Speed Through GPS Versus Old Fashioned Paddle Log

[El Pinguino]

Originally Posted by OceanSeaSpray

For the first 6 hours, you are going to get current setting due E at 5 knots. For the following 6 hours afterwards, the current will be setting you due W at 5 knots again.

So when exactly does the tidal stream run at 5 knots ( let alone 5 knots throughout the six hours ) in that area...

lookee here Kayarchy - sea & weather (5) currents about half way down

Also from somewhere above.... we used 'track mode' almost all the time in the day job. It could take us from berth to berth but we would only use it in the open water legs. It doesn't simply point you at your destination... it keeps you on your track ( ie the rhumb line or great circle...take your pick.)
Wind and tide from the west while your track is 000*? You will be steering 355* ( frinstance) to maintain track.

That was 'Kongsberg Control Systems' kit, as used by a very large percentage of the world's commercial shipping. Mr and Mrs Kongsberg seem to think that sitting on the rhumb line is the way to go... I'm sure that when they hear that by wandering all over the ocean ship owners can save billions on fuel bills every year they will be thrilled to bits.

[Dockhead]

Originally Posted by Sailor Doug

Some straight lines are slower and if you don't take the great circle you will travel further. Math just gets more complicated basic 101.

Also true, but this adds unnecessary complication for this problem. It is correct to ignore this effect for passages of less than a few hundred miles.

[OceanSeaSpray]

Originally Posted by El Pinguino

So when exactly does the tidal stream run at 5 knots ( let alone 5 knots throughout the six hours ) in that area...
...
Also from somewhere above.... we used 'track mode' almost all the time in the day job. It could take us from berth to berth but we would only use it in the open water legs. It doesn't simply point you at your destination... it keeps you on your track ( ie the rhumb line or great circle...take your pick.)

Off the water and off to school as well, El Pinguino.

It has absolutely nothing to do with the rhumb line/Great Circle. You don't understand the issue and you can't navigate either. And it is not a matter of "social proof by shipping".

I made the tidal stream alternate exactly with a constant speed to simplify the problem to the extreme and demonstrate a point. Increase the boat speed to 7 knots and you won't be steering 000 true any more. Vary the speed and the direction of the tidal stream over time and it will take a little more work to work out where to steer again.
However, if you don't grasp the simplified case, no point worrying about the real life one for now. Off to navigation school, please.

We could fully develop the matter here, but not tonight for me.

[Dockhead]

Originally Posted by transmitterdan

Ok, I'll give my answer to the problem (which could be wrong). I think the GPS will show throughout the passage that the due north heading gives the best VMG. All other headings will result in a poorer VMG.

This is what a lot of "GPS" guys who are too lazy to learn the old ways do. Is it wrong in any instance?

What you say would be correct if the current were constant. In that case, you don't need to calculate a CTS. You can just put your pilot in track mode and relax. When COG matches bearing to waypoint, everything is fine. Your GPS can solve this problem.

The problem is when the current changes. The problem is dramatic when, like in the English Channel, it changes direction. But the problem is the same if even the speed changes -- crossing the Gulf Stream to the Bahamas from Florida is exactly the same problem. In that case, your GPS does not tell you a single useful thing.



Here's a great simplified illustration of the problem:

Let's say you're trying to get right across a wide river. The river is three miles wide. The current is slack for the first mile, runs at 5 knots for the second mile, and runs at 1 knots for the last mile. The river runs due South. You are trying to get from Point A to Point B, which is due East of you. How do you navigate it?

If you're in a fast powerboat, you don't -- you just pile on the coal, keep your COG at due East, and squirt across. No reason to bother to do anything else. RWidman knows that you have to keep your speed up, but at a certain multiple of the speed of the current, you can crab across that part without any great difficulty.

If you're in a slow sailboat on a calm day, so motoring at 5 knots, you know instinctively that you won't arrive at all if you just motor due East. Because once you get into the part of the river running at 5 knots, you can no longer go East. Note that as you motor through the first mile, your GPS will be telling you that everything is fine -- VMG to waypoint is 5 knots, COG is 90, SOG is 5 knots, who needs STW? Your GPS gives you zero useful information for this case.

So in order to get across at all, you have to steer upstream from the very beginning. But how far upstream to you have to steer? Ah, there's the rub. To know that, you have to run exactly the same calculation you need for the English Channel or the Gulf Stream to get a Course to Steer, which you then have to hold the whole way across. The inputs into this calculation are heading and speed through water -- COG and SOG are irrelevant; your GPS doesn't give you a single useful datum.

[Dockhead]

Originally Posted by monte

I'm not really getting the last line there...
How does that example show that STW is important. I can't imagine a scenario where the log would assist in that particular passage. You wouldn't sail by it to try increase STW as you are sticking to your heading. The STW isn't likely to change much with the current in the beam apart from the obvious effect of increasing or decreasing app wind as the tide changes. I can imagine adjusting the heading if the speed is more or less than the anticipated 5K, but still can't see the benefit of STW there.
If heading east west I could see more benefit to confirm actual currents and possible find a better current to assist the passage.

The reason why STW is important is that speed is essential to getting the results, and STW this is the only meaningful measure of speed in this passage. Your SOG will vary wildly during the passage and tells you nothing.

Since your CTS calculation assumes a certain speed, your average speed must match the assumed value for you to arrive in the right place. So what you do in practice is log your speed through water to see how you're doing compared to your plan.

In order to evaluate effects of deviations from your planned speed, you run several scenarios before you depart. So you know, for example, that if you're making 8 knots instead of 7 knots this will change the correct CTS by 2 degrees (say). So you can make corrections along the way, and at some point you will run the whole calculation all over again based on where you got after a certain period of time.

Now I do use the GPS for one thing, and that's this -- when you do it by hand, you have hour by hour the distance you are set off. Your GPS shows "XTE" -- cross track error. That is the distance off the rhumb line. This is not always an error . XTE at any given time should equal the cumulative hourly set off. This will double check your average speed and will also double check your calculations and is a great indicator of progress. If you start to get off your calculations, then you know to run this again.

[El Pinguino]

Originally Posted by OceanSeaSpray

Off the water and off to school as well, El Pinguino.

It has absolutely nothing to do with the rhumb line/Great Circle. You don't understand the issue and you can't navigate either. And it is not a matter of "social proof by shipping".

I made the tidal stream alternate exactly with a constant speed to simplify the problem to the extreme and demonstrate a point. Increase the boat speed to 7 knots and you won't be steering 000 true any more. Vary the speed and the direction of the tidal stream over time and it will take a little more work to work out where to steer again.
However, if you don't grasp the simplified case, no point worrying about the real life one for now. Off to navigation school, please.

We could fully develop the matter here, but not tonight for me.

Oh dear.... why do I bother...... I'm amazed I have survived as long as I have.... how does it go again..2 barks for port...one for starboard?

[Dockhead]

Originally Posted by El Pinguino

.. .
You should be steering a course so that you follow the rhumb line. Continually adjusting course to point at your destination is just downright stupid and if, upon passing the needles you just steer 180 until you next hit land isn't a whole lot brighter.

But that is exactly what you do! And an illustrative case. If the cumulative set of the tide over the passage cancels out, then just steer 180. You will swept way off the rhumbline first one way, then the other, and if your calculation was right and your average speed matched your assumption, then you will be swept right into Cherbourg at the end. That's exactly how CTS works. You steer 180 all the way.

In reality, the tides don't usually cancel out so your calculation will give you 190, or 165 or whatever, and steer it the whole way. You only correct it for deviations in your speed which put out your calculation.

If, on the contrary, you try to sail down the rhumbline, you will add many miles to the distance through water you have to sail, and in a slower vessel, you will add hours to your passage time, compared to sailing a constant heading and letting yourself get swept off the rhumbline.

[El Pinguino]

Originally Posted by Dockhead

But that is exactly what you do! And an illustrative case. If the cumulative set of the tide over the passage cancels out, then just steer 180. You will swept way off the rhumbline first one way, then the other, and if your calculation was right and your average speed matched your assumption, then you will be swept right into Cherbourg at the end. That's exactly how CTS works. You steer 180 all the way.

In reality, the tides don't usually cancel out so your calculation will give you 190, or 165 or whatever, and steer it the whole way. You only correct it for deviations in your speed which put out your calculation.

If, on the contrary, you try to sail down the rhumbline, you will add many miles to the distance through water you have to sail, and in a slower vessel, you will add hours to your passage time, compared to sailing a constant heading and letting yourself get swept off the rhumbline.

Two big words in there , 'if' ( the biggest word in the english language!) and 'assumption' ( beware the bold assumption!!)

So then we have the interesting bit I had missed before...' the tides don't usually cancel out so your calculation will give you 190, or 165 or whatever,'..
So this is indeed a 'special case' with no real application in the world that real sailormen live in.

And of course no mention anywhere of how the wind would affect this 'special case'.

Ho Hum.

What ever happened to lee-bowing the tide?

Sorry about such a brief response but since losing most of my fingers to that croc up the Rio Pongo I find typing a bit of a trial...
El Pingo Negro

[Wotname]

Originally Posted by rwidman

Would it be possible for you to disagree without the insults?


And your first sentence should really read "Gentlemen, I'm sorry to say that, in my personal opinion, this is incorrect."

Well Ron, I will have to ask you to help me out here. You see, I cant find anything in my post you quoted that I consider to be an insult yet it would seem that you have found an insult there. Perhaps you might consider reposting the insulting part of it so I can better understand how you have read something insulting in that post. Meanwhile let me assure you that no insult was intended.

As to your suggested wording to the quoted first sentence, I think that such wording would be very misleading as it to my way of thinking, such wording would suggest that is was only my personal opinion that what you posted was incorrect. I sorry, but that isnt the case. I can't put it any simpler than to say that you are incorrect or just plain wrong. It isn't just an opinion.

This is not an insult, purely a statement of fact. From your posts on other threads, I know that you are a competent person in the electricial / electronic trades and probably hold very good professional expertise in these areas. You tell us that you have a reasonably large (by my standards) powerboat. I know that getting to own a decent boat takes some life knowledge so I think you must be pretty switched on. I simply have no reason to insult you.

But I do have good reason to disagree with what you posted and I don't expect you to take my word for that which is why I suggested you could revisit elementary vector maths. Using maths will show you why you are wrong. Alternativily ypu can work the examples others have posted and you will see why a straight line though water is a more accuracte method of navigating than using a staight line over ground. Once you understand this, you are then free to use whatever method works best in your case but until you can take the concept on board, you will remain blind to a truth.

Again no insult intended and if you are ever in my part of paradise, we can discuss such things over a suitable beverage.

[goboatingnow]

Originally Posted by Dockhead

The reason why STW is important is that speed is essential to getting the results, and STW this is the only meaningful measure of speed in this passage. Your SOG will vary wildly during the passage and tells you nothing.

Since your CTS calculation assumes a certain speed, your average speed must match the assumed value for you to arrive in the right place. So what you do in practice is log your speed through water to see how you're doing compared to your plan.

In order to evaluate effects of deviations from your planned speed, you run several scenarios before you depart. So you know, for example, that if you're making 8 knots instead of 7 knots this will change the correct CTS by 2 degrees (say). So you can make corrections along the way, and at some point you will run the whole calculation all over again based on where you got after a certain period of time.

Now I do use the GPS for one thing, and that's this -- when you do it by hand, you have hour by hour the distance you are set off. Your GPS shows "XTE" -- cross track error. That is the distance off the rhumb line. This is not always an error . XTE at any given time should equal the cumulative hourly set off. This will double check your average speed and will also double check your calculations and is a great indicator of progress. If you start to get off your calculations, then you know to run this again.

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

[goboatingnow]

In reality, the tides don't usually cancel out so your calculation will give you 190, or 165 or whatever, and steer it the whole way. You only correct it for deviations in your speed which put out your calculation.

If, on the contrary, you try to sail down the rhumbline, you will add many miles to the distance through water you have to sail, and in a slower vessel, you will add hours to your passage time, compared to sailing a constant heading and letting yourself get swept off the rhumbline.

The vast majority of people sail close to the rhumb line and compute CTS on maybe an hour or two basis, often ensuring they do not get too far away from the rhumb line.

This allows some advantages of CTS in making use of the tide and also ensures good safety practice , ensuring that you can assure yourself of your ground track.

outside of a few places , you rarely have access to hourly tide data anyway.

PS try your ideas sailing round the canaries for example, no tide atlas there

I know you know what you are doing and you are applying it in a special case, But I bet you dont actually navigate that way outside those few peculiar situations ,come on now

[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.

[goboatingnow]

Plus if you simply keep adjusting COG to match an ever changing BTW it is quite possible that the boat will travel unexpectedly perhaps across dangerous ground since it is in effect trying to follow a new route with an ever changing start waypoint and therefore an unpredictable ground track. I believe this is how the incident I referred to in an earlier reply happened to an American boat that was trying to enter Cherbourg by the eastern entrance which has nasty rocks to the east of it where they ended up 'parked' on the rocks whilst I believe continually trying to tweak their course in the strong cross tides to get back to their target waypoint of the harbour entrance.

If you have a vessel that cannot stem normal tides that you expect , then you have to be very very careful in sailing into such areas, usually by selecting a tidal "gate" , for example the Raz du Sein .

This has nothing to do with GPS or STW or whatever and your use of a third hand incidence, simply illustrates that you do not know how such things are handled


Close in to a coast, following the rhumb line becomes important simply because hazards are more important that efficiency of progress. Hence relying on CTS computed in often unreliable or inaccurate data and applying it to situations like closing a harbour are the actions of a neophyte navigator

on closing a waypoint under a strong tide, one needs ann understanding of course to steer, but in effect adjusting course to make a waypoint is fine as long as you understand what you are doing and that the vessel may depart from the ground track . thats the mistake the US couple made, unless they had a vessel incapable of handling the tides, in that case you have to proceed far more cautiously

it is very difficult I find to convey my intended message without confusing the reader or indeed boring them to death, but it is an important consideration in navigation and one where having boatspeed and distance travelled through the water data available is at the very least useful and even essential.

Having an accurate position fix is far more useful then any of the above , since it is ones position over the planet as opposed where one though one should be is the essence of good navigation.

I am not arguing against a log ( i.e. distance through the water ) by the way I was arguing against the uselessness of STW. ( which is different)

[goboatingnow]

For those of you who, misguidedly, believe the GPS and VMG approach, you might want to search out Dockhead's excellent thread about CTS with great details about crossing the English Channel. Anytime you sail through currents that are abeam, your GPS will seriously misguide you. Try it sometime. If you, and only YOU can the GPS can't, anticipate the current, then you'll sail a lot shorter distance. Please, think about it.

Again , you miss what was the whole issues behind all that thread nonsense

firstly of course IN THEORY, a single CTS computed for the whole journey is the most time efficient way to get from A to B. That CTS should take into account all perturbations that will occur to the vessel over the course of the computation

In PRACTICE, only an idiot , or in one of two special cases actually navigates like that. A prudent navigator will only depart from an agreed course ( course over the ground) if they are sure its safe to do. Hence , since the ground trackof a complex multi hour CTS is difficult to determine, ITS NOT A SAFE WAY TO NAVIGATE and I dont know anyone that does it so,

Most people will compute CTS over 1 or 2 hours , often ensuring that they do not stray much from the course line and also that they , in effect , return to the course line after the computed period. This allows them a sanity check that they are not getting out of position.

IN any complex tidal situation or where the tide information is sparse, no one computes multi hour CTS. To suggest that after such computation I should sail off at an obscure angle to the course line, merely to assume that in 10 hours time I will reach my destination, is simply navigation madness .

poor navigators , know the theory, good navigators know the reality and the theory, Great navigators know when to apply all of it and whats relevant when.


dave

[rwidman]

As simple as I can state it:

If point "A" and point "B" are seventy nautical miles apart and you start at point "A" and maintain a speed of seven knots over ground, it will take you ten hours to get to point "B".