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Old 02-02-2013, 06:25   #46
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Re: Single CTS or follow the Courseline?

you look at your GPS !, were not doing DR navigation !
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Old 02-02-2013, 06:33   #47
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Re: Single CTS or follow the Courseline?

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Originally Posted by goboatingnow View Post
you look at your GPS !, were not doing DR navigation !
OK, I'm looking at the GPS. I know where I am and where I've been. How do I know if this is the track my CTS was intended to follow?
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Old 02-02-2013, 06:35   #48
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Re: Single CTS or follow the Courseline?

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OK, I'm looking at the GPS. I know where I am and where I've been. How do I know if this is the track my CTS was intended to follow?

Bingo, first problem of multi hour complex CTS. you dont often know the exact ground track

2nd problem , is all the original tide data now still valid ?

third problem, skipper can you come ondeck and decide what to do with the rock ahead.
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Old 02-02-2013, 06:58   #49
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Re: Single CTS or follow the Courseline?

I suspect that the original CTS methods (Bowditch, others) were derived before the days of GPS, and designed to get you "close enough" until you were able to visually check your position. Not a bad thing to know how to do, in my opinion.

Sometimes getting there fast is important. Sailing a constant CTS (recalculating as needed when your speed changes, and factoring in your best point of sail) will get you there quickest. Just because it takes work to do the navigation does not mean it's a bad technique.

You can certainly pre-figure your track over the ground when sailing CTS. You can verify this with GPS and make mid-course corrections if you like.

Feel free to sail the rhumbline if you like. I often do. It's easy to get right and is usually fast enough. It can put you in a bad place if the current on your final approach is unfavorable, but that's one more factor you can prepare for.

I prefer logic to magic. And when using logic I try to keep my eyes open.
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Old 02-02-2013, 07:04   #50
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Re: Single CTS or follow the Courseline?

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I suspect that the original CTS methods (Bowditch, others) were derived before the days of GPS, and designed to get you "close enough" until you were able to visually check your position. Not a bad thing to know how to do, in my opinion.
Bowditch and most other US texts make no mention of multi hour CTS methodologies ( why do you think!). they stick to single hour. The RYA mentions it in passing and teaches it in class, its rarely applied with any gusto in real life.( except for x-channel crossings)

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Sometimes getting there fast is important. Sailing a constant CTS (recalculating as needed when your speed changes, and factoring in your best point of sail) will get you there quickest. Just because it takes work to do the navigation does not mean it's a bad technique.
getting there safely is far more important, with a functioning crew.
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You can certainly pre-figure your track over the ground when sailing CTS. You can verify this with GPS and make mid-course corrections if you like.
have a bash at some real life 10-12 hour non symmetrical tides, few people have the training and confidence in vector addition to be comfortable working out the ground track , and has been shown in other threads the route isnt always intuitive

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Feel free to sail the rhumbline if you like. I often do. It's easy to get right and is usually fast enough. It can put you in a bad place if the current on your final approach is unfavorable, but that's one more factor you can prepare for.
apply the 1 in 60 rule. everyone generally factors an hour or two ahead, or sit the tide out as sailing boats have done for millennia
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Old 02-02-2013, 07:31   #51
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Re: Single CTS or follow the Courseline?

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Originally Posted by bill352 View Post
I've been following this and the other threads on the subject, and I'm going to jump in with a stupid question here (yes, they exist. I've seen them.)

If you're sailing a multi-hour CTS and you do get a reliable fix, how do you know where that is compared to your planned course? As I understand it, there is no DR plot with this method. How do you know whether you're on course or not?

I understand that the prudent navigator will start anew from here, and that is certainly my practice. But there has been talk about "correcting" a course from here, and even, IIRC, of calculating the actual set and drift by comparison with some expected position. How do you do that, if you do?

Forgive me if this has been explained before. I didn't see it.
I don't believe this has been answered - it's the same question I've been asking.

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I still haven't had a very good answer as to what you do on a CTS when a fix puts you off your planned CMG? Do you steer to regain or maintain your planned CTS?
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If you get a fix, convention (read Bowditch) dictates that you start afresh. If you have been dead reckoning you can use the DR position and your fix to determine the current since your departure or the last fix. That can provide you with an indication of the variance between what ever current prediction tables you were using and the reality.
This is a "philosophical" question. With NCL, we plot our planned tracks, and can plan a CTS for an expected set, but as we proceed we fix, DR and fix again and from that can calculate the actual set, with which we can refine our CTS to regain our desired path to destination.

With a multi-hour calculation (MHCTS) you can plot your expected course over the ground (CMG), but if the fix puts you off that track, there's the conundrum. The efficiency of the MHCTS is that you are steering a straight path through the water - if you alter your course, your water path will no longer be straight and you lose that efficiency. If you take your fixes infrequently, how do you know where it went wrong - was the first hour's tide greater than expected? was it the second hour's? or was it averaged over the entire time? So assuming you recalculate a new MHCTS at the midpoint of your passage - do you continue to use the planned tidal currents, knowing that they are not correct? Or is it more efficient to stick to your planned CTS until you're within half-hour of destination, then NCL the last bit?
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Old 02-02-2013, 07:43   #52
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Re: Single CTS or follow the Courseline?

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With a multi-hour calculation (MHCTS) you can plot your expected course over the ground (CMG), but if the fix puts you off that track, there's the conundrum. The efficiency of the MHCTS is that you are steering a straight path through the water - if you alter your course, your water path will no longer be straight and you lose that efficiency. If you take your fixes infrequently, how do you know where it went wrong - was the first hour's tide greater than expected? was it the second hour's? or was it averaged over the entire time? So assuming you recalculate a new MHCTS at the midpoint of your passage - do you continue to use the planned tidal currents, knowing that they are not correct? Or is it more efficient to stick to your planned CTS until you're within half-hour of destination, then NCL the last bit?

Leaving aside the difficulty in determining for the average non maths users the ground track, You could examine the method by applying each hour of computation the error circle associated with the future EP. Hence arrving within that circle is consistent with the underlying accuracy of the method. Of course if you look at it that way the last set of error circles are quite large and can put you in trouble.

unless you have a fairly accurate predicted ground track ( which is not actually the actual ground track anyway) the method proves very difficult to use in real life. IN most caes as jackdale says , these currents occur within sight of land and you can use transists etc to correct.

There is a reason that multi hour CTS is omitted from Bowditch, Rousmarine, Annapolis Navigation etc.
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Old 02-02-2013, 07:59   #53
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Re: Single CTS or follow the Courseline?

If I were racing where the currents were a factor, I would certainly use a multi-hour CTS in my planning. My planned course would factor in the actual and predicted wind, the predicted currents, and of course my boat's performance. With any navigation method I would be checking actual vs predicted and adjusting as needed. I would most certainly *not* plan on making hourly corrections in order to stay near the rhumbline. Obstacles and hazards would not be ignored.

This is because I would have a capable crew, and be able to perform the observations and calculations necessary for prudent navigation.

You can argue that in most cases a simple single-hour approach would be easier, safer, and less likely to surprise, and I will probably agree with you.
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Old 02-02-2013, 08:42   #54
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Re: Single CTS or follow the Courseline?

The shortest distance between two points is going to be fastest, unless the current changes where it becomes a following current AND the vector component of that current towards your destination has a speed/distance greater than the additional distance added to a straight line you otherwise would have taken.

If covering great distances and you are not altering course to find better winds or are weather routing to minimize high seas, just stay on the great circle route and adjust course to vector into the current and effects of windage in order to stay on your great circle route. That's how professionals on ships do it.
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Old 02-02-2013, 09:08   #55
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Re: Single CTS or follow the Courseline?

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Originally Posted by bill352 View Post
OK, I'm looking at the GPS. I know where I am and where I've been. How do I know if this is the track my CTS was intended to follow?
Well you could do an hourly vector plot instead of adding all the tide vectors at the start, might well be full of errrors quickly though. But as Dave keeps pointing out it's hard to think of an example of where you might actually do a long multi hour plot like that. Other than a english channel crossing, where the first 3 hours of tide would show roughly how far from the rhumb line you would end up. The channel is maybe a good example of one passage where sticking to a rhum line is actually forbidden by the col regs as you won't be crossing a VTCS at 90 deg.

As this is a forum things get polorized, either this or that. In reality in northern Europe as passage might end up as a mixture of tidal calcs, some crabbing down a rhumb line and some pilotage. Whatever seems best for the conditions. But in the faster tidal areas sticking solely to the rhum line could very end up in a lot of miles and a slow passage.
Whatever works.
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Old 02-02-2013, 09:12   #56
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Re: Single CTS or follow the Courseline?

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If you're sailing a multi-hour CTS and you do get a reliable fix, how do you know where that is compared to your planned course? As I understand it, there is no DR plot with this method. How do you know whether you're on course or not?
If this is the only method of navigation available to you. You do use a DR plot and then use the expected current to establish and Estimated Position.

Quote:

I understand that the prudent navigator will start anew from here, and that is certainly my practice. But there has been talk about "correcting" a course from here, and even, IIRC, of calculating the actual set and drift by comparison with some expected position. How do you do that, if you do?
You will use you DR plot and compare it to a fix taken at the same time. The length of the line (accounting for time) between the DR and the fix represents the drift. The bearing from the DR to the fix is the set. The is the "average" of the current that you expereinced in the past.

If you need an image I will hunt one down for you.
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Old 02-02-2013, 09:20   #57
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Re: Single CTS or follow the Courseline?

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If I were racing where the currents were a factor, I would certainly use a multi-hour CTS in my planning.
Not many racing crews use paper charts as their sole means of navigation. They are using programs should a Expedition which use polars, grib files and current data to lay out the "best" route. They then use local knowledge to follow the wind, back eddies, etc.. The Van Isle 360 is classic example of this - a 14 day, 10 leg race around Vancouver Island. The inside has some very nasty currents.
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Old 02-02-2013, 10:05   #58
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Re: Single CTS or follow the Courseline?

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The shortest distance between two points is going to be fastest, unless the current changes where it becomes a following current AND the vector component of that current towards your destination has a speed/distance greater than the additional distance added to a straight line you otherwise would have taken.

If covering great distances and you are not altering course to find better winds or are weather routing to minimize high seas, just stay on the great circle route and adjust course to vector into the current and effects of windage in order to stay on your great circle route. That's how professionals on ships do it.
shortest distance through the water is always the quickest, not the shortest rhumb line
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Old 02-02-2013, 10:07   #59
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Re: Single CTS or follow the Courseline?

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If this is the only method of navigation available to you. You do use a DR plot and then use the expected current to establish and Estimated Position.



You will use you DR plot and compare it to a fix taken at the same time. The length of the line (accounting for time) between the DR and the fix represents the drift. The bearing from the DR to the fix is the set. The is the "average" of the current that you expereinced in the past.

If you need an image I will hunt one down for you.

what bill352 means is that since the ground track is not easy to compute in a long multi hour CTS, it is therefore difficult to know how you are progressing , not that you cant find where you are. its that you dont know, where you are, is where you should be?

Dave
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Old 02-02-2013, 10:23   #60
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Re: Single CTS or follow the Courseline?

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its that you dont know, where you are, is where you should be?
Brilliantly put
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