Distinct Activities: Shackled by a Common Name?

[Seaworthy Lass]

Originally Posted by Wotname

Cworthy, you either have very dry sense of humour or very little understanding of the working of the upper echelons of any "Royal Association".

I suspect the former and guess you are not standing by the mailbox with a developing SM thirst.

Keep the buggas honest

Very dry sense of humour .

Do you think I may get some action if I write to the Queen?
Maybe it will soon be just 'YA' if no action is taken.

I think Princess Anne will appreciate my information when she heads out in her new Russler 44 though. I don't think she will be using the RYA method when she reads about mine.

Maybe you will all need to refer to me as Dame Seaworthy in the future (granted in services to yachting). .

[Wotname]

Originally Posted by Seaworthy Lass

Very dry sense of humour .

Do you think I may get some action if I write to the Queen?
Maybe it will soon be just 'YA' if no action is taken.

I think Princess Margaret will appreciate my information when she heads out in her new Russler 44 though. I don't think she will be using the RYA method when she reads about mine.

Maybe you will all need to refer to me as Dame Seaworthy in the future (granted in services to yachting). .

Dam Seaworthy, I salute you. Opps sorry, didn't see the 'e' on first reading. Never mind

[Seaworthy Lass]

Originally Posted by Wotname

Dam Seaworthy, I salute you. Opps sorry, didn't see the 'e' on first reading. Never mind

I think one RYA instructor is calling me that right this minute .

[Seaworthy Lass]

Originally Posted by Andrew Troup

To those who dread any return to the status quo ante, I can only remind them of what Bob Dylan famously had to say on the topic.

OK, I'll bite .

What did Bob Dylan have to say about how things were before the war?
This post slid in a few pages ago and it went straight over my head.

Anyone figure out what Andrew means?
I am not a big Dylan fan, but I guess he would have told Dave and I to settle it on Highway 61 LOL.

[Lodesman]

Originally Posted by Seaworthy Lass

The problem with this method is that is uses the rhumb line as a reference. If we look at this problem freshly without the distraction of the RYA method, why on earth should we use the rhumb line at all? Think about it. It has nothing to do with our course over ground (your actual position) and sum of current displacement vectors could end up just about anywhere. Why reference the rhumb line at all? We need to work with B, not some line that just looks convenient.
.

I don't see it as so much of a problem - the rhumb line after all represents the desired track or the "average" of your path to destination.

Originally Posted by Dockhead

I don't know about Annapolis or Bowditch or altogether what they teach in the U.S. these days, but I was taught 30 years ago with pencil, paper, protractor and slide rule (most of which, to my shame, I have forgotten) was to always calculate a unified course to steer especially when it is a multi-hour passage. If you are affected by tides or currents which are not substantially in your direction of travel or against it, you are really screwed if you don't calculate a CTS. It does not need to be precise. Even a very rough approximation is going to be better than dumbly crabbing along the rhumb line -- in fact if the only thing you get right is to correct your heading in the right direction, this will in most cases will be better than just crabbing along the rhumb line.

Attachment 53300

I have to take exception to the idea of "crabbing dumbly along the rhumb" - it's what the Navy does
From the big ship perspective, staying on the line is the optimum - it represents the safe, approved path and any deviation from it is to be avoided if possible. In pilotage, fixes would be taken at not less than 6-minute intervals, actual set determined and CTS applied immediately to regain and maintain the line. Of course the inefficiencies of this are obvious, as easily demonstrated by your diagram - as long as the path over ground is, the water path is a straight line; mathematically the shortest and quickest route across. Crabbing for each change in tide would turn that water path into a zigzag pattern.

I've never seen the RYA method or any other to average the path, so when this topic came up, I was very interested. From my perspective the solution seemed self-evident. I don't see what could be wrong with referencing the rhumb line, as sailboaters usually plot a mean line of advance with no expectation of actually following it - the concepts are not really so different. Still I'm eager to see Seaworthy's new solution.

[Dockhead]

Originally Posted by Lodesman

I don't see it as so much of a problem - the rhumb line after all represents the desired track or the "average" of your path to destination.



I have to take exception to the idea of "crabbing dumbly along the rhumb" - it's what the Navy does
From the big ship perspective, staying on the line is the optimum - it represents the safe, approved path and any deviation from it is to be avoided if possible. In pilotage, fixes would be taken at not less than 6-minute intervals, actual set determined and CTS applied immediately to regain and maintain the line. Of course the inefficiencies of this are obvious, as easily demonstrated by your diagram - as long as the path over ground is, the water path is a straight line; mathematically the shortest and quickest route across. Crabbing for each change in tide would turn that water path into a zigzag pattern.

I've never seen the RYA method or any other to average the path, so when this topic came up, I was very interested. From my perspective the solution seemed self-evident. I don't see what could be wrong with referencing the rhumb line, as sailboaters usually plot a mean line of advance with no expectation of actually following it - the concepts are not really so different. Still I'm eager to see Seaworthy's new solution.

We reference the rhumb line in a way different, I think, from what you are thinking.

Using the classical methods of calculating CTS, we draw the vector triangle so that it touches the extended rhumb line. That's how we get our course to steer (angle of the water track line). See this thread: http://www.cruisersforum.com/forums/...hod-96579.html. That's what Seaworthy was talking about. We don't need to use the rhumb line at all if we use her modified method, which forces us to draw the triangle right to the destination.

[Dockhead]

Originally Posted by Lodesman

I have to take exception to the idea of "crabbing dumbly along the rhumb" - it's what the Navy does
From the big ship perspective, staying on the line is the optimum - it represents the safe, approved path and any deviation from it is to be avoided if possible. In pilotage, fixes would be taken at not less than 6-minute intervals, actual set determined and CTS applied immediately to regain and maintain the line.

Modern autopilots will do that automatically, if you set them on "track" mode. They will keep you right on "the line" with frequent, more often than 6 minutes, steering corrections.

This is very useful if you are steering a careful path through a bunch of obstacles (think N Brittany coast!) and have worked out one straight line which will keep you safe.

It is an extremely inefficient way to cross tidal waters, however.

[Andrew Troup]

Originally Posted by Seaworthy Lass

OK, I'll bite .

What did Bob Dylan have to say about how things were before the war?
This post slid in a few pages ago and it went straight over my head.

Anyone figure out what Andrew means?
I am not a big Dylan fan, but I guess he would have told Dave and I to settle it on Highway 61 LOL.

War? what war? the War of the Vectors?

Or were you thinking I was meaning "status quo ante bellum" ;-) ?

No, I wasn't referring to war, I was using "status quo ante" as the long form of "status quo"

In relation to expressed misgivings about the threatened return of the killer ants to this thread, I was thinking of that song Bob wrote about the ants.

You know the one ...

[Andrew Troup]

Originally Posted by Lodesman

I don't see it as so much of a problem - the rhumb line after all represents the desired track or the "average" of your path to destination.
...

Could be a geometrical problem, if the current is strong on the quarter, to the extent that the CTS approaches orthogonal to rhumb. Ships go faster than currents, typically, so this might only happen to the big boys when (say) towing a heavy rig

Originally Posted by Lodesman

I have to take exception to the idea of "crabbing dumbly along the rhumb" - it's what the Navy does
...

What did Churchill have to say about that? Something about

"R*um*, s*d*my and the l*sh ?"

(Was he suggesting a slogan for a Royal Navy recruiting poster?

Or was he suggesting a song lyric to Bob Dylan?)

[HappySeagull]

I can't figure out why nobody uses real tides here.
...see post612
http://www.cruisersforum.com/forums/...ml#post1133917
I'm going to try again, albeit I'm not good with a pen and beak
scenario 1 is (unlikely)symmetrical tides. Track is important .You don't want to be sidling over the banks...


scenario 2 (more likely) not symmetrical tides. The corrections to arrive at B are messy, but at least they are in red.

More complicated cases are real world...leave at half-tide , the rhumb line is not perpendicular to currents, etc. Also tides that "clockaround" would look better as polars.
With hundreds of posts, I thought I'd have learned something, but I am just confused.

[Dockhead]

Originally Posted by HappySeagull

I can't figure out why nobody uses real tides here.
...see post612
http://www.cruisersforum.com/forums/...ml#post1133917
I'm going to try again, albeit I'm not good with a pen and beak
scenario 1 is (unlikely)symmetrical tides. Track is important .You don't want to be sidling over the banks...
Attachment 53318

scenario 2 (more likely) not symmetrical tides. The corrections to arrive at B are messy, but at least they are in red.
Attachment 53319
More complicated cases are real world...leave at half-tide , the rhumb line is not perpendicular to currents, etc. Also tides that "clockaround" would look better as polars.
With hundreds of posts, I thought I'd have learned something, but I am just confused.

Indeed, you are rather confused. You have mixed up rise of tide with tidal currents. You can't extrapolate tidal currents from rise of tides (well, we Solent sailors do happen to know that the West-going stream starts one hour before HW Portsmouth ). We get tidal currents from tidal atlases. The problem is calculating the right course to steer, not to end up downtide with your crew cursing you, instead of in the pub, toasting your excellent passage

Read through the thread again; you'll get it

[sailorboy1]

Originally Posted by Dockhead

Modern autopilots will do that automatically, if you set them on "track" mode. They will keep you right on "the line" with frequent, more often than 6 minutes, steering corrections.

This is very useful if you are steering a careful path through a bunch of obstacles (think N Brittany coast!) and have worked out one straight line which will keep you safe.

It is an extremely inefficient way to cross tidal waters, however.

I love the track feature off my AP. I didn't think I would originally and looked at it with mistrust. But it is just so useful for maintaining boat course toward the desired way point allowing me to pay more attention to all the other "stuff" going on around me.

I don't really understand why you think it is "inefficient" unless you are saying the tide is making so much current that a direct path isn't the fastest anymore! I will say that I frequently will set numerous marks that are fairly short and track to each of them as needed to pick a route though an area.

[Andrew Troup]

Dockhead: I didn't read Happy Seagulls post as confusing them, indeed in his first diagram he commented on the coincidental similarity between horizontal and vertical displacements, for an idealised tide and an idealised channel

(perhaps a channel aligned E-W but with minimal volume and no resonance phenomena, connecting large bodies of water at each end, so that flow was a direct function of height diffs from one end to the other?)

[Lodesman]

Originally Posted by Don Lucas

I don't really understand why you think it is "inefficient" unless you are saying the tide is making so much current that a direct path isn't the fastest anymore! I will say that I frequently will set numerous marks that are fairly short and track to each of them as needed to pick a route though an area.

Picture yourself crossing a channel that is 30 miles wide. Your boat can motor at 5 kts, so it should take 6 hrs. You leave at slack, and over the first 3 hours the ebb current increases to a max of 3 kts, then back to slack; and over the final 3 hours the flood increases to 3 kts then back to slack. If you point the bow across the channel (perpendicular to it) you will get across in 6 hours - your path through the water will be a straight line, but your path over ground (and plotted by GPS) will be a curved line, taking you downstream to the middle of the channel, then curving you back to your destination. If you were to steer a course to maintain the straight track across the channel, then at max ebb and flood you will need to point your bow about 37º into the stream and your speed made good across the ground will drop to 4 kts. It will take you longer than 6 hrs to get across.

[HappySeagull]

Originally Posted by Dockhead

Indeed, you are rather confused. You have mixed up rise of tide with tidal currents. You can't extrapolate tidal currents from rise of tides (well, we Solent sailors do happen to know that the West-going stream starts one hour before HW Portsmouth ). We get tidal currents from tidal atlases. The problem is calculating the right course to steer, not to end up downtide with your crew cursing you, instead of in the pub, toasting your excellent passage

Read through the thread again; you'll get it

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.