Brain hurts - Working out course to steer

[Jim Cate]

One comment: in much of the world, there is no tidal flow info available. One is often lucky to have a tidal rise/fall station within a hundred miles, let alone hourly set data. The English channel is a place with extraordinary data richness, and there the CTS method is feasible.

In ordinary venues, lacking such data, these methods don't have any place, and simply sticking near to the rhumb line is about all you can do.

Jim

[sailingtaiyo]

Crabbing is commonly used to express the " horizontal drift " of the vessel. Crabs walk sideways. We all had to learn and we all can give back by teaching others.

Aloha,
Mike.

Originally Posted by Stu Jackson

What you really need to so is stop making up terms. CRAB?


There are definitions of all these terms. The previous poster mentioned Bowditch. It's also defined in Chapmans and any other good navigational text.


As I previously mentioned in this thread, we have seen altogether too many instances of skippers mistakenly confusing / using heading and bearing, for example.


Please learn the proper terms and what they mean. Things like heading, bearing, drift, set, leeway, etc. By doing so, your questions will be answered.

[sailingtaiyo]

Bearing, Heading, same thing in merica.

[RayRodriguez]

Originally Posted by Stu Jackson

What you really need to so is stop making up terms. CRAB?


There are definitions of all these terms. The previous poster mentioned Bowditch. It's also defined in Chapmans and any other good navigational text.


As I previously mentioned in this thread, we have seen altogether too many instances of skippers mistakenly confusing / using heading and bearing, for example.


Please learn the proper terms and what they mean. Things like heading, bearing, drift, set, leeway, etc. By doing so, your questions will be answered.

Crab angle is the term used in aviation. I am sorry that I applied that to boating.

I will ask no further questions.

[Alan Mighty]

Originally Posted by Jim Cate

One comment: in much of the world, there is no tidal flow info available. One is often lucky to have a tidal rise/fall station within a hundred miles, let alone hourly set data. The English channel is a place with extraordinary data richness, and there the CTS method is feasible.

That's largely true, Jim. But there are other forms of published data if you look.

In Queensland, I think the only tidal stream data currently published by the state govt using taxpayers' money is for Hammond Rock in Torres Strait.

The Aus govt publishes a tidal stream data for a few other sites in Aus waters.

CSIRO, the taxpayer-funded science research agency, has run projects on tidal streams. One example happens to be in my home waters of Moreton Bay. I've appended a still clipped from a movie (also available as an animated *.gif) of tidal flows in northern Moreton Bay (see Tidal Stream Moreton Bay.jpg, about 174 KB). The idea being that anyone sailing E-W in northern Moreton Bay watches the movie or animated gif and estimates the set and direction of the tidal stream at spring tides/neap tides.

The Qld govt, like a few other state govts in Aus, funds wave rider buoys at strategic locations along the coast. Some of the wave rider buoys record set and drift of the tidal stream while others are in the perfect location to capture both tidal streams and the stream of the E Aus Current. I've appended a quick graphic (see BNE Mk 4 waverider buoy data.jpg. That wave rider buoy is E of the NE tip of Moreton Island. You'll see the S set of the E Aus Current, with the tidal stream modifying the drift).

In addition to data published by govt and govt agencies, academia has produced big mobs of hydrographic data. These days, an internet search can lead one to dissertations and theses detailing tidal streams in selected locations along the coast.

A few jurisdictions in Asia through which we've cruised also publish tidal stream data, especially for harbours and straits that are important for shipping. The Singapore Strait is one that has a couple of points for which excellent tidal stream predictions are published.

Originally Posted by Jim Cate

In ordinary venues, lacking such data, these methods don't have any place, and simply sticking near to the rhumb line is about all you can do.

I'm in much less agreement with you here.

If you have tide time data, established rules of thumb to the relative magnitude (if not the absolute magnitude) of tidal drift are useful.

An earlier post in this thread alluded to such rules of thumb. I'll only mention here the Règle des Sixièmes (rule of sixths) that I learned from a French cruiser: for a mixed tidal regime or semi-diurnal tidal regime, the drift in the hour around slack water is 0; the drift in the first hour after is 3/6 of maximum; the drift in the second hour is 5/6 of max; the drift in the third hour after is 6/6 or maximum.

Another earlier post (#12 Sean Patrick) noted the technique we've used successfully: many electronic charters that collect data from both GPS/GNSS and water speed can compute set and drift. That set and drift data can be used as a basis for a vector triangle, either drawn on paper or constructed virtually on the charter using waypoints.

[Jim Cate]

If you have tide time data, established rules of thumb to the relative magnitude (if not the absolute magnitude) of tidal drift are useful.

But without info as to absolute drift velocity, how do you calculate set? And while knowing times for high and low can give you some intuitive ideas about the direction of the set, intuition fails to account for the eddies and other non-linear attributes of tidal driven currents. And how does intuition account for the data show for the wave rider buoy... which never changes direction very much at all through several days worth of diurnal cycles?

I remain skeptical about the usefulness of CTS calculations in most locations.

I wasn't aware of the Moreton Bay modeling, though. I'll have a good look at this next time we're up there, so thanks for that pointer.

Jim

[SV_Harbinger]

As a fairly proficient albeit lazy navigator, I will generally have my rhumb line and try to stick pretty near to that (near shore) by keeping an eye on both the bearing to my way point and the boats heading and correct my course until both remain fairly constant, if I wanted/needed to I could then take the difference between course steered and my intended course and make a vector diagram with that but it's not usually needed. For more of an off shore situation I tend to make a rough figure for what the initial course to steer is ( intended course +- set/drift)and start out with that but what you encounter is rarely exactly what they predicted, so even the most zealously and studiously attained figures will have some error in them, I then make course adjustments when I update my fix and once all the information is on chart (last fix, intended position, current fix, course steered vs actual) there is a ready made vector diagram that has mysteriously appeared on my chart. Is a a little sloppy? Yep. Does it work? Also yep. Am I ever too far off of my rhumb line? Not usually, no and when I do venture any significant distance* (situationally fluctuates) from my rhumb line I get back on it ASAP.
But when it come to wind and tide you; A, have been made a fool of at some time by them B, they will make a fool of you eventually C you're in denial or never venture too far from the slip.

As for the Original question I'm sure one can come up with a great way to figure CTS electronically but I've no idea how you would go about doing so, I would think a spread sheet would do a good workman like job of it, if the correct functions could be entered in, I couldn't do it myself, personally spreadsheets baffle me.

[Alan Mighty]

Originally Posted by Jim Cate

But without info as to absolute drift velocity, how do you calculate set? And while knowing times for high and low can give you some intuitive ideas about the direction of the set, intuition fails to account for the eddies and other non-linear attributes of tidal driven currents. And how does intuition account for the data show for the wave rider buoy... which never changes direction very much at all through several days worth of diurnal cycles?

My interpretation is that on those days (9-12 February 2019), the BNE Mk 4 wave rider buoy is in an S-going leg of the E Australian Current. The tide varying the set a little, but on those days never stems or reverses the EAC.

By inspection, the mean set of the EAC on those dates is about S by E, varying from S by W to SSE.

Those dates (9-12 Feb) are days 5 to 8 of the moon (if New Moon is day 1, then 9 Feb was the 5th day after the New Moon). So just after the spring tide extremes and getting closer each day to neap tides.

By inspection, the drift runs from 0.85 knots to 2.7 knots. A mean value might be around 1.8 knots. Tide and Wind together will account for most of the variation from the mean, but there must also be some variation within the current.

The rising tide runs more or less to the S (and vice versa for the falling tide) there. So inspection of the drift curve might suggest that the slowest drift is when the tide is falling; conversely the fastest S-going drift is when the rising tide is at its peak.

This morning (12 Feb) the wind at Cape Moreton has varied from about 10 knots NW (at 07 hours local) to 15 knots N (at noon). (see https://www.seabreeze.com.au/weather...ecast/brisbane scroll down for the live wind report at Cape Moreton, the wave height and period data, the tide curve, and so on).

What you do with that information depends on what your plan is.

If, for the sake of argument, you were heading from Moreton Bay to Noumea, that set and drift information might be useful in CTS calculations for a big part of your first day offshore, no?

[SeanPatrick]

Originally Posted by saltfree

Yes, this method allows me to end up at point "D" when and where I want to... but it means I was steering against the current to do it. If I don't care whether I arrive at point "D", because I'm aiming for somewhere further into the distance/future.

Point "D" is just a construction. It determines the course to steer at a given speed to make good the course to point "B" - which is beyond point "D". This particular example is for one constant current, but...

Originally Posted by saltfree

With this method, I arrive as planned at point "D", then recalculate and start crabbing the other way, steering against the new current direction when it changes. Yes, I crab along my rhumb line more or less straight, but giving up SOG to do it. In theory, if I were sailing at a perfect right-angle to the current, and sailing exactly through two full tide changes of equal strength, then I would just maintain my compass heading consistently at my destination. I would then sail a big "S" shape across the ground (much more distance), but get there much more quickly because I kept my SOG high, while allowing the power of the current to simply take me out of position, and then push me back into place.


The challenge is figuring out what the heading should be when I am not crossing the current at right angles, and where future tides may be of different strengths than the one I am currently in, between now and when I arrive on the other side.

If you happen to have an estimate of what the tides and currents will be along your route for the given time, then Dockhead's answer would be a possible solution:

Originally Posted by Dockhead

To deal with tides which are not perpendicular just do the vector triangle shown in the post you quoted -- one for every hour. Then just add up the vectors for the whole passage and steer that one course for the whole passage. That's the fastest way, because it's the shortest distance THROUGH THE WATER.

And if you're worried about adding/subtracting vectors ... don't. The same can be accomplished with the vector diagram (either on paper or in a plotting program). Consider the following:


You want to make good a course of 60° at 6.0 kts. You see that you will be traveling through the following three currents:

1. 90° at 2.0 kts.
2. 120° at 1.5 kts.
3. 190° at 1.0 kts.

So, what is the course to steer to make good a course of 60° over the whole voyage? Simple: just draw the vector diagram as usual - but draw the vector for current #2 starting from the end of the vector for current #1. Likewise, draw the vector for current #3 starting from the end of the vector for current #2. The course from the end of vector #3 to the destination is the course to steer - and, as usual, the length is the speed made good. In this case: the CTS is 23.5° and SMG is 5.2 kts. See diagram attached.


This can be verified by thinking about two equal but opposite currents. The second vector would simply double back over the first - leading right back to the original departure. The course to steer would be the same as the course to make good. Yes, your COG would be sinusoidal, but you would be steering directly toward the destination the whole time.

[SeanPatrick]

Correction: The above diagram shows what speed to use to make good a speed of 6 kts.

[Dockhead]

Originally Posted by Jim Cate

But without info as to absolute drift velocity, how do you calculate set? And while knowing times for high and low can give you some intuitive ideas about the direction of the set, intuition fails to account for the eddies and other non-linear attributes of tidal driven currents. And how does intuition account for the data show for the wave rider buoy... which never changes direction very much at all through several days worth of diurnal cycles?

I remain skeptical about the usefulness of CTS calculations in most locations.

I wasn't aware of the Moreton Bay modeling, though. I'll have a good look at this next time we're up there, so thanks for that pointer.

Jim

That's your physicist brain, Jim, craving precision . . .


In fact it ALWAYS makes sense to steer a constant heading, even if you have little data -- even a rough guess is better than intentionally crabbing (pace, Stu, that is a normal term) along the rhumbline, along a course you know is wrong.


Having a lot of data doesn't even help you all that much -- because what is unknowable to a great degree is your speed under sail. That's ok -- you correct a few times as you go along. You don't need precise data to make this work.

[Dockhead]

Originally Posted by sailingtaiyo

Crabbing is commonly used to express the " horizontal drift " of the vessel. Crabs walk sideways. We all had to learn and we all can give back by teaching others.

Aloha,
Mike.

We need the perfectly decent term "crabbing", to describe this problem. You won't find it in Bowditch, but neither will you find "ground wind". New terms were needed when GPS navigation and other technology gave us access to data we didn't have before. The term "crabbing" comes from aviation and is commonly used already in maritime contexts.

[Jim Cate]

If, for the sake of argument, you were heading from Moreton Bay to Noumea, that set and drift information might be useful in CTS calculations for a big part of your first day offshore, no?

Alan, I've made the trip from either Cape Moreton or Southport to Noumea around ten times. To be honest, I've never worried about CTS for the first day. In my world, the winds are not consistent or as forecast, the sea state varies and my boat speed varies in a somewhat unpredictable manner. Further, the winds that are in the future are pretty well unknown as are boat speeds and course... which sometimes is altered for comfort reasons (as is boat speed). Predicting these factors with any accuracy is beyond my pay grade or even interest, and this pretty well precludes worrying about CTS calculations based on tidal info (the original subject) or EAC predictions. And that is how I generally carry out my passages. Short term corrections for observed set are easy and effective when required, but these ad hoc actions are a far cry from the structured trans-channel passages that Dockhead plans and executes.

I guess I'm just a sloppy navigator, but so far I've arrived at my destinations more or less around the ETA, especially when the ETA is revised say, oh, every hour or so! Did I arrive there in the absolutely most efficient manner? Hell no, but it didn't fuss me too much. The croissants were still pretty tasty and the Camembert gooey and the wine intoxicating. Noumea is such a pleasant landfall!



None of the above change my view that there are lots of places in the world - probably MOST of the places in the world - where tidal info useful for detailed CTS calculations is just not available to the amateur navigator, and I think this is where this discussion began.

Jim

[Dockhead]

Originally Posted by Jim Cate

Alan, I've made the trip from either Cape Moreton or Southport to Noumea around ten times. To be honest, I've never worried about CTS for the first day. In my world, the winds are not consistent or as forecast, the sea state varies and my boat speed varies in a somewhat unpredictable manner. Further, the winds that are in the future are pretty well unknown as are boat speeds and course... which sometimes is altered for comfort reasons (as is boat speed). Predicting these factors with any accuracy is beyond my pay grade or even interest, and this pretty well precludes worrying about CTS calculations based on tidal info (the original subject) or EAC predictions. And that is how I generally carry out my passages. Short term corrections for observed set are easy and effective when required, but these ad hoc actions are a far cry from the structured trans-channel passages that Dockhead plans and executes.

I guess I'm just a sloppy navigator, but so far I've arrived at my destinations more or less around the ETA, especially when the ETA is revised say, oh, every hour or so! Did I arrive there in the absolutely most efficient manner? Hell no, but it didn't fuss me too much. The croissants were still pretty tasty and the Camembert gooey and the wine intoxicating. Noumea is such a pleasant landfall!



None of the above change my view that there are lots of places in the world - probably MOST of the places in the world - where tidal info useful for detailed CTS calculations is just not available to the amateur navigator, and I think this is where this discussion began.

Jim

How far is that trip?


It must be added that on a long enough passage, which covers many tides, CTS becomes easier because the currents are averaged out.


That still doesn't justify crabbing along the rhumbline -- rather, you just steer the initial bearing to your destination and let the tide do what it will (keeping clear of obstacles of course). THEN, when you're a couple of tides away, you can start thinking about this issue -- so that you don't end up downtide of your destination.


That's the other purpose of figuring out CTS -- not just the efficient constant heading, but also being careful not to end up downtide of your destination at the end, which can be miserable.

[SeanPatrick]

Originally Posted by SeanPatrick

Simple: just draw the vector diagram as usual - but draw the vector for current #2 starting from the end of the vector for current #1. Likewise, draw the vector for current #3 starting from the end of the vector for current #2. The course from the end of vector #3 to the destination is the course to steer - and, as usual, the length is the speed made good. In this case: the CTS is 23.5° and SMG is 5.2 kts. See diagram attached.

Well, I just tried working an example using this solution (which I probably should have done before posting it) and realized that it actually doesn't quite work that way.


Huh. Seemed to make sense when I thought it through.


Oh, well. Ignore all of that ... sorry. Back to the drawing (or plotting) board, I guess.