Speed Through GPS Versus Old Fashioned Paddle Log

[Wotname]

Originally Posted by sailorboy1

I like how this has now become a "you are stupid" thread. But I'm smart enough to know how to deal with the thread

Not sure this is correct either unless not knowing something equates to being stupid. If so, then I am stupid as there is a shipload of stuff I don't know and I'm sure I would have many shipmates on board with me

However when people post stuff that is factually incorrect about something as important as navigation, others will correct them. If they continue to post their incorrect viewpoint without a decent argument to back their claims, some of us ignore it and some of us try to show them the facts by way of substantive argument.

In this thread, I have suggested some to revisit their knowledge or learn for the first time, elementary vector maths as this knowledge will allow them to work it out for themselves and thus see why their claim was invalid. This doesn't mean they are stupid, it does mean they will have a better understanding of the issue.

Others has demonstated another way to see that a staight line though water when transversing varying currents will get you to your destination faster than following a straight line over the ground.

IMO, both methods will help those who don't yet believe such facts, perhaps one way suits some and other ways suits others..

Unless one is trolling, I believe we are trying to find the truth one way or another ( pun slightly intended)

YMMV

[Don C L]

I don't know if anyone is stupid, this stuff is not intuitively obvious... and GPS may have made us a little lazy and we may have put away our pencils and charts...

[monte]

Originally Posted by Dockhead

Of course it does. The shortest path from A to B over moving water is a straight line -- through water. Not over ground. That is an objective fact.



Here is the shortest path across the English Channel on a average day:



Attachment 99050



You are holding a constant heading and sailing a straight line through the water, which is the fastest way. But over ground your path can be very crooked.



The reason for all of this is that your boat goes through water. It does not have legs or wheels which reach the bottom.



And in a nutshell that is why STW is important.

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.

[rwidman]

Originally Posted by Wotname

Gentlemen, I'm sorry to say that this is incorrect.

A reasonable understanding of high school vector maths will easy demonstate that what Dockhead and others have posted is in fact, correct.

I suggest you dust off your old school books and revise vector maths or if necessary, learn it for the first time otherwise you will just continue to deny hard solid fact.

I suppose of course, you are certainly entitled to hold your own opinion but the facts will remain the facts.

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."

[bigbrownpilot]

I fly jets for a living. I have a digital readout of my INS/GPS ground speed. But I also have a big old indicated airspeed gauge and a true airspeed readout. All three are important and useful at different times. Just sayin'.


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[OceanSeaSpray]

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?

First of all, never having learned something doesn't in itself make anyone stupid. Incompetent yes, stupid no.

The issue is that many people today think in terms of GPS instead of thinking in terms of navigation. GPS is not navigation and doesn't replace navigation.
The answer to your question is no, you won't get onto the right heading by looking at any GPS data including VMG, because all it can tell you is based on what is happening now.
The guy who steered due north throughout achieved his result by looking into the future and anticipating. Sailing a lower VMG can place you into a favourable current earlier and change the picture completely. A GPS cannot see that. A full-blown routing program with all the relevant input data could, but running it in an ever-changing and adapting environment would look very much like a full-time office job and learning how to operate it efficiently would be far more involved than learning basic navigation.

Take the earlier example to the limit and you will see intuitively that VMG doesn't cut it: increase the velocity of the current to 50 knots. As soon as you cast off, you will fly off into the East at a huge distance from your destination. At this point, best VMG will be anything that reduces the rate at which your destination is disappearing away and the GPS will make you point towards it. You will be bucking the current instead of sailing north. The missing piece is that it can't integrate that little piece of information that says "later an opposite current will bring you all the way back".

[leftbrainstuff]

Originally Posted by Robin3

Nothing to do with positional accuracy per se. I had differential GPS on several marine sets years ago in Europe, they stopped bothering with dGPS when Uncle Sam removed the selective availabilty degradation of his GPS signals for non military users some years ago thereby making the basic GPS much more accurate.

The differential Gps used in Pi Research systems use a gyro to give very high resolution and they process 100s of times a second.

The change in offsets that the US used to enforce on the gps systems have no impact on the Pi systems ability to determine relative motion. (I'm referring to the 1/10th of a knot mentioned earlier)

They provide the roll, yaw, pitch and 3D displacements needed for relative motion calculations. For accurate path mapping they required additional beacons to better resolve absolute position.

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[Alan Mighty]

Originally Posted by rwidman

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

I myself personally, that is me, think that everyone understands that what I write is my opinion (and not the opinion of Pontius Pilate or Abu Zayud ibn Khaldun) and so I say that you're wrong.


No need for multiple redundancy or pretended humility.


Al

[Stu Jackson]

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.

[Stu Jackson]

OK, here's the Diluted thread, should be a link to the long original one in there somewhere:

http://www.cruisersforum.com/forums/...ad-115361.html

[El Pinguino]

All good stuff, but isn't this a 'special case'? It works because the passage time = 2 tides...one tidal stream on the beam going thisaway ... the next +/- 6 hours exactly the opposite.
What if the passage is only one of six hours and the wind is not on the beam?
This is a question...not a statement, I'm busy doing other stuff just now.

[El Pinguino]

And also... getting back to the OP....both is best, speed log for speed through the water, GPS for speed over the ground. Throw in a compass and you can have all the info you want, course steered , course made good, plus speed through water and over ground... all at least handy if not vitally important information for the recreational sailor.

Despite what was said by someone earlier this info has always been available, maybe not real time... coastwise the course and speed over the ground can be determined by inspection of two consecutive terrestrial 'fixes' while speed through the water would be a simple '100rpm gives us 15 knots... all things being equal' . Deep sea distance and thus speed over ground would be established between AM and PM ( and v/v) star sights..not instant as is demanded these days but there nevertheless.

[Robin3]

Originally Posted by Dockhead

Of course it does. The shortest path from A to B over moving water is a straight line -- through water. Not over ground. That is an objective fact.

Here is the shortest path across the English Channel on a average day:

Attachment 99050

You are holding a constant heading and sailing a straight line through the water, which is the fastest way. But over ground your path can be very crooked.

The reason for all of this is that your boat goes through water. It does not have legs or wheels which reach the bottom.

And in a nutshell that is why STW is important.

Exactly.

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.

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.

it is a valid point that motor vessels, especially the faster moving ones are much less affected by cross streams than are slower vessels such as sailing boats and this I believe is why some find the above explanations even harder to comprehend. Under sail continually adjusting the vessel's heading to compensate for a cross-current drift is difficult as sooner or later the new wind angle required becomes impossible to sail ( like straight into the wind) This is another reason why pre-planning a constant course to steer that gives the required ground track and safely clear of all dangerous obstacles is the better option, not just tweaking a simple heading manually or by using autopilot track feature is always shorter and safer. if necessary the voyage may need to be divided into several 'legs' each with a pre-planned CTS based on the expected boatspeed through the water, SOG at any one instant is interesting of course but largely irrelevant.

[Stu Jackson]

Originally Posted by El Pinguino

All good stuff, but isn't this a 'special case'? It works because the passage time = 2 tides...one tidal stream on the beam going thisaway ... the next +/- 6 hours exactly the opposite.
What if the passage is only one of six hours and the wind is not on the beam?
This is a question...not a statement, I'm busy doing other stuff just now.

Nope, not a special case. Just as important and vaid if you're going across one tidal cycle.

Again, please, think about it.

And read the link, so we don't have to retype much pertinent discussion.

Please.

In a nutshell, again, your GPS cannot anticipate current. YOU have to make the decision to OFFSET the current before you set out. Otherwise, your GPS will be constantly correcting and making the passage longer.

[El Pinguino]

Originally Posted by Stu Jackson

Nope, not a special case. Just as important and vaid if you're going across one tidal cycle.

Again, please, think about it.

And read the link, so we don't have to retype much pertinent discussion.

Please.

In a nutshell, again, your GPS cannot anticipate current. YOU have to make the decision to OFFSET the current before you set out. Otherwise, your GPS will be constantly correcting and making the passage longer.

Please don't bang your head like that, you'll do yourself an injury... no I shan't be bothering to read that thread, however I will say that the title of the link is flawed ...'Course to steer vs following the rhumb line'
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.
Oh, and btw I am well aware that GPS can't anticipate anything.
As said somewhere in this morass - you do a vector thingo and work out course to steer for next hour, then you do it again... however with GPS you can do it constantly...
Obviously a lot of far smarter people than me out there so carry on chaps, I'm going back to work on my boat....