Differences Between Ground, Apparent, and True Wind Direction

[cal40john]

Originally Posted by kaimusailing

OK, if you are racing to the windward mark, on either tack you would prefer to have current lifting your boat. The prefered tack is toward the current. Just like tacking on wind shifts, a long windward beat in a tidal current could have the prefered tack shift when the tide turns.

You're changing the rules. As I said in my previous post there are several tactical reasons for sailing one tack over the other first, but given the conditions of constant wind and constant current you will get there at the same time regardless of which tack you do first.

Quite often, but not always you would sail the long tack first. Anticipating a change in current or wind that would make this tack less favorable later will give you a gain, if you've guessed right. But that isn't what was stipulated for this example. Sailing the long tack first also keeps you more in the center. If you sail the short leg first you wind up on the layline quickly and have no options in case there is a change.

I believe one case for sailing the short tack first is when you expect the current to increase. This is similar to the persistent wind shift case.

John

John

[Dockhead]

Originally Posted by s/v Jedi

All that the discussion now shows is regular layline tacking; the myth of presenting the current your lee bow for an extra boost is clearly debunked in the diagram I linked, where the red boat does this. The blue boat, according to the theory, would have had a penalty for not lee bowing, but the diagram shows that the current has zero effect other than an equal offset for everybody. And they all have to make the short tacks somewhere.
I got this from young Olympic sailors blogs, so the new generations will fix this

cheers!
Nick.

Indeed!

The sooner these myths die, the better. But I think that electronic navigation has made it worse, not better. Now sailors look at their plotters and instantly orient themselves to the ground, which makes it extremely easy to forget that we can only sail in water. I think that's the source of many of these fallacies.

[cal40john]

Originally Posted by s/v Jedi

@Paul,

Yes, I was asking to test and isn't it amazing how many people read it but don't feel sure enough to answer?! The thing is that there is a group who is dependent on seeing their boat move over a chart on a plotter and we all (rightly so) start calling on paper charts, learning to navigate etc. But there is also this large group of old salts who know how to navigate, often incl. sextant, but have no clue what these modern instruments actually show on their dials. I hate to admit that I'm starting to be an old salt, but my career in electronic engineering drives me to learn and understand every aspect of the new electronics coming on the market. It also allows me to open my mind for new ideas, technologies and methods.

@John,

Thank you for the diagram. I fear it's gonna be a futile enterprise though. A long time ago some well known sailors came up with this lee bowing effect as secret advantage and as often the case, scientific proof of it being a myth is pushed aside without engaging the detailed proof that is put on the table. There is never gonna be a diagram and/or math in response.

All that the discussion now shows is regular layline tacking; the myth of presenting the current your lee bow for an extra boost is clearly debunked in the diagram I linked, where the red boat does this. The blue boat, according to the theory, would have had a penalty for not lee bowing, but the diagram shows that the current has zero effect other than an equal offset for everybody. And they all have to make the short tacks somewhere.
I got this from young Olympic sailors blogs, so the new generations will fix this

cheers!
Nick.

It's not new. Someone referenced the Dave Perry book earlier, it was written in 1984. It also contains a diagram that he got from Stan Honey which is pretty much what I drew.

This is a little frustrating in that I got out my copy of the North U. book and looked it up to make sure I wasn't missing something, and verified that it said a constant current just makes for a skewed course. Third time I've brought that up.

John

[s/v Jedi]

Originally Posted by cal40john

It's not new. Someone referenced the Dave Perry book earlier, it was written in 1984. It also contains a diagram that he got from Stan Honey which is pretty much what I drew.

This is a little frustrating in that I got out my copy of the North U. book and looked it up to make sure I wasn't missing something, and verified that it said a constant current just makes for a skewed course. Third time I've brought that up.

John

Exactly. The crucial details tend to go ignored in these arguments. The curves etc.: it's all on changing (tidal) currents and have nothing to do with the elusive lee bowing effect. The meaning of that term is also not understood widely, but the term once heard, gets repeated for unrelated situations nonetheless.

Back OT, I am studying my Furuno RD-33 display and it shows magnetic ground wind hahaha . In the N2K specification, I see 5(!) different wind references of basically any combination possible. So those who only want to hear about true or apparent will have some tough times ahead

cheers,
Nick.

[s/v Jedi]

Originally Posted by Dockhead

I do not think that my electronic navigation can calculate a course to steer -- maybe I also didn't read that far in the manual. But the concept is absolutely fundamental to good navigation. We do the calculation by hand. You take a tidal atlas and sketch out your intended passage hour by hour. Then you calculate the set over each hour, and calculate the net set. Then you calculate a course to steer which compensates that aggregate net set.

I don't think chartplotters can do this, as it gets very complicated to do it well. I only know of one product that can do it and that is the routing module of MaxSea. It takes your boat's polar diagram, the grib files and he tidal current information. The system calculates isochrones, which show where you can move to in a timed interval, like 3 hours. It compares every possible solution and takes the currents, eddies and windshifts and waveheights and your boat performance for different wind angles into consideration. My head starts hurting when you consider the math behind it. Here is an example for cossing the gulf stream... the yellow lines are the isochrones:


Here is another calculation with both wind and current... this is why we need quad-core i7 CPUs


EDIT: http://comen.maxsea.fr/MaxSea/Produc...o/default.aspx

ciao!
Nick.

[s/v Jedi]

Somebody stop me lol

http://www.youtube.com/watch?v=GEfAK_XGyyU

Cheers,
Nick.

[Noname]

A large bully-type Osprey from the neighborhood that likes to sit on top of my mast actually ripped my ST-50 wind transducer out of the mount and made off with it. Apparently, he didn't like the thing twirling about while he lazed atop the mast. Mr. O also bent the starboard vane of my Windex as well. So lately, since last February or so, I just sort of guess at the wind by watching some old faded telltales tied to the shrouds. I'm not so sure the investment of $400 for a new wind transducer would be wise, considering the rowdy fowl in this area.

[s/v Jedi]

Haha, may be the Osprey is trying to tell you to go for the ultrasonic sensors without moving parts instead



ciao!
Nick.

[Dockhead]

Originally Posted by s/v Jedi

I don't think chartplotters can do this, as it gets very complicated to do it well. I only know of one product that can do it and that is the routing module of MaxSea. It takes your boat's polar diagram, the grib files and he tidal current information. The system calculates isochrones, which show where you can move to in a timed interval, like 3 hours. It compares every possible solution and takes the currents, eddies and windshifts and waveheights and your boat performance for different wind angles into consideration. My head starts hurting when you consider the math behind it. Here is an example for cossing the gulf stream... the yellow lines are the isochrones:


Here is another calculation with both wind and current... this is why we need quad-core i7 CPUs


EDIT: MaxSea Routing by Brice Pryszo

ciao!
Nick.

I LOVE it!!!! I could never do that calculation by hand! I want that! Look at that weird curved path across the ground crossing the gulf stream! That will drive some people crazy!

[Dockhead]

Originally Posted by s/v Jedi

Back OT, I am studying my Furuno RD-33 display and it shows magnetic ground wind hahaha . In the N2K specification, I see 5(!) different wind references of basically any combination possible. So those who only want to hear about true or apparent will have some tough times ahead

cheers,
Nick.

But of course, why not? We can express any wind, any way we like. It's even possible to express apparent wind in compass degrees, if we like. Of course the data would be completely useless in that form Well, nearly useless. You would need to compare your heading to the apparent wind in compass degrees in order to understand what is going on.

For reasons of usefulness, we inevitably see Apparent and True in degrees off the bow; Ground in compass degrees (and inevitably in true, not mag). But that doesn't mean that someone might want to see the data presented differently, for whatever reason. Since every instrument on my boat gives magnetic compass degrees, and I instinctively work with magnetic degrees on my charts, I might very well be one who would also like to see ground wind in magnetic degrees.

For me that's hypothetical so far -- I don't have a weather display like yours. I want one! I hope my new nav system will let me put up a screen like yours with complete weather info. In fact, I would really like to have a separate screen at the nav table which shows only that -- barograph, (ground) current and history over 24 hours, temps, etc. That would be really useful.

It's a layer of complexity, that's all, and people stumble on it. The only thing which was important in the OP was the question of what wind is referenced to what.

[Ex-Calif]

Originally Posted by cal40john

You're changing the rules. As I said in my previous post there are several tactical reasons for sailing one tack over the other first, but given the conditions of constant wind and constant current you will get there at the same time regardless of which tack you do first.

Quite often, but not always you would sail the long tack first. Anticipating a change in current or wind that would make this tack less favorable later will give you a gain, if you've guessed right. But that isn't what was stipulated for this example. Sailing the long tack first also keeps you more in the center. If you sail the short leg first you wind up on the layline quickly and have no options in case there is a change.

I believe one case for sailing the short tack first is when you expect the current to increase. This is similar to the persistent wind shift case.

John

John

Originally Posted by Ex-Calif

If you are on a passage with 000 rhumb line, true wind 000 and going to pass through a period of 090 degree cross current. Which tack is favored 045 or 135?

Through a period cross current. I statd this many posts ago.

And once again in case the is any confusion, I am not defnding th so called lee bow effect. I am defending thenfact you sail into the currrent in a period of current.

[Noname]

Originally Posted by s/v Jedi

Haha, may be the Osprey is trying to tell you to go for the ultrasonic sensors without moving parts instead



ciao!
Nick.

I bet he could build a nest on top of one of those!

[Ex-Calif]

Originally Posted by Dockhead

Ground Wind = Meteorologist's True Wind simply means that what our instruments call "Ground Wind" is the same thing as true wind for someone on land. It is ground-referenced true wind instead of water-referenced True Wind. Some people are uncomfortable with the idea that True Wind on our instruments is not calculated according to the definition used by the Met Office. Calculated using a different reference point. They say "meteorologists own the term 'true wind'" and don't like the fact that we use the term differently.

So ground wind is calculated on the boat and when it is referenced it is refenced to true magnetic north?

Originally Posted by Dockhead

In changing currents, sailing by COG and crabbing the rhumbline can increase passage time by 30% -- 40% or more. And "point and shoot navigation" -- well, we've all seen it, haven't we?

Are you saying it is longer to crab the rhumb line than to sail straight across a current?

[noelex 77]

Originally Posted by Seaduction

So lately, since last February or so, I just sort of guess at the wind by watching some old faded telltales tied to the shrouds.

Do you guess true, apparant or ground wind

[Paul Elliott]

Originally Posted by s/v Jedi

I don't think chartplotters can do this, as it gets very complicated to do it well. I only know of one product that can do it and that is the routing module of MaxSea. [good screenshots of MaxSea's weather-routing]

There are several other charting programs that can do wind and current routing using your boat's polars. Expedition is a good one (which I use). Raymarine has or had one called "Raytech", but I don't know if they included current in the calculations. There are several routing techniques, but as Jedi mentioned, calculating "isochrones" seem to be the standard. BTW, isochrones are similar to the isobars we see in the WFAX plots: Isobars are lines of constant pressure, and isochrones are lines of constant time.

Weather routing is only as good as the accuracy of your polars (and you may have to adjust these for sea-state), the accuracy of the wind and current forecasts, and your ability to sail your boat as well as your polars indicate. I use weather routing as a first-cut at a route, and adjust to compensate for how lazy I'm feeling, how well we've been sailing the boat, how much we want to get beaten up, and what's going to happen if the forecasts are wrong. Some people are really good at this -- for me it's more of a toy.