Distinct Activities: Shackled by a Common Name?

[HappySeagull]

Don Lucas, of course real life is course changes, but this is a straight haul across- no obstacles.If the current is going to change direction ,say halfway as Lodesman and I are modelling, it 's kind of odd to fight one way then the other when you could just tootle along steering blythly acrooss the channel.Your ground track is a big s but you are heading straight across! faster.
But, still, I have a vague recollection (and you may too?) that the original question was, If the current is only one way, Is it still more efficient to steer a single summed vector? (and accept the curved track-it don't matter out in the open where there are no obstacles)...or is it more efficient to continuously alter as your autopilot would?

I'd always have the model include a vessel capable of only 5 knots where max current is also 5 knots......gives it urgency
but it peeves me that I can't agree with the models presented....not knowing where I are going to be, is the beginning of being lost....(which almost heads back to the original thread)

[Andrew Troup]

Happy Seagull

Earlier in this interminable thread, at the instigation of the gallant CaptForce, we modelled an ocean current which varied from zero at each boundary to 4 knots midway, always from the same direction.

I'm not sure if the 'one way' scenario you are thinking of was at varying rates; I presume so because otherwise there's no difference between constant heading and rhumb line track across ground, or time taken.

I never posted the most interesting cases, because at that time we were arguing about the boring ones, eg 5 knots for both boats (Rhumb Line vs Constant Heading)

If anyone's interested I could work out the times, and distances swept up and down current (and maybe even plot out a few of the more interesting ground tracks) for slower and faster options.

I did a quick and dirty run-through, and the constant heading boat still struggled across when it could only manage 2 knots, (admittedly, it did take several days!) whereas the rhumb line boat's time zoomed up to literally forever if it could only manage double that, ie 4 knots, matching the current midstream.

Now at 50 knots, there was (I think) less than half a second in it. If you want to know who won, you need to pretend to be interested, because I'll have to dust off the spreadsheet and work out how to drive it again !

[Dockhead]

Originally Posted by HappySeagull

I know very well that current in the channel does not agree with the tide on the shore.
I'm thinking you are not understanding.
It's current.That is a current curve on the 0 line,above and below flood and ebb. Have you never seen one? All current predictions can be graphed. Funnily enough they are NEARLY sin curves.
Current flood+ebb= one tide rise and fall.
Look again at the upper example-I only mention that the resultant track looks like a tidecurve. Is it a suprise that it does?

It will look like a tide curve if you choose a single CTS based on resultant of current sum.
It will not look like a tide curve if you sail the range, the rhumb.

I have entirely forgotten what we were arguing about I apologize if I misunderstood anything you said.

But a couple of points related to this:

1. The tide flows at six and something hour intervals just like the tide goes up and down, but the relationship is attenuated. In the Solent, the West going stream starts one hour before HW Portsmouth. So you don't think about rise of tide when you're working out tidal streams -- you look at the atlas. The streams are referenced to HW of a standard port (HW Dover in the English Channel), but there is no visible relationship in the timing. Half way down the Channel, the tide is "coming in" (flowing "upchannel") even as the tide is "going down" in Dover, for example.

2. The usually or generally sine wave form of the tide going up and down, and streams ebbing and flowing generally following sine wave patterns is actually extremely significant, although there are a lot of anomalies (double high water, etc., in some places). I have a theory that we can calculate very accurate tidal vectors if we only know the limits and timing of the limits of the streams. I don't think we need to know the hourly streams at all. I will be working on this.

[Dockhead]

Originally Posted by HappySeagull

Don Lucas, of course real life is course changes, but this is a straight haul across- no obstacles.If the current is going to change direction ,say halfway as Lodesman and I are modelling, it 's kind of odd to fight one way then the other when you could just tootle along steering blythly acrooss the channel.Your ground track is a big s but you are heading straight across! faster.
But, still, I have a vague recollection (and you may too?) that the original question was, If the current is only one way, Is it still more efficient to steer a single summed vector? (and accept the curved track-it don't matter out in the open where there are no obstacles)...or is it more efficient to continuously alter as your autopilot would?

I'd always have the model include a vessel capable of only 5 knots where max current is also 5 knots......gives it urgency
but it peeves me that I can't agree with the models presented....not knowing where I are going to be, is the beginning of being lost....(which almost heads back to the original thread)

This may help:

captforcescenario_distancesliced1.xls

You can plug in any parameters you want; the formulae are all open.

It's a model of the Capt Force scenario: one-sided current, starting at 0, peaking at 4 knots, and going back down to 0 at the other side.

Once again would be appropriate to acknowledge Andrew Troup's key insight in distance-slicing rather than time-slicing, to analyze the GPS track boat passage, which allowed the model to attain mathematical perfection

[sailorboy1]

I'm not really gong to join in a thread argument here.

But if you are in track mode on your AP and steering to a waypoint you are traveling a straight line (not really but in effect) over ground far as I'm concerned.

[Dockhead]

Originally Posted by Don Lucas

I'm not really gong to join in a thread argument here.

But if you are in track mode on your AP and steering to a waypoint you are traveling a straight line (not really but in effect) over ground far as I'm concerned.

Not through water.

Try to think of it like this:

If the water is moving at varying rates and/or directions, you can't have a straight line through water AND a straight line over ground. One or the other of them will be crooked.

With me so far?

So, what is the most efficient way to get to your destination? You have to choose: straight line over ground a crooked through water, or vice versa.

How you choose should depend on what kind of vessel/vehicle you are in. If you are in a canoe and poling off the bottom, then you want to go straight across ground, because your propulsion method doesn't feel the water track.

If, however, you are in a regular boat propelled by sails or motor, you have no attachment to the bottom and on the contrary don't feel the ground track -- you can only sail through water. Your speed is speed through the water; SOG depends on the current. So calculating a single course to steer will get you there faster, because it's a direct path through the water. A straight ground track is slower, because you have to keep changing headings to stay on it, which means you have to zig-zag through the water, adding water miles.

Got it now? There are a lot of illustrative examples further back in the thread, and a spreadsheet where we precisely calculate the time it takes to do a certain passage (the Capt Force challenge) using different methods.

[HappySeagull]

I apologize if I misunderstood anything you said.
You needn't apologize. Apparently, what I see as so simple, is not.

streams ebbing and flowing generally following sine wave patterns is actually extremely significant,

They are extremely significant. That's why I drew the model.
There are two examples...


I wish I hadn't remarked that the track is so much like a tide graph. It is apparently confusing .I say again. This picture is not about the tide on the shore. It is a current model. Time =slack to slacks for simplicity. Boat speed is a constant A->B.


Thank you for your replies, but they are no "help".

[HappySeagull]

Originally Posted by Don Lucas

I'm not really gong to join in a thread argument here.

But if you are in track mode on your AP and steering to a waypoint you are traveling a straight line (not really but in effect) over ground far as I'm concerned.

Of course! Who disagreed? Not me! No argument there.

[Dockhead]

Originally Posted by HappySeagull

streams ebbing and flowing generally following sine wave patterns is actually extremely significant,

They are extremely significant. That's why I drew the model.
There are two examples...

You made a model of current flows based on sine waves? That sounds interesting. Link? Sorry I lost the thread of the conversation somewhere among the 727 posts here , or maybe just a result of general mental confusion