Speed Through GPS Versus Old Fashioned Paddle Log

[Ericson38]

Originally Posted by valhalla360

It's not bad have to have speed thru the water and it does tell you some information but if you are alert, it doesn't tell you much more than you can figure out if you pay attention.

Could also just put it neutral and after awhile measure the set and drift off the GPS SOG and COG and keep a mental note of what is going on (at that moment), but doing a 180 or what I just offered is not the way to get somewhere in a limited width channel in a strange area with degraded visibility to what is floating around.

The STW sensor is going to allow you to continually measure the difference between STW and SOG, as opposed to a one time measurement. Situational awareness is what this type of cruising is about, using diverse data.

Open ocean, the STW sensor has much less value, I would certainly agree. Same for charted depths in open ocean versus soundings.

[sailorboy1]

Originally Posted by Dockhead

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.
.

I don't believe the shortest path is a straight through the water, I don't even know what that means. The shortest path is a straight line over the ground. Regardless a straight lines is, well a straight line!

[OceanSeaSpray]

So, tell me something rwidman: when you are heading somewhere, do you just "maximise your speed over the ground" or do you use something like a HEADING??

The typical "GPS navigator" like you keeps pointing the boat towards the destination, trying to follow a straight line over the ground, which results in a much longer distance sailed and a slower trip. When I say "GPS navigator" I am referring to people who in fact can't navigate: chuck the magic gizmo overboard and they are hopelessly screwed. Incompetent. They don't even know enough to realise what they don't know.

Dockhead is entirely correct and I happened to teach navigation - in the English Channel - for Les Glenans Sailing School, some 25 years ago. I have made that passage many, many times over. It is classic example for planning with the current. We taught that.

The fact is that unless you have speed and course information in both reference frames, over the water and over the ground, you have no idea of what the current is doing. "GPS navigators" haven't got a clue about the current they are sailing in. When the compass and the GPS courses disagree, at best they say "uh, there must be some current". What is its direction, strength? No clue.
In the days when we just sailed with a speed/log, we used to take bearings, fixes and other independent positions and worked out what the current was doing from there. Knowing what the current is doing allows to anticipate. "GPS navigators" on the other hand not only have no idea of what the current is doing, but are also ignorant enough to claim they don't need to care.

I know of a few instances when by the time they realised how strong the tidal stream was, it was also too late to get out of a situation they should have never got into.

Now if you don't have your distance travelled over the water because it is supposedly so unnecessary to have an instrument for that, you can't possibly retrieve that back if you ever need it. You are in a much weakened position with no way to alleviate it. If you don't have ground data, you can get that back easily enough by observing something.
The old paddlewheel log is still the starting point. It is ultra-low powered, doesn't rely on external infrastructure and its electronics are incredibly simple and robust compared to a GPS.

Offshore I take one documented position each day and compare fix with dead-reckoning. It tells me what the ocean current has been doing, it influences my decisions. I also keep an eye on sea temperature. I do the same in coastal navigation, just more often. It is called navigation.

[Wotname]

Originally Posted by rwidman

Again, if you are sailing from point "A" to point "B", speed over ground is most important. Your speed over ground, not speed over water is what tells you how soon you should get to your destination.

Sure you are travelling through the water but there is ground under it and the speed of the water (current) doesn't change the actual distance (over ground) between point "A" and point "B".

Originally Posted by sailorboy1

I don't believe the shortest path is a straight through the water, I don't even know what that means. The shortest path is a straight line over the ground. Regardless a straight lines is, well a straight line!

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.

[Sailor Doug]

Just think if we have another strong solar flair it might disable gps. Knot log looks really good then, if you have one and know how to use it. One of the keys to safety at sea is to have back up systems. I have a bunch of gps units and a knot log, three compasses and paper charts, would have a sexton if I knew how to use it.


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[sailorboy1]

Originally Posted by Wotname

Gentlemen, I'm sorry to say that this in 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.

So in your math book the shortest path between objects isn't a straight line?

You guys are now in a fight about the shortest path and the fastest time to an object. They don't have to go together.

I believe what some are really trying to say is that starting from a given distance from something, and taking a bearing to that object, and then setting a heading matching the bearing, doesn't mean you are going to the object.

[Dockhead]

Originally Posted by rwidman

Again, if you are sailing from point "A" to point "B", speed over ground is most important. Your speed over ground, not speed over water is what tells you how soon you should get to your destination.

Sure you are travelling through the water but there is ground under it and the speed of the water (current) doesn't change the actual distance (over ground) between point "A" and point "B".

Nope. Indeed not. Every single proposition in this post is false. Think about this with an open mind and you'll see.

Speed over ground tell you absolutely nothing. The rhumb line of the passage above is 60 miles -- Needles to Cherbourg. The distance over ground under the "S" curve is probably closer to 80 miles. The distance sailed through the water is about 63 miles.

So the average speed over ground, if it takes 10 hours, is 8 knots.

But you only sail 63 miles through the water -- average 6.3 knots.

Your speed over ground as shown by your instruments (I've done that exact passage several scores of time) will vary wildly, from 2 or 3 knots to 12 or 13 knots. But you will be sailing a constant 6.3 knots, say, in a straight line through the water. And your COG also swings around wildly. If you try to think in land-referenced terms you will go crazy crossing the English Channel or any other body of water with strong reversing tides.

If what you are trying to say is that your speed down the rhumb line is what is important -- now that's a different story -- that's correct. That's called speed of advance in technical maritime language. Because points A and B are both land-referenced. But your instruments don't tell you speed of advance, which is not at all the same as SOG from your instruments -- because in moving water, you're not going in a straight line at all in relation to land, much less down the rhumb line.

I sincerely hope that that helps?

[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.


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[Don C L]

I am really enamored with my new GPS in my new radio, but I still want a Walker Log to go with the one already installed... by the way maybe I missed something but with GPS, if my heading is one thing and my compass says another, is it not safe to assume I am under the influence of a current? Of course I won't know the vectors which are way more important, but at least I can see something is going on with the water I am floating through then I can start (start??) tracking my course on paper to see... wait, do we still use paper charts and pencils? Maybe I am too old.

[Wotname]

Originally Posted by sailorboy1

So in your math book the shortest path between objects isn't a straight line?

You guys are now in a fight about the shortest path and the fastest time to an object. They don't have to go together.

I believe what some are really trying to say is that starting from a given distance from something, and taking a bearing to that object, and then setting a heading matching the bearing, doesn't mean you are going to the object.

My maths book is very clear. Understanding and applying vector maths demonstates that when navigating, the shortest distance between points is a staight line though water. It is not a staight line over the ground.

This is fact that was established a very long time ago, at least a century back, probably many centuries back. It may be counter intuitive if one hasn't learnt the maths but it can be demonstated by trial and error if one doesn't care for book learning

[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

[Sandero]

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.


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There are no "straight lines" on the surface of a sphere

[OceanSeaSpray]

Sailorboy1, you are another one of them. Off the water, off to school please.

I am going to simplify the English Channel crossing problem to show you - and the many others - that you are completely wrong.

Your passage is going to be due N, 72NM. Your boat will be doing 6 knots throughout. 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.

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. In that time you will cover 20NM towards your destination. Next you will be doing the same for another 6 hours bucking the W-going current heading 056 true and covering another 20NM. So after 12 hours, you will have covered 40NM out of 72NM to reach your destination.

Another boat, with a real navigator, left in the same time as you did. The guy said: "I have 72NM to cover and I can make 6 knots, that would be 12 hours with no tide. I am going to get 6 hours of tide East at 5 knots, that is 30NM drift to the east, and then the same to the west, so it cancels out. As a result, I am going to steer due N and just track the drift."
After 6 hours, he has travelled 36NM north and 30NM east due to the tide. His position shows him 47NM in the 040 true, 30NM off your route. In the following 6 hours, the tide carries him back west by 30NM and he makes another 36NM north. He has been steering N all the time and he is home. You have barely covered half of the distance, trying to stick to your waypoints and straight line, heading northwest and then northeast when you should have been going north.

He can navigate, you can't. That's the GPS revolution.

[transmitterdan]

But if he had put that route into his GPS before he left he could tell in an instant whether or not he was following his plan.

Here is an interesting question:

Assume the example of OceanSeaSpray ('cuz I like its simplicity), is there a course to steer that makes the GPS show a higher "Velocity Made Good" but is actually not the fastest VMG. Steering due north is the fastest course to steer. But does the GPS know that?

[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?