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

[Hoofsmit]

Ok apples to apples. I think this is the problem, the variants are to broad to compute.
All I know is that most craft balance on one tack better and achieve better results on that tack. You both seem to miss the obvious that a craft that is planeing has very little reaction to current than a heavy displacement larger wetted area craft. Let a lone waves!
You have tried to compute the affect of increased wind on the sails on certain vectors but the other factors of the boat may well out way any conclusion maths give you.
We all know constants rarely if ever exist on a passage or even a leg. The way to do this lee bow discussion is to sail it in practice. That's why we use xte , sog , the quickest time from A to B is all that matters.

[Dockhead]

Originally Posted by AndytheSailor

My only comment, while agreeing with Seaworthy, is that if you are crossing a tide on a tack, if the tide is on the lee bow then you can lay you course less close to the wind, thus sail faster and still make your destination. I know this doesn't deal with the maths but it does offer the logic.

Oh, we all got that from the very beginning. My drawings in Post #76 lay out that argument, which is simply that the change of tide will make the wind freerer if you tack so as to keep the lee bow into the current.

But there seem to be two different things wrong with that on two different layers, one of which seems to be uncontroversial, and the second of which Seaworthy is about to poke holes in.

So we don't know for sure yet whether it works or not, but I think we're making rapid progress.

[Dockhead]

Originally Posted by Seaworthy Lass

Hey, that's cheating. That is not how this game works .

I need a cuppa, maybe address what I have just written while I make one

If it hasn't sunk in I will then go and poke holes.

Sure it works that way If you have a smart person who disagrees with you, that saves half your work. Because if you didn't have that person, you would have to poke the holes yourself, or try to, in order to really know whether you were on to something or not.

[Hoofsmit]

Am I talking b..locks or am I missing the point, I really don't see the point of coming to a mathmatical equation if the other variables I have mentioned out way any theoretical advantage, the proof is in the pudding not in the recipe book !

[Dockhead]

Originally Posted by Seaworthy Lass

I am not LOL.



Yes, you are sailing to point B on the water. This is your destination point on the water. This will wander all over the ground when you have any current. However, when you arrive at your destination point on the ground, point B will at that instant coincide with it



Yes, your heading to point B does not depend on current at all. We both agree on this. But when you can't steer the CTS to B because you are close hauled, then it is a case of coordinating with the current, as the current alters the direction of your true wind. If the true wind is due to shift because the ground wind changes, then you have a favoured tack. The same applies if it due to change because of current.

OK -- that's all clear and logical and agreed. So far, we are absolutely on the same page.

But now show the next bit -- what is that wind shift going to look like to your sails, sailing on a constant heading or tacking up a constant average heading?

And most importantly: Can you exploit that wind shift, or not?

And going to my last proposition, which you're going to poke holes in: Can you even feel the wind shift?

Traps not to fall into:

1. You're not sailing constant COG like in my first set of drawings. I don't think you've missed this.

2. It's devilishly hard to keep the frames of reference straight. Don't mix up a ground-referenced parameter with a water-referenced one. I now feel like I might have done this myself with my last proposition.


Now have at it. I have to do some work, so won't be able to contribute for a few hours.

[Dockhead]

Originally Posted by Hoofsmit

Am I talking b..locks or am I missing the point, I really don't see the point of coming to a mathmatical equation if the other variables I have mentioned out way any theoretical advantage, the proof is in the pudding not in the recipe book !

Bzzt, thanks for playing!

Sorry, just joking! Indeed, you are talking bullocks a bit -- you should read through this thread: http://www.cruisersforum.com/forums/...ine-97251.html. You will not get across moving water fastest by using SOG and COG, if the current is changing during your passage. You have to calculate a Course to Steer, and hold a constant heading.

So neither can you use SOG, COG, XTE data from your GPS to solve this question. We are discussing other ways to solve it.

[Seaworthy Lass]

Originally Posted by Dockhead

OK -- that's all clear and logical and agreed. So far, we are absolutely on the same page.

But now show the next bit -- what is that wind shift going to look like to your sails, sailing on a constant heading or tacking up a constant average heading?

And most importantly: Can you exploit that wind shift, or not?

And going to my last proposition, which you're going to poke holes in: Can you even feel the wind shift?

Traps not to fall into:

1. You're not sailing constant COG like in my first set of drawings. I don't think you've missed this.

2. It's devilishly hard to keep the frames of reference straight. Don't mix up a ground-referenced parameter with a water-referenced one. I now feel like I might have done this myself with my last proposition.


Now have at it. I have to do some work, so won't be able to contribute for a few hours.

Yes, you will of course feel the wind shift. The true wind has shifted.

I will answer all this later. Just anchored, dashing ashore before the shops shut for the usual four hour siesta. Been without shops for a month now!

[Hoofsmit]

Ok .... This is getting silly, " can you feel the wind shift" if you cannot feel it on the heading you need then the current is in your favour ! But the reality of wind shift and current change simultaneously ain't gonna happen ( unless you would like to do a mathmatical equation of probability)

Lee bow is the question ....... I think we are going back to last years thread

[Hoofsmit]

Originally Posted by Dockhead

Bzzt, thanks for playing! Sorry, just joking! Indeed, you are talking bullocks a bit -- you should read through this thread: http://www.cruisersforum.com/forums/...ine-97251.html. You will not get across moving water fastest by using SOG and COG, if the current is changing during your passage. You have to calculate a Course to Steer, and hold a constant heading. So neither can you use SOG, COG, XTE data from your GPS to solve this question. We are discussing other ways to solve it.

I never said don't plot a CTS , I ment that the variables will cause you to adjust you CTS on your passage .
I love doing the blind nav in a YM course , but you soon find out that some things you cannot calculate , when doing a CTS , so that's when you use other aids to find out where you are (or not ' as is more important ) thus the reference to xte etc
adding more maths to a standard CTS with current , leeway, already worked in is pointless because it will change on route, the topic is lee bow affect.

Please don't think I am being negative , I was trying to bring some practical in put to consider ,with the question of lee bowing a current

[Dockhead]

Originally Posted by Hoofsmit

I never said don't plot a CTS , I ment that the variables will cause you to adjust you CTS on your passage .
I love doing the blind nav in a YM course , but you soon find out that some things you cannot calculate , when doing a CTS , so that's when you use other aids to find out where you are (or not ' as is more important ) thus the reference to xte etc
adding more maths to a standard CTS with current , leeway, already worked in is pointless because it will change on route, the topic is lee bow affect.

Please don't think I am being negative , I was trying to bring some practical in put to consider ,with the question of lee bowing a current

Not at all; all contributions to the topic will be very welcome.


The Lee Bow Effect has a number of problems in common with CTS since both deal with crossing moving water.

[estarzinger]

I have seen no mention about boat polars in this thread. That seems to be a serious omission.

You all seem to be assuming that optimal vmg upwing sailing angle is constant. But that is (usually) not true

Most boats can sail higher upwind angles in higher wind (wave state being the same, and in 'less than gale force' winds).

Thus you can (normally) sail higher when lee bow.

It's not 'pinching'. It's optimal polar vmg sailing

[transmitterdan]

Originally Posted by estarzinger

I have seen no mention about boat polars in this thread. That seems to be a serious omission.

Yes, I mentioned it very early on and you are right it is very important. For that reason alone simple algebra cannot solve the problem. It's a differential equation problem. Think of it like this;: as soon as the drift induced wind creates forward thrust on one polar the same induced wind changes it's vector heading and speed. So then a new polar is in play and the boat will find a new speed and direction. This continues until a stable condition is found. The trick is to find that stable condition which I would guess to be near impossible to do by hand calculation as it will take nearly as long as the passage itself.

Is there an advantage or disadvantage created by current? Yes, the same as if you had more or less wind. How much is it and what course should I steer to get all of it? Very hard question to answer.

[Dockhead]

Originally Posted by estarzinger

I have seen no mention about boat polars in this thread. That seems to be a serious omission.

You all seem to be assuming that optimal vmg upwing sailing angle is constant. But that is (usually) not true

Most boats can sail higher upwind angles in higher wind (wave state being the same, and in 'less than gale force' winds).

Thus you can (normally) sail higher when lee bow.

It's not 'pinching'. It's optimal polar vmg sailing

We are indeed assuming that the optimal VMG upwind sailing angle is constant.

But I think we established that this is actually a reasonable assumption. The question of lee bowing does not arise unless the wind direction is perpendicular or nearly perpindicular to the direction of the current flow. Therefore, effect of your choice of tack on wind speed is limited.

The main question is the shift in True Wind direction caused by the change of tide. That is what we are struggling with now.

The limited effect on wind speed of lee bowing can be expressed like this: you will get a slight lift from increased wind speed by lee bowing IF increasing wind speed does give you lift (above a certain speed it might not). The degree of increase of wind speed depends on the deviation from perpendicular of the angle between Ground Wind direction and direction of current flow. This increases as you approach an angle which would allow you to lay your mark on one tack. This effect also increases in the case of boats with wide tacking angles, as the lee bow problem continues for these boats into areas where racing boats will already be sailing on a single tack. On a racing boat with a very narrow tacking angle, I think this effect is almost nil.

So the effect of current on wind direction is what is key.

[Dockhead]

Originally Posted by transmitterdan

Yes, I mentioned it very early on and you are right it is very important. For that reason alone simple algebra cannot solve the problem. It's a differential equation problem. Think of it like this;: as soon as the drift induced wind creates forward thrust on one polar the same induced wind changes it's vector heading and speed. So then a new polar is in play and the boat will find a new speed and direction. This continues until a stable condition is found. The trick is to find that stable condition which I would guess to be near impossible to do by hand calculation as it will take nearly as long as the passage itself.

Is there an advantage or disadvantage created by current? Yes, the same as if you had more or less wind. How much is it and what course should I steer to get all of it? Very hard question to answer.

The effect of lee bowing on wind speed ranges from nil to very slight -- see the post above. It's best to leave this aside for a moment so that we can concentrate on figuring out whether it is possible to exploit the change in wind direction to get a lift, by lee bowing. The jury is still out. Seaworthy is struggling manfully to put together a case that you can. I am blundering around a bit but seem to be stumbling towards the conclusion that you can't. Stay tuned. When we make a little more progress with the abstract principles, then we will start doing maths, I think.

[Dockhead]

Originally Posted by transmitterdan

For that reason alone simple algebra cannot solve the problem. It's a differential equation problem. Think of it like this;: as soon as the drift induced wind creates forward thrust on one polar the same induced wind changes it's vector heading and speed. So then a new polar is in play and the boat will find a new speed and direction. This continues until a stable condition is found. The trick is to find that stable condition which I would guess to be near impossible to do by hand calculation as it will take nearly as long as the passage itself.

I think it can be done by hand with trig, actually. Leave aside wind speed variations, and assume a constant optimum maximum VMG to windward angle to the wind. We can work out the change in True Wind direction caused by any given current with trig. From that, simple arithmetic gives us our possible heading. Assuming constant Ground Wind speed and direction, we can reasonably assume constant speed through the water. These assumptions are slightly artificial, but good enough to prove or disprove the concept. Changing Ground Wind would be a Superfluous Variable so a violation of correct procedure for this kind of analysis.

Then we can test a lee bow tacking strategy versus a windward bow strategy over a given set of hypothetical assumptions, or any other kind of tacking strategy, and calculate elapsed time to lay the mark for every scenario.

Then, if we like, we can add back in changes in wind speed caused by the change in current, which again, can be calculated with trig. Unless we see that the difference in wind speed is so much that it would move the polar around (dollar to doughnuts, can't be, in any realistic scenario), then it will be a simple correction to the previous calculations, which I'm betting will have negligible influence.

The whole business is easily analyzable with relatively simple math, I think. No calculus required.