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Old 03-02-2013, 06:35   #106
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Re: Plotting the expected ground track

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Originally Posted by bill352 View Post
SWL - That is a beautifully clear presentation, and thank you for taking the time to respond. Just to clarify, I never doubted that something like this would be done by the prudent navigator, but it was never obvious that this step was part of the calculation and implementation of the complete CTS method; and, judging from other posts, it may not be so in all cases.

Since apparently you did explain all this at the beginning, I'm going to be embarrassed now when I go back and see that it just went right over my head. I can only plead confusion caused by the immediate and vigorous introduction of peripheral issues following your original post.

Thanks to you, I am quite satisfied now on the procedure, although I see that the discussion may not be entirely over:

Your post #95 - "With the CTS method, the 'expected ground track' is also plotted on the chart. "

And Dockhead's #99 - "You do NOT plot any ground track in order to check your progress except in case of assumed perpendicular tides (more about anon)."

But I think I'll sit this one out for now. Thank you again for taking the time and trouble to respond.

Bill
You're welcome Bill . Really glad that helped to clarify things.

Dockhead and I disagree only slightly I think. He feels in open water where there are no hazards you are not concerned where the current takes you. That is not unreasonable.

I feel though that in this situation if you are not racing, then rather then recomputing the CTS frequently, you can just look at your deviation from the expected ground track that you plotted before you started. You would only then need to recompute if this varied significantly, saving nav work along the way.

When sailing a course surrounded by hazards and if the current is not perpendicular and particularly when it varies in direction, I think (hope, LOL) Dockhead may agree that it is important to plot the expected ground track as this is not intuitive and hazards need to be checked for.
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Old 03-02-2013, 06:55   #107
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Re: BSP = Vboat = S

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Originally Posted by bewitched View Post
So, if you believe you can meet the 'ifs' then the CTS boat wins hands down.

If there a small fraction of error or omission in the calculation data or a small difference between the conditions expected and the actual passage itself and I would be wouldn't be putting money on either.
The bigger the advantage of an idealized CTS passage, the bigger an error can be tolerated. Even if you're 5 degrees out, on the passage above whose Neptune solution I posted, and don't discover it until you are an hour off, you are still more than an hour ahead of a rhumb line boat.


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The longer the leg, the more I'd be thinking about backing the NCL boat.
I think you have this backwards. The longer the leg, the more essential an efficient path is. Besides that, I don't think you have listened at all to what people have said about course corrections on a CTS passage. You are allowed to do this. If you have poor tide data * you can start out with a pretty wild guess and still come out way ahead if you keep correcting.

This stops making sense only where the advantage is very small, like Lodestar's example above where the current varied only by 1 knot in the middle. Where you encounter a 4 knot current or more at some point in a passage where you have zero or one knot current some hours, say, and especially where the current changes direction, the advantage of CTS is huge. This is a very ordinary and common situation in tidal waters.



* And I don't even know where that is; in tidal parts of European waters, and in U.S. waters, at least, we have extremely precise tidal data, expressed with precision in tenths of a knot and probably accuracy at worst a couple of tenths of a knot, and with resolution of 5 minutes if you want it, at least in European waters.
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Old 03-02-2013, 06:59   #108
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Re: BSP = Vboat = S

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Sailing using the CTS method does not mean one CTS the entire way.

Both for the RYA method and mine you will be constantly recomputing the CTS when you are racing.
So now it's not a single course to steer, it's multiple courses to steer?.....or is that constantly changing courses to steer?


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Sailing using the NCL method does not mean not deviating from the rhumbline the entire way. .
Well, we have been talking about different things then, because I've been talking about moving over the ground in a straight line between two waypoints.
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Old 03-02-2013, 07:17   #109
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Re: BSP = Vboat = S

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So now it's not a single course to steer, it's multiple courses to steer?.....or is that constantly changing courses to steer?
Sorry for the confusion .
In an ideal world with ideal data the CTS method means following a constant heading the whole way. In the real world it needs to be recomputed, so yes, you are following several different headings along the way.

The difference is that unlike the NCL method, you are NOT following close to the rhumb line unless the cross current is constant the whole way. You are following some other headings.

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Originally Posted by bewitched View Post
Well, we have been talking about different things then, because I've been talking about moving over the ground in a straight line between two waypoints.
You are very unlikely to move in a straight line when following the NCL method when racing. You need to avoid other boats and when you do, you don't try and get back to the original course line do you? You recompute a new one and follow that (or at least you should). Also if you chase better breeze etc each time you deviate you recompute. So your ground track is unlikely to be a straight line for the NCL method, just as your CTS will vary with the CTS method.

Hope that makes sense .
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Old 03-02-2013, 07:55   #110
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Re: Making corrections when following a computed CTS

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Originally Posted by Seaworthy Lass View Post
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).
Thanks for the answer Seaworthy. For the record (and if DeepFrz is paying attention) I had no problem with you being able to plot the planned CMG from the data you have - the question was always "what do you do when you are off the planned CMG?"

With NCL there are three choices normally when you find yourself off your navtrack - (1) regain the track quickly (6 or 12 minutes usually) then steer to maintain; (2) steer a course to intercept the navtrack at the next waypoint; or (3) plot a new navtrack directly to destination. It would depend on how far off course you are and distance left on the track/passage as to which option would be employed. In the big ships, the navigator or master are responsible for the navigation plan, and the watchkeepers are (usually) restricted in their allowances to deviate from it (collision avoidance and correcting for set basically).

Quote:
Do you: (a) plot the expected CMG for each tidal set and change course to maintain that?;
(b) take periodic fixes and recalculate a new CTS based on previously-planned and possibly incorrect tidal data;
(c) take periodic fixes and recalculate a new CTS based on tidal data adjusted by observation
;
(d) stick to the planned CTS with faith that it will all even out by the end, fixing and running DRs/EPs to ensure navigational safety;
(e) stick to the planned CTS and only pansies fix; or
(f) other?
Notwithstanding the divergence between yourself and Dockhead on the issue of plotting CMGs, you both seem to go with (b) or (c)? Do you stick with the tabulated currents, or do you calculate the set you experienced over the past interval (hour or whatever) and apply it to your calculations?
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Old 03-02-2013, 08:22   #111
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Re: Single CTS or follow the Courseline?

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Well, a ship making 16 knots will feel much less effect of current vectors, than a yacht making 6 or 7. The effect of 2 knot currents on a ship making 16 knots will be very slight and maybe not worth calculating.

And anyway, I bet professional navigators on a ship crossing the Gulf Stream, for example, will certainly do a CTS calculation. There you can have a lot more than 2 knots. I know for sure they do it in the English Channel.

As Dave said, the shortest distance is a straight line -- through the water.
Just stay on your great circle route by adjusting your steering compass heading accordingly. It really is that simple. This is being made much more complex than is necessary. The quickest way is to stay on a straight line between two land based points. The reference is land.
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Old 03-02-2013, 08:43   #112
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Re: Single CTS or follow the Courseline?

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This is being made much more complex than is necessary. The quickest way is to stay on a straight line between two land based points. The reference is land.
Sorry David .
I hate to disagree with you, but the quickest way between two points is in a straight line through the water, not on a straight line between two land based points (ie not a straight line on the chartplotter unless the current is constant with zero current included). This is the whole reason the CTS method evolved.
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Old 03-02-2013, 09:24   #113
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Re: Single CTS or follow the Courseline?

This is not impossible where I am ....
Bewitched, you have not addressed the issue at all of strong current relative to boat speed that is predicted in the latter portion of a long leg. The boat sailing on a CTS will have allowed for this and will still be making progress, while you will be furiously crabbing and making little or no progress or worse still going backwards if you can't anchor!

from Single CTS or follow the Courseline?

OR, in the hypothetical condition where a cross current floods one way and ebbs in the opposite over the period.
Steering the rhumb will have you "fighting back" both ways... while a longer view single CTS may allow blithely steering "your easting down" without the constant struggle against the current d'heure...

The choice of method will always rely on the trip plan.
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Old 03-02-2013, 09:31   #114
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Re: Single CTS or follow the Courseline?

A quick comment on leeway.

Treat it separately from the calculation of CTS via currents.

If you are using the variance between your wake and heading, you will only know that once you are sailing.

If you are using John Rousmaniere's table, you need to know both wind speed and point of sail, which requires being on the water.

Once you know your leeway, head up that much. Or if you are close hauled already, subtract the leeway from your heading.
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Old 03-02-2013, 09:35   #115
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Re: Plotting the expected ground track

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Let me clarify, and harmonize these different statements.

You do NOT need any ground track plots for checking and correcting your CTS.

You DO need to plot a ground track, as SWL suggests, in case there is any question that you may run into any hazards.
Can't argue with that, and won't, especially since you're crossing the Channel and I'm not. But, aside from safety, I see two advantages to having the ground track plotted:

1. You can get real-time information about set and drift, for whatever that's worth; and

2. If the fix is close to your DR (don't laugh - it could happen) you save yourself an extra calculation, as SWL points out in another post.

The down side is that you always wind up doing the original calculation, which, as you point out, is not always needed. But, being clear on the concept, I'll just do what everyone else does - whatever feels right on the given day.

Thanks for the response and the clarification.

Bill
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Old 03-02-2013, 12:35   #116
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Re: Plotting the expected ground track

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Originally Posted by bill352 View Post
Can't argue with that, and won't, especially since you're crossing the Channel and I'm not. But, aside from safety, I see two advantages to having the ground track plotted:

1. You can get real-time information about set and drift, for whatever that's worth; and

2. If the fix is close to your DR (don't laugh - it could happen) you save yourself an extra calculation, as SWL points out in another post.

The down side is that you always wind up doing the original calculation, which, as you point out, is not always needed. But, being clear on the concept, I'll just do what everyone else does - whatever feels right on the given day.

Thanks for the response and the clarification.

Bill
On your practical advantages to calculating ground track -- I agree with you.

With assumed perpendicular tides, you've already got it because you know what XTE you need every hour. You can easily plot it on the chart, but then again, for simply checking whether you're on track or not, you don't even have to do that.

If the tides are not perpendicular, you have to separately plot every hour -- a bit tedious. Or you can use a program like Neptune and you have it.

For just checking whether you're on track, it will be simpler just to work up a fresh CTS calculation.

Concerning SWL's multiple headings -- absolutely, if you're correcting from time to time, you will have corrections of heading from time to time. The ideal CTS passage will have one single heading the whole way until you sail up to the fairway buoy of your destination. In reality you almost always need to correct at least once. A minor correction, or a few of them, or even a major correction in time, will still put you far ahead of a rhumbline boat in most cases. The other thing which is really important, is that you don't actually plan to sail your CTS all the way to the destination anyway -- you abandon your CTS an hour or so off and just home in.

I think those who are arguing that using CTS is useless because you can't achieve perfection are really missing this. You don't hope to collide with the fairway buoy at the end without every touching your pilot -- doesn't happen. You hope to arrive a mile or so uptide of your destination (or half a mile, or 5 miles -- it all depends), and that's what you steer towards -- your aim is to arrive in a strategic spot within a certain circle of probability which allows you then to just glide in. To use golf as a metaphor -- you don't drive expecting to make a hole in one -- you drive in order to get onto the green in the right position to comfortably putt it in. You can't criticize Lee Trevino's drive by saying "oh, you'll never get a hole in one that way". That's not the point of his driving technique. If he gets onto the green every time, and upslope of the hole, he will kick your a*s if you're driving a different way, which is calculated only to avoid the problems which make it hard for Lee to get a hole in one. It's an illogical approach.

You create your own margin of error in planning your passage depending on whether the conditions favor an exact passage, or not. If it's blowing F7 and the sea is rough, and you're hard on the wind; you give yourself lots of miles (or stay in the pub, maybe). If it's a dead calm and a glassy sea and you're motoring, you might not even need that mile. In any case, you're miles ahead of some poor soul crabbing and laboring along the rhumbline, intentionally sailing a long and winding path through the water just because he can't figure out how to plot a course to steer, or because he's afraid of the uncertainty of his ground track.

It works really well in practice -- these are not armchair, hypothetical opinions.
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Old 03-02-2013, 15:07   #117
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Re: Leeway

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Originally Posted by Seaworthy Lass View Post
Addressing the issues raised regarding leeway:



It is critical to allow for leeway in any CTS computation.

Leeway is dependent on the shape of the boat (underwater and above), so it is very specific to the boat you are on. As well, numerous outside factors will affect it.

The RYA Navigation Handbook has two pages dealing with leeway.
It gives a formula to enable it to be estimated when sailing:
Leeway = ( Heel / speed squared) x K
where Heel is measured by an inclinometer and "K is a constant primarily related to the boat's design that can be found by measuring the leeway in one set of conditions, and then put into the formula to calculate leeway in other circumstances".

I don't know how reliable this formula is, but yes, leeway can certainly be estimated!

The better you can determine the amount of leeway for your boat for the expected conditions, the more accurate your CTS will and therefore quicker you will get to your destination. If you are racing it is important to take the time to get a good estimate for K.
I believe that early in this thread there was an assumption that the CTS method did not account for leeway. Calculating the CTS is but one step in the planning process. One must also check that the CTS is safe to follow. Once the this has been done then it is necessary to apply variation, leeway and deviation. After that has been done you will have a compass course to steer for the helmsman.

Most of us will probably have a good feel for how much leeway to apply for our boats for the conditions we are expecting (5 degrees below F4 or 10 degrees above as a rough start). Will the winds allow it or not? Plan B....
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Old 03-02-2013, 15:15   #118
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Re: BSP = Vboat = S

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Last (for now ), but not least:





Of course you go as fast as you possibly can in each boat!
We are cruising here not racing. The admiral sets limits for me at times that result in having to accept less than hull speed at times. There are other times that you want to have a set arrival time. There have been several nights that I have slowed 6 kts to make a daytime landfall.

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As the BSP increases unexpectedly, the advantage of the CTS method diminishes. At some point the increase may even even result in the same time taken as the NCL method, or it may be worse (even if you recompute your 'expected ground track' and remember Expedition is probably doing this instantaneously for you).

But, even if the BSP increases unexpectedly the CTS method may still have you ahead .

However, if the BSP drops, using the NCL method can be a huge disadvantage as your only option is to continue navigating the straight line to the destination. In light wind you may end up going backwards if the water is too deep to anchor! You may well still be making progress in the CTS method as you are sailing on a different heading.
That said, if your boat speed increases adjust your heading a degree or two toward your destination and recalculate.
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Old 03-02-2013, 15:19   #119
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Re: BSP = Vboat = S

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+1 to all of this.

Why in the world would sailing an intentionally longer path possibly be inherently faster in any way? I find this idea to be simply bizarre.

Of course you push your boat as hard as you can. You NEVER slow down to make your plan, as I already posted a couple of pages back -- where did anyone get such an idea? When making your initial calculations, you forecast your average speed, and often in three variants (I always do 7, 8 and 9 knots for Channel crossings). If you get out there and see you're averaging 9 instead of 8, you just punch the "+1" or "-1" on your pilot a couple of times and carry on.

It will be only cases of very weak currents or very consistent currents, that you can come even close crabbing down the rhumb line. I think SWL actually understates this case!

If you pick up speed and your ETA is suitable why would you slow down. If you compare plans for varying boats speeds then it should be easy to adapt when conditions change.
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Old 03-02-2013, 15:27   #120
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Re: BSP = Vboat = S

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If you pick up speed and your ETA is suitable why would you slow down. If you compare plans for varying boats speeds then it should be easy to adapt when conditions change.
Absolutely. Picking up speed is all good. More speed means less time in each tide, less set, less correction, and a better passage. It is always a happy correction, correcting for higher than expected speed -- what they call a "high class problem" .

It's otherwise with slower than expected speed. This is all bad. It means more tidal set and it requires you to correct with a greater amount of course correction, and passage time increases disproportionately. But it's even worse for a rhumb line boat -- your COG falls disproportionately, as your speed falls in relation to the current speed. To put that another way -- the slower your boat speed in relation to the current, the bigger the advantage of a CTS passage.
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