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

[Paul Elliott]

Originally Posted by Hoofsmit

FS:::: OR ANYBODY ELSE !

Could I ask for your opinions

We have suggested that the current/tide creates a wind by 'pushing' the boat.
This has then been worked in as a angle and speed to come to an answer to show the affect of the wind that hits our sails ( in simple terminology)

Why is the full current rate applied ? we have resistance from the water do we not?

We seem happy to add leeway knowing that it differs dependant on sailing angle

If we look at being on a table cloth the dynamics of resistance the affect of hull shape, keel shape and wetted area are completely ignored.

is this correct ?

If we had no resistance we would just be blown downwind. This resistance is what gives us our leeway. That is where it shows up in the calculations.

This is another reason that the water-referenced True Wind can be useful when calculating these effects.

[Hoofsmit]

Originally Posted by Paul Elliott

If we had no resistance we would just be blown downwind. This resistance is what gives us our leeway. That is where it shows up in the calculations.

This is another reason that the water-referenced True Wind can be useful when calculating these effects.

so how do you calculate for that resistance? as in the length of your vector?

[Seaworthy Lass]

Originally Posted by europaflyer

As your post #223 shows, there is either a lifting or knocking effect for any direction of tide. I think we agree on this point.

Hi, sorry it has taken me so long to reply.
Glad we have common ground .

Originally Posted by europaflyer

I am saying that the points when the tide direction changes from producing a beneficial effect on one tack to beneficial on the other are at 360 and 180 degrees tidal stream relative to apparent wind, as shown on your diagram. These are the points when there is no effect of tide on apparent wind direction.

Apparent wind depends on boat speed (and that in turn depends on boat characteristics). The effect has nothing to do with the boat. It is not centred on something that has boat related properties. It is centred on the ground wind as shown on my cone diagram, NOT apparent wind (and it can't be both LOL). If you agree with my diagram, then you are agreeing it is centred on ground wind and therefore can't be centred on apparent wind.

Originally Posted by europaflyer

You are saying that current lifts when forward of the beam ('bow effect') and knocks when aft. I dispute this, and here is a diagram to show a scanario where a tide aft of the beam is shifting the apparent wind aft (ie. favouring the boat). Although the creation of a favoured tack does not in this case follow your 'bow theory', it does follow my 'apparent wind current angle theory' - the current is between 360 and 180 apparent, and so creates a favourable port tack.

Sorry, your diagram does not dispute my theory at all LOL. Check out the other tack. The current is even further around. In the example you have shown even though the current is coming from astern it is less astern than the knocked tack.

I challenge you to find one single example where the lifted tack is not the one with the current coming more onto the bow compared to its partner .

Originally Posted by europaflyer

I don't see how the angle of the current relative to the boat is relevant to the creation of a shift in apparent wind. This will be felt regardless of where the boat is pointing. Imagine if the boat is stationary in the water - where the bow and stern are doesn't matter a bit, but the apparent wind will still be shifted by the current. If the destination is exactly in the eye of the true (ground) wind, then the apparent wind now creates a favoured tack - because of the effect of current. When the current changes direction, the favoured tack will also change.

This is where you are going wrong. It is not the angle relative to the boat, it is the angle relative to the ground wind that the effect is centred on. This just happens to coincide with being close to the bow.

I have stated that looking at how close the current is to being on the bow is just an approximation. I agree it has nothing to do with the boat. Have a look at the come diagram though. When you are tacking the lift happens to be maximum roughly when the current is on the bow.

Originally Posted by europaflyer

By my continuing to use the term 'lee bow effect' I am not trying to say that what matters is that the current is on the leeward side, or on the bow. I am just using the generally accepted term for what is a generally accepted effect that any good book on sailing theory will discuss. It happens that the effect is strongest (ie. biggest shift in apparent wind) when the current is at 90 or 270 degrees to the apparent wind. The effect is only relevant tactically when closehauled, which for this most important case of the effect would place the current on the lee bow. I have no complaint with this nomenclature, even though it my create the misleading impression that the CAUSE of the effect relates to the angle of the current relative to the boat rather than to the wind. The name is just due to the most common rule of thumb for the use of the effect.

If a book discusses "lee bow" as some kind of effect then I am afraid the book is incorrect. And the effect has nothing to do with the apparent wind (in fact I throw in "nonsense" again ). As you have stated it is NOT due to the boat (apparent wind is due partly to the boat). It is not impossible texts are wrong. You sometimes see some really stupid things printed in otherwise well respected texts.

The effect by the way is maximum roughly at 90 or 27O degrees to the GROUND wind, NOT the apparent wind (is alters a bit towards the ground wind when the current is high compared to the ground wind. My cone diagram shows beautifully how it is centred. And it does actually apply when gybing downwind as well, except then it is a 'stern affect'. The current being closer to the stern is the more favourable of the two gybes.
---------------------------

How do you explain this diagram? The lifted tack is clearly on the windward side. It also follows my rule of thumb (it is the tack with the current close to the bow that is lifted more with current):

[goboatingnow]

Originally Posted by Dockhead

You tack around it, striving for average heading equal to CTS. You can't do this problem without it.

I've never seen that situation , you work up a course to steer taking into account all variables. Including sailing angle. That's the definition of CTS , if you can't steer it your doing it wrong

Dave

[Seaworthy Lass]

Originally Posted by goboatingnow

I've never seen that situation , you work up a course to steer taking into account all variables. Including sailing angle. That's the definition of CTS , if you can't steer it your doing it wrong

Dave

Dave, the situation occurs when the course to steer is into wind. You cannot sail the CTS then, you must tack.

[goboatingnow]

Originally Posted by Seaworthy Lass

Dave, the situation occurs when the course to steer is into wind. You cannot sail the CTS then, you must tack.

That's not how to go about selecting a CTS.( or more correctly a route ) If you cannot lay the destination then you , compute a CTS for a course You can sail. A CTS is always a course a Helmand steers.

Dave

[goboatingnow]

Originally Posted by Seaworthy Lass

Hi, sorry it has taken me so long to reply.
Glad we have common ground .

Apparent wind depends on boat speed (and that in turn depends on boat characteristics). The effect has nothing to do with the boat. It is not centred on something that has boat related properties. It is centred on the ground wind as shown on my cone diagram, NOT apparent wind (and it can't be both LOL). If you agree with my diagram, then you are agreeing it is centred on ground wind and therefore can't be centred on apparent wind.

Sorry, your diagram does not dispute my theory at all LOL. Check out the other tack. The current is even further around. In the example you have shown even though the current is coming from astern it is less astern than the knocked tack.

I challenge you to find one single example where the lifted tack is not the one with the current coming more onto the bow compared to its partner .

This is where you are going wrong. It is not the angle relative to the boat, it is the angle relative to the ground wind that the effect is centred on. This just happens to coincide with being close to the bow.

I have stated that looking at how close the current is to being on the bow is just an approximation. I agree it has nothing to do with the boat. Have a look at the come diagram though. When you are tacking the lift happens to be maximum roughly when the current is on the bow.

If a book discusses "lee bow" as some kind of effect then I am afraid the book is incorrect. And the effect has nothing to do with the apparent wind (in fact I throw in "nonsense" again ). As you have stated it is NOT due to the boat (apparent wind is due partly to the boat). It is not impossible texts are wrong. You sometimes see some really stupid things printed in otherwise well respected texts.

The effect by the way is maximum roughly at 90 or 27O degrees to the GROUND wind, NOT the apparent wind (is alters a bit towards the ground wind when the current is high compared to the ground wind. My cone diagram shows beautifully how it is centred. And it does actually apply when gybing downwind as well, except then it is a 'stern affect'. The current being closer to the stern is the more favourable of the two gybes.
---------------------------

How do you explain this diagram? The lifted tack is clearly on the windward side. It also follows my rule of thumb (it is the tack with the current close to the bow that is lifted more with current):

I don't understand that diagram , since there is no apparent wind vector how can a windward tide cause You to be lifted. Doesn't make sense the term " lifted " or headed refers to effect of changes in cog due to changes in ground wind relative the a ground referenced destination , ie a racing mark.

Dave

[europaflyer]

Hi Angela,

Originally Posted by Seaworthy Lass

Apparent wind depends on boat speed (and that in turn depends on boat characteristics). The effect has nothing to do with the boat. It is not centred on something that has boat related properties. It is centred on the ground wind as shown on my cone diagram, NOT apparent wind (and it can't be both LOL). If you agree with my diagram, then you are agreeing it is centred on ground wind and therefore can't be centred on apparent wind.

I will try to use your nomenclature as used on the diagram in this post. Note that what you call true wind I have previously called apparent or 'water referenced true' wind, which is a potential source of confusion.

Your diagram shows both winds - ground wind (what I call true wind) as the thick black central line, and the true wind (what I call apparent wind) as the red lines. It clearly shows the effect that the current has on altering the true wind from the ground wind. This is the effect I am talking about - the effect on the ground wind by the current to create a new true wind. It is a great little diagram, and I hope you don't mind my copying it below to save people the trouble. It shows when the current lines up with the apparent wind (which in this case, with no boat speed, is equal to the true wind) there is nil effect on the direction of the wind (360 and 180 deg. apparent). The two red lines which are most different angle wise from the ground wind are, you can see, at exactly 90 degrees and 270 degrees to the current. A great feature of the diagram which you probably didn't even realise you included, which shows conclusively that what matters here is the angle of the current relative to true wind (in zero boat speed scenario) or apparent wind (if there is boat speed) and NOT the ground wind. The point at 90/270 deg. to ground wind is at a slightly lower angle. A subtle but important distinction.

The diagram shows that when the current is between 0 and 180 degrees to the true wind then it creates a favourable port tack, and when it is between 180 and 360 degrees it creates a favourable starboard tack. At exactly 360 and 180 degrees there is nil effect, and the ground wind is the same as the true wind. Every six hours, the current would flip 180 degrees and in so doing alter the favourable tack. This is all the 'lee bow effect' is.

Originally Posted by Seaworthy Lass

This is where you are going wrong. It is not the angle relative to the boat, it is the angle relative to the ground wind that the effect is centred on. This just happens to coincide with being close to the bow.

The effect by the way is maximum roughly at 90 or 27O degrees to the GROUND wind, NOT the apparent wind (is alters a bit towards the ground wind when the current is high compared to the ground wind.

Nope. As I say above, your diagram shows the biggest angular change in true wind when the current is at 90/270 degrees to the TRUE wind. In fact, taking boat speed in to account as well (which your diagram does not) this extends to it being relative to the APPARENT wind.

Your turn.

[goboatingnow]

Originally Posted by europaflyer

Hi Angela,

I will try to use your nomenclature as used on the diagram in this post. Note that what you call true wind I have previously called apparent or 'water referenced true' wind, which is a potential source of confusion.

Your diagram shows both winds - ground wind (what I call true wind) as the thick black central line, and the true wind (what I call apparent wind) as the red lines. It clearly shows the effect that the current has on altering the true wind from the ground wind. This is the effect I am talking about - the effect on the ground wind by the current to create a new true wind. It is a great little diagram, and I hope you don't mind my copying it below to save people the trouble. It shows when the current lines up with the apparent wind (which in this case, with no boat speed, is equal to the true wind) there is nil effect on the direction of the wind (360 and 180 deg. apparent). The two red lines which are most different angle wise from the ground wind are, you can see, at exactly 90 degrees and 270 degrees to the current. A great feature of the diagram which you probably didn't even realise you included, which shows conclusively that what matters here is the angle of the current relative to true wind (in zero boat speed scenario) or apparent wind (if there is boat speed) and NOT the ground wind. The point at 90/270 deg. to ground wind is at a slightly lower angle. A subtle but important distinction.

The diagram shows that when the current is between 0 and 180 degrees to the true wind then it creates a favourable port tack, and when it is between 180 and 360 degrees it creates a favourable starboard tack. At exactly 360 and 180 degrees there is nil effect, and the ground wind is the same as the true wind. Every six hours, the current would flip 180 degrees and in so doing alter the favourable tack. This is all the 'lee bow effect' is.

Nope. As I say above, your diagram shows the biggest angular change in true wind when the current is at 90/270 degrees to the TRUE wind. In fact, taking boat speed in to account as well (which your diagram does not) this extends to it being relative to the APPARENT wind.

Your turn.

This debate cannot be resolved when issues such as your terminology are wrong.

For sailors we have

Apparent Wind . The only real wind , ie the one you experience on a boat

True wind : incorrectly applied in marine logic, but now means the wind felt by a person not moving on he surface of the water

ground Wind , the wind felt by a person not moving in respect of the ground. Ie anchored etc

You need to restate your argument in those terms for clarity

Dave

[europaflyer]

Originally Posted by goboatingnow

This debate cannot be resolved when issues such as your terminology are wrong.

For sailors we have

Apparent Wind . The only real wind , ie the one you experience on a boat

True wind : incorrectly applied in marine logic, but now means the wind felt by a person not moving on he surface of the water

ground Wind , the wind felt by a person not moving in respect of the ground. Ie anchored etc

You need to restate your argument in those terms for clarity

Dave

This is what I did in my last post. I have previously been using 'true wind' wrongly, or at least differently!

[goboatingnow]

Originally Posted by europaflyer

This is what I did in my last post. I have previously been using 'true wind' wrongly, or at least differently!

That diagram is not correct. It purports to show the boat in a current induced apparent wind. In this case the boat will move with the current. Hence the apparent wind will move forward and the boat is headed. Irrespective of anything , if a boat moves forward by whatever means it is always headed , ie the apparent wind always moves forward. , never aft as that diagram attempts to show.

Dave

[europaflyer]

Originally Posted by goboatingnow

That diagram is not correct. It purports to show the boat in a current induced apparent wind. In this case the boat will move with the current. Hence the apparent wind will move forward and the boat is headed. Irrespective of anything , if a boat moves forward by whatever means it is always headed , ie the apparent wind always moves forward. , never aft as that diagram attempts to show.

Dave

True for one tack. On the other tack the boat will be going against the current ('lee bowing') and will be freed ('lee bow effect').

[goboatingnow]

Originally Posted by europaflyer

True for one tack. On the other tack the boat will be going against the current ('lee bowing') and will be freed ('lee bow effect').

Are you suggesting that a boat attempting in conditions of zero true wind, can make ground into the current. I know of no polars that suggest that's possible.

The position of the wind vectors makes no sense. The apparent wind is in the wrong position relative to the boat orientation.

Every increase in boat speed causes a header. Irrespective of wind direction or tack


Dave

[Paul Elliott]

Originally Posted by goboatingnow

That's not how to go about selecting a CTS.( or more correctly a route ) If you cannot lay the destination then you , compute a CTS for a course You can sail. A CTS is always a course a Helmand steers.

Dave

Dave, look at my post discussing the CTS term
This CTS is not the CTS you are thinking of. Sorry, it's one of those definition things. I didn't invent the term, or the usages, but there is some context you have missed.

[goboatingnow]

Originally Posted by Paul Elliott

Dave, look at my post discussing the CTS term
This CTS is not the CTS you are thinking of. Sorry, it's one of those definition things. I didn't invent the term, or the usages, but there is some context you have missed.

That may be your interpretation. But CTS has nothing to do with tacking , CTS is the commanded course to steer , the navigator in computing CTS, will take into account ability for the vessel to be able to proceed in any given direction and will modify his CTS calculations accordingly

In a multi hour current cts scenario , it's makes no sense to tack around your CTS , as the wind direction may result in undoing all the benefits of say a multi hour CTS. If one computes a CTS that one cannot achieve or one finds that a previously CTS is now un achievable , then one recomputes a new CTS using the appropriate data at hand.


But CTS has nothing to do with that diagram it's just wrong IMHO
Dave