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

[europaflyer]

Originally Posted by foolishsailor

Europa,

I see you are using a boat speed of 6 kts which is different to the 10kts Im using - which raises something very intersting as well interms of STW for this situation that I hadn't thought about.

however your headings dont seem to be correct assuming a 90 tacking angle relative to the wind. The wind shifts for the different current speeds are 6, 9 and 12° respectively.

I will finish working out the other tack and then redo it with 6kt boat speed to see the difference

You posted this while I was writing my post above. Nice to know we're both reaching the same conclusions pretty much simultaneously.

My headings probably aren't exactly correct - as I said, accurate but not precise, IE. they are OK relative to each other for making a point, but not good if we are working to the nearest degree!

[foolishsailor]

This is covered without calculating by the idea that you always go toward the area of least current first. It is used primarily if you have to tack upwind in current to get to your destination.

The phrase

"Beam to Big Current then Bow to little Current" as a rule works well to get you upwind and up current faster.

However I may have created a problem where that doesn't actually work - we will see. Ill start on the other tack now.

See the attached image from much earlier. It represent a winning decision by a boat in Cowes Week. He went running to the light current and none of us thought the current was going to give him enough relief to offset the huge loss he made by beam reaching across.

But it did, he was a last plae and slow boat the rest of the Series and in this race he beat the top Pro boats by over 18 minutes.

[europaflyer]

Originally Posted by foolishsailor

This is covered without calculating by the idea that you always go toward the area of least current first. It is used primarily if you have to tack upwind in current to get to your destination.

The phrase

"Beam to Big Current then Bow to little Current" as a rule works well to get you upwind and up current faster.

However I may have created a problem where that doesn't actually work - we will see. Ill start on the other tack now.

See the attached image from much earlier. It represent a winning decision by a boat in Cowes Week. He went running to the light current and none of us thought the current was going to give him enough relief to offset the huge loss he made by beam reaching across.

But it did, he was a last plae and slow boat the rest of the Series and in this race he beat the top Pro boats by over 18 minutes.

Surely this is just a throw of the dice scenario? I don't see any intrinsic advantage in either course.

[Dockhead]

Originally Posted by foolishsailor

This is covered without calculating by the idea that you always go toward the area of least current first. It is used primarily if you have to tack upwind in current to get to your destination.

The phrase

"Beam to Big Current then Bow to little Current" as a rule works well to get you upwind and up current faster.

However I may have created a problem where that doesn't actually work - we will see. Ill start on the other tack now.

See the attached image from much earlier. It represent a winning decision by a boat in Cowes Week. He went running to the light current and none of us thought the current was going to give him enough relief to offset the huge loss he made by beam reaching across.

But it did, he was a last plae and slow boat the rest of the Series and in this race he beat the top Pro boats by over 18 minutes.

Jesus, what kind of boats are those? They are tacking through less than 90 degrees over ground despite the adverse current.

Sorry for the thread drift

[foolishsailor]

Dockhead - sorry to get you excited - it is not a real ground track. I couldnt get the ground track to print an image so I estimated.

Ignore the tacking angles - the only relevent info the short tacking on the two angels that were used to cross at top and bottom in red and yellow.

If I wanted to be really precise the short tacking in the yellow whould have been almost invisible - like a fuzzy line of tacking - they said they did over a hundred tacks to stay in current relief

[foolishsailor]

Ok, my head is going to explode - here is the fully updated image.

I may leave it to someone else to calculate the exact ETE for both boats - but it should be pretty clear...

One boat is sailing into 3kts of current, the other is sailing into 2kts of current.


The port tack start
19nm @ 7kts = 2.71hrs
1.5nm@ 6kts = .25hrs
6.5nm @ 10.7kts = .6 hrs
4nm @ 10.2kts = .39
31nm Total

ETE = 3.95hrs
Average Speed = 7.84

The starboard tack start
4nm @ 10.45kts = .38hrs
7nm @ 10.7kts = .65hrs
1.4nm @ 10.2 = .13hrs
21.5nm @ 8kts = 2.68hrs
33.9nm Total

ETE = 3.84hrs
Average speed = 8.83kts

Delta gain .11 hour or 6 minutes.

That is a dramatic gain...

...for a racer

...maybe not so much of any real benefit to a cruiser.

However there are other scenarios where you are required to tack upwind against a current (the situation I originally proposed to SL) and it is worth while to go across a tidal stream to tack up a current that is less than the side you are on.

[Seaworthy Lass]

A zillion and one thoughts here LOL.
After a day offline I cant keep up with reading the posts LOL. My only comment is that my head is now hurting, but I must be a sucker for punishment as I am enjoying it LOL.

The strategy proposed by Europaflyer is almost the same as I initially proposed when Foolish set the example several days ago. Foolish was proposing the exact opposite (along with the entire fleet with experienced tacticians LOL), so I knew I must be wrong suggesting it, but I still went ahead and did it.

After a day I talked myself into the reason why I was wrong LOL . I will now look at it again freshly as there seems to be support for my proposal.

JC, could we please have some figures for the width of each segment of current and the expected boat speed for the three lots of true wind and the angle between tacks for each true wind amount?

Rather than guesswork we could then play with figures.

Note: CTS is a very complicated determination in this case as the boat speed varies over the leg.

[Dockhead]

Originally Posted by foolishsailor

Dockhead - sorry to get you excited - it is not a real ground track. I couldnt get the ground track to print an image so I estimated.

Ignore the tacking angles - the only relevent info the short tacking on the two angels that were used to cross at top and bottom in red and yellow.

If I wanted to be really precise the short tacking in the yellow whould have been almost invisible - like a fuzzy line of tacking - they said they did over a hundred tacks to stay in current relief

I was going to say, you'd need a wing sail to do that against the current.

[foolishsailor]

Originally Posted by Seaworthy Lass

A zillion and one thoughts here LOL.
After a day offline I cant keep up with reading the posts LOL. My only comment is that my head is now hurting, but I must be a sucker for punishment as I am enjoying it LOL.

The strategy proposed by Europaflyer is almost the same as I initially proposed when Foolish set the example several days ago. Foolish was proposing the exact opposite (along with the entire fleet with experienced tacticians LOL), so I knew I must be wrong suggesting it, but I still went ahead and did it.

After a day I talked myself into the reason why I was wrong LOL . I will now look at it again freshly as there seems to be support for my proposal.

JC, could we please have some figures for the width of each segment of current and the expected boat speed for the three lots of true wind and the angle between tacks for each true wind amount?

Rather than guesswork we could then play with figures.

Note: CTS is a very complicated determination in this case as the boat speed varies over the leg.

Its all on there - each grid line is one nautical mile. The currents are deliniated by the change in color.

We assume in the case I present above that the boat maintains a STW of 10kts. SOG for each segment i outlined in my post above. you can download the image, print it if you like and measure it for a very precise calc - it is to scale.

[europaflyer]

Great work by Foolishsailor. What you say makes sense, I hadn't really considered it in terms of maximising time in the lesser current. Which is funny, because it would be more obvious in real life, it's just that this theory is killing me.

Anyway, we are now firmly outside the realm of the lee bow effect/bow effect and in to more conventional navigation. The true LBE situation needs a current change to swap over the favoured tack.

I'm pretty sure we are all in agreement here about what I shall carefully call 'the effect'. Staying on the favoured tack is what it boils down to. If the current can alter the favoured tack as it changes, then you need to stay on the tack with the current nearer the bow (thanks Angela)... or use the 'true wind current angle' formula I proposed earlier (same thing).

'Bow effect' is a good name for this. As no-one outside this forum uses it, I will continue to use the questionable term 'lee bow effect'. As I said earlier, just because a rope is called a halliard doesn't mean it has to haul a yard...

It's been a great ride people. Thanks!

[foolishsailor]

I should have just stuck with my original scenario - I was just feeling too lazy to calculate all the tacks for an upwind situation.

But here is another = maybe someone else can work out a starboard port tack scenario between two boats.

I would start by crossing the whole way on starboard tack and then tack up the far side staying in the 2kt boundary.

[Hoofsmit]

Originally Posted by Seaworthy Lass

Hoof, I did see this and did get it. I agree the strips are different if current is different around the course at any particular moment. This does not invalidate the tablecloth analogy though . And yes, I agree CTS cannot be used if the current us changing with position rather than time. I have stated that after my initial brain freeze .

This does not invalidate the tablecloth , i just cut it into 50 ! To use a table cloth analogy is can be misleading to your navigation.

My point was purely that you only address sailing to a destination with your CTS ( which I do consider is one which I will adoubt for passage making ) method of thought,

the whole point of my previous posts have been to prove to you that in the environment which I described as a rule of thumb for the lee bow discussion 2. In my # 271 post sailing by the seat of your pants
or pilotage,
with visual ground base features to reference To ...say a small strong tidal harbour after a trip across the English Channel
your point of reference help considerably if you can change your mind Set away from a passage plan approach of CTS

Possibly I think you may now see my point

Pretty shore DH is sticking to his ego ( tongue in cheek)

All I would say is....... When he entered Yarmouth yesterday did he plot a cts with all vectors and keep popping to chart table every 30 seconds to change his tidal vector as he can't lay his Wpt because the wind veered and he missed the flood at 1450 and now he is heading on a 1.32 knt ebb tide
" hell I better turn the tablecloth around"

I. Think not !
He probably looked for the radio mast on the bend and the shore, a cable to each side of his hull ,referenced the tide to his hull going through the current and thought " hell it's against me I better look on the surface of the water ( tablecloth) to find out if the current is reduced to my starboard because I need to stem this tide as much as I can if ,this wind the that was coming from the hospital is now coming from west key and I will be going backwards if the dame idiot pulling this dame tablecloth doesn't get tired soon!

[europaflyer]

Originally Posted by foolishsailor

I should have just stuck with my original scenario - I was just feeling too lazy to calculate all the tacks for an upwind situation.

But here is another = maybe someone else can work out a starboard port tack scenario between two boats.

I would start by crossing the whole way on starboard tack and then tack up the far side staying in the 2kt boundary.

Dead simple. No choice but to start on starboard, any extra beating that has to be done wait until in the 2kt area.

We're just on to more 'basic' (ha) strategy now. Not to do with lee bow anything.

In fact, I think this is pretty much exactly the same as your Cowes Week example, except that doesn't make the differences between the tides near the shore clear.

[Seaworthy Lass]

Originally Posted by foolishsailor

Its all on there - each grid line is one nautical mile. The currents are deliniated by the change in color.

We assume in the case I present above that the boat maintains a STW of 10kts. SOG for each segment i outlined in my post above. you can download the image, print it if you like and measure it for a very precise calc - it is to scale.

Ah, yes, sorry, missed we were working on your grid now.

The boat speed will vary in each segment as true wind is altering. This can't be dismissed. The devil may be in the detail with examples like this.

(Dockhead in an example like this CTS is not a simple calculation as boat speed is varying).

[Dockhead]

Originally Posted by europaflyer

Great work by Foolishsailor. What you say makes sense, I hadn't really considered it in terms of maximising time in the lesser current. Which is funny, because it would be more obvious in real life, it's just that this theory is killing me.

Anyway, we are now firmly outside the realm of the lee bow effect/bow effect and in to more conventional navigation. The true LBE situation needs a current change to swap over the favoured tack.

I'm pretty sure we are all in agreement here about what I shall carefully call 'the effect'. Staying on the favoured tack is what it boils down to. If the current can alter the favoured tack as it changes, then you need to stay on the tack with the current nearer the bow (thanks Angela)... or use the 'true wind current angle' formula I proposed earlier (same thing).

'Bow effect' is a good name for this. As no-one outside this forum uses it, I will continue to use the questionable term 'lee bow effect'. As I said earlier, just because a rope is called a halliard doesn't mean it has to haul a yard...

It's been a great ride people. Thanks!

This thread has been a remarkable lesson on how much easier it is to solve complicated problems when you have several smart people all collaborating. For me, it's been a great leap forward in understanding these issues.

I think "Lee Bow Effect" is probably an ok is not totally precise term for the one flavor of it which, by consensus view, actually works. That's because of this:

Geometrically, the wind shift caused by a tide change is going to be greatest when the current is just perpendicular to the ground wind. When the current is aligned with the ground wind, there is zero wind shift.

When the current is perpendicular to the ground wind, then in fact keeping the lee bow into the current is what exactly does the trick. So I think it's not an accident that it was called like this. This covers every conceivable Channel crossing situation.

As the direction of the tide deviates from perpendicular to the ground wind, the magnitude of the wind shift falls off. Of course, the magnitude of the wind shift also depends on the relative velocities of wind and current, so the effect will be more profound at springs and light wind, than it will at neaps and strong wind.

If you imagine the tide cocking around off the perpendicular, I can see that on one favored (lifted) tack, the tide might be on your lee quarter or lee beam. But I'm struggling to imagine a case where you will have a favored tack -- a tack lifted by a tide change -- with the tide on the weather side of the boat at all. Maybe Angela, who has been a step ahead on this, could help.