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Old 11-05-2011, 16:11   #76
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Re: What Heading Am I Really On ?

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Originally Posted by DotDun View Post
let the current take you north, expect the lighter south bound current to assist later all while maintaining the autopilot heading
Exactly.

I would assess the tidal stream for each hour of the expected passage (using tidal stream atlas or vectors lifted from chart etc as in your case), then plot them onto the chart starting at the departure point, joining them nose to tail - see red lines on pic.

From the tail of the last vector, I would then plot the course line to destination, which will give course to steer and expected distance through water - see green line on pic.

I would expect the ground track to appear similar to the orange line in pic, albeit a bit more curved in places.

Alternatively, you can calculate the net tidal vector - pink line - and use that to calculate course to steer... there are a few methods - I prefer diagrams!

Don't forget to apply leeway where applicable.

Yes your bearing to destination will vary by a few degrees throughout and your XTE will change too, but your heading will remain constant.

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Old 11-05-2011, 17:18   #77
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Re: What Heading Am I Really On ?

A technical correction for those who are interested:

The rhumb is not the shortest geographical distance between two waypoints, rather it is simply a means of determining a single (constant) bearing between two points on the earth's surface. It is a straight line on a chart (mercator projection).

The shortest distance over ground (but not always through water) is the great circle route (GC).

The older GPS units I have used have all given the bearing to the next waypoint as a GC bearing and this can change (depending on actual waypoints) by several degrees as you progress along the track.

Maybe current marine chart plotters determine the rhumb line bearing?

Of course in the real world of currents tides, leeway, obstacles and short passages, it hardly matters much whether you use a rhumb line or a GC route.

Does anyone know what their GPS / chartplotter determines as a bearing or course line between waypoints (rhumb or GC)?
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Old 11-05-2011, 18:23   #78
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Re: What Heading Am I Really On ?

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Does anyone know what their GPS / chartplotter determines as a bearing or course line between waypoints (rhumb or GC)?
On the Furuno NavNet3D, GC or Rhumbline Navigation is an option.
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Old 11-05-2011, 18:46   #79
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Re: What Heading Am I Really On ?

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Originally Posted by bewitched View Post
I can't remember the last time I actually worked out which direction the boat was moving in using the binacle compass. Does anyone with a gps actually do this any more?
I do. All the time. And when someone takes the helm, I specify a magnetic course to sail. When someone can't steer a magnetic course, that tells me all I need to know about their skill level.

I'm from the old school where you tack on a header. And when you tack back later on, if you haven't been monitoring the binnacle compass, you don't know whether you've been lifted or headed in the exchange. Which means you are now lacking an important data point regarding the wind patterns.

Unfortunately, a lot of our new GPS-era sailors don't know a lift from a header.
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Old 12-05-2011, 01:19   #80
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Re: What Heading Am I Really On ?

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But the bottom line is that XTE has many more positives than negatives. On the recent passage I cited, without XTE, one of two things would have happened:

1) The Yucatan Channel current (3.5kts) and then the opposite direction from the Gulf Stream loop current (1.5kts) would have pushed us way off course and added significant distance to the trip.

2) The grueling work for a helmsman to continually correct the heading to match the ever-changing current and maintain the rhumbline probably would have created a mutiny from my crew.
The rhumbline was certainly not the shortest or fastest way to go on this particular passage. Crabbing along the rhumbline might be the fastest way if you have a constant current along the whole passage, but only in that case. If you have different currents, and especially if they push you in different directions, the rhumbline cannot be the shortest or fastest way.

The fact that you cannot calculate the currents precisely doesn't change anything. Even a rough idea of the currents will give you a better path than just fighting the current and crabbing along the rhumbline.

In the passage you mentioned, you should have tried to calculate a heading which would let the first current carry you to just that point where the second current would carry you back towards your destination.

Let’s say for example the passage was 120 miles, and you are maintaining a 6 knot average, and you have 60 miles with a 4.5 knot current carrying you off to starboard, and then 60 miles with a 1.5 knot current carrying you off to port.

The first current will carry you off 45 miles to starboard over the 10 hours you need to sail 60 miles.

The second current will carry you off 15 miles to port over the second 10 hours of your passage.

If you sail the rhumbline, you will steer fully 45 degrees to port of your waypoint bearing just to stay on the rhumbline. It means you will sail a very long distance further just to end up in the WRONG place when you hit the second current.

In fact, your 30 degrees of wrong course will cost you exactly 9.28 miles of extra sailing by the time you get to the half-way point of your passage. Then you will alter course to start fighting the second current, to stay on the rhumbline. At the halfway point you will be right on the rhumbline – that is, the bearing to your destination will be the same as when you started. But now with the current flowing in the other direction, you will need to steer now 15 degrees to starboard of your waypoint bearing. You will lose another 2.12 miles.

The right way to do it is to steer 15 degrees, not 45 degrees to port of your destination's original bearing, and to maintain this heading the whole way. That way, when you hit the second current, you will be 30 miles to starboard of the rhumbline, which is just the right spot so that the second current will carry you right back to your destination while maintaining the same heading of 15 degrees to port of your destination's original bearing.

By sailing on one heading the whole way, you will sail the shortest path THROUGH THE WATER, that is, 120 miles. Crabbing along the rhumbline you will sail 131.39 miles through the water.

Your helmsman will not do anything except steer a constant magnetic heading. He will ignore completely COG. Sitting at the nav table you can check your XTE every hour to make sure you are experiencing the same set and drift you assumed.


If your data about the currents is not precise, this will not work as well, but even the crudest guess about the currents will do better than intentionally sailing the wrong path, that is, the rhumbline.
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Old 14-05-2011, 04:58   #81
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Re: What Heading Am I Really On ?

Just completed a run from the Isle of Man to Liverpool. Wind was SW allowing the me to sail a reach. I allowed the ebb tide to push the boat to the south of the line, in the knowledge that that flood would take me back towards the destination waypoint. This allowed me to sail the same compass course with minimal trim corrections. If I had followed the rumb line, I would have ended the leg on a beat, hard work, spray over the boat on a cold night. As it was, I had a enjoyable and comfortable reach the whole leg.
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Old 24-05-2011, 05:53   #82
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Re: What Heading Am I Really On ?

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Heading can be a crucially important datum.

We just got back Sunday from an English Channel crossing, where the tide sweeps back and forth every six-odd hours at up to four knots and more.

If you steer by Course Over Ground you'll never get there, or, if that is a slight exaggeration, you'll certainly experience a world of frustruation trying to.

The way to cross a body of water with cross currents is to calculate, using a tidal stream atlas, how far you will be pushed off course along the way first in one direction, then in the other. You add all this up to get a result like -- if I steer directly towards my waypoint, I will end up 12 miles east of there. Then you calculate how many degrees different you need to steer in order to correct out that 12 mile error. Then you steer that heading for the whole passage, ignoring COG completely, and not paying attention to the fact that you are being swept way over this way, then way over that way. Your course looks like a big "S" on your chart plotter. You can check how accurate your calculations were by watching the XTE function on the plotter hour by hour.

COG becomes relevant ONLY in the last hour or two. Meanwhile, you MUST know your heading in order to navigate successfully. I guess this is pretty basic information for anyone who sails in tidal waters, but for anyone who actually thinks that heading is a useless piece of information -- here you are.
Well there are many ways to skin a cat. You could in as you say get a fairly good idea of how far west you would need to point the boat in order to account for what you believe the tide will be and you could use the binacle compass and tide tables for the basis of that. Or you could use the real time information generated by your instruments. In other words keep pointing the boat west until you are traveling in the direction you want to move in. Which would be COG.

So i guess we do things differently. I've just arrived in Samui after 8 days sailing into current from Hong kong. I'm pretty sure none of us looked at the binacle compassfor more than a cursory glance. We certainly didn't do any navigation by it. Chartplotter and instrument readouts is how we navigated. I think its how most people do it. But, no, its not the only way.
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Old 24-05-2011, 06:52   #83
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Re: What Heading Am I Really On ?

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Well there are many ways to skin a cat. You could in as you say get a fairly good idea of how far west you would need to point the boat in order to account for what you believe the tide will be and you could use the binacle compass and tide tables for the basis of that. Or you could use the real time information generated by your instruments. In other words keep pointing the boat west until you are traveling in the direction you want to move in. Which would be COG.

So i guess we do things differently. I've just arrived in Samui after 8 days sailing into current from Hong kong. I'm pretty sure none of us looked at the binacle compassfor more than a cursory glance. We certainly didn't do any navigation by it. Chartplotter and instrument readouts is how we navigated. I think its how most people do it. But, no, its not the only way.
Well, sailing a single calculated magnetic heading through waters with cross-currents is not the only way to get there, of course, but it's the best way. You can also get to your destination by sailing a reciprocal heading around the other side of the world -- that's one way to do it -- you'll get there eventually . That would be even less efficient than crabbing over COG, of course!

The geometry of all this is explained in previous posts. If you force your boat to stay on the rhumb line by steering COG to your destination, crabbing this way and that, you will sail more miles through the water and it will take you longer -- with the sole exception of a case where you have exactly the same current over the entire route (in that case, the course steered will be exactly the same by both methods).

The shortest distance to any destination is the shortest distance through the water, not over ground. The shortest distance through the water is a single heading, without any change of heading, which gets you to your destination after the effect of currents in different spots. The rhumb line will never be the shortest way unless either there is no current at all, or if the current vector is exactly the same over the whole passage.


If you're steering by magnetic compass, there is also the effect of leeway (just to make it a little more complicated still). If you spend an equal amount of time on both tacks, you can ignore this. If not, then you also need to calculate a net leeway offset to correct your course to steer.
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Old 24-05-2011, 07:07   #84
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Re: What Heading Am I Really On ?

I have noticed this kind of discrepancy as well. It usually happens in high current areas. Being that the compass shows where you are pointing and the GPS shows course over ground (as heading), these two are almost always going to be out of synch. I think the unnerving discrepancies come from differences in current set as you progress along a course. There are often short sections on your course where the current varies considerably from the averages shown on current charts. The GPS is showing a huge change in heading at these points. This is why the GPS course is NOT always the best way to navigate strong changes in current. (The hull is more efficient letting the current drag you off course so as to not try to buck heavy current): an S course.

Before the days of GPS, when dead reckoning was the norm, we probably paid more attention to dealing with current and wind drift. When navigating on clear days, when you could use sight and compare it to your charted course, it was clear what current was doing to your plan.

As mentioned above, if you think your compass is in error, just put your boat on a known heading and see if the compass reads true. Then do it again somewhere around 90 degrees from that to see if your compass needs the compensation adjusted.

With multiple GPS units, radar and computer charting, I still keep a paper chart right in the cockpit with my course on it.
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Old 24-05-2011, 11:22   #85
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Re: What Heading Am I Really On ?

Wow, the thread drift on this topic was really helpful to me. I sail in an extremely tidal area, but amongst many islands with very small crossings, so you're always sailing across a more or less constant tidal stream. There's no time for the tide to change part way across and use the methods you guys are talking about.

That said, however, in cases where you have wind against current and the current covers only part of your journey, an equally valid tactic is to point the boat at 90 degrees to the current to get across as quickly as possible. You'll be off the rhumb line on the other side, and will also be increasing your miles through the water, but you may actually be able to sail faster in the calmer water and the crew will have a more pleasant trip.
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Old 24-05-2011, 19:13   #86
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Re: What Heading Am I Really On ?

Yes, that's the S-course idea. Large displacement hulls are very inefficient going against current, therefore, if you make your way up-current before and after crossing it is faster than crabbing across. So if the total drift is 1 mile, go upriver 1/2 mile before crossing and 1/2 mile on the other side, taking the current at 90 degrees as you said. It's really helpful crossing the Gulf Stream.
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Old 25-05-2011, 02:04   #87
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Re: What Heading Am I Really On ?

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Well, sailing a single calculated magnetic heading through waters with cross-currents is not the only way to get there, of course, but it's the best way. You can also get to your destination by sailing a reciprocal heading around the other side of the world -- that's one way to do it -- you'll get there eventually . That would be even less efficient than crabbing over COG, of course!

The geometry of all this is explained in previous posts. If you force your boat to stay on the rhumb line by steering COG to your destination, crabbing this way and that, you will sail more miles through the water and it will take you longer -- with the sole exception of a case where you have exactly the same current over the entire route (in that case, the course steered will be exactly the same by both methods).

The shortest distance to any destination is the shortest distance through the water, not over ground. The shortest distance through the water is a single heading, without any change of heading, which gets you to your destination after the effect of currents in different spots. The rhumb line will never be the shortest way unless either there is no current at all, or if the current vector is exactly the same over the whole passage.




If you're steering by magnetic compass, there is also the effect of leeway (just to make it a little more complicated still). If you spend an equal amount of time on both tacks, you can ignore this. If not, then you also need to calculate a net leeway offset to correct your course to steer.
you wouldn't be putting words into my mouth would you, I'm pretty sure I didn't advise sticking to the rumb line.
I fully understand how to establish a course to steer on paper beforehand. It does bring back memories of interpolating between tidal stations and drawing little vector diagrams. But I stopped that years ago. VPPs have been available for a long time now, I'd even go so far to say that they are a standard feature on most chartplotting software. They utilise real time information and are so much more accurate than anything I could do by hand. And of course they update the calcs by the second.

The output is a target COG to be achieved, which updates continuously. I can achieve this with a few clicks of a mouse. So I don,t bother with the old Course to Steer calc but use the computing power of the electronics on board. It's not the only way, but for me it is quicker, easier and more accurate.
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Old 25-05-2011, 06:03   #88
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Re: What Heading Am I Really On ?

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you wouldn't be putting words into my mouth would you, I'm pretty sure I didn't advise sticking to the rumb line.
I fully understand how to establish a course to steer on paper beforehand. It does bring back memories of interpolating between tidal stations and drawing little vector diagrams. But I stopped that years ago. VPPs have been available for a long time now, I'd even go so far to say that they are a standard feature on most chartplotting software. They utilise real time information and are so much more accurate than anything I could do by hand. And of course they update the calcs by the second.

The output is a target COG to be achieved, which updates continuously. I can achieve this with a few clicks of a mouse. So I don,t bother with the old Course to Steer calc but use the computing power of the electronics on board. It's not the only way, but for me it is quicker, easier and more accurate.
Your plotter (and indeed your pilot) can steer a course - or rather a bunch of different courses - which will hold you on a constant COG. But sailing a constant COG - while it may be the simplest way to understand - will make you sail further through the water. 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.
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Old 25-05-2011, 06:29   #89
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Re: What Heading Am I Really On ?

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Your plotter /and indeed your pilot can steer a course - or rather a bunch of different courses - which will hold you on a constant COG. But sailing a constant COG - while it may be the simplest way to understand - will make you sail further through the water.
In a worst case scenario where the cross current is greater or equal to the boat speed through the water, steering a COG corrected over time will result in you never getting there. At some point on a long enough crossing you will be pointed upstream with a boat speed less than the current. The idea of sailing up stream before the crossing is moot and in this case impossible. Crossing the Gulf Stream is a good example of how this can really happen.

You can get sucked into this dilema if you set the autopilot to steer to a way point. The quickest course is always to compute the drift and steer a constant heading. So for equal current and boat speed you would steer 45 degrees upstream when the crossing was as wide as the current to be crossed and the COG was dead ahead perpendicular to the current. With a faster current it would be more than 45 degrees. At some increased current velocity you won't hold steerage and thus never be able to cross to the opposite shore 90 degrees to the current under any corrected course. Holding steerage is why you don't break / slow down in a cross current and why some rivers can not be crossed.
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Old 25-05-2011, 07:01   #90
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Re: What Heading Am I Really On ?

I haven't sailed in any areas of of significant current for any amount of time. It will probably be a bit interesting for the first few trips, and then damn boring! LOL

Like anything else though, its probably going to be a bit of experimentation whislt rattling the theory abuot in my head.
For example "lee bowing" a current has always been the 'go' but some write that it doesn't work (especially where speed is less than current like in Pblais post) But thats really time for the anchor and a book.

In the 1 to 2 knot currents I have been in I like the COG and XTE, but it doesn't mean experiments can't be carried out, even to the detriment. We can learn lots by doing the wrong thing.

Most importantly is not to become frustrated at the sailing either...
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