Inaccurate RYA Teaching : CTS - Quest For a New Method

[Dockhead]

Originally Posted by Andrew Troup

Jackdale wrote:

<< Most currents are sinusoidal.

The drop in [the scenario's] current speed is less in the first hour than the second.

Not realistic.

Lets us try using a current atlas, current tables or the real numbers from from current table on a chart.>>


I've graphed the stream rates for representative diamonds from the first chart which came to hand, as noted.

The tidal streams are nothing close to 'sinusoidal' here, in other waters around these parts, and in many other parts of the world.

Particularly where, as the current does in most of the cases for which the RYA method would be warranted* and as it does at the locations of the diamonds I've instanced below: the direction of the stream boxes right around the compass, and there is typically no such thing as slack water.

*(rather than a simpler aggregated set as recommended by Dockhead, which can be done without recourse to the chart)

Every day of every passage deals up challenges which are not typical or mundane.

The typical ones are trivial, to anyone with some experience.

If the others are somehow off the table, is there much point in discussion?

If you choose to discuss a scenario on the forum, it may be pushing the boundaries to expect to dictate terms to the person who devised it.

It's right and proper, of course, that different rules would be applicable in your classroom.

Those graphs are really useful, Andrew! First I've seen what they really look like.

I think I may have to scrap my idea -- I still think the velocity of the current can be closely approximated with math between the limits -- between every time the tide stops or changes directions -- not necessarily 6 and something hour cycles. But DIRECTION is a whole different question, dammit. Places like this with rotary currents are not that common, but this method will be useless if it can't deal with them. Hourly averages CAN deal with them within the limits of the resolution.

Looks like there is at least one software package specifically intended to solve this problem:

PASSAGE PLANNER PLUS

This one released just this month.

Interestingly, it calculates not only CTS for a tidal passage, but will spit out an optimized departure time. Interesting whether it is available for whole world coverage or not. Their tidal streams come from the UK Hydrographic Office's high res tidal streams, which are published for the whole world.

I might download the trial version.

[jackdale]

Originally Posted by Andrew Troup

I've graphed the stream rates for representative diamonds from the first chart which came to hand, as noted.

The tidal streams are nothing close to 'sinusoidal' here, in other waters around these parts, and in many other parts of the world.

Particularly where, as the current does in most of the cases for which the RYA method would be warranted* and as it does at the locations of the diamonds I've instanced below: the direction of the stream boxes right around the compass, and there is typically no such thing as slack water.

*(rather than a simpler aggregated set as recommended by Dockhead, which can be done without recourse to the chart)

Every day of every passage deals up challenges which are not typical or mundane.

The typical ones are trivial, to anyone with some experience.

If the others are somehow off the table, is there much point in discussion?

If you choose to discuss a scenario on the forum, it may be pushing the boundaries to expect to dictate terms to the person who devised it.

It's right and proper, of course, that different rules would be applicable in your classroom.

I guess I should have said somewhat sinusoidal. I also live with mixed semidiurnal tides. But even the examples that you posted show the drop following the maximum is generally less than the drop later on. Otherwise the curve would be concave rather than convex.

Check out the tidal streams in the St. Lawrence in this material. (http://www3.telus.net/jackdale/navle...t_Lawrence.pdf) As a river, it is not a prefect example.

In Canada we use current tables more often. http://tides-marees.gc.ca/Content/ti...100_public.pdf
We use sine waves to do the interpolation. See the graph in the lower left corner.
http://www.swiftsuresailing.com/documents/current.pdf
Or I have also seen a 50/90 rule applied.


(David Burch Navigation Blog: Starpath 50-90 Rule)

What I am seeking is realistic scenarios. I am not the one doing the dictating by using unrealistic, extreme examples.

[goboatingnow]

RYA don't even calculate the partial hour. Incidentally, RYA will produce EXACTLY the same result as SWL if you were to do two of them, one through the whole hour before, and one through the whole hour after, and calculate a proportionate share of the difference. The only thing is that this is vastly more laborious than SWL's ingeniously simple method.

Typically , you have to compute at least the last tide anyway, even if your using the RYA method. Hence its a simple matter of either say inflating from 2 tide or deflating from 3 tides, This can actually usually be done quite accurately by eye. all all in all its just a second swipe of a compass arc.

Furthermore as was incorrectly pointed pointed out by anothr poster, we do have to actually get the tide at whatever hour we intend to pass through the area. hence we interpolate from the HW point. This brings an increase in perceived accuracy , but has the disadvantage that the interpolation can be wrong and in fact we might never be at point in crossing that the tidal time applies.!!

dave

[HappySeagull]

jackdale, try this and tell me what you think...

http://www.cruisersforum.com/forums/...ml#post1141272
seems handy for us anyways.

[goboatingnow]

BTW looking at Andrews example, The RYA method doesn't need a start point per say. All it does is plot a water vector ARC at the time time quantum's as the plotted tidal vectors. I often use it as a poor mans CTS, just plot the tides and then sweep the compass through the required arc. If the arc passes close to the destination , then you can eyeball CTS, etc. There is not requirement to cut the rhumb line, thats just a simplification that is applied because it works in the majority of cases

Dave

[HappySeagull]

http://www.cruisersforum.com/forums/...ml#post1141567

"Are you the person who confused tide heights with streams? I can't remember, after what must now be approaching 1000 posts in two threads.

In any case, tidal stations have nothing to do with it. The stream data is a grid produced By the tide model and available in one hour and five minute resolution. The tide model also gives limits and timing of the streams, but I don't know whether this data is published in the Admiralty Hydrographic system....."

gee. I was just trying to help. Current data has been a concern brought up. Extrapolation of inside the last hour has also been mentioned.
Ignore it, if you don't want to try it. Improve it, if you like. As to the lecture, it speaks for itself, but if you tried the formula, which I think you didn't, you will see it offers a value for any particular time:

"...I think we will have much better resolution and accuracy by assuming a sine wave development of the current. Even though this is artificial and will also not be identical to reality, it should be much, much, much closer than one hour averages."
in your same post.

And no,that wasn't me that confused tide heights with current from my drawing here...(from http://www.cruisersforum.com/forums/...ml#post1136563) I merely thought it was interesting aside.

[HappySeagull]

sin[(eT/mT)*90)]*MAX= Current at eT

where MAX=maximum current
where eT= time past slack
where mT= time "MAX->Slack" or viceversa

Is easy to draw on paper, with a protractor and ruler OR just a compass, ruler and square.
Scale Current on y, scale Time on x or vice-versa.
If MAXCurrent=mT in your scale, you just draw a 1/4 circle from origin, plot time(eT) from slack along x, then draw a vertical to the circle to find the current at eT.

but most computers have calculators.

[Dockhead]

Originally Posted by HappySeagull

sin[(eT/mT)*90)]*MAX= Current at eT

where MAX=maximum current
where eT= time past slack
where mT= time "MAX->Slack" or viceversa

Is easy to draw on paper, with a protractor and ruler OR just a compass, ruler and square.
Scale Current on y, scale Time on x or vice-versa.
If MAXCurrent=mT in your scale, you just draw a 1/4 circle from origin, plot time(eT) from slack along x, then draw a vertical to the circle to find the current at eT.

but most computers have calculators.

Your math is a lot better than mine, and that was helpful. Superb! Thanks!

[Seaworthy Lass]

What! No replies to Andrew's challenge in post # 483
Too easy or too hard LOL?
I came back on board after a long day ashore and eagerly checked CF to read other people's solution to Andrew's problem.

There was none apart from LJH's that was posted 19 hours ago .
Is no one game to put forward their opinion?
Computations are very quick if you can download Andrew's sheet (very easy way of trying out the SWL method).
Thinking about what to do takes longer
Go on, have a go

[jackdale]

Originally Posted by HappySeagull

jackdale, try this and tell me what you think...

http://www.cruisersforum.com/forums/...ml#post1141272
seems handy for us anyways.

Shall do. I am not sure that level of significant figures is realistic

[Dockhead]

Originally Posted by jackdale

I guess I should have said somewhat sinusoidal. I also live with mixed semidiurnal tides. But even the examples that you posted show the drop following the maximum is generally less than the drop later on. Otherwise the curve would be concave rather than convex.

Check out the tidal streams in the St. Lawrence in this material. (http://www3.telus.net/jackdale/navle...t_Lawrence.pdf) As a river, it is not a prefect example.

In Canada we use current tables more often. http://tides-marees.gc.ca/Content/ti...100_public.pdf
We use sine waves to do the interpolation. See the graph in the lower left corner.
http://www.swiftsuresailing.com/documents/current.pdf
Or I have also seen a 50/90 rule applied.


(David Burch Navigation Blog: Starpath 50-90 Rule)

Sine waves would work to give a superior tidal rate prediction. You need the limits. And -- drum roll please -- you can get them. They are reported by the Admiralty H.O., and you can get them out of programs like the Neptune Planner:



However, without stream direction, this is useless -- a dead end.

I have fallen in love with this Neptune Planner, which uses 5 minutes tide data to not only calculate an extremely accurate CTS, but gives you optimum departure time (and will graphically compare departure times), AND on top of all that, will take your polars and GRIBS and do wind for you, too.

So I have come to the conclusion, after this excellent thread: For quickly plotted multi-hour CTS, SWL is the way to go. RYA is also ok IF you follow the unpublished user notes which Dave has so kindly provided just now ("RYA CTS Calculation: The Missing Manual").

For more or less perpendicular tides, the traditional Channel pilot math method is good.

BUT -- if you have a computer on board, why bother? I just bought Neptune, and ironically after all this work on CTS theory and practice, I may never do one by hand again. I ran some Channel crossings and found out that the non-perpendicular element of the tide, which we are trained to ignore, has a BIG influence on your passage.

And Neptune will do the rotary tides in the Channel Island, which NO hand method can deal with. I'm in lurrrvvvv -- for the first time all day!

[Seaworthy Lass]

Originally Posted by Dockhead

Sine waves would work to give a superior tidal rate prediction. You need the limits. And -- drum roll please -- you can get them. They are reported by the Admiralty H.O., and you can get them out of programs like the Neptune Planner:

Attachment 53636

However, without stream direction, this is useless -- a dead end.

I have fallen in love with this Neptune Planner, which uses 5 minutes tide data to not only calculate an extremely accurate CTS, but gives you optimum departure time (and will graphically compare departure times), AND on top of all that, will take your polars and GRIBS and do wind for you, too.

So I have come to the conclusion, after this excellent thread: For quickly plotted multi-hour CTS, SWL is the way to go. RYA is also ok IF you follow the unpublished user notes which Dave has so kindly provided just now ("RYA CTS Calculation: The Missing Manual").

For more or less perpendicular tides, the traditional Channel pilot math method is good.

BUT -- if you have a computer on board, why bother? I just bought Neptune, and ironically after all this work on CTS theory and practice, I may never do one by hand again. I ran some Channel crossings and found out that the non-perpendicular element of the tide, which we are trained to ignore, has a BIG influence on your passage.

And Neptune will do the rotary tides in the Channel Island, which NO hand method can deal with. I'm in lurrrvvvv -- for the first time in a whole day!

I had a look at Neptune.
Fantastic to have info like this instantly at our fingertips! No use at the moment for me in the Med, but I can imagine you will find it invaluable.

You will miss sitting in your favourite spot at the nav table with your chart and dividers planning a journey though . It just won't ever be the same doing this from your laptop at home.

First loves are always hard to forget .

[Dockhead]

Originally Posted by Seaworthy Lass

I had a look at Neptune.
Fantastic to have info like this instantly at our fingertips! No use at the moment for me in the Med, but I can imagine you will find it invaluable.

You will miss sitting in your favourite spot at the nav table with your chart and dividers planning a journey though . It just won't ever be the same doing this from your laptop at home.

First loves are always hard to forget .

Lol. Not so fast -- I'm not planning to give up paper charts or chart work Just tidal vector calcs . . .

Why would you need tidal stream planning in the Med? I thought that lakes have no tides?

Neptune stops doing tidal streams at Kattegat -- no more point once you get into the Baltic . . .

[Seaworthy Lass]

Originally Posted by Dockhead

Why would you need tidal stream planning in the Med? I thought that lakes have no tides?

LOL, I said it would be of no use here in the Med.

Someone forgot to tell the Med that it was only a lake. Conditions can be WILD!
We were anchored at Paros when his storm came through in Dec 2010:

YouTube

[Dockhead]

Originally Posted by Seaworthy Lass

LOL, I said it would be of no use here in the Med.

Someone forgot to tell the Med that it was only a lake. Conditions can be WILD!
We were anchored at Paros when his storm came through in Dec 2010:

YouTube

The lake bit was a joke. I have done a fair bit of sailing in the Med and it can be challenging. The Meltemi in the Aegean can be wild, especially in the acceleration zones between islands!

For sheer sh*tty weather, the Black Sea is hard to beat -- the sea state can be horrendous with the long fetch and shallow depth . . .


Before we finish our conversation about the RYA method of calculating CTS, I though I would actually put up the pages!

It would be an exaggeration to say that I sleep with the RYA navigation handbook, but I always have it in my briefcase, even here in Helsinki in my hotel room:



Here's all it says about multi-hour CTS "course shaping":







So how the instructor teaches it is obviously key!