Distinct Activities: Shackled by a Common Name?

[carstenb]

Originally Posted by CaptForce

It's difficult for me to accept that I would stand in such complete opposition to someone that seems to present such intelligent discourse. I sense that we must have some basic agreement, but I can't find it. Basic geography would present the shortest passage between two points crossing over the sea as a straight line and also maintaining a straight line over the ocean floor. It would be the mistake of maintaining a constant compass heading and keeping the bow pointed in the same direction regardless of variations in current set that would result in the inefficient sigmoid curve.

I'll chime in here wiht Capt Force. Assuming your current is from one direction only (not reversing), then you would figure current at say, average 2 knots, passage time 4 hours = make your compass bearing 8 NM upstream of where you want to go.

Assuming reversing current (tides) then you use same formula as above but figure the counteracting current/time against the above-mentioned 8NM and make your bearing this.

If my assumptions above are what you are saying Dockhead - then I agree with you completely (buy you a beer sometime when we are in same harbor). Otherwise, I'm at a total loss

[Seaworthy Lass]

Originally Posted by CaptForce

It's difficult for me to accept that I would stand in such complete opposition to someone that seems to present such intelligent discourse. I sense that we must have some basic agreement, but I can't find it. Basic geography would present the shortest passage between two points crossing over the sea as a straight line and also maintaining a straight line over the ocean floor. It would be the mistake of maintaining a constant compass heading and keeping the bow pointed in the same direction regardless of variations in current set that would result in the inefficient sigmoid curve.

CaptForce, sorry, but Dockhead is correct. You need to look at vector diagrams showing tide, speed over water and speed over ground. The quickest journey is to calculate the correct heading for the passage and stick to this, moving in a straight path relative to the water and a wavy pattern relative to the ground .

[foolishsailor]

Dockhead is correct.

One way to visualize is to think of an ant crossing a treadmill.

The tread mill is one meter wide and 3 meters long. Lets assume a simulation of a symetrical tide for simplicities sake.

The ant starts crossing the treadmill and it starts moving to the left 2 meters. As the ant reaches the mid point of crossing the treadmill moves back to the right 2 meters.

The ant, no matter how far the treadmill moves each way, has only travelled 1 meter across the surface of the treadmill to get to the other side. He is attached to the surface. He has however travelled over ground in a sinewave a substantial added distance.

The movement of a boat in tidal waters is the same, you are attached to the surface. The shortest distance is the distance you travel through the water regardless of how that water shifts you around.

A good way to see this for yourself is to calculate a travel distance, find your cts and then go. Compare your distance travelled as calculated by your knotmeter and your gps. If the tide is symetrical the knotmeter will be very close to the actual distance.

[noelex 77]

My high school maths teacher said first draw a good diagram.

Well i never could draw a good diagram but here is bad vector diagram I drew.

I have assumed the simplistic case where the two currents cancel each other out and the current is at right angles to direction of travel. It's easiest to imagine a constant current that suddenly reverses half way along.

You can see the red line is longer meaning maintaining zero cross track error is the slowest / longest way. The quickest way is to allow the tide to push you to one side then the other.

[goboatingnow]

what Dockhead is talking about is I suppose textbook CTS ( Course to Steer) calculations. That is, you sit down in advance and knowing all the tidal movements that are taking place through out your journey ( it matters not that the tide is symmetrical or otherwise). You then compute all the tidal vectors, vector add them and then that results in a single course to steer, ie ONE compass direction. This is the most efficient way to sail a rhumb line ( leaving aside great circle).

Of course in real life it has a number of problems

(a) Exact tidal streams are not always available
(b) in large tides the XTE from the ground track is very large , often many miles, so you have to watch for unexpected route hazards, one of the problems is that the vector addition on the chart doesn't show the ground track , just the summation. Or you can laboriously lay out each hourly vector to determine each hourly ground track ( even that isnt actually right either)
(c) Changes in real life conditions have quite significant consequences, especially speed. forcing passage recalculations
(d) For complex tides, theres is quite a bit of work.

The classic case is in symmetrical tides, crossing the irish sea for example tends to put you through two tides, so you basically ignore the tide and sail the compass course to the destination. often you save upto 25% of the time using a proper CTS. Similar effects are typical on say the Needles to Cherbourg.

I personally tend to follow a more intuitive route as I have a limit to how much XTE I will tolerate , on a typical channel crossing, if you set up a full single CTS, you can get XTEs of over 12 miles , which I dont like. So I tend to use the track and Bearing to estimate hourly CTS,s which limits my XTE to about the tidal set.

The worse way in these circumstances is to just blindly follow a "rolling road" type display


in effect I use a version of the Noelex diagram above.
Dave

[carstenb]

This jibes with what I said, so I did understand Dockhead, and I agree with him (after all this is just basic navigation for course setting allowing for offset of current/wind). When I took basic navigation, this was called a "current triangle" (in Danish of course). So nothing new here.

However, you do need to "guestimate" somewhat accurately to maintain a constant compass heading. And I would suggest that you also need to figure in the wind offset.

Setting the plotters cursor to the point and then sailing the given bearing will mean a longer sail (but as cruisers - do we care?).

If anyone wants a longer discussion, we can take a run at "Great Circle Sailing".

How many of you have a great circle chart?

How many know how to use it?

How many can plot a great circle without having to use reference books?

jes sayin'

[goboatingnow]

The default for most GPS units is to use great circle calulations as bearing to waypoint, and its easily verified on a mercator ocean chart

You dont need to delve into cosecants and haversines

Heres an approximation ( sailing chords of the great circle)

A Great Circle always starts off curving away from the Equator, but don’t start off exactly on the Great Circle initial bearing or you will start heading off into the wild blue yonder in a few hours. Start your route on a heading about 2º closer to the Equator and after a few hours check the actual route sailed on the chart plotter and trim your course to get onto the course you have chosen. Each day, give the GPS a new GOTO the same waypoint and when there is a change in the bearing, trim your course again to a couple of degrees closer to the Equator. Of course it is wise to go around rocks, reefs and land masses, then do a GOTO again! When you get near the Equator, keep to the GPS bearing because the Great Circle starts flattening out and will curve the other way after crossing the Equator, when you will again start trimming a couple of degrees towards the Equator. When you are about 400 NM from your destination, follow the GPS daily GOTO exactly.

In practice you sail chords anyway, since you dont have a constant changing heading.

dave

[conachair]

Originally Posted by carstenb

If anyone wants a longer discussion, we can take a run at "Great Circle Sailing".

How many of you have a great circle chart?

How many know how to use it?

How many can plot a great circle without having to use reference books?

Yes to the first 2, no to the third. But google earth does that for you in a few seconds.

But never have done, other factors always seen much higher up the list, sailing way off course to get to better wind or quite often just getting the boat on a point of sail where it seems happiest, which hopefully is in roughly the right direction. All from a cruisers perspective, for a successful passage the time spent at sea matters much less than how many things there are to fix afterwards

[goboatingnow]

Originally Posted by conachair

But never have done, other factors always seen much higher up the list, sailing way off course to get to better wind or quite often just getting the boat on a point of sail where it seems happiest, which hopefully is in roughly the right direction. All from a cruisers perspective, for a successful passage the time spent at sea matters much less than how many things there are to fix afterwards

Absolutely +1, This is why I personally dont hold to much with huge CTS workups in advance, often you cant sail the course anyway, or the wave motion means taking alternatives etc. On long passages, keeping the boat safe and comfortable is far preferable to speed to the destination.

Its the appliance of theory, and the output of tools, be that crayon or LCD, that distinguish a good navigator, not what actual tools are being used.

Dave

[barnakiel]

Originally Posted by carstenb

(...)

If anyone wants a longer discussion, we can take a run at "Great Circle Sailing".

How many of you have a great circle chart?

How many know how to use it?

How many can plot a great circle without having to use reference books?

jes sayin'

Getting the drift here. Alas ...

> we do (e.g. Imray 100 Atlantic),

> I do,

> I do not plot the circle, I plot a series of rhumb lines that connect the pivot points which are on the circle,

> BTW in open water, a sailing boat will rather follow the wind shifts, not the circle, nor the rhumb line,

> BTW2 the only way I can see sailing the great circle is by following the gps directions, ours is set to show the great circle direction, not the rhumb line direction(/course).

Then again I think we may both agree, in the real world cruising aspect, how does all that matter?

Big hug,
b.

[carstenb]

LOL - I never expected anyone to say yes to all three. I can plot a great circle (but need my reference books).

And yes - comfortable sailing is more important than hitting the great circle line

[nigel1]

Originally Posted by carstenb

LOL - I never expected anyone to say yes to all three. I can plot a great circle (but need my reference books).

And yes - comfortable sailing is more important than hitting the great circle line

Yes to the first two, and back in 1985 (Mates), and 1988 (Masters), yes to the third, as we had to calculate GC tracks, waypoints along a GC, and composite sailings (limiting latitude), calculators and reference books not permitted. Bugger if I can remember them now, but thats what the laptop is for, and at a last pinch, Norries Tables.

I've got an old edition of Ocean Passages of the World, which includes a large section on sailing ship routes, and a nice chart showing recommended routes for various times of the year. Guess with all this climate change, the information will need to be used with caution,

[carstenb]

Originally Posted by daddle

If the luddites here want any respect from me they should set to sea with no charts whatsoever. Just a belief that the earth is round, a compass, and a man in the crows nest. Now that was navigating. Everything from paper charts to chart plotters is for sissies. End of thread.

A compass is for all you techie nerds. Real men only watch the sky and ocean. Everything else is for pussies.

[Hudson Force]

Originally Posted by foolishsailor

Dockhead is correct.

One way to visualize is to think of an ant crossing a treadmill.

The tread mill is one meter wide and 3 meters long. Lets assume a simulation of a symetrical tide for simplicities sake.................

Ah, now I think I see the problem and why so many seem to stick to an intelligent answer that I can not agree with. It's all because of the "for simplicities sake" and the simple example from the high school math teacher that does not wish to calculate the complexities of the real world. All those that are following the plan of maintaining a constant compass heading while crossing a current may remain satisfied with this procedure "for simplicities sake", but in the real world they will be taking a longer course on the sigmoid curve because the drift, if not the set also, will vary. Let me explain with one of the most common passages taken across a current. If you were to procede on a plan of crossing in a direction of 90 degrees M from Palm Beach, Florida across the Gulfstream to the Bahamas, you might, for simplicities sake determine that you will need to take a constant course of 115 degrees M at five knots, due to the average current set to the north at 2.5 kts, in order to reach your destination in the same manner as many suggest. The method is simple, but during the begining of the trip, when the current is less than the 5 kts at the center of the Gulfstrem, you would be making headway to the south of the rumbline course. Half way across the passage, when you are in the full five kt current, you will meet your rhumbline and then be moved north of your most efficient path. Finally, as you approach your Bahamian landfall and the current has weakened, you will arrive at your destination on the heading of 115M. This method works for "simplicities sake", but if you had elected to remain on the couse determined by your GPS you would arrive sooner and traveled in a straight line as you would constantly be recieving updates to change your required course vector as the strength of the current changed. Kudos to all the math teachers and all those correctly planning to cross an imaginary current that remains at a constant velocity for the passage. There is nothing wrong with taking the longer course if you value the method that gives you a simple answer. By the way, that treadmill analogy is excellent. Now, just imagine that the center of the treadmill is moving faster than the edges!

[Andrew Troup]

Better still, Capt Force, is to imagine that the speed of the treadmill belt is varying all the time.

I say this because if you can imagine a treadmill belt which runs faster at the center, your imagination is so lively that it might be making it hard to see what lies at the core of your disagreement with Dockhead et al, which is actually rather simple: you're trying to sail a straight line on the chart; they're trying to sail a straight line on the water.

It doesn't achieve anything to imagine a complicated belt, because the speed of the belt where the ant is not, doesn't matter.

From the point of view of the ant, they don't need to know anything about changes to the speed of the belt, provided they have been given a point on the far side of the belt to aim at. The shortest route across the belt to get to that point is at a constant heading, in other words, keep aiming at it.

If, when they arrive, the point they were aiming at happens to coincide with the desired exit, then the data and the maths from which the aiming point was derived were correct.

If the ant was dipped in ink, it would trace a straight line on the belt, hence the shortest distance across the medium the ant is travelling on.

If it was glowing, a time exposure taken from above from a camera fixed to the surrounds, rather than the belt, would show the ant heading in all sorts of different directions, and make it look as though it had travelled too far. However this is an illusion

Imagine a second ant who has been given, instead of an aiming point on the far side of the belt, a laser line 'painted' on the belt (stationary relative to the surroundings) to walk along. This laser is much easier to set up than the aiming point was: all you have to do is make it pass through the entry and exit points, neither of which move.

He leaves at the same time and speed as the first ant. Which ant gets there first?

(Has anyone else noticed this has turned into a thread discussing navigation? I couldn't be happier)