Distinct Activities: Shackled by a Common Name?

[goboatingnow]

Originally Posted by David M

At the maritime academy it was drilled into our heads for four years to use all of your sources for obtaining a fix, and I still believe this. As good as GPS is, it is still just one source.

This debate now has nothing to do with position fixing. It's that merely following the ground track ( by whatever means) is not an efficient method where there is a cross track tidal component.

Dave

[Andrew Troup]

Well, like, you know, ... when he says "really concrete", he doesn't really mean concrete.

Even sailors don't necessarily mean it, to pick another example of sanctioned doublespeak, when they say "I was literally blown away" ;-)

But you might be right: perhaps he was taking a Tarantino view of the problem of preventing the ants from larking about ... 3 pair of little concrete gumboots (preparatory to sleeping wit da fishes)

[Jammer Six]

Tarantino runs hot and cold. You gotta wonder about a guy who thinks Samuel L. Jackson can act.

[Seaworthy Lass]

"Ant-he-climb-axe"
"Antputee"
"Maya-nnaise"

I am still wiping away the tears! I have read that laughter is the best medicine, but some pills are just too hard to swallow before breakfast .

Curiosity is getting the better of me though. Did Stephen Jay Gould's proposed mineshaft have anything to do with the maritime academy's drilling explorations?

[Jammer Six]

I've been trying to ignore the puns. Antputee was so bad I hit myself in the head with a hammer.

[Seaworthy Lass]

Originally Posted by Jammer Six

I've been trying to ignore the puns. Antputee was so bad I hit myself in the head with a hammer.

?

[carstenb]

Yep, the puns were terrible. In fact so bad they could be classified as being the antithesis of good taste and grammar. Andrew, being defiant, will no doubt resist the efforts of the antiant ( how about that!) crusade.

[Hudson Force]

Originally Posted by goboatingnow

This debate now has nothing to do with position fixing. It's that merely following the ground track ( by whatever means) is not an efficient method where there is a cross track tidal component.

Dave

I agree that the dabate is not a question of position fixes or the use of navigation tools or of ant behavior. The debate is about calculating elapsed time. We can not dispute that the variables of speed and distance are direcly proportional to time. If speed and distance are increased or decreased proportionally, then time remains the same.

The debate is wether the choice of the constant heading or the shortest distance course GPS track results in a disproportional change in speed an distance.

I remain with the opinion that the time elapsed would be the same; however, as with any valid endeavor, I would be eager to hear convincing opposing opinions supported by good data.

[Dockhead]

Originally Posted by CaptForce

I agree that the dabate is not a question of position fixes or the use of navigation tools or of ant behavior. The debate is about calculating elapsed time. We can not dispute that the variables of speed and distance are direcly proportional to time. If speed and distance are increased or decreased proportionally, then time remains the same.

The debate is wether the choice of the constant heading or the shortest distance course GPS track results in a disproportional change in speed an distance.

I remain with the opinion that the time elapsed would be the same; however, as with any valid endeavor, I would be eager to hear convincing opposing opinions supported by good data.

All right, let's try again.

1. Distance sailed through the water does not necessarily equal distance over the ground. If the water is moving, it will ALWAYS be different.

2. The shortest path between any two points is a straight line.

3. Since we sail in water, we care about distance through the water, not over ground. Therefore, the shortest distance across moving water for a boat is a straight line through the water.

4. A straight line through water is a constant heading. NOT a constant COG, which will vary according to the motion of the water.

Therefore, the shortest distance across moving water is a constant heading calculated to put you in that bit of water which will be in front of your destination when you get there.

A corollary to this -- a straight line across the ground, following the GPS approach, will ALWAYS be the wrong way across, unless either (a) the water is not moving; or (b) the water is moving at a constant speed and direction for the entire passage (in both cases, constant heading and straight track across ground coincide).


OK, is that more clear? It boils down to this -- you have a choice to go straight with regard to the ground, or straight with regard to the water. You need to choose the latter if you want to sail less distance and get there sooner. Sailing a straight line over ground means sailing a crooked path through water, which will add miles to your passage. Except only exceptions (a) and (b) above, where the shortest paths through water and over ground are in the same place.


There are specific examples in Post 253 by Noelex, with calculations showing the difference in time and distances:

http://www.cruisersforum.com/forums/...ml#post1126538

And in Post 237: http://www.cruisersforum.com/forums/...ml#post1126273


Let me give another example showing how different water and ground distances can be:

You are crossing a river one mile wide with a 6 knot current which runs due North and South. You are on the W bank of the river in a boat capable of making 6 knots. You want to sail to a dock on the E bank of the river which is one mile downstream from your point of departure.

You point your bow directly across the stream, and in 10 minutes you are there. You sailed 1.4 miles over ground but only 1 mile through the water, which is why you got there in 10 minutes although you were only going 6 knots.

That is to illustrate the difference in water distance and ground distance. In that example, you never feel the 0.4 miles extra over ground -- you don't sail in the bottom.

Now another example -- exaggerating the effect to underline -- let's suppose you're crossing a tidal body of water, 60 miles across, on Mars, say, with the tide exactly perpendicular to your course. On Mars the tides run only for two hours at a time, then an hour pause, then two hours in the other direction, then 10 hours of slack water. They run at 10 knots. You are sailing W to E and the tide runs N and S.

What's the fastest way to get there? Go full speed ahead against the tide, when it's running, in order to stay closer to the rhumb line and minimize distance over ground? Or forget about the tides since they cancel each other out? If you hold a constant heading of 90, you will sail 60 miles through water and get there in 10 hours at 6 knots, although you will have sailed 100 miles over ground as the tide swept you back and forth. Sailing to your GPS and staying on the rhumb line you will sail a shorter distance over ground, but longer through the water. If you can make 10 knots at full revs and stem the tide when its running, then you will have sailed 60 miles over the ground, but you will have added 40 miles of useless sailing through the water, at redline to boot, and you will get there 4 hours later, by following the GPS approach.


In Post 237, you have a choice between sailing 60 miles through the water on a constant heading (although this is a huge "S" curve over ground, taking you 12 miles off the rhumb line at one point), or 75 miles through water on the rhumb line and 60 miles over ground. Since your boat can only sail in water, the constant heading saves you 15 miles and three hours of sailing, compared to staying on the rhumb line over the shortest distance over ground following the GPS approach.


Again, the shortest distance over ground will NEVER be the shortest distance through the water except for the two cases above. You never feel ground distance; you only feel distance through the water since that's what your boat sails in. The shortest distance across a moving body of water is ALWAYS a constant heading.

[Hudson Force]

Originally Posted by Dockhead

All right, let's try again.

1. Distance sailed through the water does not necessarily equal distance over the ground. If the water is moving, it will ALWAYS be different..................

How can I possibly be motivated to read the rest of this post when you begin by changing the definitons of distance and speed? A boat does not transit a greater distance because it's speed is reduced while traveling against a current. Without an agreed operational definiton of speed and distance that conforms with the international norms of physics we have no means to communicate.

I'll add again that, even if you were to be correct, there's no convincing if we are speaking a different language.

[carstenb]

This ia actually an interesting excercise. It may be view as foolws:

Assume a channel 5 NM wide. Current exactly 2 knots. We want to go from E to W which coincidentally falls on the 90-180 degree line. We will label these points A and B. Current is exactly from N to S. Our boat has a sailing speed of 5 knots.

Dockhead does his calculations, and realizing a 2 knot current will result in a 2 NM offset, points his bow T a point (which we label C), 2 NM north of B and off he goes.

Sometime later, he arrive at B. What distance did he travel? He traveled 5 NM across the ground, however he did not travel 5 NM through the water! He actually traveled the distance from A to C through the water, which we can calculate. The water that the water was moving while he was sailing has no bearing on the fact that he had to travel the distance. So how far? A to C is a rifht angle triangle so A-b squared plus B-C squared will equal a-c squared or 5 squared (25) plus 2 squared (4)equals 27. Square root of 27 is 5.2(rounded).

Dockhead then traveled 5.2 NM to go 5 NM. Meaning his travel time is 5 hours, 2 minutes and 24 seconds.

Capt force starts out saying "damn the torpedos, full speed ahead" and sets a COG directly at point B. If he stays on this he will travel the C squared from above plus the B squared or 7 NM, since he will be offset by 2 NM on the way. Or he can realize his error half way over and make a course correction. Now he (use the pythagrian theory) will travel 5.4 NM.
Final alterntive is to make continual course correction along the way, but he will in reality end with the same bow angle as Dockhead.

I ANTicipate your remarks

[Lake-Effect]

Just a small voice saying that I got a book on sextants this Christmas, and I am going to buy a Davis Mk 15 and learn the damn thing, for the same reasons I like to build small radios using tubes: cos it's interesting and a link to the past.

I do see too many weekend boaters (stinkboats mostly) driving with chartplotters like they were car units (" -beep- turn left here"). We like to try to use a combination of navigation methods, and the GPS is most often fired up just to check the other methods or as situation warrants.

Carry on

(no ants on our boat! I feel left out)

[Dockhead]

Originally Posted by CaptForce

How can I possibly be motivated to read the rest of this post when you begin by changing the definitons of distance and speed? A boat does not transit a greater distance because it's speed is reduced while traveling against a current. Without an agreed operational definiton of speed and distance that conforms with the international norms of physics we have no means to communicate.

I'll add again that, even if you were to be correct, there's no convincing if we are speaking a different language.

A boat's speed through the water is not reduced while travelling against a current. Its speed over ground is.

Enlightenment in this question lies in understanding that our boats move through water, not over ground. It's absolutely essential to understand the concept of distance through water versus distance over ground. I thought there were some examples which really well illustrated this difference.

So definitions of speed and distance:

SOG, COG, and distance over ground in miles -- all ground-referenced

STW, heading, and distance through water in miles -- all water-referenced.

You cannot navigate efficiently without the water-referenced concepts. Because -- once again -- that's all our boats know how to do.

Like I said in the last post -- distance through the water and distance over ground are always different if the water is moving. You can navigate in order to minimize the distance over ground -- that would be putting your pilot on "track mode" and letting it keep you on the rhumb line, sailing in a straight line from "A" to "B" referenced to the ground. Result -- minimum distance over ground. But you will sail further than necessary through the water.

OR, you can choose to minimize the distance through water -- that would be calculating CTS and sailing a constant heading. You will sail the shortest possible distance through water, but the distance over ground will be increased. Since your boat sails through water, you will get there faster by choosing to sail the shortest distance through water. You will sail further over ground, but you don't care in the least. The ground distance is irrelevant if your boat doesn't have legs which reach down to the bottom.

[wolfenzee]

There is a thread in Sailnet.com " The future of the sailing world. Why are there so few young people cruising" which has evolved into talking about American's fascination of gadgets and/or the marketing hype that causes/controls that. One of the posts read:
"...I'm primarily referring to the fascination so many have for gadgets. The iPhone5, iPadx, the latest 24MP digital camera with those various accessories, or the most scary-looking firearm with the 50-round magazine - the list goes on and on.

Apparently, re that 'grandma gear' fetish, you've led a sheltered existence? Good if true. But I link 'gear' fetishes with the gadget fascinations; the phenomenae may be more than just a "lure of the 'new' and 'different'," and something deeper psychologically.
Marketing hype hooks into this psychological opening?"

[Andrew Troup]

I frankly fail to see how anyone could remain unconvinced by Dockhead's great explanations, but everyone's brain is wired slightly differently, and often it's a matter of finding the exact key to unlock a hidden assumption which is blocking things, so here's another angle which just occurred to me:

0 – 0 – 0 – 0 – 0 – 0 – 0 – 0 – 0 – 0 – 0 – 0 – 0 – 0 – 0 – 0 – 0

What's the quickest way to swim across a river: is it to stay on the shortest line between where you start and the opposite side? This would mean staying on a perpendicular, which is not the same as swimming at a perpendicular.
Consider a river which is flowing almost as fast as you can swim. If you are trying to stay on a perpendicular, you will have to swim almost due upstream. You will edge out into the river, staying over a notional perpendicular line drawn on the bottom.

If you get tired or the current is stronger in the middle, you will have to angle directly upstream to try and stay on the line, until the point is reached where it's all you can do to stay over the line, and you are no longer crossing the river at all.

Whereas if you simply swim perpendicular to the current, it will take the same time to swim across the river as it would if it stopped flowing.

This is true even if the river flows much faster than you can swim, whereas by adopting the other strategy, you cannot make any headway at all across a river which merely matches your speed.

On the perpendicular heading (rather than perpendicular 'Course over Ground') you will cover a much greater distance across the ground than the width of the river, but that increased distance is entirely due to the river's flow: you have not had to contribute anything to covering the increased distance. I

f, on reaching the other side, you encounter another river flowing at the same speed in the opposite direction, you should adopt the same strategy.

In other words, swim at the same heading which worked last time: perpendicular to the current. You will be swept downstream by the new current, which incidentally will spit you out on the bank directly opposite the point at which you entered the first river. But on the map it will look as though you covered a great deal of unnecessary distance in travelling across the equivalent of a tidal current which reversed when you were half way across.