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Old 13-01-2013, 12:34   #331
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Re: Distinct Activities: Shackled by a Common Name?

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Originally Posted by carstenb View Post
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.
What else did you expect, from down here in the antipodes?
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Old 13-01-2013, 12:43   #332
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Re: Distinct Activities: Shackled by a Common Name?

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.... The debate is about calculating elapsed time. .... If speed and distance are increased or decreased proportionally, then time remains the same.
Hmmm... short of buying a new suit of sails or a longer boat, I'm not sure how to go about increasing speed in order to offset the increased distance through the water of the tactic you propose.
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Old 13-01-2013, 12:47   #333
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Re: Distinct Activities: Shackled by a Common Name?

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Originally Posted by carstenb View Post
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
I humbly suggest you check your calculations. The sum of five squared and two squared is 29 not 27 (easy error to make if you are in a hurry).

The reason distances travelled by both boats is the same (and the time taken to reach the destination is the same, and both will follow the same heading along the way) is that there was no reversal of current along the way. The boat following the straight GPS track from A to B will have to make a correction very quickly once he starts, but if he does that super quickly both boats will essentially follow the rhumb line for the entire journey .

We need an example with current reversing (or at least changing in strength) to demonstrate when following the rhumb line is not a good idea.
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Old 13-01-2013, 13:18   #334
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Re: Distinct Activities: Shackled by a Common Name?

I thought the whole idea of this was are we shackled by the gadgets and gizmos that figure all that. In threads that were essentially and/or had evolved into chart plotter vs paper charts (electronic vs manual). I had tried to point out the importance of knowing how to navigate with "analog navigation systems" that even if your electronic gizmos never went down the knowledge and resulting though patterns of how to navigate was still helpful. This idea is regularly shot down by the people whose confidence in "push button navigation" was such that they felt it was not necessary and a total waste of time.
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Old 13-01-2013, 13:32   #335
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Re: Distinct Activities: Shackled by a Common Name?

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Originally Posted by wolfenzee View Post
I thought the whole idea of this was are we shackled by the gadgets and gizmos that figure all that. In threads that were essentially and/or had evolved into chart plotter vs paper charts (electronic vs manual). I had tried to point out the importance of knowing how to navigate with "analog navigation systems" that even if your electronic gizmos never went down the knowledge and resulting though patterns of how to navigate was still helpful. This idea is regularly shot down by the people whose confidence in "push button navigation" was such that they felt it was not necessary and a total waste of time.
Wolf, this discussion is a classic example of why following the chartplotter to get from A to B in a straight line is not always a good idea regardless of how super accurate the chartplotter is - in this case if the current alters along the way in a manner that could have been predicted and allowed for before the start of the journey, the trip would be quicker if a straight line was not followed. On a long trip cross current this may make a difference of hours in the arrival time .
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Old 13-01-2013, 13:59   #336
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Re: Distinct Activities: Shackled by a Common Name?

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Originally Posted by carstenb View Post
......
Dockhead then traveled 5.2 NM to go 5 NM. Meaning his travel time is 5 hours, 2 minutes and 24 seconds.
.......
I think Dockhead needs a new boat (maybe an Oyster?) if his current (no pun intended) boat is taking him five hours and two minutes to travel 5.2 nm!

The time taken for both boats is an hour and five minutes, Dockhead's boat just nudging in by seconds as CaptForce had to make a course correction initially .

Calculation for this:
Boat speed through the water is 5 knots at a diagonal relative to the water towards the north. Current is 2 knots.
Using Pythagoras' rule (it applies as the current is perpendicular to the line between A and B), the speed made good towards B is 4.58 knots (square root of 5 squared less 2 squared). The distance between A and B is 5nm, so it will take 1.09 hours (= 1 hour and five minutes rounding off to the nearest minute).
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Old 13-01-2013, 14:52   #337
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Re: Distinct Activities: Shackled by a Common Name?

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Originally Posted by Seaworthy Lass View Post
I think Dockhead needs a new boat (maybe an Oyster?) if his current (no pun intended) boat is taking him five hours and two minutes to travel 5.2 nm!
Umm, those were hypothetical examples intended to be something most cruisers can relate to

Here's Dockhead's actual boat, on an actual recent Channel crossing:

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Oysters are nice, but no Oyster under 60' has a handicap like Dockhead's boat . . .
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Old 13-01-2013, 15:03   #338
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Re: Distinct Activities: Shackled by a Common Name?

A very entertaining and informative discussion. First I would like to thank Dockhead for his enlightening explanation. the next time I cross the Strait of Georgia i will pay more attention and not sail in a straight line (or do I mean not aim directly at Vancouver?). Mind you the straits don't have near the current running that the English Channel does. Also thanks to whomever used the word sclerotic it will come in handy next scrabble game.

But I do have a question you all can help me with. I was crewing on a boat delivery from Sinapore to Australia ( I got off at Cairns). Due to the complete lack of wind, and what light breezes did come up always being on the nose we were constantly concerned with running out of fuel. One morning while the cap'n was sleeping (only the two of us on board and I was on my 6 am to noon watch) a light breeze came up that seemed useful in propelling us in the right direction (we were hoping to sail to Kavieng at the time to get fuel, and as we had fuel for about 260 nautical miles and 450 miles to go I was a bit concerned). So off went the engine - what a relief, and up went the sails. We were making 3 knots, not great but at least movement in the right direction without using precious diesel. the breeze dropped off some, and our speed was reduced to about 1.5 knots. But still movement in the right direction. After a bit I went below to check our position - our only gps being on a laptop. Much to my chagrine we were going backwards at .5 knots! Rats! A practical lesson of SOG versus SOW!

Anyway, my question is this - without gps would I have had any way of actually determining that I was going against a 2 knot current, and secondly how the heck did Cap'n Cook handle these type of situations?
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Old 13-01-2013, 15:22   #339
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Re: Distinct Activities: Shackled by a Common Name?

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Originally Posted by carstenb View Post
Let's try another analogy in the real world. BAck when I used to do these things, I worked a bit with a customer who manufactured plate glass, including window panes etc.

For those who don't know, glass is made by extrusion, meaning a machine produces an endlessly long sheet of glass continuously. This sheet is say 3 meters wide and moves continuously on a roller belt.

Now it is no problem to imagine cutting the sheet into ribbons lengthwise. You set a couple of glass cutters and as the sheet rolls under them they cut and you now have 3 ribbons 1 meter wide and endlessly long (it is still being extruded at the other end.). So how do we cut to get 1 meter X 1 meter panes?

We have to cut while the glass is still moving, since we can't stop it (this is = to current). The solution is to set a glass cutter on a roller going ACROSS the sheet of glass (= sailboat crossing channel).

Now if the cutter is set to cut at right angles then in the time it takes for the glass cutter to move from one side to the other, the angle will cease to be 90 degrees. (= current offset) This is because the sheet of glass underneath it is moving.

To get square wind panes, the cutter is set at an offset angle towards the direction the glass is coming from (= offset against the current) If the math has been done correctly, the glass cutter now moves across the ribbons of glass at an angle which in conjunction with the movement of the glass produces a right angle. (which it does and that is why you can buy square window panes at home depot)

Now just to make things a bit more interesting, the extrusion process can have varying speeds (= more or less current), to compensate, the roller arrangement's angle across the glass can be adjusted by the computer, thereby resulting in always having square panes.

The point of all the above is to show that even though the glass cutter is SET at an angle which is offset against the current, at any given point on its travel across the ribbons (water) it actually is traveling at a 90 degrees angle to the glass (water.)

Phew. Anyone understand the analogy?

clear .. Crystal clear...
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Old 13-01-2013, 15:31   #340
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Re: Distinct Activities: Shackled by a Common Name?

CaptForce, given the strength of your convictions, you are probably thoroughly sick of reading all this by now, but I will have a go at illustrating an example, as I think it is a very important concept.

The details:
- You and Dockhead have a channel to cross that is 20nm wide going directly south to north (magnetic) from A to B.

- The current is flowing exactly east-west (magnetic) at 2 knots for the first two hours of the journey, then it instantly flips and is going 2 knots the other way for several hours.

- You and Dockhead are both motoring at 2400 revs and in still water this gives each of you a speed of 5 knots (I know that's slow, but lets make the numbers simple). Neither of you changes revs for the entire journey.

- You are sticking to the straight line between A and B thinking this is the quickest route. You will actually find you need to initially motor at a compass heading of 24 degrees to be able to plot a straight course on the chartplotter.

Dockhead had worked out that a compass heading of 360 (0) degrees will get him from A to B the quickest.

I have drawn your tracks as shown by the chartplotter and marked your positions at the 1,2, 3 and 4 hour marks.

At the one hour mark the current would have swept Dockhead off to the west, and increased his speed over ground as he is not fighting against it. He has made progress of 5 nm towards the other shore, but not directly towards B.
You have had to point into the current and to maintain a straight line towards B on the chartplotter and this has slowed you down to 4.58 knots as the VMG on the chartplotter.

At the two hour mark Dockhead is even further from the line between A and B, but he is actually closer to the other shore than you are. Suddenly the current flips and is going 2 knots the other way.
Dockhead placidly does nothing, and keeps his compass heading 360.
You frantically need to alter your compass heading from 24 degrees to 336 degrees to maintain that straight line on the chart plotter.

At the three hour mark Dockhead is being swept back towards your line. His speed over ground is still better than 5 knots as the current is pushing him back to the rhumb line. You are 13.75 nm from your departure point at this stage and your VMG has been a steady 4.58 knots the entire way.

At the four hour mark Dockhead has just arrived. You still have 1.68 nm to go. You arrive about 22 minutes after Dockhead.

This is the charplotter track for each of you:
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Old 13-01-2013, 15:47   #341
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Re: Distinct Activities: Shackled by a Common Name?

I am a cruising sailor and am not particularly in a hurry to get anywhere, it will take me as long as it takes me. Granted having an idea of when you are going to get there, how much progress you making is good, it also helps you to figure where you are (yeah I can pull up the data on the chart plotter, but just knowing how things work allows you to do that without looking down).
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Old 13-01-2013, 15:52   #342
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Re: Distinct Activities: Shackled by a Common Name?

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Originally Posted by bytownboy View Post
A very entertaining and informative discussion. First I would like to thank Dockhead for his enlightening explanation. the next time I cross the Strait of Georgia i will pay more attention and not sail in a straight line (or do I mean not aim directly at Vancouver?). Mind you the straits don't have near the current running that the English Channel does. Also thanks to whomever used the word sclerotic it will come in handy next scrabble game.

But I do have a question you all can help me with. I was crewing on a boat delivery from Sinapore to Australia ( I got off at Cairns). Due to the complete lack of wind, and what light breezes did come up always being on the nose we were constantly concerned with running out of fuel. One morning while the cap'n was sleeping (only the two of us on board and I was on my 6 am to noon watch) a light breeze came up that seemed useful in propelling us in the right direction (we were hoping to sail to Kavieng at the time to get fuel, and as we had fuel for about 260 nautical miles and 450 miles to go I was a bit concerned). So off went the engine - what a relief, and up went the sails. We were making 3 knots, not great but at least movement in the right direction without using precious diesel. the breeze dropped off some, and our speed was reduced to about 1.5 knots. But still movement in the right direction. After a bit I went below to check our position - our only gps being on a laptop. Much to my chagrine we were going backwards at .5 knots! Rats! A practical lesson of SOG versus SOW!

Anyway, my question is this - without gps would I have had any way of actually determining that I was going against a 2 knot current, and secondly how the heck did Cap'n Cook handle these type of situations?
Piolot charts do have the more consistent currents.
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Old 13-01-2013, 16:10   #343
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Re: Distinct Activities: Shackled by a Common Name?

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.... So off went the engine - what a relief, and up went the sails. We were making 3 knots, not great but at least movement in the right direction without using precious diesel. the breeze dropped off some, and our speed was reduced to about 1.5 knots. But still movement in the right direction. After a bit I went below to check our position - our only gps being on a laptop. Much to my chagrine we were going backwards at .5 knots! Rats! A practical lesson of SOG versus SOW!

Anyway, my question is this - without gps would I have had any way of actually determining that I was going against a 2 knot current, and secondly how the heck did Cap'n Cook handle these type of situations?
The more interesting answers are to the Captain Cook scenario, with no access to tabulated current data (which in any case, as wolfenzee alludes to, are unreliable in some parts of the world, as ocean current eddies interfere with the more predictable tidal component)

I'm presuming you were out of sight of any land)

By day, it would have been impossible if it's rough, or overcast. In good conditions, if two heavenly bodies were visible, eg sun and moon, or sun and Sirius: a couple of fixes with a three hour spread, compared with a careful DR, would (if the angle of cut of the position lines for each fix was substantial, and conditions suitable for accurate shots) reveal the rate and direction of the nett current over that period.

By night, starsights allow good position fixing, because you can select stars which give three position lines at roughly 120 degrees apart for each fix, and check by adding a couple more stars if you're a stickler.
But you'd have to suspect there was a problem in the first place; Captain Cook would have been rather more fatalistic, I suspect, in the general case. Unless it was feasible to anchor, a period of being carried in the wrong direction would not be anything they weren't very accustomed to.

ON EDIT: Stars can be a tough ask on a small boat unless the conditions are very good: smooth sailing, clear horizon. You generally only get a clear horizon near dawn or dusk, so it was still problematic for Captain Cook.

One thing, though: these methods do not require that you have an accurate chart, or even a chart: you could simply put your position lines on graph paper and it would work just as well.
In that sense, Captain Cook was not disadvantaged by the fact that in many places, the only charts which would be available for decades after were ones he hadn't yet made.
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Old 13-01-2013, 16:12   #344
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Seaworthy. I understand DH vectors. How did you compute CF progress.


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Old 13-01-2013, 16:31   #345
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Re: Distinct Activities: Shackled by a Common Name?

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Seaworthy. I understand DH vectors. How did you compute CF progress.
Dave
CF's speed relative to the water is 5 knots (fixed throughout the journey as he is keeping constant revs). The current is pushing the boat at 2 knots perpendicular to the line between A and B. So we can use Pythagoras' triangle to work out CF's VMG (see diagram below).

The VMG squared plus two squared = five squared.
So VMG squared = 25 - 4 = 21
So VMG = 4.58

The formula for the angle of deviation (and therefore the compass heading in this case as the destination point is magnetically north of the departure point) is as follows:
Sinθ = 2/5
where θ is the angle
This works out to be 24 degrees.
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