongs in this picture -- it is extremely confusing and probably leads to grave errors. Forget the ground for a minute.

You point exactly the same way, relative to the True Wind Direction, on either tack.

Bring the ground back into the equation to see whether your Course to Steer (Yes! Back to that!) has properly taken account of the current vectors to put you at your destination.

So if this is the formulation of Lee Bow Effect we are discussing (there are several slightly different ones), then I think I could now say that I am ready to take a position -- saying with some confidence that it is nonsense -- it's confusing sailing tactics with Course to Steer calculation, which must be done separately.

[Dockhead]

Notice that this formulation of the Lee Bow Effect is different from the classical tacking across the Channel formulation -- there, the question is how to tack in relation to the changing tide -- do you want to be on the tack which lets you be swept further downtide, or the tack which keeps you closer to the rhumb line, during any given phase of the tide?

Of course it is a devlish additional layer of complexity added on top of the Course to Steer principles -- already complicated enough -- we were discussing last year.

[Dockhead]

Originally Posted by Seaworthy Lass

OK, we agree on this

So lets take this one step at a time and go further.

If the current is relatively strong and only just off your bow and you are close hauled, your ability to point will depend on whether the current is on your windward or leeward side. Any agreement with this?

So here's my answer:

With a non-changing current, you first:

1. Calculate your Course to Steer to get to your mark.

2. Then you tack however you like to get an average heading corresponding to your Course to Steer.

There is no difference from one tack to the other as long as you have in mind your Course to Steer, rather than Bearing to the Mark. There are no magic lifts or mysterious knocks, other than the knocks you will inevitably get if you try to sail according to Bearing to Mark.

Thinking about Bearing to the Mark in moving water, when working up your sailing tactics, will fornicate your brain. It will lead you off into all kinds of idiotic ideas. The key to this particular problem -- which may not be the whole Lee Bow Effect problem -- is to throw away Bearing to the Mark, and substitute Course to Steer, then just sail as usual.

OK, so now I've taken a position

[Seaworthy Lass]

Originally Posted by Dockhead

Notice that this formulation of the Lee Bow Effect is different from the classical tacking across the Channel formulation -- there, the question is how to tack in relation to the changing tide -- do you want to be on the tack which lets you be swept further downtide, or the tack which keeps you closer to the rhumb line, during any given phase of the tide?

Of course it is a devlish additional layer of complexity added on top of the Course to Steer principles -- already complicated enough -- we were discussing last year.

That one is easy. It is all part of the "non existent" lee bow effect .

I will get back to this later. I just need to finish with the pinching issue.

[jongleur]

Oh where is Max Ebb and Lee Helm
when you need 'em.

[Paul Elliott]

Originally Posted by Seaworthy Lass

OK, we agree on this

So lets take this one step at a time and go further.

If the current is relatively strong and only just off your bow and you are close hauled, your ability to point will depend on whether the current is on your windward or leeward side. Any agreement with this?

If there is zero ground wind, then the True wind will be coming from the direction the current is moving. But your boat is *in* this current, so unless you are looking at landmarks, or the GPS, you can't even tell that you are in a current. All you know is that there is some wind, coming from some direction. I'm not even sure I understand what you mean by this:

If the current is relatively strong and only just off your bow and you are close hauled, your ability to point will depend on whether the current is on your windward or leeward side.

Your ability to point? Relative to the wind? To the ground??? Can you clarify?

[Seaworthy Lass]

Originally Posted by Dockhead

I don't think the ground belongs in this picture -- it is extremely confusing and probably leads to grave errors. Forget the ground for a minute.

You point exactly the same way, relative to the True Wind Direction, on either tack.

Bring the ground back into the equation to see whether your Course to Steer (Yes! Back to that!) has properly taken account of the current vectors to put you at your destination.

So if this is the formulation of Lee Bow Effect we are discussing (there are several slightly different ones), then I think I could now say that I am ready to take a position -- saying with some confidence that it is nonsense -- it's confusing sailing tactics with Course to Steer calculation, which must be done separately.

OK forget about ground wind. The point I was making was that true wind depends on current (amount and direction). We have agreed on that.

What about this statement:

Originally Posted by Seaworthy Lass

If the current is relatively strong and only just off your bow and you are close hauled, your ability to point will depend on whether the current is on your windward or leeward side. Any agreement with this?

(I don't disagree with anything you said in the post you made after the one I quoted above, but this is a separate issue)

[Dockhead]

Originally Posted by Dockhead

I don't think the ground belongs in this picture -- it is extremely confusing and probably leads to grave errors. Forget the ground for a minute.

You point exactly the same way, relative to the True Wind Direction, on either tack.

Bring the ground back into the equation to see whether your Course to Steer (Yes! Back to that!) has properly taken account of the current vectors to put you at your destination.

So if this is the formulation of Lee Bow Effect we are discussing (there are several slightly different ones), then I think I could now say that I am ready to take a position -- saying with some confidence that it is nonsense -- it's confusing sailing tactics with Course to Steer calculation, which must be done separately.

I hate to quote my own posts, but I'm adding to it here:

Another corollary of this is the following:

Pinching in a moving body of water will never give you any kind of lift.

If you think you need to be pinching, you have gotten to the problem in a backwards way and made a mistake along the way.

To win the race, you need maximum average VMG to mark over the leg.

You will get maximum VMG by sailing optimally according to your polars.

You have to set up your tactics according to your Course to Steer, however. So if the Bearing to Mark is dead upwind, you are not actually sailing dead upwind on your Course to Steer, and vice versa, and the tactics will be different. If you confuse these, you will lose to anyone who doesn't. But the optimal tactics will never involve pinching.