The "lee bow effect"

[bdh198]

Now I know this is a perennial question and debates over whether there is such a thing as a “lee bow effect” have been going on for decades, but I’m nevertheless finding it quite difficult to fully understand what is supposed to be happening and whether there is ever any benefit from being lee bowed.

First of all I understand there are several definitions of “lee bow effect”, but it is only the last one (below) I am struggling with:

1. During a race when you tack downwind and just in front of an opponent and spill your foul air over them – I understand this and it is not what I’m concerned about.

2. When sailing an upwind passage where the tide strengths/direction varies and you tack to keep the tide on the lee bow thereby staying closer to the rhumb line – again, this I understand and it’s not what’s confused me.

3. The idea that by keeping a tide/current on the lee bow when sailing upwind enables you to either sail faster through the water or sail a shorter distance (presumably by pointing higher) – now this is what I’m keen to get clear.

I am familiar with the arguments by the likes of Dave Perry and others who say the idea of an advantage being gained by keeping the tide on the lee bow is a myth that doesn’t stand up to mathematical scrutiny (looking at vectors etc) and that the tide affects all objects on the water exactly the same (conveyor belt analogy); however, there are a number of people who I’ve raced with who nevertheless feel there is an advantage to be had by keeping the tide/current on a lee bow when on an upwind beat.

Irrespective of Dave Perry arguments, is there any advantage that can be gained by keeping the tide/current on the lee bow when there is an even tide at a constant rate for the duration of the beat?

 

[mainsail1]

In my experience it seems to be that way but I too would like a definitive answer to this question.

 

[bedouin]

As I understand it the lee bow effect only applies when you expect the tide to change during a beat. In that case there is no doubt that planning as far as possible to keep the tide on the lee bow will get you there faster. The point being that the tide will move the apparent wind aft and so allow you to point higher.

If you expect constant tide and constant wind it makes no difference.

 

[RupertW]

I'm looking forward to the answer to this as I get that a tide on the Lee bow will push you windwards and therefore towards you derivation - but it will also pinch you into the wind so you will have to steer away to get the ideal wind angle to your sails.

 

[bedouin]

I'm looking forward to the answer to this as I get that a tide on the Lee bow will push you windwards and therefore towards you derivation - but it will also pinch you into the wind so you will have to steer away to get the ideal wind angle to your sails.

Wrong way round - adding the tide to the wind will move the apparent wind aft

 

[lw395]

The lee bow effect is all about people not being able to think in vectors.
Most of the arguments about it are all about people not understanding the effect the other person is describing, which usually feeds into a disagreement about whether the effect exists.
"here are a number of people who I’ve raced with who nevertheless feel there is an advantage to be had by keeping the tide/current on a lee bow when on an upwind beat."
What does this mean exactly?
Are you asking whether it's worth pinching to keep the tide under the lee bow?
If so, in general no. But not always!
Take for instance sailing off the Squadron line at Cowes. SW wind, so close hauled is roughly parallel to the shore. Tide pissing East.
It will often be worth pinching to keep the tide under the lee bow, because that gets you into weaker tide.

But in general, just do the vector maths. Your best course upwind will be the same apparent wind angle regardless of where the tide is. That may mean you lose ground on one tack in order to make more ground on the other.

 

[lpdsn]

3. The idea that by keeping a tide/current on the lee bow when sailing upwind enables you to either sail faster through the water or sail a shorter distance (presumably by pointing higher) – now this is what I’m keen to get clear.

The key component is what is sometimes called the tide wind. That is the effective wind felt by the boat as the tide pushes it relative to the ground. The true wind (now there's another debate but let's call it the wind over the ground, which is not the same as the 'true wind' given by most on-board instruments) is lifted by the tide wind. That's where you gain.

Of course in the real world the true wind will change substantially over a whole tidal cycle, so blindly keeping going with the tactic of keeping the tide on the lee bow regardless of what the wind actually does won't win you races. Not to mention that you'll cover a fair bit of ground in that time and the various directions and strengths of tide will often be significantly different in those new locations.

 

[RupertW]

Wrong way round - adding the tide to the wind will move the apparent wind aft

You may be right but i can't see it. If a tide pushes you from the Lee side it is pushing you to windward. So now your boat has an apparent wind shifted forewords so you have to .... oh I see ...it's a slightly stronger apparent wind with angle more from the side, hence aft

 

[bedouin]

You may be right but i can't see it. If a tide pushes you from the Lee side it is pushing you to windward. So now your boat has an apparent wind shifted forewords so you have to .... oh I see ...it's a slightly stronger apparent wind with angle more from the side, hence aft

Just think about the situation when there is no true wind - it is obvious that the impact of the tide is to put the wind more on your beam, hence move it aft when beating

 

[flaming]

In my experience it seems to be that way but I too would like a definitive answer to this question.

The definitive answer is this.

There is no advantage to pinching to get the tide the other side of your keel.

The reason that people sometimes get confused is that they are misreading the situation. Sometimes there are tactical reasons to pinch, such as not sailing out into greater tide or reducing the number of tacks needed to get round a mark that you are nearly at. But if you’re in constant tide and a long way off the mark, then just sail your normal upwind angles.

If you still get confused then think of it this way. Instead of tide coming towards you, imagine that you are sailing on a lake, but the mark is a rib that is motoring slowly away from you. If you’d pinch briefly to chase the rib and nip round it without tacking, then do that. But if you’d just tack up to it, then do that.

 

[mainsail1]

Thank you.
Now I am really confused...... and will go for a lie down!
My experience is not about better or worse pointing but more about better boat speed when lee bowing a tide.

 

[flaming]

Thank you.
Now I am really confused...... and will go for a lie down!
My experience is not about better or worse pointing but more about better boat speed when lee bowing a tide.

Boat speed through the water when referenced to true wind as measured by your wind instruments (Best thought of as the difference in velocity between the air and the water) is completely uneffected by tide.
So if you see 10 knots true on your wind instruments you will always be aiming for the same STW (speed through the water) regardless of whether the tide is against you, under you or across your path. And regardless of whether is is 1 knot or 10.

 

[Mark-1]

The Dave Perry article really spells out why it's a myth. I can't think of a better explanation.

EDIT TO ADD THE FULL TEXT:

THERE IS NO LEE-BOW EFFECT - Dave Perry

One of the most fascinating and timeless controversies in our sport is over what effect current has on
how we sail and race our boats. Beginning in early 1979, Peter Isler and I filled hours of time debating
the effects of current, and it wasn't until mid-1980 that he finally parted my clouds and shook me loose
from years of misconceptions and incorrect assumptions. Here then is my understanding of the effects
of current, substantiated by several of my more mathematically-clever friends.

Assuming that we're sailing in constant current direction and strength, No! As we've determined, the
direction and strength of the current created wind is the same no matter at what angle the boat is aiming
or at what speed it is moving. The presumption of the lee-bow effect is that if you are sailing directly into
the current you can pinch slightly, putting the current on your leeward bow, and the current will push you
up to weather. This is obviously false because the only direction the current can move you is in the
direction it is going (the stick on the river).

The presumption of those who believe that in current a boat will have a different apparent wind direction
and strength on opposite tacks, is that on one tack the boat will be slowed more by the current than on
the other. The extreme example is when port tack takes you right into the current, and starboard tack
takes you across it. The illusion is that on port tack it would seem that the boat is still going forward
toward the wind, but that on starboard the boat is being swept away from the wind by the current.
Therefore, the apparent winds must be different on the two tacks.

The fallacy here, though, is that the judgment of going toward the wind and being swept away are made
in reference to fixed objects such as the mark, an anchored boat, or land. In reality, both boats are being
affected equally by the current and the wind "sees" both boats in the same way. In other words, if you
were following the race in a motorboat and were in the ocean where you couldn't see any land for
reference, the boats would look identical on either tack, and in fact you would have no clue that there
even was current unless you knew from charts or perhaps from the surface condition of the water. Put
another way, if you're sailing on a boat with apparent wind strength and direction instruments, they'll
read the same on both tacks because the boat is affected in the same way by the current on either tacks
(the stick in the river again).

Winning in One-Designs by Dave Perry

 

[Halo]

The key component is what is sometimes called the tide wind. .

+1
If there is no ground wind at all and you are being driven at 4 knots by a tide there will be an apparent wind of 4 knots. If you point across the tide then you will be pointing across this wind and able to sail very slowly. The wind will be from the opposite side of the lee bow. Lee bowing is tactically making the best use of this tide wind which can add to or reduce the apparent wind. As others have pointed out it may not be possible to secure any advantage from this effect but there must be times when you can use it.

 

[Halo]

Boat speed through the water when referenced to true wind as measured by your wind instruments (Best thought of as the difference in velocity between the air and the water) is completely uneffected by tide.
So if you see 10 knots true on your wind instruments you will always be aiming for the same STW (speed through the water) regardless of whether the tide is against you, under you or across your path. And regardless of whether is is 1 knot or 10.

The above is true - so you are right - BUT "true wind" by your definition varies with the strength of the tide - so in an area with a 4 knot tidal flow the true wind can vary by 8 knots over a 6 hour period simply due to tidal flow. The difference between sailing an 8 knot true wind and a 16 knot true wind is massive. Tactically using this must make sense.

 

[JumbleDuck]

Boat speed through the water when referenced to true wind as measured by your wind instruments (Best thought of as the difference in velocity between the air and the water) is completely uneffected by tide.

I feel nervous about even asking, you being a whizz-bang racing skipper and me being a pottering cruiser,but are you sure about that definition of "true wind"? The first ten Google hits agree with what I thought, which is that it is the velocity of the wind relative to the earth.

 

[BelleSerene]

I feel nervous about even asking, you being a whizz-bang racing skipper and me being a pottering cruiser,but are you sure about that definition of "true wind"? The first ten Google hits agree with what I thought, which is that it is the velocity of the wind relative to the earth.

True wind is relative to the water.

Ground wind is relative to a point on the chart.

Apparent wind is relative to you - but you knew that.

Sources that call the wind over the ground ‘true wind’ are ignoring any tide.

 

[flaming]

The above is true - so you are right - BUT "true wind" by your definition varies with the strength of the tide - so in an area with a 4 knot tidal flow the true wind can vary by 8 knots over a 6 hour period simply due to tidal flow. The difference between sailing an 8 knot true wind and a 16 knot true wind is massive. Tactically using this must make sense.

Yes. Knowing how this works, and using the predictable wind strength and direction changes as the tide changes or you sail into or out of the strong tide is an essential part of tactical sailing in tidal waters.

 

[flaming]

I feel nervous about even asking, you being a whizz-bang racing skipper and me being a pottering cruiser,but are you sure about that definition of "true wind"? The first ten Google hits agree with what I thought, which is that it is the velocity of the wind relative to the earth.

I’ve had this discussion more than once on this forum!
Ground wind = wind over ground or anchored boat
True wind = wind as seen by boat drifting with tide
Apparent wind = wind as measured by boat that is Sailing.

 

[lpdsn]

I’ve had this discussion more than once on this forum!
Ground wind = wind over ground or anchored boat
True wind = wind as seen by boat drifting with tide
Apparent wind = wind as measured by boat that is Sailing.

That's the historic usage which comes from pre-GPS instrumentation where it was impossible to calculate what the true (ground) wind was actually doing. Problem with that usage is that it misleading implies the true wind is true, whereas it is the 'ground' wind which is true. I'm sure the usage of calling the ground wind the true wind will finally predominate.