Distinct Activities: Shackled by a Common Name?

[goboatingnow]

Originally Posted by Dockhead

Constant compass heading = straight water track. So that's always the shortest and fastest way to get anywhere, assuming of course you know the correct heading. That's in the RYA training materials, even.

If the tide is unidirectional with a constant speed, your CTS = COG to your waypoint -- the two approaches converge. So your ideal constant heading happens to take you over the rhumb line.

If the tide is unidirectional with varying speeds, like the Gulf Stream for example, then you will go wrong with steering to COG -- that is, staying on the rhumb line. Here you get back to the classical CTS calculation to get your ideal way across, which is a straight line through the water, which is a constant compass heading.

And the approach is exactly the same -- you just add up the set and drift for every hour and calculate a CTS to get you to the right spot at the edge of the treadmill, to use the ant analogy, to get to the right spot by the shortest possible path.

If you change headings every hour, then you are sailing an erratic course through the water and losing miles. However straight the line may appear over ground. The key thing is really to just forget about the ground, which is not relevant when you are crossing a moving body of water, even if it is moving in only one direction.

Yes I unsertand all tat and its what i do, what you say. But the symmetrical tide is the most advantageous situation. In a unidirectional tide of varying speed I like to see the actual difference between a single CTS and say. Series of 1 hour or 30 min triangles. After all an autopilot is still computing CTS. Just on a quite small triangle.

As I said I prefer a limit to my excursion from the rhumb line. Except in the clearly advantageous situations you and I are familiar with.

Dave

[Andrew Troup]

Originally Posted by Dockhead

Constant compass heading = straight water track. So that's always the shortest and fastest way to get anywhere, assuming of course you know the correct heading. .....

I agree with this constant compass heading solution for all the examples given so far in this thread*, where the current is flowing one way, then the other (at 180deg to the original direction) but not necessarily at the same speed, or for the same time.

I also agree when it is flowing in the same direction at different rates, as you just alluded to.

*Except in the real world situation corresponding to the revised model I introduced in my second-to-last post, where the conveyor belt could swing to intermediate angles, say 70 deg :

I think this scenario invalidates the constant compass heading tactic.
It is no longer consistent with the ant following a straight line on the belt.

That straight line, viewed from above, is now at a new angle with respect to North, whereas simply speeding up, slowing down, or reversing the belt does not affect that angle.

-------

From your other post, you make the point "But not necesssarily perpendicularly across." ... I was never under that illusion - I understood that the perpendicular solution only works when the current sets cancel out over the duration of the passage.

What I'm saying is that if the belt slews through an intermediate angle for part of the crossing (lets say it slews 30 deg CW), in order to follow a straight line drawn on the belt, an ant who is unfortunate enough to be relying on a compass will have to correct his heading by adding 30 degrees for the duration of the slew.

An ant has the option of simply disregarding the compass and following the line, but we do not have that option, and I'm not aware of any technology solution at present.

However there may well be niche software solutions available - although it occurs to me that vessels expensive enough to afford them are possibly generally fast enough to get across such tricky bodies of water - eg Cook Strait - before it really becomes an issue. An exception would be rigs under tow, and such...

(In Cook Strait, between NZ's Nth and Sth Islands, there is no period of slack water; the tidal currents box progressively around the compass throughout the entire tidal cycle. They flow virtually the entire time, in a variety of directions and at various reasonably significant rates.)

I do not recall this (admittedly rather uncommon) situation being dealt with in any navigational texts, but my library is currently in container storage so it's not easy to check.

[carstenb]

Andrew

I see your point, and albeit I don't know the waters whereof you speak, will agree with based on you description.

However, the example you provide is (as you admit) perhaps an extreme situation, which most of us, will not encounter.

A more likely situation, would be one, where you have wind against current. This occurs here in the Øresund region around Copenhagen and it is possible to experience water levels with up to 2-3 meters of difference in the space of 1 NM. In essence, you end up sailing across the slope of a hill. The calculation for crossing here in a straight line (on the water) becomes needlessly complicated. Fortunately the Straits in this area are only 14 -18 NM wide so it is more a question of cracking on some sail and going.

[noelex 77]

The simple rule is that is always quicker, all other things being equal, to calculate the average current for the voyage and then maintain that constant heading as the current changes.
Relying on the chart plotter to maintain zero cross track error is always less efficient.

The only exceptions occur when the current is the same thought the voyage. (Then the average current is identical to instantaneous current and the constant heading will maintain zero cross track error, so the results are identical).The second exception is when there is no sideways component to the current and once again a constant heading will maintain zero cross track error so the results are identical.

[Dockhead]

Originally Posted by noelex 77

The simple rule is that is always quicker, all other things being equal, to calculate the average current for the voyage and then maintain that constant heading as the current changes.
Relying on the chart plotter to maintain zero cross track error is always less efficient.

The only exceptions occur when the current is the same thought the voyage. (Then the average current is identical to instantaneous current and the constant heading will maintain zero cross track error, so the results are identical).The second exception is when there is no sideways component to the current and once again a constant heading will maintain zero cross track error so the results are identical.

Indeed, but those are not exceptions. Those are simply two cases where the efficient track through the water coincides with a straight track over ground. There is no exception to the rule that the straight track through the water will be the most efficient way there -- it's an axiom -- depending only on the assumption that you are dealing with a body of water moving all together.

A case not like that -- where all bets are off -- is if for example you are navigating a river with a strong adverse current in the middle and less current along the sides. You need to bug out of the channel and skirt the banks. But that's already a completely different problem.

[Dockhead]

Originally Posted by Andrew Troup

I agree with this constant compass heading solution for all the examples given so far in this thread*, where the current is flowing one way, then the other (at 180deg to the original direction) but not necessarily at the same speed, or for the same time.

I also agree when it is flowing in the same direction at different rates, as you just alluded to.

*Except in the real world situation corresponding to the revised model I introduced in my second-to-last post, where the conveyor belt could swing to intermediate angles, say 70 deg :

I think this scenario invalidates the constant compass heading tactic.
It is no longer consistent with the ant following a straight line on the belt.

That straight line, viewed from above, is now at a new angle with respect to North, whereas simply speeding up, slowing down, or reversing the belt does not affect that angle.

-------

From your other post, you make the point "But not necesssarily perpendicularly across." ... I was never under that illusion - I understood that the perpendicular solution only works when the current sets cancel out over the duration of the passage.

What I'm saying is that if the belt slews through an intermediate angle for part of the crossing (lets say it slews 30 deg CW), in order to follow a straight line drawn on the belt, an ant who is unfortunate enough to be relying on a compass will have to correct his heading by adding 30 degrees for the duration of the slew.

No, not indeed.

In any case, however the belt slews around, the ant needs to walk a straight path across the belt. Any wandering around in relation to the belt loses him miles (inches).

His navigational task, which is exactly the same as ours, is to calculate the sum of all the vectors, however wild they are, which apply during his odyssey. He will get a point on the edge of the belt which will be in front of his destination upon arrival. He needs to steer directly for that point and not deviate from a straight line however the belt slews around.

That's really exactly what we do when we calculate our CTS. Our CTS is the bearing to that bit of water which will be in front of our destination when we get there. That is the bit of water we need to get to. It matters not whether the water moves towards us, away from us, left, right, up, or down, or any combination of any of those. The object of calculating CTS is really just finding the spot in the water which will be in front of our destination upon arrival, so that we can steer towards it on a constant heading.

To give another example, proving that this method is not limited to a case with tides running both ways:

You have a high speed mobo which can make Cherbourg from Needles in one tide, in six hours.

You depart Needles at slack tide.

The first hour you will be set W for 0.25 miles
The second hour you will be set W for 1 miles
The third hour you will be set W for 1.5 miles
The fourth hour you will be set W for 2.5 miles
The fifth hour you will be st W for 1 miles
The sixth hour there is no set.

To navigate correctly you add up all the motion -- 6.25 miles.

That means if you steer towards a point which is presently 6.25 miles E of Cherbourg, the sum of all the streams will set you W cumulately 6.25 miles, and if you motor a constant bearing towards where that point was at the beginning of your passage, you will arrive right in front of Cherbourg 6 hours later. So you calculate a course to steer + 6.25 degrees from your bearing to Cherbourg or 186.25 (in reality, you will of course aim to be a mile or so uptide of Cherbourg to give yourself a margin for error). You steer that 186.25 or probably rather 188 the whole way.

If however you change course every hour, you will sail a bunch of wasted miles. Especially during the fourth hour, you will be uselessly crabbing hard against the tide in order to get to a spot at the end of the hour you don't want to be. Your VMG towards Cherbourg will fall, and for nothing.

So you see -- still exactly the same as the first example. It's still true, as it always is, that the shortest distance to your destination is a straight line through the water, that is, a constant heading.

If the stream is not perpendicular to your course, it's still the same, but I bet everyone has got it by now, surely!!

[goboatingnow]

I don't beleive anyone argues the theory Dockhead. I do think that the practice ( vector summing) is rarely used in any sort of complicated tidal situation. I've certainly not seen it routinely done.

I still prefer to sail within a reasonable distance to the rhumb. Reasonable being a flexible concept ( the exception being your typical symmetrical tide crossings) so I trade absolute speed to destination for other factors. But then again on a sailing yacht there are lots more trade off going on as well.

What I was interested was in a single varying current , it would be interesting to graph the CTS leg length, for various times ,against the use of an overall CTS. Just to see the amount of loss or gain of time . I wouldn't mind showing that to few of my chart class. Obviously its zero for an inline CTS. But it' would be interesting to see the graphs. I must try if I have some time to draw some vectors and compute the advantages/ disadvantages.

This s because you get bandying about 25% etc, Whereas the worked example was about 10%

The other issue especially in a slow sailing boat is you contend to have wide variations in speed and the effect of wind shifts. This forces recomputes anyway.

I think also that many people often do CTS in their head rather like your mobo example making the assumption of perpendicular set

Dave

[Dockhead]

Originally Posted by goboatingnow

I don't beleive anyone argues the theory Dockhead. I do think that the practice ( vector summing) is rarely used in any sort of complicated tidal situation. I've certainly not seen it routinely done.

I think crossing the Channel or crossing the Gulf Stream, this is the only reasonable way to do it. The consequence of getting it wrong is ending up downtide of your destination with a hard, hard slog to finally get there. So it behooves every navigator to know how to calculate a single CTS for such a passage.

The maximum XTE on such a passage is really nothing other than the distance uptide you need to be when the tide changes in order to stay on your efficient course. For a margin of error, you actually need a bit more, not less, XTE at the maximum, to ensure that you really will be uptide, and not downtide, when you get there allowing for some variations in speed. I don't know why you are nervous about big XTE's -- there is one right XTE, which if you fail to achieve, will mean a very unpleasant end to your passage. I can tell you from the experience of many, many Channel crossings, that it is one thing to glide into Cherbourg or Roscoff with the tide bringing you home, and entirely another to have to claw your way uptide at the end of an 8 hour or 10 hour sail, inevitably bringing with it also having to harden up on the wind -- ugh. That's what you get by not being far enough uptide when the tide changes -- that is, not having enough XTE in the middle of the passage.

If you're tacking, then of course it becomes much more complicated, and you also have to account for leeway.

What you might have in mind as a "complicated tidal situation" is something like the passage from St. Malo to St. Heliers in the Channel Islands. Whew, that is complicated. The tides don't just sweep back and forth, they sweep all around in all different directions in a rotary manner. Picking the right time to depart, much less finding the right CTS, and with all kinds of obstacles like the Minkies, makes my brain overheat.

As you suggest, I do this entirely by feel, by hook, by crook, which is really not the right way to do it, but I don't have any other tools. I do it in a way which I instinctively know puts me uptide of the destination as I approach the end of the passage. What is really needed for this is a good computer program with tidal streams in it which can run a large number of scenarios to find the best departure time and the best route. Here a computer is really needed because you really need to crunch a ton of numbers in order to have a clue about the right way to sail, much more than you could feasibly do by hand.


I am actually right now on the Red Funnel ferry to the IOW with an entire new B&G navigation system in my suitcases, on my way to my boat to start installing it. One thing I am really looking forward to playing with is the capability of this system to calculate and display set & drift in real time based on a real time comparison between heading and water speed with COG & SOG. For this to be reasonably accurate, you need good heading data (which you need for so many other things as well) and you need very accurate STW data. I have bought the Airmar 2183 three-axis gyro stablized heading sensor (after toying with the idea of buying the Furuno GPS heading sensor, rejecting the idea finally because it is too physically huge to mount anywhere on my boat), and I am hoping that the new Airmar ultrasonic N2K speed, depth, temp sensor will be shipping, prior to my splashing the boat. It should be interesting.

[Stu Jackson]

This has been fascinating. I have been sailing with a friend for many years. He's an engineer, too. i explained "current sailing" to him, and he first disagreed, thinking that using the GPS heading would get us there quicker.

So we did an example: heading east 3 nm across a current that was running north.

In all of the examples that have been discussed here, some "nitpicking" seems to have creeped in, and Dockhead had repeatedly and correctly come back and given explanations of how varying currents and directions do NOT change the basic facts, that these are simply VARIABLES in the basic calculations of current sailing.

Back to the example. Regardless of whether the current changes during the voyage, for THIS example, simply assume it is CONSTANT. If we followed the GPS heading - and we did - we ended up sailing a longer course since it "sagged" - think about it. If we "compensated" for the current, we got there quicker.

Try re-reading Dockhead's great explanations again, and do some "homework" on "current sailing."

Great subject, thanks. Remember, all the "buts..." are simply changes in the variables, which need to be part of the basic current vector/calculations.

[GrowleyMonster]

The best approach is to indeed calculate a cts and the waypoints you should pass over on an ideal passage, which mifht give you a curved or sinusoidal track or whatever, but remember it is JUST A LINE, and you may want to depart from it as wind, forecast wind, traffic, etc dictate. You dont serve the gps; it serves you. Look at the whole picture not just the shoortest possible route. Drive the boat, not the little arrow on the screen.

[Andrew Troup]

Originally Posted by Dockhead

... however the belt slews around, the ant needs to walk a straight path across the belt. Any wandering around in relation to the belt loses him miles (inches).....

I agree entirely with this statement.

I'm not sure how carefully you're considering my argument, because I've already agreed with it several times.

My point is that if you draw a stripe across the belt (not necessarily perpendicularly) representing the straight path you refer to, that stripe cannot possibly point in the same compass direction when the belt is slewed through any angle other than 360 degrees.

But I think the problem is probably with the "belt" model itself - or at least the 'enhancement' to the model which I suggested. I think that slewing the belt takes the model into a realm where it no longer reflects reality.

I first started to smell a rat when I realised that slewing the belt 180 degrees did not deliver the same outcome as reversing the direction of belt travel. It seems to me that it should, for the model to be true and useful.

Here's the clincher, though:

Consider an ant trying to get North on a belt which is initially travelling East, at the same speed he can walk. After an hour, when he's half way across, the belt slews 90 deg and starts travelling North, which it continues to do for the next hour.

The optimum strategy for the ant is surely to walk for the first hour on a heading roughly NW, so that at the end of that hour he's directly south of the destination. This means he's actually travelled north "across the ground". Then he needs to turn and walk north. This does not take him across the belt at all: he's now walking up the centreline of the belt, but both his legs and the travel of the belt are taking him directly to the destination.

Which, I think, exposes the silliness of the slewable belt metaphor: it doesn't make any sense if you can reach your destination without reaching the far edge of the belt

Mea Culpa.

I suspect a better enhancement to the model would have been to always keep the belt at the same angle, but translate the whole belt laterally (ie sideways to the direction the belt is travelling) to represent any "cross belt" component of the current flow.

This is consistent with the "constant heading" solution which we are taught works in all situations, which is a relief.
Having said that, I haven't yet explored whether it's a true or useful abstraction.

- - - - -

As you say, in practice, it's probably unrealistically hard to try to nail a solution in a seriously rotary tidal current scenario, especially if the data is patchy (as it usually is in such locations)

If there is good data perhaps it might be worthwhile if you are towing a rig whilst expending multiple megawatts, or some other situation where the old adage "TIM IS MONY" holds spectacularly true.

It would be great having 'real time' data, as you are hoping to achieve, but I guess that only helps to verify (and recalculate) the plan, given that to arrive at a CTS, your knowledge of current set has to travel into the future.

[Hudson Force]

Originally Posted by GrowleyMonster

.......................... Drive the boat, not the little arrow on the screen.

This characteristic dismisal of the GPS track as a device for simpletons drives much of the inability for people to properly use the device. All through the 70's and 80's I crossed the Gulfstream between Florida to the Bahamas quite satisfied with the vector summation method supported by Dockhead and I backed up my dead reckoning with an RDF. I was always satisfied with my fixes and not unexpectedly found my route to follow the sigmoid curve representing a longer distance over the chart and the constant heading. I did refine my constant heading by preparing a table in advance that would allow me to change my heading if my speed changed. I say this in order that no one thinks I'm supporting the GPS heading over vector summation to be a plan for the "airhead". It should be recognized that there are scenarios when the constant heading plan and the GPS track are identical. This would always occur if the current set and drift were to remain at a constant velocity throughout the passage or even if the drift changed, but the set was parallel to the desired course. As Dockhead suggested, with the trip from Needles to Cherbourg, or the crossing that I am more accustomed to across the Gulfstream, currents are often stronger at the center of a passage than the margins. In a case such as this the shortest distance (and here I'm speaking of the direct straight line from coordinates at start to finish while dismissing any distortion of a mercator projection and concerns about "great circle" routes) will be available on the line presented by the constant data updates of the GPS. Here, it has been suggested that the "crabbing" at the maximum current in the middle of the passage would decrease the speed. This is absolutely correct; however, there will be the compensating increase in velocity made good during the first and final third of the passage while those on the constant heading path would be veering away from the shortest route. If we employ the standard (time = distance/speed) formula, then the GPS track decreases the distance and speed while the constant heading track increases the distance and the speed. This, actually, would cause someone to conclude that the advantages and disadvantages of each plan cancel out. This thought that the plans final outcome would be identical would also be supported by the fact that they can be considered mathmatically the same. One plan takes a summation of just two vectors,- vessel velocity and current velocity) and the other plan takes in account the summation of many vessel and current velocities. In the post #261 example from Needles to Cherbourg there can be seen an example of six different velocities. The summation of many small vectors or individual larger vectors does not affect the final relationship between speed, time & distance. So, I'm changing my position from the thought that the less distant route is faster. Mathmatically they come out the same. This causes me to consider other thoughts. If I take the constant heading plan and sacrifice distance I may do better if the wind is constant and I'm not changing the trim of my sails as I would with the GPS track and a changing heading. If I'm on the GPS track an there is a change in the wind conditions, then I am more likely, as I am on the rhumbine, to be able to take advantage of the wind shift. As I ponder all the qualities of using the GPS track and the constant heading method I'm convinced that they are inherently the same. This is far different from my original stance that favored the shorter distance, but I'm at a block by physics and math to see an advantage of one system over the other without bending the rule of speed equalling distance divided by time. I should add in closing that I rarely now cross to the Bahamas from the Palm Beach area with the current perpendicular to my course. Now, retired and without time constraints, I cruise further south to 'Lauderdale or Miami and take the current ride off my starboard quarter. Ironically, I'm increasing my speed and my distance with this route and keeping my time about the same.

[wolfenzee]

Anyone who is so much of an idiot so that would apply doesn't belong on the water anyway.

[Andrew Troup]

Hmm ...the ant-based example I gave in my last post, transposed into the real world, seems on the face of it to invalidate the constant heading strategy, in a situation of rotary tidal currents.

Needs further thought and discussion, perhaps.

[wolfenzee]

The best answer I can see for the "thread starter":
People in this culture are "shackled" by what marketing says we should do/buy. It is the foundation of our economy. It is also why boating is thought to be so expensive. Boating is not a "rich man's sport", those of us who cannot afford the latest greatest fanciest name brand, realize we can function quite comfortably on extremely limited budgets with out...and have no problem doing so.