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

[Seaworthy Lass]

Originally Posted by Dockhead

?....making it an "advantageous tack"? I still don't quite understand the terminology).

At any point when beating to windward you always have a choice of two tacks.

If sailing with variable current, your time taken to get to the destination will depend on which of these two tacks you chose to take (and not just because you are leaving the decision of when to make the final tack until you are close to the destination, making it easier to judge, but a real effect).

The tack that will actually get you to your destination is not necessarily the favoured one ie it is not necessarily the tack that is closest to your CTS. This is the tack I would call the advantageous tack.

[Seaworthy Lass]

Originally Posted by Dockhead

The only thing I would object to and be unable to accept would be a description of the effect of current as creating "apparent wind", as someone above did. This is false. If the boat is dead in the water, and there is zero ground wind, but you can feel 4 knots of wind because a current is running, it would be a perversion to call this "apparent wind". We can't get around the fact that there are two flavors of what used to be called true wind.

That is still a stumbling block for you then .

Apparent wind is the wind you feel. On the water it is the sum of the effect of your speed and true wind.
True wind is the sum of ground wind and the wind induced by current (and leeway).

So apparent wind is the sum of wind due to boat speed (heading and leeway) plus ground wind plus wind induced by current.

If your boat speed and ground wind are zero, then your apparent wind is the wind induced by current .

It is very important to get your head around that. You can sail to your apparent wind. If your ground wind is zero and you have current, you can sail.

[Paul Elliott]

Seaworthy and Dockhead, you are arguing about something you both agree with.

Apparent Wind is indeed the wind you feel (or measure) from the deck of your boat. What Dockhead is saying is that we shouldn't call current-induced wind (which includes Ground Wind as a component) Apparent Wind (as I recall someone else recently did). This wind is the True Wind. Yes, if your boat is drifting with the current "dead in the water" then the True and Apparent Wind will be equal.

True Wind does not include boat speed-induced wind. Apparent Wind does. I think you are both saying this as well.

I am still waiting for a new definition of lee-bow effect that is anything other than an obvious restatement of basic sailing geometry. If you want to work out a rule of thumb for optimum tacking strategy in a changing current/wind, then please call that something else. The lee-bow effect that I have seen described is supposed to apply in an unchanging current and wind.

[Seaworthy Lass]

NOT THE LEE BOW EFFECT AT ALL:

No one at all has comment on my discovery LOL. I woke up keen to see responses from who had been awake while I slept and there was nothing. You lot are a tough crowd .

OK, this is it with technicolor drawings if that helps. It is so beautifully elegant and simple. The change in true wind direction has nothing to do with whether or not the current is on your lee bow.

It is purely which side of the ground wind the wind induced by current falls (this may be the lee bow, but it may also be the windward bow, it depends where your heading fits in this picture - am I about to be burned at the stake for blasphemy again LOL?).

I have drawn a simple diagram with just 8 points around the compass (any more lines and it becomes cluttered.

[Paul Elliott]

SWL, that diagram is just fine. For that matter, replace current with boat speed/direction and Ground Wind with True Wind, and your diagram now shows how True Wind turns into Apparent Wind.

It's all triangles! (And the proper frame of reference!)

[Seaworthy Lass]

Originally Posted by Paul Elliott

SWL, that diagram is just fine. For that matter, replace current with boat speed/direction and Ground Wind with True Wind, and your diagram now shows how True Wind turns into Apparent Wind.

It's all triangles! (And the proper frame of reference!)

Paul, that does mean that it is not the 'lee bow effect' then at all. It is a stupid term if can occur on the windward side as well and not always on the lee side!

Edited to add: the term 'Lee Bow Effect' needs to be ditched. If anyone really thinks we should include the bow in the term, then call it 'The Effect That Often But Not Always Occurs On the Lee Bow and Sometimes Occurs On the Windward Bow' .

I propose a new name altogether . Any suggestions?

[Dockhead]

Originally Posted by Seaworthy Lass

That is still a stumbling block for you then .

Apparent wind is the wind you feel. On the water it is the sum of the effect of your speed and true wind.
True wind is the sum of ground wind and the wind induced by current (and leeway).

So apparent wind is the sum of wind due to boat speed (heading and leeway) plus ground wind plus wind induced by current.

If your boat speed and ground wind are zero, then your apparent wind is the wind induced by current .

It is very important to get your head around that. You can sail to your apparent wind. If your ground wind is zero and you have current, you can sail.

I don't think you understood what I wrote --

I said that if you have zero ground wind and the boat is dead in the water, but there is 4 knots of true wind -- the way you and I and Raymarine use the terminology, right? --

That it would be a perversion to say that this is an effect of apparent wind -- that you have no true wind but 4 knots of apparent wind. What I am saying is that you can't talk about this without the two flavors of what used to be called true wind. The perversion is 0 boat speed 4 knots of apparent wind 0 knots of true wind.

[Dockhead]

Originally Posted by Paul Elliott

Seaworthy and Dockhead, you are arguing about something you both agree with.

Apparent Wind is indeed the wind you feel (or measure) from the deck of your boat. What Dockhead is saying is that we shouldn't call current-induced wind (which includes Ground Wind as a component) Apparent Wind (as I recall someone else recently did). This wind is the True Wind. Yes, if your boat is drifting with the current "dead in the water" then the True and Apparent Wind will be equal.

True Wind does not include boat speed-induced wind. Apparent Wind does. I think you are both saying this as well.

The proverbial nail on the head.

[Seaworthy Lass]

Originally Posted by Dockhead

The proverbial nail on the head.

Sorry :shamefaced:. Misunderstood what you wrote.

[Dockhead]

Originally Posted by Seaworthy Lass

NOT THE LEE BOW EFFECT AT ALL:

No one at all has comment on my discovery LOL. I woke up keen to see responses from who had been awake while I slept and there was nothing. You lot are a tough crowd .

OK, this is it with technicolor drawings if that helps. It is so beautifully elegant and simple. The change in true wind direction has nothing to do with whether or not the current is on your lee bow.

It is purely which side of the ground wind the wind induced by current falls (this may be the lee bow, but it may also be the windward bow, it depends where your heading fits in this picture - am I about to be burned at the stake for blasphemy again LOL?).

I have drawn a simple diagram with just 8 points around the compass (any more lines and it becomes cluttered.

Superb That's a very helpful visualization.

I am tentatively sticking with "lee bow" as a term, however, because I haven't yet seen the situation where it works with the current on the windward bow.

My mind is open; need to start getting into the limits of the effect so we start to see what happens at the borders.

But so far the cases we have seen have the current on the lee bow exactly because that shifts the wind further around the windward bow -- making the wind freerer. If the wind were to shift towards the lee bow, you are being knocked.

But my mind is open as to what that looks like at the borders -- my mind hasn't gotten that far yet.

[Dockhead]

Originally Posted by Seaworthy Lass

Sorry :shamefaced:. Misunderstood what you wrote.

You are not the first person, and won't be the last, to misunderstand some poorly expressed idea of mine. No shamefaces necessary

I was surprised, however, that you would jump to the conclusion that I would misunderstand something about the different kinds of wind

[Seaworthy Lass]

Originally Posted by Dockhead

Superb That's a very helpful visualization.

I am tentatively sticking with "lee bow" as a term, however, because I haven't yet seen the situation where it works with the current on the windward bow.

My mind is open; need to start getting into the limits of the effect so we start to see what happens at the borders.

But so far the cases we have seen have the current on the lee bow exactly because that shifts the wind further around the windward bow -- making the wind freerer. If the wind were to shift towards the lee bow, you are being knocked.

But my mind is open as to what that looks like at the borders -- my mind hasn't gotten that far yet.

Easy to draw. Give me a sec. When things are really easy to show, I just get a gut feeling they must be right.
Will show an example where it works on the windward bow and where it doesn't work on the lee bow.

By the way where did the term "Lee Bow Effect" originate? Did the RYA make it up .

[Seaworthy Lass]

Originally Posted by Seaworthy Lass

By the way where did the term "Lee Bow Effect" originate? Did the RYA make it up .

Folks, before you think too harshly of me, let me fill you in. I had a very hard time in the hands of two instructors (one very senior, one RYA, both of whom are rightly very respected CF members) early this year who refused to budge an inch on something the RYA was preaching incorrectly regarding CTS. The comment was basically if the RYA had been publishing it for decades, who was I to argue (particularly as I had not been familiar with the method prior to the thread, so was a total beginner in their eyes). It frustrated me to no end. I was not on my best behaviour as a result. I wrote extremely politely and humbly both to the RYA and to Tim Bartlett, the author of the RYA manual, with not a shred of response.

In the end I just simply dropped it. I didn't have the energy for it any longer and they were simply refusing to take off their blinkers or even just peek from behind them.

This was the thread:
http://www.cruisersforum.com/forums/...hod-96579.html

Any good organisation should welcome being challenged and improving, not bury its head in the sand when someone suggests something major is incorrect. How else do we advance?

[Seaworthy Lass]

These are simple examples of being lifted when the current is on the windward bow and knocked when the current is on the lee bow.

Before anyone gets too excited about the amount of true wind increasing or decreasing in the two examples (hope you noticed ), this can change if the amount of current relative to ground wind is different.

[IdoraKeeper]

In theory I can see how choosing the correct CTS can average out the advantage/disadvantage between tacks with respect to the current. ( Assuming constant wind speed/direction as a sum of all the relevant vectors.)
What would be useful (to me at least) would be an understanding of how to best compute when/at what location to tack to take best advantage of proper CTS. When any of the variables changes the spot to tack moves. Sometimes I can sense it. More often I see the other guy get closer to correct.