Single CTS or follow the Courseline?

[Paul Elliott]

The direct California to PNW trip is tough slog. The current isn't the big thing, although it's probably 1/5 to 1 kt against you. There are some nearshore counter-currents in paces, but I don't usually like being that close to the rocks. What makes it tough is that the seas and wind are usually on your nose. I tried it one time, and decided to turn around just shy of Cape Mendocino. We had been pounding into 20-25 kt winds and 10-15 ft seas for a couple of days, and the forecast was for it to get a lot worse. We would have had to duck into Eureka for a week to wait until things calmed down, but I was tired of the abuse and didn't feel like waiting that long. We turned around and ran back to San Francisco. I eventually sailed San Francisco to Hawaii (in the Pacific Cup race), then sailed from there to Cape Flattery. This was a much nicer sail, although the coastal approach can be brutal.

People who want to sail up the coast usually do a lot of motoring, and wait to catch the back-end of a low-pressure system so they have favorable winds. In the summer and fall there is usually a semi-permanent gale offshore of the California coast north of San Francisco. Some people will hug the coast so they can head for shelter (even though there aren't many safe harbors when the conditions are bad), and others will head offshore several hundred miles hoping for calmer conditions.

[Lodesman]

Originally Posted by Dockhead

If you get off plan using Andrew's method, you have to throw all the waypoints out and start all over again, just like you have to do with a regular CTS approach.

Not necessarily. If a fix finds you off-track, you can immediately calculate the actual set experienced and calculate a course to steer to regain the track at the next waypoint. At that point you could then revert to your planned CTS and hope that some of your planned data is correct. Or you could make a judgment call and modify your expected set by the same ratio your current set is off. While it may not retain all the efficiency of the original plan, it should get you as close as you are liable to achieve underway.

[Dockhead]

Originally Posted by Lodesman

Not necessarily. If a fix finds you off-track, you can immediately calculate the actual set experienced and calculate a course to steer to regain the track at the next waypoint. At that point you could then revert to your planned CTS and hope that some of your planned data is correct. Or you could make a judgment call and modify your expected set by the same ratio your current set is off. While it may not retain all the efficiency of the original plan, it should get you as close as you are liable to achieve underway.

Maybe, but why bother? Can quickly be self-defeating.

If you are off track now, it means in 99% you will be in the next hours as well -- usually means you are going faster or slower than planned, usually because of more or less wind than forecast. In the vast majority of cases, it means you need to rerun all the numbers.

[Andrew Troup]

Originally Posted by Lodesman

Not necessarily. If a fix finds you off-track, you can immediately calculate the actual set experienced and calculate a course to steer to regain the track at the next waypoint. At that point you could then revert to your planned CTS and hope that some of your planned data is correct. Or you could make a judgment call and modify your expected set by the same ratio your current set is off. While it may not retain all the efficiency of the original plan, it should get you as close as you are liable to achieve underway.

"If a fix finds you off-track" - are you thinking of the case where the GPS track mode is simply unable to keep you on the interim rhumbline? (Which the unit itself will tell you by reporting a CTE: no need to take a fix, unless you're keeping the GPS honest, which I applaud)

This implies to me that the boat is not going fast enough to stem the tide, and the early symptom would be a heading tending towards perpendicular to the bearing to the next waypoint, suggesting one of two quite extreme cases: either the data must be significantly different from reality, or the boatspeed chosen is well below what's needed.

In either case, I don't see the need to calculate, let alone rework anything, if I had more speed available ... and I would always make a plan which did not require full speed)

I don't personally think a complicated CTS plan - ie one involving strong rotary tides - is realistic when sailing except in very particular circumstances, so I'm assuming a boat motoring. However on particular legs of such a plan, it may be perfectly valid to put up the sails if the planned ground track and speed can be maintained.

I would simply go faster, press "GO TO" on the GPS to set up a fresh interim rhumbline to the currently scheduled next waypoint, reengage Track mode on the pilot, and see how things went.

If the boat still couldn't crab along the interim rhumb, I would probably abandon the plan altogether because a CTS strategy (which this fundamentally is) relies on being able to predict the current for the entire passage.

It really doesn't seem to me to be suited as an adaptive strategy, in fact I think that is a potentially unhappy marriage.

If the data is this wrong on this leg, it's likely to be wrong on the others, and probably in unexpected ways.

However, it's possible that the SHAPE of the vector diagram is correct, but the size is wrong (in other words, the tides are running harder, but in the predicted directions) and if you have strong reasons to believe this is the case, then reworking the plan based on what happened on the first leg might be worthwhile.



Or, Lodesman, are you thinking that the boat gets "off-track" in the sense of running behind or ahead of schedule?

(given that the GPS and autopilot should keep the boat on the planned ground track).

Once again, if the difference was minor I'd probably simply adjust the speed accordingly; if major, I'd reassess as in my first post on this proposal.

[Andrew Troup]

Thanks for your kind appraisals of my hybrid proposal, Dockhead.

I have a few minor points of clarification.

You mentioned that the "overview of the ground track" was the big advantage - and I agree it is a big advantage, because it tells you at a glance both where you are and where you should be, the two key elements of navigation.

Remembering that the rhumbs may all be different lengths: quarter of an hour into the second hour of the plan, you should be quarter of the way along the second interim rhumb.

It seems quite an easy way of keeping track of progress simply by eyeballing the GPS display, without needing to consult either chart or plotter. (Not that I'm suggesting ignoring these, of course.)

However I think there's another advantage which is similarly important: using the method I proposed, the execution of the plan is somewhat self-adjusting, which seems to me useful, given that the data is never perfect.

This is because the GPS and autopilot will adjust the heading continually to compensate for minor differences between prediction and reality... particularly the sideways component... but also things like leeway (if you decide you can sail for any portion of the distance) or deviations around ships using a fresh "GO TO" per last post - and all you need to do is adjust speed if need be to keep suitably close to plan.

- - - -

I have a minor quibble with another point you made a few posts before I reentered the discussion, on a related matter:

You said it didn't matter, in executing a CTS plan, if there were deviations from the planned speed on individual legs, as it was the average speed which mattered.

I disagree, unless you're unencumbered by fixed obstacles.

I might have a philosophical point of difference with you here, because I'm also unhappy with the prospect of setting off on a "constant heading" CTS programme, even in open water, on a 'discovery' basis.

In other words, without pre-plotting the ground track, but letting the boat travel where it will, and monitoring where it is.

However I have no strong argument that my way is better, let alone correct.

But I'm sure you'll agree that once we are faced with fixed obstacles the picture changes:

Even if we assume that the entire sheet of water travels as if it were inelastic, provided the current varies throughout in strength and direction, we will move away from the planned ground track if we deviate from the planned speed. This will be true even if we make up the lost time at a later stage in the execution of the plan.

- - - -
Another related topic is a CTS strategy involving PLANNED speed changes.

Plotting a ground track becomes a bit more laborious in the general case.

And if the tidal streams at a given time are different at various geographical locations along the route, the problem becomes altogether more difficult if we change from a planned constant speed: in this event, it's not just a difficulty plotting the ground track: we have to get into a recursive process to decide what currents we'll encounter, for which purpose we need foreknowledge of where we will be, and when.

Which of course depends on the currents we'll encounter .....

[Lodesman]

Originally Posted by Andrew Troup

"Or, Lodesman, are you thinking that the boat gets "off-track" in the sense of running behind or ahead of schedule?

(given that the GPS and autopilot should keep the boat on the planned ground track).

Some of us don't like to leave the navigation or driving to the machines

[Seaworthy Lass]

Originally Posted by Andrew Troup

Why not take the best out of both the main schools, in cases where it makes no sense to stick to a straight path across the ground ("crabbing along the rhumb"):

Here's what I'm thinking:

Work out a CTS using the best tidal data you have, and plot the theoretical ground track which would result from steering it.

Make the usual 'step-change' assumption, that the tide is constant during each hour (or half hour, if you have better data)

The result will be the series of straight lines we're familiar with from SwL's plots of ground track. It's just not that difficult.
Check it keeps you well away from dangers.

Then set a GPS waypoint at each kink in that theoretical ground track (eg hourly)

Now "crab along" each hourly "rhumbline" between successive waypoints, using the GPS.
Your heading will always be roughly what you calculated, if the data are good.

If the data predict current reality perfectly (unlikely, given their "step change" nature) then your heading (or more pedantically, the reciprocal of your wake bearing) will be exactly as calculated, for the entire duration.

What if the data are somewhat inaccurate but correct in 'character'?

(In other words, the current vector diagram is recognisably similar in shape and size to a diagram drawn with perfect foreknowledge)

Surely you're still way better off, in any case where that diagram is big enough and meandering enough to justify a CTS strategy in the first place, than the person crabbing along the single straight rhumbline.

If you have to make a course correction to avoid an unplanned obstacle, you would simply continue heading for the next waypoint. No drama; and no need to sidle back onto the mini-rhumb, unless you would otherwise pass unacceptably near to a danger

It seems to me that with this method you're largely immunised from problems with the data, unless these problems are so severe that

a) you probably shouldn't be attemping the passage at all with the data you have*, and

b) Crabbing along a single rhumbline would simply not get you there.


I wouldn't bother recalculating the intended ground track unless the boat's heading was wildly at variance with the CTS figure: and the first thing I would suspect would be my calculations or plot.

If these were OK, I'd check I'd entered the waypoints correctly, and if no problems there, I'd reconsider the plan, possibly involving abandoning the projected arrival point in favour of a more achievable one, or even turning back.

If I arrived at particular waypoint twenty minutes early or late, I'd certainly be reviewing the plan: it would suggest a major component of tide with or against the CTS was very different from what the data predicted.

* (unless it's entirely free of dangers, or other special circumstances: eg you are greatly slowed because you're towing a dispensable load, you're trying to conserve a small reserve of fuel, you're trying not to arrive at the beginning of a tide race too early ..... in circumstances, generally, where you can "put the hammer down" and greatly exceed the target boatspeed, AND at least match the maximum current speed, if things get ugly)

Hi Andrew
As usual it is great to read your fresh perspective on things.

Despite all the favourable response to your suggested modification of the implementation of the CTS method, I would not be comfortable doing as you suggest .

Using the CTS method often means a significant deviation from the rhumb line. Eg In the English Channel crossing example Jackdale gave in post #383 of http://www.cruisersforum.com/forums/...-96579-10.html
at the halfway point the tidal stream would have swept you 6.5 nm away from the rhumb line (and I think during spring tides this would be even more).

You propose deliberately sticking to this expected track, even if it means you are modifying your CTS. If you are going faster than expected this will mean you are unnecessarily heading further from the rhumb line and lengthening your journey. If you are going slower than expected, you are again lengthening your journey by trying to head closer to the rhumb line than you should.

If you are not sticking reasonably to your expected ground track (and before you start, for several reasons I advocate plotting this on the chart, or most commonly on the chatplotter or using a program such as OpenCPN), something is very wrong with your CTS and you need to modify it.

Your track during each hour of the plot will not be a straight line as you have drawn so you should not attempt to chase this straight line by constantly modifying your course. At the end of each hour you should, however, be at the expected point you have marked and if you are out by a significant amount, this is the appropriate time to recalculate your CTS, not to head off to the next waypoint you have marked on your expected track. You could be deviating unnecessarily and chewing significantly into any time advantage you may have had.

In Jackdale's English Channel crossing example, the boat following the rhumb line was only 2.7nm behind the boat following the computed CTS at the end.
Zig zagging around unnecessarily trying to follow a track that may be taking you more than 6nm from the rhumb line as it did in this example, is in my opinion not a good idea .

If you want to modify the CTS slightly without recomputing, then make logical changes. If your speed is greater than expected (and you do not think it will drop lower than expected later in the journey to compensate), then adjust your CTS so that it is a little close to the bearing of B from A and vice versa. At some stage for greater accuracy, you need to recompute your CTS.

If your speed is exactly as predicted and you are significantly off your expected track, it means your tidal steam data does not match reality (or leeway was not correctly estimated) and the decision needs to be made about whether or not this will persist. This is a tougher decision.

Personally, I think that if you are simply cruising, and over a 60 nm journey there is only 2.7 nm difference (less than half an hour at the expected 6 knots) then I would personally just crab along the rhumb line instead of deviating over 6nm from the rhumb line. Particularly in a busy shipping lane, where following a CTS does not have you following a straight ground track making it highly confusing to other vessels who are trying to avoid you. In my opinion maintaining a straight line track in these conditions contributes to a much safer passage.

Where the average current displacement for the journey is high relative to your boat speed (it was only about 0.2 knots relative to a boat speed of 6 knots in the English Channel example Jackdale gave), this is where following the CTS method has huge advantage over crabbing.

The burning question is which method are you using to compute your CTS? The time you most need an accurate CTS method, is the time the RYA method is most likely to fail.

In the channel crossing example, the RYA and SWL methods gave the same result (there was very little total current displacement over 10+ hours).

In other cases, the error in the RYA method may easily be 5 or 10 or 20 or more degrees out. The errors can become particularly large if the current is strong relative to boat speed and either half the last hour of tidal data applies and worse still the current is also close to zero at the end of the journey.

If your computed CTS method is not accurate for whatever data you input, any advantage following a CTS will quickly disappear and you may even be worse off!!!

I have not had time yet, but tomorrow I will post step by step diagrams demonstrating the new method and I will post these on the other thread.

[Andrew Troup]

Seaworthy Lass:

For the sake of simplicity, I'll temporarily refer to a pure constant heading strategy as "your" method, and the hybrid method I've proposed as "my" method.

Let's first see if we can set aside things we already agree on, because it would make my brain hurt to try to address your post without first clearing these considerable elements out of the way:

1) An accurate method should be used to calculate CTS.

We would both be naturally inclined to use such a method, and for the purposes of this discussion, let's agree not to make plotting or calculating errors.
Our CTS, and out theoretical expected ground track, will therefore be the same.

I don't think this is a 'burning question' between us, and I'm not sure why you brought it up in discussing my proposed method.

2) The rhumb line from departure to destination is not a helpful construct, when deciding how far we travel through the water.

Can we agree that the gold standard, equally applicable to judging the wasted distance arising from our respective methods, is a dead straight line through the actual water which departs (at T1) and arrives (at T2) at the correct ground positions?

I don't think there's a designated label for that idealised virtual line, so I'll refer to that as the DSL (dead straight line).

Of course it can be determined only with hindsight (given the absence of perfect and unachievable foresight).

3) It seems we agree that a constant heading, CTS strategy is only appropriate when the substantial advantages of 'crabbing the rhumb' are more than outweighed.

You re-stated some of these advantages in your post, but I'm not sure why, since they don't seem to bear on a comparison of the respective merits of "your" vs "my" proposals.
　
I won't launch into analysing the relative merits of acknowledged points of difference between our methods until you tell me whether you think I'm taking any liberties as to what we can agree on.

[Seaworthy Lass]

Originally Posted by Andrew Troup

Seaworthy Lass:
For the sake of simplicity, I'll temporarily refer to a pure constant heading strategy as "your" method, and the hybrid method I've proposed as "my" method.

I have not any any stage suggested following a pure constant heading all the way once the CTS has been calculated.
What I suggest doing was was explained in this post:

Originally Posted by Seaworthy Lass

You plot your 'expected ground track' before the start of the journey and you examine your track relative to this as you would the rhumb line in the NCL method.

As soon as you deviate significantly, a new track must be computed, just as it is for the NCL method (you do NOT try and get back on the old track).

If you are racing this will need to be done frequently, particularly if the deviation is due to an unexpected variation in speed (Expedition probably does this automatically for you). Otherwise pick your duration of time to recompute this (eg hourly? - I think the most appropriate interval of time depends on the distance left to go).

Onto other points you raise:

Originally Posted by Andrew Troup

Let's first see if we can set aside things we already agree on, because it would make my brain hurt to try to address your post without first clearing these considerable elements out of the way:

1) An accurate method should be used to calculate CTS.

We would both be naturally inclined to use such a method, and for the purposes of this discussion, let's agree not to make plotting or calculating errors.
Our CTS, and out theoretical expected ground track, will therefore be the same.

Agreed

Originally Posted by Andrew Troup

I don't think this is a 'burning question' between us, and I'm not sure why you brought it up in discussing my proposed method.

It is not a burning question between us, but it is a burning question in general, as the RYA is teaching something quite different. A lassie needs to use any opportunity she can to alert people to this

Originally Posted by Andrew Troup

2) The rhumb line from departure to destination is not a helpful construct, when deciding how far we travel through the water.

Agreed

Originally Posted by Andrew Troup

Can we agree that the gold standard, equally applicable to judging the wasted distance arising from our respective methods, is a dead straight line through the actual water which departs (at T1) and arrives (at T2) at the correct ground positions?

Agreed

Originally Posted by Andrew Troup

I don't think there's a designated label for that idealised virtual line, so I'll refer to that as the DSL (dead straight line).

Of course it can be determined only with hindsight (given the absence of perfect and unachievable foresight).

Agreed

Originally Posted by Andrew Troup

3) It seems we agree that a constant heading, CTS strategy is only appropriate when the substantial advantages of 'crabbing the rhumb' are more than outweighed.

Agreed , but I did not understand you were stressing this previously. My error if you were.
　

Originally Posted by Andrew Troup

I won't launch into analysing the relative merits of acknowledged points of difference between our methods until you tell me whether you think I'm taking any liberties as to what we can agree on.

No liberties at all should be taken with married lassies
The only liberty you seem to be taking is that I am suggesting following "a pure constant heading strategy", so perhaps it is best to just skip this bit

[Seaworthy Lass]

I am heading ashore soon to catch the bus to the "big smoke" for the day.

Andrew, I look forward to reading your reply when I get back, although I have a sneaking suspicion your clear logical way of thinking may have me reconsidering my position on this topic when I get back LOL. Let's see .

Have a great day/evening everyone.

[LJH]

Originally Posted by Andrew Troup

I don't personally think a complicated CTS plan - ie one involving strong rotary tides - is realistic when sailing except in very particular circumstances, so I'm assuming a boat motoring. However on particular legs of such a plan, it may be perfectly valid to put up the sails if the planned ground track and speed can be maintained.

I don'think it matters if you are sailing or motoring, if you can calculate a CTS using a realistic boat speed. I prefer to let my old Perkins sit quietly and trim the sails. If done well it should use less battery power than crabbing in the Track mode on the autopilot. If conditions are such that I need to motor excessively, I might consider waiting for a better wind. Of course if conditions change underway and I am concerned about my arrival time I may motor sail if my boat speed drops much below 6 kts.

Originally Posted by Andrew Troup

I would simply go faster, press "GO TO" on the GPS to set up a fresh interim rhumbline to the currently scheduled next waypoint, reengage Track mode on the pilot, and see how things went.

Originally Posted by Lodesman

Some of us don't like to leave the navigation or driving to the machines

If you have spend the time to do the planning and have made a comparative ground track it would be a shame to disregard some of the effort and let the autopilot follow the rhumb line for you. If you have the opportunity, monitor you navigation planning with reference to the ground track realizing that you will not be exactly following it and sail the CTS. Make small corrections if you aren't comfortable. It would be the best way to get confidence in your planning and the method you used. Then if conditions change drastically you can use the GO TO to get back to your plotted ground track.

There is only one way to know it works. Do it, and then..

Originally Posted by Dockhead

But one aspect of the classical CTS method which is unquantifiable, and is not a matter of efficiency at all, is this -- when it comes off well, it simply feels so cool that you had the skill to do the numbers right and as a result, you just arced right into your destination hardly having touched your helm. It is really incredibly satisfying -- the power of navigation and your own skill over the chaos of moving water -- something like the feeling after a well-executed tack or a well-executed docking maneuver -- one of the joys of sailing.

[conachair]

Originally Posted by Seaworthy Lass

Personally, I think that if you are simply cruising, and over a 60 nm journey there is only 2.7 nm difference (less than half an hour at the expected 6 knots) then I would personally just crab along the rhumb line instead of deviating over 6nm from the rhumb line. Particularly in a busy shipping lane, where following a CTS does not have you following a straight ground track making it highly confusing to other vessels who are trying to avoid you. In my opinion maintaining a straight line track in these conditions contributes to a much safer passage.

Might be wise to bear in mind that if the shipping lane is a TSS then sticking to the rhumb line with some cross tide running could put you in contradiction of irpcs rule 10

A vessel shall, so far as practicable, avoid crossing traffic lanes but if obliged to do so shall cross on a heading as nearly as practicable at right angles to the general direction of traffic flow.

[Dockhead]

Originally Posted by LJH

I don'think it matters if you are sailing or motoring, if you can calculate a CTS using a realistic boat speed. I prefer to let my old Perkins sit quietly and trim the sails. If done well it should use less battery power than crabbing in the Track mode on the autopilot. If conditions are such that I need to motor excessively, I might consider waiting for a better wind. Of course if conditions change underway and I am concerned about my arrival time I may motor sail if my boat speed drops much below 6 kts.





If you have spend the time to do the planning and have made a comparative ground track it would be a shame to disregard some of the effort and let the autopilot follow the rhumb line for you. If you have the opportunity, monitor you navigation planning with reference to the ground track realizing that you will not be exactly following it and sail the CTS. Make small corrections if you aren't comfortable. It would be the best way to get confidence in your planning and the method you used. Then if conditions change drastically you can use the GO TO to get back to your plotted ground track.

There is only one way to know it works. Do it, and then..

I agree. Speed variations are not the problem, in practice, that some of you think, and certainly never enough to make you want to slow down intentionally or motor.

As I wrote before, in practice, if you are sailing well and exceeding your plan, it's all good. Your course correction you used to get your CTS is too much and you can ease off a little and enjoy the ride. There is no urgency to make any correction because your error takes you further uptide. To be headed for a spot further uptide than you planned is not a problem. This is not a video game -- you are not aiming to collide with the fairway buoy. An hour or at least a half hour off you will change course anyway, and an error of a couple of miles uptide will be unnoticeable at the time of a final course correction an hour off. If you are sailing across strong currents, speed is life. The faster you go, the less the currents are knocking you away from your destination. Your speed made good will go up disproportionately to your speed through water. Therefore, you really don't care about the error that you will get from this -- it is easily washed out at the end and you gain far more than you lose from the error.

If you end up going slower than planned, then you have a problem, because your error puts you downtide, and by not anticipating the slower speed, you didn't correct enough, which means you have to make up for it with a disproportionate correction later. This is inefficient. So the error compounds the loss of speed and disproportionate loss of speed made good. Here you need to react sharpish, calculate a new CTS and get onto a new heading, because every minute you sail with too little correction for your real speed, will hurt you disproportionately in the end.

To put this in perspective, a slower than planned speed will hurt a rhumbline boat even much more -- the advantage of CTS is greater, with slower boat speeds, to the extent of even being unable to reach the destination at all, at a certain combination of current and boat speed, where a well executed CTS passage might still get you there comfortably, through the same currents.

A corollary of this is that you would prefer to somewhat underestimate your speed, than overestimate it. As I wrote before, I always do three sets for a Channel crossing, based on 7, 8 or 9 knots average speed. If the wind is decent, is use 8 as the base case and know that I usually achieve an average of about 8.5. If something goes wrong with speed, I have instant recourse to the 7 knot calcs.


Andrew, it is true that any combination of speeds to get a given average speed, will not necessarily have the same effect here, so my statement about that was an oversimplification. If you sail 4 knots and 8 knots for an average of 6, if your 4 knot part is in the strongest part of the current, then obviously the result will be different.

But since speed doesn't usually change in big jerks and neither does the tide, you don't notice this usually so much, so practically speaking you can ignore this unless it is really dramatic for some reason.

Using your method, you really don't want to get back on an hourly rhumbline if you get knocked off, unless it is a tiny XTE, but in that case, why bother? Why not just head for the waypoint directly? Your method will give you passages very close, almost indistinguishable from a regular CTS passage, in efficiency, if everything goes to plan, but breaks down quickly if not, unless you are ready to throw out the waypoints and produce a new set every time you get substantially off plan. This is not really a weakness of your method if you have a computer and don't mind replotting those waypoints, but any rigidity in relation to the waypoints will ruin it.


I actually think that the better application of your method is not hourly waypoints closely simulating a regular CTS passage, but a single waypoint in the middle of a 6 to 8 hour passage which put you in an advantageous position to make the last leg home. For some passages it will not be hugely worse than regular CTS, and will be vastly better than rhumb line crabbing. It will have a genuine advantage in simplicity and won't need to be recalculated much. The hourly waypoint version will work extremely well in theory, but I think in practice for most people will be somewhat fiddly and awkward, and more sensitive to plan deviations.

If you run some numbers, you will prove, I'm sure, that the hourly method will give results in some cases unmeasurably different from regular CTS -- well done!


These Gedankenexperimente have been especially useful, I suppose, for people who haven't done much of this kind of navigation in real life. What we haven't seen so far is the flexibility of the CTS method -- remember this -- even with errors in input information (poor tide information), errors in methodology (uncritical application of the RYA method), and deviations from your speed plan, even a very approximately calculated CTS will put you off at least in the correct general direction. This is already good!! It's pretty easy to check how well you're doing as you sail along, and you will see the errors. You can easily rerun the RYA method at some point in your passage where you've got whole hours, and the disadvantages of this method disappear. If you are headed off generally in the right direction, you can easily correct as you go along. It's much better to sail, say 065, then an hour or two later recalculate and -- oops, from here I actually need to be on 069 -- and you have lost very little of anything -- a couple of hours of steering four degrees wrong has very little effect on your overall passage, whereas being on the rhumb line instead of 12 miles uptide where you should be (say; Channel passage on a slowish boat at springs) can be a disaster which can conceivably mean you can't even arrive at all.

[Lodesman]

Originally Posted by Dockhead

Maybe, but why bother? Can quickly be self-defeating.

If you are off track now, it means in 99% you will be in the next hours as well -- usually means you are going faster or slower than planned, usually because of more or less wind than forecast. In the vast majority of cases, it means you need to rerun all the numbers.

If you're going slower or faster than planned, that should be apparent on your log - you would want to rerun all the numbers anyway. Unless you're going the planned speed, the plan is garbage.

[Lodesman]

Originally Posted by conachair

Might be wise to bear in mind that if the shipping lane is a TSS then sticking to the rhumb line with some cross tide running could put you in contradiction of irpcs rule 10

This also applies to following any CTS(heading) which isn't perpendicular to the lane. The rule does have the 'as near as practicable' proviso - wind direction may prevent strict adherence by sailing vessels.