Single CTS or follow the Courseline?

[Seaworthy Lass]

Originally Posted by Seaworthy Lass

Destination (B) is 6.5 nm due east of A
Boat speed is 5 knots (log reading)
Average current for the first hour is 3 knots to the south
Average current for the second hour is 1 knot to the south

Edited to add: motoring in calm water, so no leeway to complicate the issue.

RYA METHOD RESULTS:

CTS = 53 degrees true
SMG = 4 knots
Time taken = 1.6 hours

The ground tracks are shown below as a dotted red line.
The "expected" one takes you to B in 1.6 hours.
If you actually follow 53 degrees true for 1.6 hours, then you arrive somewhere north of B.

In this case D is just within half an hour of B, making it a viable calculation according to the RYA.

In my opinion getting to D is by no means adequate. You are still 2.5 nm away from your destination for a journey that is only 6.5 nm long. I do NOT believe this is close enough.

Regardless of the accuracy of the data, the method used to compute the CTS should be reasonably accurate. The RYA method is flawed in this respect.

[goboatingnow]

Originally Posted by Dockhead

We're kind of going around the houses here, so sorry for repeating what has been said before.

The difference between RYA and SWL is very simple -- they deal with the last partial hour in different ways. RYA subsitutes the average of the passage, where SWL substitutes the average of the last hour. Neither, of course, is extremely precise, but SWL is obviously better, ranging from somewhat better to vastly better, depending on whether or not you run into the limitations of the SWL method.

Now that I have 5 minute tide data I can show it with examples -- just need a bit of quiet time to do it. Maybe tomorrow.

Whether or not its vastly better or not is only relevant if in real life the error circle of SWLs method reduces the overall real life error ( taking into account all real errors) such that it exposes the RYA error. If it doesnt and the total real error remains larger then either method, then nothing has been gained

The fact is Dockhead, that nobody but nobody uses either method. Nobody sails any sort of complex multi hour tidal journey without re-working their CTS several times. Hence by doing this you are compensating for any errors in both the method and the underlying data. A bit like the mph gauge always gets it right in the end.

Dave

[jackdale]

One other big advantage of the RYA method, is that it is an extension of a single current vector. They use similar principles to get an ESTIMATE of where to point the boat to get from A to B using the variables that are somewhat known. Current can be forecast within normal parameters such as mean barometric pressure, no wind, no strong river outflows (snow pack affects currents in both Johnstone Strait and the Strait of Georgia, not a big issue in The Channel between Southampton and Cherbourg). It cannot be predicted when those parameters change as they will.

The variables that are unknown and unpredictable during the planning process will throw off any estimates calculated by any means.

The determination of CTS and SMG cannot be taken in isolation. Once you depart you should be using all of your navigation skills to get positions that can be plotted and noted in the logbook. And courses will be adjusted accordingly.

[Dockhead]

Originally Posted by goboatingnow

Whether or not its vastly better or not is only relevant if in real life the error circle of SWLs method reduces the overall real life error ( taking into account all real errors) such that it exposes the RYA error. If it doesnt and the total real error remains larger then either method, then nothing has been gained

The fact is Dockhead, that nobody but nobody uses either method. Nobody sails any sort of complex multi hour tidal journey without re-working their CTS several times. Hence by doing this you are compensating for any errors in both the method and the underlying data. A bit like the mph gauge always gets it right in the end.

Dave

The implications of rerunning the numbers over the passage are very significant. And I think you are right that they will wash out the errors in both SWL and RYA method. Probably it reduces the differences between them.

But still, it would be stupid to forgo the advantages of a method which starts you off so much better, and at no cost in extra trouble. I personally think the RYA should be teaching the SWL method.

[goboatingnow]

Originally Posted by Seaworthy Lass

RYA METHOD RESULTS:

CTS = 53 degrees true
SMG = 4 knots
Time taken = 1.6 hours

The ground tracks are shown below as a dotted red line.
The "expected one takes you to B in 1.6 hours.
If you actually follow 53 degrees true for 1.6 hours, then you arrive somewhere north of B.

In this case D is just within half an hour of B, making it a viable calculation according to the RYA.

In my opinion getting to D is by no means adequate. You are still 2.5 nm away from your destination for a journey that is only 6.5 nm long. I do NOT believe this is close enough.

Regardless of the accuracy of the data, the method used to compute the CTS should be reasonably accurate. The RYA method is flawed in this respect.

Correct me if Im wrong but the RYA method would use two hours of tide as the basis here as B is more then 30 minutes away from D. You would not apply the RYA method correctly in the first case. Hence in my class I would use the two hour CTS as a better average.

In practice of course what you would do is merely compute the second CTS on arriving at D. slightly less efficient but better seamanship.

[Seaworthy Lass]

Originally Posted by Seaworthy Lass

Destination (B) is 6.5 nm due east of A
Boat speed is 5 knots (log reading)
Average current for the first hour is 3 knots to the south
Average current for the second hour is 1 knot to the south

Edited to add: motoring in calm water, so no leeway to complicate the issue.

SWL METHOD RESULTS:

CTS = 62 degrees true
SMG = 4.4 knots
Time taken = 1.48 hours = 1 hour 29 minutes

The ground tracks are shown below as a dotted red line.
My expected track will have you arriving at B in 1 hour and 29 minutes.

The results are of course only as good as the data used.
Unlike the RYA method, the SWL method will at least give you an accurate CTS each and every time if the data is good.

[Dockhead]

Originally Posted by jackdale

One other big advantage of the RYA method, is that it is an extension of a single current vector. They use similar principles to get an ESTIMATE of where to point the boat to get from A to B using the variables that are somewhat known. Current can be forecast within normal parameters such as mean barometric pressure, no wind, no strong river outflows (snow pack affects currents in both Johnstone Strait and the Strait of Georgia, not a big issue in The Channel between Southampton and Cherbourg). It cannot be predicted when those parameters change as they will.

The variables that are unknown and unpredictable during the planning process will throw off any estimates calculated by any means.

The determination of CTS and SMG cannot be taken in isolation. Once you depart you should be using all of your navigation skills to get positions that can be plotted and noted in the logbook.

One big advantage of the RYA method over what? I didn't quite understand. I don't see how that can be an advantage of SWL, which is also an estimate, just with much less inherent error.

[goboatingnow]

Regardless of the accuracy of the data, the method used to compute the CTS should be reasonably accurate. The RYA method is flawed in this respect.

No there is no point in computing an answer that attempts to give greater accuracy then is in the underlying "measurements" what you are doing is confusing precision with accuracy. Your methods are more precise, but I will contend that both methods are "inaccurate" with comparison to real life.

[Dockhead]

Originally Posted by Seaworthy Lass

RYA METHOD RESULTS:

CTS = 53 degrees true
SMG = 4 knots
Time taken = 1.6 hours

The ground tracks are shown below as a dotted red line.
The "expected one takes you to B in 1.6 hours.
If you actually follow 53 degrees true for 1.6 hours, then you arrive somewhere north of B.

In this case D is just within half an hour of B, making it a viable calculation according to the RYA.

In my opinion getting to D is by no means adequate. You are still 2.5 nm away from your destination for a journey that is only 6.5 nm long. I do NOT believe this is close enough.

Regardless of the accuracy of the data, the method used to compute the CTS should be reasonably accurate. The RYA method is flawed in this respect.

One flaw in this, dushenka moya, is that you are using your method as the baseline. There is also error in your method, so the difference between your results and RYA is not necessarily 100% the error of RYA.

We need 5 minute tides to give us a precisely calculated baseline, against which to compare both methods, and I swear I will find some time in the next two days to run some scenarios

[goboatingnow]

Originally Posted by Dockhead

One big advantage of the RYA method over what? I didn't quite understand. I don't see how that can be an advantage of SWL, which is also an estimate, just with much less inherent error.

There is only inherently less error if that error can be shown to be real ( and less). Even then, the errors in real life almost always forces mid stream recomputation and adjustment, rendering multi CTS where it should be , in the theory books and internet forums

Dave

[goboatingnow]

Originally Posted by Dockhead

One flaw in this, dushenka moya, is that you are using your method as the baseline. There is also error in your method, so the difference between your results and RYA is not necessarily 100% the error of RYA.

We need 5 minute tides to give us a precisely calculated baseline, against which to compare both methods, and I swear I will find some time in the next two days to run some scenarios

of course Dockhead if one has 5 minutes tides, one runs either method ( on 5 minute tide time quantum's) one end up with little theoretical error in either case.

[Dockhead]

Originally Posted by goboatingnow

There is only inherently less error if that error can be shown to be real ( and less). Even then, the errors in real life almost always forces mid stream recomputation and adjustment, rendering multi CTS where it should be , in the theory books and internet forums

Dave

Mid-passage recomputations will wash the errors out of both methods, but why in the world would you not want to start out with less error? We will need some scenarios to see how it really looks.

[jackdale]

Originally Posted by Dockhead

We need 5 minute tides to give us a precisely calculated baseline, against which to compare both methods, and I swear I will find some time in the next two days to run some scenarios

Two things

1) If you are using 5 minutes currents you are using a level of "prediction" that is not used by hydrographic services, I expect that you are using algorithms produced by that are not the same as those used by hydrographic services. One of the charting programs I used provided the references ports that they used to compute currents and it had no bearing those used by CHS. X-tides used algorithms provided through crowd sourcing.

2) If you are using 5 minute tides, I expect that you are using an electronic source. In which case, why not just use advanced electronic navigation and forget the vectoring process.

[goboatingnow]

Originally Posted by Dockhead

Mid-passage recomputations will wash the errors out of both methods, but why in the world would you not want to start out with less error? We will need some scenarios to see how it really looks.

Really in most cases a helms can keep +-5 degrees often +-10 ( and the errors dont cancel). add leeway, wind direction, sea state and speed variations are the resulting EP error circle over say many hours is very large . And thats is what we are trying to acomplish , establish a future EP.

Hence it matters not that the maths say 'X', since X lies within a much greater circle of uncertainly ( the say 10 hour CTS EP error).

Which is why all this simply doesn't advance CTS accuracy one bit. In all multi hour CTS on a slow sail in boat, recomputation of teh CTS is a given . Only the really stupid would sail off on a single CTS and rest their fate in a hypotectical solution . Hence as you say recomputation removes errors.

Ultimately you end up probably recomuting the last tide ( or even two tide) anyway, and in reality in a 30 -40 minute tide, you transit your way in using fixes.

This argument is purely theoretical, even if you show inaccuracies, in real life re-computation is a given hence the results are meaningless.

[Seaworthy Lass]

Originally Posted by goboatingnow

Correct me if Im wrong but the RYA method would use two hours of tide as the basis here as B is more then 30 minutes away from D. You would not apply the RYA method correctly in the first case.

Sorry, you are wrong. D is not more than 30 minutes away in this example (it is exactly on the limit).
If you used two hours of tide data you would be more than 30 minutes away, so I do not think anyone using the RYA method would use 2 lots of tide data. If you did though, you would not be much better off. The CTS would compute as 66 degrees, still inaccurate and if you follow it you will not arrive at B.

Originally Posted by goboatingnow

In practice of course what you would do is merely compute the second CTS on arriving at D. slightly less efficient but better seamanship.

Compute a second CTS?
The whole point of determining a CTS it to get you to your destination on one heading, not recompute a totally different heading when you are not even 2/3 of the way there and nothing has changed with the data.

Dave, you have kept repeating the RYA method will get you to B on the computed heading. It does NOT unless a few limited conditions apply.