Speed Through GPS Versus Old Fashioned Paddle Log

[Dockhead]

Originally Posted by goboatingnow

not to re=open a whole CTS debate

Firstly its important to distinguish between regular symmetrical right angle reversing tides, backed up by good detailed tidal stream detail , like exists in the English Channel and also the Irish Sea , no real CTS computation is needed as everyone knows just steer a fixed heading etc


IN real life STW is far less useful. Firslty in complex tides over many hours, NO-one in reality computes multi hour ( like 6 or 8 hours cts) . This is the key fallacy of the theorists.

In reality navigation books like Bowditch, look at only ONE hour CTS. in that case your ground track is directly at the objective ( one hour later)

The primary reason for this approach is two fold

(a) Unless you determine the complex ground track , you cannot be sure if you have plotted a course safe from hazards

(b) Inaccuracies in boat speed, tidal data, progress due to wave action , poor trim etc , mean that long CTS rapidly get-ridiculously in accurate and at time dangerously so


IN reality instantaous speed ( either SOG or STW) on a boat are virtually useless and never existed in the past


That was my point, INSTANTANEOUS STW is only really useful for sail trim


distance through the water , or over the ground is useful. I would argue that distance over the ground is FAR more useful then distance thorough the water.


Note that CTS calculations, dont really depend on knowing STW. No prudent mariner, computes CTS without determining the route over the ground. In the Irish sea for example, as you approach each coast, where you are on such approach is more important then saving a few minutes .

IN CTS , you establish a course based " assumptions". then you maintain a plot on your chart and you revaluate as you go. IN no case do you need STW, what you need is a FIX.


Again, you have to ignore examples of peculiar situations such as an english channel crossing , where you have few ground hazards and good tidal data


Personally outside of a few well known cases like above, I rarely maintain a CTS for any period longer then a constant tide, that often 1 hour, Then I replot based on a DR, EP or preferably a Fix. ( I draw appropriate circles of error )

All this nonsenses about symmetrical reversing tides is in fact nonsense. its taking a tidal party trick and using it to assume thats how it done all the time.

Again , in my opinion, people that attempt to calculate complex multi hour tides and as a result end up with a complex and UNKNOWN ground track are NOT navigators. They have traded theoretical efficiency for safety

Dave

Dave, you have a deep understanding of all of the issues, and I respect that. You have a totally ground-referenced view of all of these problems which does work mathematically, and I respect that, too. We know all of this from our earlier discussions on the lee bow effect.
However, I must disagree with a couple of your points here.
First of all, CTS navigation is not a “party trick”. It is applicable to a wide range of real life situations, and profoundly useful in many of them. The usefulness of CTS navigation does not depend on high precision of the data inputs, either. You would see it instantly and instinctively if you could see the world in water-referenced terms (where that is appropriate), as many of the rest of us do. When you “get” the water-referenced frame of reference, then you realize that even a very imperfect CTS will usually be better than intentionally going off in the wrong direction by trying to crab along the rhumb line.
The main limitations of CTS navigation are twofold:
1. It is easy to calculate by hand only if the currents are close enough to perpendicular for you to ignore drift. If not, then you lose the option of a useful hand calculation which is fairly close to right. You are left with only two practical ways to do it: a. an “eyeball” CTS, where you just guess based on a series of current data, and you head off in that direction and correct as you go along; and b. a computer calculated CTS.

2. It loses its relevance with a big difference between boat speed and current speed. This is why Ron Widman and other motorboaters can’t even imagine what we’re talking about – it’s because he doesn’t need to. The whole problem doesn’t even exist in his world.

The “eyeball CTS” is profoundly useful, and is something used by nearly all good Channel sailors I know. It is amazing how good the results can be, and this contradicts the idea that CTS navigation is only useful with very precise data. I have used this technique for passages like St. Malo to St. Helier through complex rotary Channel Island tides where it is completely impossible to do any kind of real calculation, with spectacular results. With rotary tides, the problems are orders of magnitude more complex than for a Channel crossing, because besides course to steer, you also have timing. If you can visualize a water-referenced path from A to B, you can navigate even these complex waters far better than you could crab along the rhumb line. But you have to feel the water reference, to do it.

A great way to feel the water reference – and this gets back to our original topic in this thread – is to watch your instruments during a Channel crossing. You see SOG and COG an VMG to waypoint varying wildly – the data is garbage. But your compass is steady, and your STW is steady. That’s because you’re sailing a straight line through the water, which is always – this is mathematically demonstrable; mathematically, this proposition is trivial – the shortest way to your waypoint. Watching the instruments during a Channel crossing in the “Eureka!” moment for many sailors coming to mental grips with this type of navigation.
The real point and essence of the matter is that the ideal path to your waypoint across moving water is a constant heading. Like most ideals in life, this ideal is unattainable in practice. But that’s ok – because sailing in the generally correct direction is almost always – other than in the most extreme cases – better than sailing in the intentionally wrong direction, and it’s natural and efficient to correct it as you go along.

Now those are the big points – now a couple of nits.
1. You say that STW is not relevant to CTS navigation. But this is not right – the whole principle of CTS navigation is to calculate the course to steer at a certain speed through water which will get you to your waypoint without a change in heading. You can’t calculate it any other way – speed through the water is one of the operands of the equation. And so it is essential to keep up with average STW during a CTS passage so that you can correct if for deviations from plan. You NEVER use a fix, contrary to what you wrote! That’s because of the complexity of calculating your desired ground referenced position over a CTS navigated passage. Your ground referenced position in the middle of a passage is external to the CTS system, awkward, and unnatural (really relevant only when you need to check for hazards). What you care about is your average speed through the water, which is why experienced CTS navigators carefully log speed and miles run through water. In the Channel where the tides are perpendicular, a fantastic method is to compared XTE on the GPS with the cumulative tidal set at any given moment – gives you an instant check on how you’re doing.

2. You make much of the dangers of the “unknown ground track”, which results from CTS navigation. I don’t really understand why this seems so dangerous to you. If you have hazards along the way, you can calculate whether you will get near them or not. You must watch your plotter in any case, which is always correct seamanship. You steer around when you need to. After a certain density of hazards, you abandon CTS navigation – and the rhumb line too! And set up specific waypoints you need to pass in order to get through a really dangerous area. That does not reflect at all on the usefulness of CTS navigation.

3. You talk about “reversing symmetrical tides” – as if that’s the only application of CTS navigation. Since I know you thoroughly understand the principles involved, I know you don’t believe that, but it’s misleading to others. Symmetry has nothing to do with it, first of all. A reversing tide increases the effectiveness of CTS navigation (because it increases the stupidity of crabbing along the rhumbline), but CTS navigation is profoundly useful for many other cases – like crossing the Gulf Stream. If you want to boil it down to its essence – CTS navigation is actually always the right way to navigate. It’s just that whenever the water is not moving, or whenever it is moving consistently, you don’t really need to think about CTS, because you can easily use the ground as a reference, and let your GPS do all the work. But in fact you are always sailing in water –even when the water is still. Not over the ground! So when the water is still, it is just an accident that GPS navigation coincides with what you would get from CTS navigation.

4. This statement: “Inaccuracies in boat speed, tidal data, progress due to wave action , poor trim etc , mean that long CTS rapidly get-ridiculously in accurate and at time dangerously so “ is simply false. On the contrary, even a crude CTS determined by eyeball without even calculating anything, almost always puts you ahead of crabbing down the rhumbline, and you refine and correct it as you go along. This is almost always the most effective way across moving water when you don’t have precise tidal or because of the wind, you are tacking or sailing an unpredictable course because you’re hard on the wind, and so can’t calculate it precisely. To give a concrete example – a whole knot of boat speed makes a difference of usually 2 or 3 degrees on a typical Channel crossing. I know that, because like most experienced Channel sailors, I run several scenarios before setting out, so that I know at a glance how to correct the CTS as my average speed starts to become clear.


One thing which has changed my life totally with regard to CTS navigation is having acquired the Neptune Planner Plus program. I now never do hand calculations of any CTS. This program will give you a precise CTS – based on 5 minute resolution tidal maps of UK waters – even where there are complex rotary tides impossible to calculate by hand. Using this program, you get a precise ground track, in case you want to check for hazards. You can rerun the numbers in a second and with no effort, so you can update your CTS as often as you want to. It is absolutely totally cool.

[Dockhead]

Originally Posted by transmitterdan

Ok, I get the "guessing where the tide will take us" scenario. Sorry for being dense. But I agree with Dave that in all this discussion no one has explained how STW would affect their calculations or decision making. Also, I have not read anywhere that SOG would lead you onto the rocks. I believe that someone who does not plan for tidal drift can end up on the rocks. But how checking their knot log can prevent that escapes me.

BTW, lacking intelligence and being stupid are basically the same thing. It's one thing to be stupid (incapable of learning) and another to be ignorant. No one in this thread intends to imply anyone is stupid. I, for one, appreciate the reduction in ignorance though.

I'll answer the question of "how STW would affect their calculations or decision making."


Because STW is one of the operands in the CTS equation, without which you can't reach any conclusions. To get any tidal vector, you need to know heading and STW, on the one hand, and set and drift, on the other.

If you go slower or faster through the water, the tide will affect you more or less, and change the CTS which is needed to get you to your waypoint.

That's why we log STW on a long CTS passage.

[Stu Jackson]

Originally Posted by goboatingnow

Forget crossing the English channel, why, because symmetrical reversing tides in effect cancel out errors, in real life most tidal journeys are not symmetrical nor as simple as a channel crossing

I agree. I also mentioned just a single crossing with current in only one direction.

If one sails only based on the GPS heading to the next waypoint, as in Dockhead's river example, WITHOUT calculating the effect of the current, WHICH THE GPS CANNOT KNOW or anticipate, then you'll sail a further distance. That's the whole point. iT's called navigation and is explained in Dutton's - current sailing.

I understand that many have not read Dockhead's excellent article that I provided the link to. It's long, it's complicated, it's effective.

I've tried it in many cases here on SF Bay, for example crossing west to east in the South Bay with a strong building to up to 3 or 4 knot current ebbing north in a sailboat going 5 knots. It DOES make a BIG difference over even a four to six mile distance.

Your boat, your choice.

[Ericson38]

Originally Posted by transmitterdan

I have done some calibration curves of my Raymarine paddle wheel. At 6.5 knots it is accurate +/-0.2 knots. At 3 knots it reads high by 1 knot. At 1 knot it reads zero. There is no calibration function to correct the nonlinearity.

Is that on both tacks, or powering along with no heel ? If tack dependent, have you rotated the sensor slightly (if there is a difference) to get about the same output on either tack ?

My H2000 B&G system does not support but a single Hz per knot calibration factor, but the H3000 will take in the heel sensor data on either tack, and allow multiple adjustments to the basic Hz-knot parameter on either tack, based on heel angle on that tack. Basically two of these tables that you would enter data into. Program interpolates as needed based on the entries.

Heelº Boat Speed (Knots)
5 10 15 20 25 30

0º 0.0 -2.0 -3.9 -6.0 -7.8 -9.3

10º -0.2 -2.3 -4.0 -6.5 -9.6 -11.0

20º -0.4 -3.9 -6.1 -8.5 -11.5 -13.3

[goboatingnow]

Originally Posted by Dockhead

First of all, CTS navigation is not a “party trick”. It is applicable to a wide range of real life situations, and profoundly useful in many of them. The usefulness of CTS navigation does not depend on high precision of the data inputs, either. You would see it instantly and instinctively if you could see the world in water-referenced terms (where that is appropriate), as many of the rest of us do. When you “get” the water-referenced frame of reference, then you realize that even a very imperfect CTS will usually be better than intentionally going off in the wrong direction by trying to crab along the rhumb line.

Never said that CTS was a party trick, I said that using symmetrical reversing tides is a party trick

I say again Dockhead, multi hour CTS is complex tides , is nonsense , you cannot easily determine ground track and you cannot hence decide on safety of the route. Single or 2 hours is acceptable , especially where tides dont significantly change as you will in fact proceed along the ground track.

The main limitations of CTS navigation are twofold:
1. It is easy to calculate by hand only if the currents are close enough to perpendicular for you to ignore drift. If not, then you lose the option of a useful hand calculation which is fairly close to right. You are left with only two practical ways to do it: a. an “eyeball” CTS, where you just guess based on a series of current data, and you head off in that direction and correct as you go along; and b. a computer calculated CTS.

You can of course do a chart work up.

The “eyeball CTS” is profoundly useful, and is something used by nearly all good Channel sailors I know. It is amazing how good the results can be, and this contradicts the idea that CTS navigation is only useful with very precise data. I have used this technique for passages like St. Malo to St. Helier through complex rotary Channel Island tides where it is completely impossible to do any kind of real calculation, with spectacular results. With rotary tides, the problems are orders of magnitude more complex than for a Channel crossing, because besides course to steer, you also have timing. If you can visualize a water-referenced path from A to B, you can navigate even these complex waters far better than you could crab along the rhumb line. But you have to feel the water reference, to do it.

no argument, once you do not stray too far from the course line

A great way to feel the water reference – and this gets back to our original topic in this thread – is to watch your instruments during a Channel crossing. You see SOG and COG an VMG to waypoint varying wildly – the data is garbage. But your compass is steady, and your STW is steady. That’s because you’re sailing a straight line through the water, which is always – this is mathematically demonstrable; mathematically, this proposition is trivial – the shortest way to your waypoint. Watching the instruments during a Channel crossing in the “Eureka!” moment for many sailors coming to mental grips with this type of navigation.

Yes Yes, lets not do the suck eggs issues here. The fact remains that in a complex non reversing , angular tides, the ground path cannot be easily computed using a single CTS and hence ITS NOT A SAFE WAY TO NAVIGATE

The real point and essence of the matter is that the ideal path to your waypoint across moving water is a constant heading. Like most ideals in life, this ideal is unattainable in practice. But that’s ok – because sailing in the generally correct direction is almost always – other than in the most extreme cases – better than sailing in the intentionally wrong direction, and it’s natural and efficient to correct it as you go along.

Indeed, but multi hour complex CTS has you sailing off initially at sometimes bizarre headings

Now those are the big points – now a couple of nits.
1. You say that STW is not relevant to CTS navigation. But this is not right – the whole principle of CTS navigation is to calculate the course to steer at a certain speed through water which will get you to your waypoint without a change in heading. You can’t calculate it any other way – speed through the water is one of the operands of the equation. And so it is essential to keep up with average STW during a CTS passage so that you can correct if for deviations from plan.

NO not correct, you merely uses an estimated speed ( which is actually ground distance by time ) to estimate the number of tidal hours you will pass through to therefor extract the tidal information. This isn't actually STW its actually a form of SOG. ( actually distance over the ground) remember all of this stuff predates GPS.

You NEVER use a fix, contrary to what you wrote! That’s because of the complexity of calculating your desired ground referenced position over a CTS navigated passage. Your ground referenced position in the middle of a passage is external to the CTS system, awkward, and unnatural (really relevant only when you need to check for hazards). What you care about is your average speed through the water, which is why experienced CTS navigators carefully log speed and miles run through water. In the Channel where the tides are perpendicular, a fantastic method is to compared XTE on the GPS with the cumulative tidal set at any given moment – gives you an instant check on how you’re doing

.

I see you are suggesting a navigator does not fix his position for the duration of a multi hour CTS !!!!!!!!!!!!!!

The reality Dockhead is that multi hour complex CTS ( i.e. a combination of tides resulting in a combined CTS vector) is a complete nonsense because you cannot determine in advantage the ground track and hence evaluate the dangers

2. You make much of the dangers of the “unknown ground track”, which results from CTS navigation. I don’t really understand why this seems so dangerous to you. If you have hazards along the way, you can calculate whether you will get near them or not. You must watch your plotter in any case, which is always correct seamanship. You steer around when you need to. After a certain density of hazards, you abandon CTS navigation – and the rhumb line too! And set up specific waypoints you need to pass in order to get through a really dangerous area. That does not reflect at all on the usefulness of CTS navigation.

Very bad navigation to only determine haxxards as you come among them. ( or use a chart plotter exclusively )

The fact Dockhead ( and try this yourself ) if you work up a course to steer over many hours , firstly you cannot determine your ground track , hence you cannot determine in advance what hazards are important. You could for example stray in shallows, sand banks.


Its no use them dumping the whole CTS calculation, AT this stage you are in effect in the wrong place to begun with. You may find that you now have to set an uncomfortable direction or even have to stem a tide, simply to remove yourself from the hazard

Sorry this is not navigation

3. You talk about “reversing symmetrical tides” – as if that’s the only application of CTS navigation. Since I know you thoroughly understand the principles involved, I know you don’t believe that, but it’s misleading to others. Symmetry has nothing to do with it, first of all. A reversing tide increases the effectiveness of CTS navigation (because it increases the stupidity of crabbing along the rhumbline), but CTS navigation is profoundly useful for many other cases – like crossing the Gulf Stream.

1-2 hour CTS calculations are useful, I never argued CTS as a whole isn't useful

If you want to boil it down to its essence – CTS navigation is actually always the right way to navigate. It’s just that whenever the water is not moving, or whenever it is moving consistently, you don’t really need to think about CTS, because you can easily use the ground as a reference, and let your GPS do all the work. But in fact you are always sailing in water –even when the water is still. Not over the ground! So when the water is still, it is just an accident that GPS navigation coincides with what you would get from CTS navigation.

well yes , obvious is obvious

4. This statement: “Inaccuracies in boat speed, tidal data, progress due to wave action , poor trim etc , mean that long CTS rapidly get-ridiculously in accurate and at time dangerously so “ is simply false. On the contrary, even a crude CTS determined by eyeball without even calculating anything, almost always puts you ahead of crabbing down the rhumbline,

The first priority of navigation is safety. Multi hour CTS on complex tides can have you sailing over a ground track you cant easily determine. Thats NOT good navigation

The second is efficiency of progress.

One does not have to crap along the rhumb, you can calculate simple 1-2 hour CTS as you go along, on such single triangular vectors, you will therefore remain in the course line , i.e. the designated ground track. IT is NOT good navigation to deviate into an unknown ground track


( or not uses fixes)

and you refine and correct it as you go along. This is almost always the most effective way across moving water when you don’t have precise tidal or because of the wind, you are tacking or sailing an unpredictable course because you’re hard on the wind, and so can’t calculate it precisely. To give a concrete example – a whole knot of boat speed makes a difference of usually 2 or 3 degrees on a typical Channel crossing. I know that, because like most experienced Channel sailors, I run several scenarios before setting out, so that I know at a glance how to correct the CTS as my average speed starts to become clear.

I personally run 1-2 CTS as I progress , thats effectively the same as what you do ( because all CTS's become out of date fairly rapidly in a small boat)

One thing which has changed my life totally with regard to CTS navigation is having acquired the Neptune Planner Plus program. I now never do hand calculations of any CTS. This program will give you a precise CTS – based on 5 minute resolution tidal maps of UK waters – even where there are complex rotary tides impossible to calculate by hand. Using this program, you get a precise ground track, in case you want to check for hazards. You can rerun the numbers in a second and with no effort, so you can update your CTS as often as you want to. It is absolutely totally cool.

Great more computers to rely on, personally I dont want to be left on an unknown ground track by a computed CTS, and then find I have to rebuild everything by hand



Dockhead

This is the single point Im making about multi hour CTS . You cannot easily determine you ground track nor can you determine where you should be at any given point in your journey.

Hence (a) you may stray into an areas with hazards that you have not considered

(b) you cannot use positions fixes to determine your progress as a function of your CTS plan. SO you cannot easily evaluate whether your CTS is correct


Eye CTS, 1-2 hours CTS, trick reversing tides , great use of CTS.

Long passages , with unknown ground track - Madness


dave

[goboatingnow]

Originally Posted by Stu Jackson

I agree. I also mentioned just a single crossing with current in only one direction.

If one sails only based on the GPS heading to the next waypoint, as in Dockhead's river example, WITHOUT calculating the effect of the current, WHICH THE GPS CANNOT KNOW or anticipate, then you'll sail a further distance. That's the whole point. iT's called navigation and is explained in Dutton's - current sailing.

I understand that many have not read Dockhead's excellent article that I provided the link to. It's long, it's complicated, it's effective.

I've tried it in many cases here on SF Bay, for example crossing west to east in the South Bay with a strong building to up to 3 or 4 knot current ebbing north in a sailboat going 5 knots. It DOES make a BIG difference over even a four to six mile distance.

Your boat, your choice.

Im not arguing against using CTS, just like you said , 4-6 miles, 5 knots , 3-4 knot current, Thats a simple one hour classic triangular CTS plot

IN all simple triangular CTS plots , while you face ( head) in the CTS direction , The Boat actually proceeds along your ground course track. you actually crab along the rhumb lines as Dockhead misleadingly put it.

BUT. Where this all breaks down, is where you compute ONE CTS heading, to take into account a multi hour complex tidal crossing. What you end up with is a heading that often bears no intuitive sense as to where you are going, and MORE IMPORTANTLY, you cannot determine the ground track in advance that you will pass over ( well not this side of a computer as Dockhead does). Hence you cannot evaluate the safety of the route proposed.

This is NOT navigation. ( and you'll search for example BowDitch from one end to the other looking for multi hour CTS}

You'll also search the RYA exam papers fruitlessly for anything over a three hour CTS ( and even that I have issues with and I used to teach it )


in any navigation exercise , you should only depart from your intended " ground track " , " course" or " rhumb line" where its very safe to do . A classic case is Great Circle sailing for example. Otherwise you are not being safe.


The fact is people just dont understand the issues around multi hour CTS, Its not actually taught at all ( for good reasons ) no prudent mariners would use it outside of a few trick cases


dave

[goboatingnow]

Originally Posted by Dockhead

I'll answer the question of "how STW would affect their calculations or decision making."


Because STW is one of the operands in the CTS equation, without which you can't reach any conclusions. To get any tidal vector, you need to know heading and STW, on the one hand, and set and drift, on the other.

If you go slower or faster through the water, the tide will affect you more or less, and change the CTS which is needed to get you to your waypoint.

That's why we log STW on a long CTS passage.

The issue here is that people tell you a "method" as if its an explanation.

CTS is a vector computation, any method that computes the effect of steering to compensate for a tidal set and drift can be used.You can simply judge the steering angle from experience

In a simple triangular CTS plot , you tend proceed down the rhumb line OK ( Dockhead seems to have forgotten this). In fact in practice a 1-2 hour plot will also be basically triangular in shape and you will proceed down the rhumb line.


You can then use GPS fixes to determine the accuracy of your chosen course to steer. ( or XTE for example) and adjust your heading accordingly

At no time in this process have you actually required STW.

This is in fact what many do electronically , you drop a waypoint ahead, and calculate a simple CTS ( maybe over an hour or 2) . at the end of that process you should be back on the course or rhumb line and you can begin the CTS process again, using the tidal atlas or a combination of whats actually under you etc.

AT no time , do you make extensive excursions from the rhumb line.

[transmitterdan]

Originally Posted by Ericson38

Is that on both tacks, or powering along with no heel ? If tack dependent, have you rotated the sensor slightly (if there is a difference) to get about the same output on either tack ?

It is powering with no current and no heel. On our boat the sensor is pretty far below the waterline.

[Dockhead]

Originally Posted by goboatingnow

The issue here is that people tell you a "method" as if its an explanation.

CTS is a vector computation, any method that computes the effect of steering to compensate for a tidal set and drift can be used.You can simply judge the steering angle from experience

In a simple triangular CTS plot , you tend proceed down the rhumb line OK ( Dockhead seems to have forgotten this). In fact in practice a 1-2 hour plot will also be basically triangular in shape and you will proceed down the rhumb line.


You can then use GPS fixes to determine the accuracy of your chosen course to steer. ( or XTE for example) and adjust your heading accordingly

At no time in this process have you actually required STW.

This is in fact what many do electronically , you drop a waypoint ahead, and calculate a simple CTS ( maybe over an hour or 2) . at the end of that process you should be back on the course or rhumb line and you can begin the CTS process again, using the tidal atlas or a combination of whats actually under you etc.

AT no time , do you make extensive excursions from the rhumb line.

All technically correct, but again we are torturing a water-referenced system with ground references. Which is possible, mathematically sound, etc., but makes it all very hard to understand. Why would you try to check your progress on a CTS passage with ground coordinates, which as you said yourself are fairly hard to calculate? When living in the water-referenced world of CTS all you need to do is check your miles run through the water (or average STW) against your plan, without any calculations at all?

Concerning one hour CTS -- this is taught in Bowditch etc. as a pre-GPS navigation technique. I don't think that it has any application today. Hardly anywhere has a big change in a current over one hour; thus you can just use the Ron W method and just adjust course until COG matches the bearing to waypoint. Or put the pilot in track mode. That's what I do. If you intend to stay on the rhumb line -- which is a ground-referenced concept, there is no need to use any water-referenced concepts -- just let the GPS calculate everything. You say that on a one-hour CTS plot you tend to stay on the rhumb line -- correct! That's why you don't even need CTS navigation for this problem, or STW.

CTS navigation only has practical application, today, on passages long enough for there to be appreciable changes in the current over the passage so that the correct path deviates far from the rhumb line. Especially, but not exclusively, currents which change direction. Once you step into this water-referenced world, STW becomes, with heading, one of the two meaningful metrics, and your GPS starts to talk gibberish.

[El Pinguino]

Originally Posted by OceanSeaSpray

Trying to follow a straight line on the ground will see you bucking the E-going tide for 6 hours with your bow pointing 304 true.

'Bucking the tide'? No wind has been mentioned so what difference will it make what direction you are pointing... you aren't bucking anything. If there is a wind.. lets say from the west... you are going to have a lovely flat water ride while steering 000*. Unfortunately when the tidal stream comes away from the east you are going to be a long way down wind from your destination with an interesting wind over tide situation, esp in this 5 knot stream of yours.....

This has been interesting exercise in theory... in the real world that most of us inhabit not of much practical use. In 99% of the world tidal information such as is available in the English Channel just does not exist.
It has limited application elsewhere, SF Bay it seems, and also when crossing the Rio Pongo 20 miles up stream where you know that being swept down river is not an issue as when you near the northern shore the eddies below Bing Bong will lift you back up to the village.

This is not the Rio Pongo but somewhere else in the 'world of limited info'...

[transmitterdan]

Originally Posted by Dockhead

I'll answer the question of "how STW would affect their calculations or decision making."

Because STW is one of the operands in the CTS equation, without which you can't reach any conclusions. To get any tidal vector, you need to know heading and STW, on the one hand, and set and drift, on the other.

If you go slower or faster through the water, the tide will affect you more or less, and change the CTS which is needed to get you to your waypoint.

That's why we log STW on a long CTS passage.

Dockhead,

I get how you use an estimate of STW to plan a course. And I get that you will adjust your future CTS calculations based on your previous STW by assuming that in the future the STW will be similar to the recent past. That's not unreasonable. But I would think you can get a sufficiently accurate historical STW by looking at the recent track and current estimates. One can easily compute STW from the track if the estimate of current can be assumed to be accurate.

I believe there is a way to use a standard GPS position log along with current tables instead of a paddlewheel in every real world case. And the deviation from planned route will be known much more accurately.

To rephrase Dave's either or proposition let me phrase it this way, which would you rather log, accurate STW or accurate position? I think one can get accurate enough STW from a GPS track and estimated current but not the other way around.

I think the implied premise of this topic is that paddle wheels fail often, GPS not so much. So can you live without a paddle wheel if you have GPS? I think you can even in 5 knot current regions. I am willing to learn why that is not the case though.

[Dockhead]

Originally Posted by goboatingnow;1778772. . .

The fact is people just dont understand the issues around multi hour CTS, Its not actually taught at all ( for good reasons ) no prudent mariners would use it outside of a few trick cases . . .

I was taught it -- in the U.S. even ("it's not taught at all"). And all of the experienced Channel sailors I know use it, and use it instinctively, and use it all the time -- they will use it with the "eyeball" method for a passage which is hard to calculate. Since even a rough approximation of the correct CTS is better than an intentionally wrong course. Of course the Channel is a pretty rigorous laboratory for learning the method, since you really just can't navigate without it -- but the Channel is not a "trick case", or even a unique one.

This is simply not true, that "no prudent mariners would use it outside of a few trick cases." It is a basic technique wherever you have moving water on any scale.

[Dockhead]

Originally Posted by transmitterdan

Dockhead,

I get how you use an estimate of STW to plan a course. And I get that you will adjust your future CTS calculations based on your previous STW by assuming that in the future the STW will be similar to the recent past. That's not unreasonable. But I would think you can get a sufficiently accurate historical STW by looking at the recent track and current estimates. One can easily compute STW from the track if the estimate of current can be assumed to be accurate.

I believe there is a way to use a standard GPS position log along with current tables instead of a paddlewheel in every real world case. And the deviation from planned route will be known much more accurately.

To rephrase Dave's either or proposition let me phrase it this way, which would you rather log, accurate STW or accurate position? I think one can get accurate enough STW from a GPS track and estimated current but not the other way around.

I think the implied premise of this topic is that paddle wheels fail often, GPS not so much. So can you live without a paddle wheel if you have GPS? I think you can even in 5 knot current regions. I am willing to learn why that is not the case though.

Well, of course you can live without the paddlewheel, as many sailors do.

But when doing CTS navigation, your ground-referenced position is somewhat hard to calculate, as Dave mentioned several times. So it is a much more awkward way to check your progress, than just checking to be sure you're on plan with regard to miles run through the water. It's possible, of course, to use, but awkward since you have to step out of the water-referenced world and translate it into ground-reference.

SOG is not a proxy for STW when you have varying currents, for obvious reasons. This -- also VMG -- will vary erratically over a CTS passage.


One excellent GPS method -- a trick, I guess -- when you're crossing perpendicular currents is to check XTE against cumulative tidal set. This ground-referenced concept is convenient, as an exception to the rule, because the concept of tide set exactly relates water to ground.

[Robin3]

Originally Posted by goboatingnow

put it simply this way folks


If you had to pick between loosing your GPS or loosing your paddlewheel , which would you pick.


I think that might end this debate

I had the Raymarine dGPs go tits up on one boat but the then ST50 instruments network automatically went into DR mode continuing to give a Lat/Log position based on the log and heading data from the instruments, tide or current drift obviously not allowed for as not available electronically, this could be manually included when plotting the position on the paper chart (remember those?) using of course a mk1 2B pencil ( remember those too??)

ActuallY I had two such Raymarine GPS heads go out, turned out there was a snafoo in the factory assembly that pinched a wire and caused the failure.

I guess nowadays one would simply ask the smartphone navigation App where we were and hope we remembered to bring the pencil and chart to plot it.

[transmitterdan]

These days a boat can have 5-6 GPS receivers without trying hard. It's hard not to have that many.