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

[Paul Elliott]

Originally Posted by Dockhead

[...]
There is a lot of argument in here about the frame of reference. In fact, of course, there is no right or wrong frame of reference. If you make no mistakes in computing the vectors, and don't mix up the sequence of any of them, you can describe any problem with any frame of reference. We could reference everything to the core of the earth, or the sun, if we like, adding a set of vectors for every new reference point which is moving in relation to the other entities that we are describing.[...]

Exactly! It doesn't matter to me which frame of reference someone uses. What matters is that they use terms that we understand, and that the results are correct.

I prefer using water-referenced True Wind, boat speed, and heading when calculating sailing performance. This is the simplest method. Current is brought in and out of the analysis as we convert to and from the ground-reference frame. This is where we get SOG and COG.

goboatingnow, this entire side-track was started when you posted that ground-referenced vector diagram showing the current-affected wind shifting wildly when the boat's heading was changing almost imperceptively. I still dispute this and invite you to provide some numbers. I also invite you to work out the simple "zero ground wind, 10 kts current" scenario for which I posted my solution. There are some fundamental issues we need to resolve before any further discussion can bear fruit.

[Dockhead]

Originally Posted by Paul Elliott

Hoofsmit, if you want to figure leeway, just use 5 degrees on all points of sail, other than (say) within 20 degrees of dead downwind). Use zero for the that.

The only significant difference you will see is that your course through the water will be different than your heading (leeway) and the Apparent Wind / True Wind calculations will have to account for the heading offset. I believe that accurately-done polars will take leeway into account and give you boat speed as a function of actual True Wind.

Ultimately, it's only going to make a fairly small difference in your tacking angles.

I can only add to that, that the most important thing for these problems is total tacking angle, which includes leeway, true-to-apparent conversion, etc.

However, it is very important not to forget leeway in your heading calculation.

[Hoofsmit]

Originally Posted by Paul Elliott

Hoofsmit, if you want to figure leeway, just use 5 degrees on all points of sail, other than (say) within 20 degrees of dead downwind). Use zero for the that.

The only significant difference you will see is that your course through the water will be different than your heading (leeway) and the Apparent Wind / True Wind calculations will have to account for the heading offset. I believe that accurately-done polars will take leeway into account and give you boat speed as a function of actual True Wind.

Ultimately, it's only going to make a fairly small difference in your tacking angles.

so if you want some credibility, do it .
they are after all variables that make the difference.
unfortunately these conceptualised vectors will cause you a big problem when you try to prove the lee bow effect does not exist.
but first make sure those polars are conceptualised to the correct reference
I love concepts

[Paul Elliott]

Originally Posted by Hoofsmit

so if you want some credibility, do it .
they are after all variables that make the difference.
unfortunately these conceptualised vectors will cause you a big problem when you try to prove the lee bow effect does not exist.
but first make sure those polars are conceptualised to the correct reference
I love concepts

I am perfectly comfortable with whatever credibility I may or may not have, thank you very much. But why don't you go ahead and suggest a particular problem.

Look, the leeway can vary significantly between different types of boat. My fat shoal-draft cruiser has more leeway than my buddy's hot-shot racer with the 12-ft keel. The polars are completely different. But the fundamental effect will be on tacking angles and speed through the water. Once you have these you really don't care about leeway anymore.

We have tried to show you that there is no current-sourced hydrodynamic effect. The current sweeps the entire boat downstream. For all practical purposes, when you are adrift the water at the surface is moving exactly the same speed and direction as the water down by your keel (which is zero, relative to your boat). Once you start sailing the current still has nothing to do it.

If you want to claim something different, I suggest that you explain to us what you think is going on. Please be specific. To be honest, while I enjoy a good debate, I am growing weary of hand-waving. I don't care if you make them up, but show me some numbers.

[estarzinger]

Originally Posted by Hoofsmit

but first make sure those polars are conceptualised to the correct reference

Just to expand on that a little . . . .

Dockhead has tried to eliminate the ground reference by creating a fixed CTS.

However, for the lee bow question that does not work . . . Because CTS assumes a boat speed (thru the water), and the lee bow question has boat speed as a variable. Boat speed depends on the sailing angle - whether you are sailing high (even pinching) or low - which is exactly the lee bow question. Your optimal CTS will vary depending on your sailing angle. This is why you can't solve the problem analytically without polars.

[Seaworthy Lass]

Originally Posted by estarzinger

Just to expand on that a little . . . .

Dockhead has tried to eliminate the ground reference by creating a fixed CTS.

However, for the lee bow question that does not work . . . Because CTS assumes a boat speed (thru the water), and the lee bow question has boat speed as a variable. Boat speed depends on the sailing angle - whether you are sailing high (even pinching) or low - which is exactly the lee bow question. Your optimal CTS will vary depending on your sailing angle. This is why you can't solve the problem analytically without polars.

I agree your CTS will need to be calculated using an average of the variable speed as the current changes, which adds to the complexity. For these calculations your ground wind is whatever is predicted. Your true wind can then be calculated by adding the wind induced by current vector. Your close hauled speed is estimated from that figure.

Your quickest route to windward is always tacking close hauled. Your quickest route if not tacking is always a straight line through the water (a single CTS). Neither of these depend on your boat's polars except for your estimation of boat speed.

Sailing low will never get you there faster if your heading is into wind. Pinching will never get you there any faster either, except if you have misjudged your final tack.

Pinching to get the current on your lee bow is useless. The lifted tack is the port tack if the current direction is between 0 and 180 relative to the ground wind and starboard tack if the current is between 180 and 360. It just happens that for most of the time this places the current on your lee side and closer to the bow than on the alternative tack. Pinching does absolutely nothing to create more of a lift from the current.

[I.Grind]

Originally Posted by Seaworthy Lass

I agree your CTS will need to be calculated using an average of the variable speed as the current changes, which adds to the complexity. For these calculations your ground wind is whatever is predicted. Your true wind can then be calculated by adding the wind induced by current vector. Your close hauled speed is estimated from that figure. Your quickest route to windward is always tacking close hauled. Your quickest route if not tacking is always a straight line through the water (a single CTS). Neither of these depend on your boat's polars except for your estimation of boat speed. Sailing low will never get you there faster if your heading is into wind. Pinching will never get you there any faster either, except if you have misjudged your final tack. Pinching to get the current on your lee bow is useless. The lifted tack is the port tack if the current direction is between 0 and 180 relative to the ground wind and starboard tack if the current is between 180 and 360. It just happens that for most of the time this places the current on your lee side and closer to the bow than on the alternative tack. Pinching does absolutely nothing to create more of a lift from the current.

Nicely summed up.

[estarzinger]

SL

you seemed to miss my point . . . Which was simply that you cannot answer the lee bow question analytically without including polars because (a) your course to steer depend on boat speed thru the water (and current), (b) boat speed thru the water depends on sailing angle/polar, (c) sailing angle is exactly the lee bow question.

Your speed will vary a lot depending if you are sailing at 27 AWA or 30 AWA, exactly how much will depend on your polar. You assert that you can not win by pinching . . . But is not that exactly the pure lee bow question? If I pinch the CTS changes.

>>Your quickest route to windward is always tacking close hauled. Your quickest route if not tacking is always a straight line through the water (a single CTS). Neither of these depend on your boat's polars except for your estimation of boat speed.

No . . . Downwind your quickest route is often/usually NOT a straight line (especially in light winds)... But you would not know that without the polar. And what your optimal close hauled (and downwind) angle and speed is depends exactly on the polar.

[Seaworthy Lass]

Originally Posted by estarzinger

SL

you seemed to miss my point . . . Which was simply that you cannot answer the lee bow question analytically without including polars because (a) your course to steer depend on boat speed thru the water (and current), (b) boat speed thru the water depends on sailing angle/polar, (c) sailing angle is exactly the lee bow question.

Your speed will vary a lot depending if you are sailing at 27 AWA or 30 AWA, exactly how much will depend on your polar.

I agree that you need to look at polars to estimate speed. I disagree that that you can get anywhere upwind faster than being close hauled with sails perfectly trimmed.

Sailing angle is NOT the lee bow question. Pinching does nothing but slow you down. The only "lee bow" effect to discuss is which tack to be on to get a lift from the current. It is often (but not always) the tack with the current on your lee side and closer to the bow when comparing the two tacks. Pinching will never suddenly make the tack you are on the lifted one.

Originally Posted by estarzinger

>>Your quickest route to windward is always tacking close hauled. Your quickest route if not tacking is always a straight line through the water (a single CTS). Neither of these depend on your boat's polars except for your estimation of boat speed.

No . . . Downwind your quickest route is often/usually NOT a straight line. And what your optimal close hauled (and downwind) angle and speed is depends exactly on the polar.

Sorry, yes dead downwind is an exception. Of course gybing is then faster.

[estarzinger]

Originally Posted by Seaworthy Lass

I agree that you need to look at polars to estimate speed.I disagree that that you can get anywhere upwind faster than being close hauled with sails perfectly trimmed.

Sailing angle is NOT the lee bow question. Pinching does nothing but slow you down.

It's fine for you to assert that. It may even be true. My simple point was that you cannot prove it analytically without including polars.

By the way, you do know that racers have upwind "fast/low mode" and "slow/high mode" and they do in fact use both. They do not have the one single upwind mode you seem to be suggesting. Usually these are used tactically (with reference to other boats) But there are strategic uses. For instance, in the AC, fast mode was used to get to favorable current and then a switch to slow mode when in the current.

[Seaworthy Lass]

Originally Posted by estarzinger

It's fine for you to assert that. It may even be true. My simple point was that you cannot prove it analytically without including polars.

Your CTS will depend on your boat speed, which will depend on your polar. It will not, however, alter the lift induced by the current. The lifted effect on one tack is purely due to the change in true wind angle induced by the current. It has absolutely nothing to do with the boat or its polars.

Originally Posted by estarzinger

By the way, you do know that racers have upwind "fast/low mode" and "slow/high mode" and they do in fact use both. They do not have the one single upwind mode you seem to be suggesting. Usually these are used tactically.

I am not discussing tactics, I am discussing strategy to get you to your destination in the quickest possible manner .

[I.Grind]

Originally Posted by Seaworthy Lass

yes dead downwind is an exception. Of course gybing is then faster.

Usually, but not always, I crew on a boat that seems faster going DDW (or very close to it) than gybing. New boats with an asym, for sure, but not ALL boats ALL the time. Just had to throw that in.

The only rule to which there is no exception, is that there is an exception to every rule.

Sorry, I'll shut up now.

[Seaworthy Lass]

Originally Posted by I.Grind

Usually, but not always, I crew on a boat that seems faster going DDW (or very close to it) than gybing. New boats with an asym, for sure, but not ALL boats ALL the time. Just had to throw that in.

The only rule to which there is no exception, is that there is an exception to every rule.

Sorry, I'll shut up now.

OK, it is boat dependent .

But there is no escaping the fact that there is a particular tack relative to the ground wind that will be lifted more by the current. Once you are on the water and in a constant current, all you are doing it tacking to the true wind. There is no lifted tack relative to the true wind. And certainly pinching so that you can achieve some kind of lifting effect from the current is nonsensical.

[estarzinger]

Originally Posted by Seaworthy Lass

Your CTS will depend on your boat speed, which will depend on your polar.

Terrific, we agree.

So your CTS depends on your polars. One sailing angle/speed will give a different CTS than another. Might the CTS at one angle/speed be shorter to the mark (thru the water) than at another angle/speed?

I am not discussing tactics, I am discussing strategy to get you to your destination in the quickest possible manner

Well you rather unfairly clipped off my comment that there are also strategic uses for fast and slow modes

OR, in this AC, had the boat set up to have different modes on port and starboard, for instance with different boards. And this was done for strategic reasons, not tactical, given the predictable nature of the SF course..


.

.....

[Seaworthy Lass]

Originally Posted by estarzinger

By the way, you do know that racers have upwind "fast/low mode" and "slow/high mode" and they do in fact use both. They do not have the one single upwind mode you seem to be suggesting. Usually these are used tactically (with reference to other boats) But there are strategic uses. For instance, in the AC, fast mode was used to get to favorable current and then a switch to slow mode when in the current.

Well you rather unfairly clipped off my comment that there are also strategic uses for fast and slow modes.

Sorry, I did not mean to cut you off .

What you are discussing here is something completely different - current you can reach varying in strength in different positions on the course.

You do not use a CTS then at all. You decide which bits of current you must cross most quickly to get to patches of favourable current. FoolishSailor was discussing this when he described the tacking strategy of "bow into low current and beam onto strong current". The lifted tack relative to ground wind still exists, but you may well ignore it entirely. The CTS does not apply then, it is only valid for current changing over time, not current changing according to where you are on the course.