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Old 25-05-2011, 08:02   #91
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Re: What Heading Am I Really On ?

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Originally Posted by bewitched View Post
you wouldn't be putting words into my mouth would you, I'm pretty sure I didn't advise sticking to the rumb line.
You advised steering so that your COG as calculated by the plotter is equal to the bearing to your destination and letting the electronics figure out what heading is needed to keep your COG constant.

That exactly puts you on the rhumb line, and keeps you there. If you steer that way, you will crab along the rhumb line -- you will move along the rhumb line while heading this way and that to compensate for the cross-current (you said "keep steering West until the boat is going in the desired direction" -- that's called "crabbing"). So it's one and the same thing.


Well, there's nothing wrong with this method as long as either (1) there is no cross-current; or (2) there is a cross-current which is consistent over the whole passage. I use this method all the time myself -- it's dead simple -- just put the boat on autopilot, and keep plussing and minusing the autopilot until COG equals Bearing to Waypoint. Simples.

But it doesn't work if you're dealing with varying cross currents, and especially cross currents that change direction like the English Channel. As Paul pointed out -- and I had not thought it through that far ahead -- this method can even ensure that you never reach your destination -- under some extreme circumstances.
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Old 25-05-2011, 09:06   #92
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Re: What Heading Am I Really On ?

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Originally Posted by Pblais View Post
The quickest course is always to compute the drift and steer a constant heading.
Yes, and that's the usual way to get from A to B. It CAN be improved though if you know you will be coming into a current and then correcting for the drift ahead of time which keeps you close to perpendicular to the current where it is strongest, therefore negating the speed loss where it would be greatest. Same when exiting: allow the current to take you past your A to B course and then make up the distance when the current is weakening. Of course wind direction and maintaining hull speed at the best point of sail will often negate any time gain using this tactic. In any case, using a GPS plotter and simply setting a waypoint as you have pointed out, may NEVER get you there.
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Old 25-05-2011, 09:23   #93
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Re: What Heading Am I Really On ?

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Originally Posted by smurphny View Post
........Of course wind direction and maintaining hull speed at the best point of sail will often negate any time gain using this tactic. In any case, using a GPS plotter and simply setting a waypoint as you have pointed out, may NEVER get you there.
Well, well. Someone finally said it. Correcting or not correcting for existing or expected set and drift is all well and good, but it's not a slam dunk unless you take into account wind direction and boat speed. Sometimes falling off the wind a bit can give a remarkable increase in boat speed.

Bill
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Old 25-05-2011, 09:47   #94
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Re: What Heading Am I Really On ?

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Originally Posted by btrayfors View Post
Well, well. Someone finally said it. Correcting or not correcting for existing or expected set and drift is all well and good, but it's not a slam dunk unless you take into account wind direction and boat speed. Sometimes falling off the wind a bit can give a remarkable increase in boat speed.

Bill
That's an entirely different can of worms, which adds another layer of complexity which we had not yet overlaid over the original problem.

If you're doing it up wind and tacking, there's the question of whether you should be lee bowing or not.

I faced this problem last fall crossing the Channel and for the life of me couldn't figure it out. I don't understand it. Can anyone enlighten?

See for example: http://www.j105.org/docs/noleebow.pdf
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Old 25-05-2011, 10:17   #95
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Re: What Heading Am I Really On ?

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lee bowing I don't understand it. Can anyone enlighten?
My internet connection is the pits so I can't open that link, or others.

Like many things discussed here(!!!) there is the simple correct answer that then dim-witted folks manage to make into a 1,000 reply thread!

There is, consequently, writings out saying lee bowing ain't what its cracked up to be.

But simply: Have the current coming from the lee bow of your boat and you will find it acts like extra lift as you work to windward.

Obviously then one says, on just one tack you will never arrive where you want. Thats correct. But it gives you to oportunity to decide which is your winning tack. And then, for example, put in your losing tacks close to the shore when the current is less / or when the tide is less etc.

However, as I said in an earlier post, I haven't done enough of it to work out how well it works.

The added bit of fun is if you Lee Bow you don't need to worry about COG as you are close hauled. So how do you know if its working? LOL


Certainly things to enjoy making a night watch go quicker
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Old 25-05-2011, 10:23   #96
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Re: What Heading Am I Really On ?

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Originally Posted by MarkJ View Post
My internet connection is the pits so I can't open that link, or others.

Like many things discussed here(!!!) there is the simple correct answer that then dim-witted folks manage to make into a 1,000 reply thread!

There is, consequently, writings out saying lee bowing ain't what its cracked up to be.

But simply: Have the current coming from the lee bow of your boat and you will find it acts like extra lift as you work to windward.

Obviously then one says, on just one tack you will never arrive where you want. Thats correct. But it gives you to oportunity to decide which is your winning tack. And then, for example, put in your losing tacks close to the shore when the current is less / or when the tide is less etc.

However, as I said in an earlier post, I haven't done enough of it to work out how well it works.

The added bit of fun is if you Lee Bow you don't need to worry about COG as you are close hauled. So how do you know if its working? LOL


Certainly things to enjoy making a night watch go quicker
Well explained!

But I knew that. What I would like to know and which busts my brains is -- why is it supposed to work? What is the principle?
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Old 25-05-2011, 10:33   #97
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Re: What Heading Am I Really On ?

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Originally Posted by Dockhead View Post
- why is it supposed to work? What is the principle?
The principal is quite simple if you keep things simple

The cross current pushes your boat up to windward.

So if you are close hauled on the starboard tack - wind from the starboard bow - and if the current is anywhere on the port side of the boat, then you will be pushed a little to starboard. Meaning you can either have a more advantageous tack, or if really lucky come off a few degrees.

So that basic bit is easy to understand. So just apply the rule whenever you can and it should help. The problem being if example gets confusing or some smart ass tries to put in some exceptional case thinking it should disprove the whole rule.


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Old 25-05-2011, 11:02   #98
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Re: What Heading Am I Really On ?

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Originally Posted by Dockhead View Post
I don't understand it. Can anyone enlighten?

Well whole books have been written about this topic alone, so I predict more thread drift (and set?)...

Try thinking about the tidal stream as a component of apparent wind; a 5 knot current pushing your boat in one direction creates a 5 knot apparent wind component over the deck from the opposite direction. You can use that apparent wind component to modify the overall apparent wind to your advantage.

Lee-bowing can move the apparent wind aft and increase its strength. It therefore allows you to sail closer to the true wind and increase your boat speed.

Does that help?

Cheers,
Neal
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Old 25-05-2011, 11:19   #99
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Re: What Heading Am I Really On ?

Did anyone mention WAVES? that'd be my smartass comment..
Since the whole sytem wind+current = one resultant wind-you might as well sail properly ..Oh,sure the counterwindcurrent helps...to generate a shorter steeper chop!...It depends on your boat size and rig versus wind strength,fetch,current. ie some kind of three line graph.So,for me the help is only good in a light breeze.
When a fishboat chum said,"I guess the current helps sailing a lot!"well,he knew better.Just hadn't thought it through...."No,not really but sometimes,yes but, depends on..."

Pinching against a current (= wind direction) can be even more ridiculous what with that loss of resultant wind PLUS the drift...hopeless.get across! Sometimes ..that is.,because you may be sailing into a lee or less wind generally.

But motoring,same place,you might think,what a lovely smooth place!Pour it on!Is it possible a contrary current +contrary wind is better than contrary wind + fair current? It helps the efficiency of my MV?There's less mean chop!

So Flying across the middle contrary "strong flow" then clawing up patiently in a counter-current or less tide works too...sometimes.Because your apparent wind drops a bit.
If you can work up coastwise to a good slant,at least there's a chance of getting round a point a....if nothing else,the tide'll have changed when you get up there...
Oh,and then there's a lift of wind into a bay..is it worth "crossing over" to get in it?...on and on and on.That's my sailing done for the day.I'm done!
Cripes,we're way off OP here!
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Old 25-05-2011, 12:21   #100
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Re: What Heading Am I Really On ?

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Originally Posted by wolfaroo View Post
Well whole books have been written about this topic alone, so I predict more thread drift (and set?)...

Try thinking about the tidal stream as a component of apparent wind; a 5 knot current pushing your boat in one direction creates a 5 knot apparent wind component over the deck from the opposite direction. You can use that apparent wind component to modify the overall apparent wind to your advantage.

Lee-bowing can move the apparent wind aft and increase its strength. It therefore allows you to sail closer to the true wind and increase your boat speed.

Does that help?

Cheers,
Neal
Thanks for trying, but I'm still confused.

Since true wind is measured in relation to the water, not the ground, I don't understand how you can sail closer to the true wind. If you forget about the water moving -- that is, forget about the land -- and think about your boat, the wind, and the water, what difference does it make? You've got one true wind speed and direction on both tacks. You tack as normally, not so? It's just that the current is carrying you off somewhere different in relation to the land, but we deal with that by the methods discussed in this thread. How does it make one tack different from the other?

The tide is not "pushing" you anywhere in relation to the wind, because your true wind already has the tide factored into it.

In other words, if you're sitting dead in the water and have 15 knots of wind from N, then true and apparent wind speed and direction are identical, and if you are out of sight of land, neither you nor your boat could possibly notice that you are being swept off to the E by the tide at 4 knots.

You start sailing, trying sail directly upwind to N, and you tack. Why would one tack be any different from the other? If you forget about the land, then surely you are just tacking upwind as normal, no?

Now your COG will be different from the course you are tacking along, and the geographic wind is going to be different from true wind -- both by factors implied by the tidal vector. But that shouldn't affect how you sail, should it? It should rather affect the course you steer. If you are trying to make a destination due N into true wind of due N you will nevertheless have to steer somewhat W depending on the tidal vector. That means in effect that you are not beating dead upwind -- your correct course will be somewhat W of N, so it means that you will spend more time on starboard tack than on port tack and you'll make better progress than if you are actually beating dead upwind. But that is simply because the circumstances combine to allow you to navigate not dead upwind, but in a more favorable direction -- it has nothing to do with your boat's sailing performance.

And in fact, although true wind is N, the geographic wind is somewhat W of N. So I guess you ARE sailing closer to the GEOGRAPHIC wind. But I still don't see how it affects the way the boat sails. Where is that "puzzled" smiley when you need it?
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Old 25-05-2011, 12:25   #101
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Re: What Heading Am I Really On ?

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Originally Posted by HappySeagull View Post
Did anyone mention WAVES? that'd be my smartass comment..
Since the whole sytem wind+current = one resultant wind-you might as well sail properly ..
These are what is known as "additional layers of complexity". All valid points, but it's important to understand how to create the optimum course through cross-current first. Assume that the water is glassy smooth and you are motoring. No questions of waves or point of sail or lifts from bay entrances.

Once you perfectly understand THAT, then you can see how you might modify your optimum course to take advantage of those factors.
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Old 25-05-2011, 13:42   #102
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Re: What Heading Am I Really On ?

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Thanks for trying, but I'm still confused.
No worries - I'll try to help but I might not be the best at explaining this... hopefully between me and some other more enlightened posters we can help shed some light.

Since true wind is measured in relation to the water, not the ground, I don't understand how you can sail closer to the true wind. If you forget about the water moving -- that is, forget about the land -- and think about your boat, the wind, and the water, what difference does it make?

In that sense, I guess not much. But why would you forget about the water moving or the land? I don't understand what you are trying to achieve in that case. You are right that wind tactics would make little difference if we forgot about the land but aren't we always enroute to/from somewhere connected to the land?

You've got one true wind speed and direction on both tacks. You tack as normally, not so? It's just that the current is carrying you off somewhere different in relation to the land, but we deal with that by the methods discussed in this thread. How does it make one tack different from the other?

Since the tidal stream modifies the apparent wind, one can use it to sail closer to the true wind on a given tack. First, lets establish how the tidal stream affects apparent wind. See attached image as an example:

That boat (heading 000 deg) is moving forward at 5 knots (creating 5 knots induced wind from ahead (000)). The tidal stream across her bow from port (270deg) is 3 knots (creating 3 knot induced wind from starboard (090deg)). Her combined induced wind would therefore be the resultant of these (somewhere around 040 degrees and ~6 knots). If you then add in a true wind of 5 knots from 090 deg, it combines with the induced wind so to speak, creating an apparent wind of 9 knots just forward of the beam.

In this example, the tidal stream increased the apparent wind and moved it aft.

The tide is not "pushing" you anywhere in relation to the wind, because your true wind already has the tide factored into it.

Yes you are right, apparent (not true) wind has tide factored in already. But if we understand how the tide affects the apparent wind, we can use it to our advantage on a passage. For example, while the tidal stream is against the wind, we can sail on the tack which allows us to sail closer to the wind (and make more ground to windward).


In other words, if you're sitting dead in the water and have 15 knots of wind from N, then true and apparent wind speed and direction are identical, and if you are out of sight of land, neither you nor your boat could possibly notice that you are being swept off to the E by the tide at 4 knots.

Not quite right. Apparent wind (15 knots from N) will have a tide-induced component of 4 knots from E already factored in. Take away the tide and the apparent wind is altered.

In any case, since the tidal stream changes with time, you can choose which tack to sail through a given stage of tidal stream to gain the best advantage.

Sorry if I haven't explained any of this very clearly!
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Old 25-05-2011, 19:24   #103
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You guys are just talking semantics. I have only ever seen two winds mentioned in any book or course: true and apparent. Some talk about a third: geographic. Unfortunately, true wind has two different meanings depending on which system you ascribe to. To me, true wind is the real wind with respect to the land. To some, true wind is with respect to the water and wind with respect to the land is called geographic wind.

Not sure why I'd want to know what the wind is with respect to the water? Also, did I get the definitions right for the "third wind?"
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Old 25-05-2011, 19:37   #104
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Re: What Heading Am I Really On ?

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You guys are just talking semantics. I have only ever seen two winds mentioned in any book or course: true and apparent. Some talk about a third: geographic. Unfortunately, true wind has two different meanings depending on which system you ascribe to. To me, true wind is the real wind with respect to the land. To some, true wind is with respect to the water and wind with respect to the land is called geographic wind.

Not sure why I'd want to know what the wind is with respect to the water? Also, did I get the definitions right for the "third wind?"
I agree with you. There are only two winds. The TRUE (geographic, if you will) wind and the APPARENT wind.

The APPARENT wind represents the combined vectors of the TRUE wind and the wind caused by the movement of the boat. Whether that movement is caused by sailing, leeway, current, or motor is totally irrelevant. To talk about a "third wind" is just unnecessarily complicating things.

IMHO,

Bill
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Old 25-05-2011, 20:41   #105
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Re: What Heading Am I Really On ?

[QUOTE=Dockhead;693811 Your plotter only understands ground. But your boat sails through water. Your plotter cant calculate the best path through the water where you have varyimg cross-currents. For that you have to do it yourself, and you need a tidal atlas or at least some idea about the strength and direction of the currents along your route. And a compass.[/QUOTE]

My plotter understands far more than ground. It understands my boat because it has learned its polar, it understands the weather forecast because it reads the grib file, it understands tides and currents forecasts because this.information can be loaded too. And of.course it has access to real time.wind and current information from the instruments.

So it can plan my most efficient route beforehand based on the polar, weather and current information and update that most efficient route in real time as I travel.along it.

The output is a line on the chart, in other words a course over the ground. Obviously this very rarely a straight line. The COG can be sent to a readout on deck.

This is not some high end system only found on race boats, most major chartplotting software will have this functionality.

But this has all drifted far from my original query about how many still use the binacle compass as the basis for steering from one position to.another.
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