Inaccurate RYA Teaching : CTS - Quest For a New Method

[Seaworthy Lass]

Originally Posted by Andrew Troup

I can see a few wee problems with your from-the-hip analysis, Seaworthy, but lets see what others come up with independently, before we start interrogating any suspects.

Please remember the scenario is not the usual sort of textbook simplified abstraction, and may have some curly aspects.

One assumption we have to make for simplicity is that the tide diamonds tell the same story, at any given time, everywhere the ground track might go.

In terms of a previous abstraction: the checkered tablecloth representing the ocean is inelastic and does not rotate. All the squares remain the same size, oriented the same way.

-- - - --

It might help to try for the mindset of someone who's actually trying to make it to shelter in earnest, using their wits instead of their diesel.

The amount of fuel in the daytank is one of the things our intrepid sailors will have to assess against the favoured scenario; the brief here is to come up with the best preliminary options in terms of minimum fuel burn.

When they do take that further step based on our analysis they may consider the reliability of outcome, and that might tip them into choosing an option which our analysis would suggest is not optimal.

You are not being asked to evaluate such considerations, and that would need more info, but feel free to discuss the issues, if they interest you.

The calculations themselves were easy if I understood the situation correctly (it would have been easier in real life unless I had been woken from a very deep sleep and asked to perform calculations LOL). I suppose this is why the kettle needed to be put on for coffee before commencing looking at the data.

The hard bit for me was deciding what to do. I could do most of the planning apart from plotting the ground track fully.

The ground track plot at the end could at least be considered immediately, as could the current at the end. The current was adverse after the nominated arrival time at Island One. This is where a short burst of extra power may have been needed if things hadnt gone to plan.

I really didnt fancy spending five plus hours travelling to the second island just for the sake of saving an extra 1.6 units of fuel although after the nominated arrival time the current was not as adverse.

If I was close to land at that stage (we dont know where the islands actually are) the affect of the current may have been less in shallower water.

My actual desicion would depend on the size of the fuel tank and of course any hazards along the way (the islands themselves to start with) once I had plotted the ground track.

The forecast wind direction for the following day or two would be important info to gain as well, as one of the islands may have provided better shelter.

Very little info was given with the data, and being forced to make a decision without this is difficult to say the least . One thing I would have done is suggest the other person slow down from his present 6 knots (even though it meant more time in the shipping lane) and conserve fuel (10 whole units was to be used in the next hour travelling at 6 knots)

Two heads are better than one, so all this would have been discussed with the other person, even as it meant getting rugged up and joining him in the cockpit after computations had been made.

Someone needed to post first with a solution, so I have stuck my neck out LOL.
What would others have done?
It is an interesting dilemma.

I guess Andrew will give his solution once a few more people have chimed in with responses.

[Seaworthy Lass]

Originally Posted by LJH:
"As I have had no formal training, please correct me if this is assumption is wrong."

Originally Posted by Dockhead

That had me laughing. NONE of us has any formal training -- we are a bunch of amateurs stumbling together towards some kind of understanding. Blowing cobwebs off high school trig, in some cases (like mine).

Very much an amateur here as well and the cobwebs were thick with the passage of time LOL, altough I always did enjoy trigonometry.
Nice to throw around ideas and learn heaps in the process, starting with Andrew Troup's original thread

Originally Posted by Dockhead

The problems in all the methods arises if you have a half hour (say), so a large partial hour. SWL takes the average of the whole hour, but the first half hour will be different from the second half hour, so there's a built in error.

Yes, you have nailed it on the head. That is the main limitation with my method I think.

[HappySeagull]

http://www.cruisersforum.com/forums/...ml#post1140743
"As I've said before, I think there is a third, almost absolutely precise method lurking in our unformed thoughts, having to do with sine wave development of currents, and requiring even less information -- just limits and timing of limits of the streams."

Whether or not this is better than the "Piloting-Network-of-Wide-Canals-where-Waterflow-is-controlled" models I think I imagine here, is a moot point best left to Moots to figure out...

but Why not,

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

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


Example: MAX current is 6 knots and this happens 3.2 hours after slack.
What is current 1.3 hours after slack?

(sin((1.3/3.2)*90))*6 =3.574

so, current 1.3 hours after slack=3.574 knots
This can be plotted if you don't like calculators....
.
Minutes could be used instead of annoying decimal hours.
Creeping Errors beyond the ones in this simple concept..best used from max->slack or slack->max. Gets worse if it's slack->slack.......also, it's only applicable near the station you got MAX current from.Extrapolation up or down channel ,yada yada.
Might as well cast off into the stream with a calculation slightly "updrift" at "B" than spend another 15 minutes on exquisite accuracy?...include Windrift+/or Leeway if you must.

[Seaworthy Lass]

Originally Posted by HappySeagull

http://www.cruisersforum.com/forums/...ml#post1140743
"As I've said before, I think there is a third, almost absolutely precise method lurking in our unformed thoughts, having to do with sine wave development of currents, and requiring even less information -- just limits and timing of limits of the streams."

Whether or not this is better than the "Piloting-Network-of-Wide-Canals-where-Waterflow-is-controlled" models I think I imagine here, is a moot point best left to Moots to figure out...

but Why not,

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

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


Example: MAX current is 6 knots and this happens 3.2 hours after slack.
What is current 1.3 hours after slack?

(sin((1.3/3.2)*90))*6 =3.574

so, current 1.3 hours after slack=3.574 knots
This can be plotted if you don't like calculators....
.
Minutes could be used instead of annoying decimal hours.
Creeping Errors beyond the ones in this simple concept..best used from max->slack or slack->max. Gets worse if it's slack->slack.......also, it's only applicable near the station you got MAX current from.Extrapolation up or down channel ,yada yada.
Might as well cast off into the stream with a calculation slightly "updrift" at "B" than spend another 15 minutes on exquisite accuracy?...include Windrift+/or Leeway if you must.

Would have to think about that with a clearer head (it is past midnight here now). At first glance it looks reasonable.

The only problem is that few cruisers will do calculations like this. The beauty of the RYA method and mine is that they don't require calculators, although my method will speed up the calculation of the 'proportion of the last hour needed' if you have one.

[Andrew Troup]

Anyone who read this from an email:

Please re-read here, as I hit the "Post message" prematurely, and the post was not fit for release.

IT HAS BEEN EDITED

Originally Posted by Seaworthy Lass

....
The hard bit for me was deciding what to do. I could do most of the planning apart from plotting the ground track fully..

Shortlisting the options crudely by CTS and transit time, was all you strictly HAD to do.
Anything more is optional (but perhaps it didn't seem that way, to a hard-plotting Lassie with the bit between her teeth)

Originally Posted by Seaworthy Lass

....
The ground track plot at the end could at least be considered immediately, as could the current at the end. ..

Hmmm - I THINK I see where you're going with the first part of that: now that's something I hadn't realised we could do with the info given. Seaworthy, polish that damehood!..

Originally Posted by Seaworthy Lass

....
Very little info was given with the data, and being forced to make a decision without this is difficult to say the least .

Completely agree. This is why my formal question is limited to providing CTS and duration info, to enable shortlisting the alternatives, so that the guys in the scenario can start bringing in other factors (which we don't have) to enable them make their pick of islands and refine the plan.

They might well decide to 'break the rules' I have set: the point of those rules was to enable a first pass at a range of solutions

I brought in a crude fuel curve, just to add interest for those who still found the problem, as initially presented, a bit text-booky.

I'm not angling for a job with the RYA, or anything like that.

Originally Posted by Seaworthy Lass

....
One thing I would have done is suggest the other person slow down from his present 6 knots (even though it meant more time in the shipping lane) and conserve fuel (10 whole units was to be used in the next hour travelling at 6 knots)

I tried telling him that, too .... and he said actually it was more like half an hour or 5 units (read the scenario AGAIN, Lassie!) and went on to tell me last time he crossed this traffic separation zone sailing at four knots, even though at 90deg to the median strip, he got sternly rebuked by traffic control and had difficulty making his course changes sufficiently apparent to ships he was crossing.

Originally Posted by Seaworthy Lass

....
I guess Andrew will give his solution once a few more people have chimed in with responses.

My solution? You mean, I have to solve it too?

I was just going to pick my favourite bits out of other peoples' ....
surely there has to be some perk for the scenario setter ?

[LJH]

Originally Posted by Andrew Troup

For anyone who has got their head thoroughly around SwL's method, and preferably has some familiarity also with the RYA method:

- - - - -
What he wants to know ASAP is this: Can you provide the information he asks for with what he's left you with, or will you need his chart as well?

I have an additional question: would the RYA method or the SwL method be a better choice in this particular situation, and why?

Or do you have a better method than either of these, for the unusual and even intriguing situation this undoubtedly is?
　
- - - - - - - - - -
If this is too easy, and you want a small extra challenge:
which island minimises the fuel usage ?

Assuming you burn
2 units of diesel per hour at 2 knots
(2.8 units of diesel per hour at 3 knots)
4 units of diesel per hour at 4 knots
(7 units of diesel per hour at 5 knots)
(10 units of diesel per hour at 6 knots)
(15 units of diesel per hour at 7 knots: Black smoke pouring forth
Alright, we know already, our prop's pretty dirty, along with our bottom)

Dropping below 2 knots would not be considered prudent, to maintain a semblance of steerage way (eg to correct for current variations from "as predicted"), assist collision avoidance, and delay the onset of terminal "are we there yet" syndrome.

Given that we have full speed available (for emergency manoeuvres) and provided we are not trying to stem the tide thereby wasting precious fuel standing still, we feel 2 knots is perfickly OK.
In the words of a skipper I once navigated for, "It's not your boat, it's not your problem." <tongue poke>

If you want to get really fancy, you could check whether there's a better tabulated speed than either of the values already suggested.

Interesting problem.

Yes we have all the info we need for the SWL method. Since A is not on our chart we would not be able to use the RYA method unless “D” coincides with “B”. As the tide coming onto our chart is perpendicular to the line between the two islands we do not need any other data until we want to plot a ground track and check for hazards.

You will need 4 kts to reach reach island one with the minimum fuel burn of 12 units. However you will be encountering an adverse 4 kt tide when you get there.

I would head down tide to Island 2. Not knowing the capacity of our day tank, I would probably make a compromise between fuel and time and travel at 3 kts. This would be 2.2 hrs quicker than 2 Kts and would only burn .8 units more fuel. Increasing the speed to 4 Kts will save an additional 45 minutes, but will cost 3.8 more units of fuel!

Well thought out problem Andrew.

[Seaworthy Lass]

Originally Posted by LJH

Interesting problem.
Yes we have all the info we need for the SWL method. Since A is not on our chart we would not be able to use the RYA method unless “D” coincides with “B”. As the tide coming onto our chart is perpendicular to the line between the two islands we do not need any other data until we want to plot a ground track and check for hazards.

Yes , I had it pointed out to me after I posted that the starting point was not on the chart (I had misread Andrew's lengthy tale and rushed in and marked it as (3,4). Reading back on what Andrew had written, he stressed that this point was on the chart that was up in the cockpit, so all my RYA calculations need to be ignored. The SWL computations all still apply .

Originally Posted by LJH

You will need 4 kts to reach reach island one with the minimum fuel burn of 12 units. However you will be encountering an adverse 4 kt tide when you get there.

Yes, the adverse tide bothered me. My suggestion was to speed up a bit at the end if things were not going to plan, so you would not encounter this. While crossing the shipping lane you were burning heaps of fuel and you could have slowed down a bit then and therefore had this extra fuel in reserve.

Originally Posted by LJH

I would head down tide to Island 2. Not knowing the capacity of our day tank, I would probably make a compromise between fuel and time and travel at 3 kts. This would be 2.2 hrs quicker than 2 Kts and would only burn .8 units more fuel. Increasing the speed to 4 Kts will save an additional 45 minutes, but will cost 3.8 more units of fuel!

I get 4.35 hrs of travel time at 3 knots burning 12.2 units of fuel, so to me Island One still looks more appealing with 3 hrs taken at 4 knots burning 12 units (over an hour and a half quicker and slightly lees fuel used).

[Seaworthy Lass]

Just looking at things more carefully now that I know the start point is not (3,4).

I suspect the start point is about 10nm from the anchorage/mooring at Island One, and probably Island One destination is at a bearing of about 60 degrees from the start point and the shipping lane runs roughly north south.

How close am I Andrew?

[jackdale]

Originally Posted by Seaworthy Lass

Example 5

CURRENT
1st hour: 3.6 knots 270 T (pushing you west)
2nd hour: 1.6 knots 270 T
3rd hour: slack

Boat speed 4 knots
Destination is 8.4 nm due south

All perfectly reasonable realistic figures

Catching up after my tournament.

Most currents are sinusoidal. The drop in 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.

[jackdale]

Originally Posted by Paul Elliott

Trust me, Expedition knows all about tacking on headwinds! Expedition also knows all about the (forecast) winds ahead. The forecasts don't handle the conditions we see in nearshore sailing, where the headlands create those interesting eddies, funnels, and dead-zones, but at sea they usually get the big patterns right. The navigator and crew are always fine-tuning for actual conditions, but Expedition (and other programs) can do a good job of working out your strategy through the next 3-5 days. You will re-run the simulation whenever you get data from a new wind forecast model run, or you deliberately diverge from the suggested route.

Of course, there are times when I decide that I know more than expedition, and adjust a route to minimize the risk when a forecast might miss a shift in the highs / lows. Or, when I just say "Screw it! I want to sail in *that* direction!". Then again, I'm not a rock-star pro racer, I just like to sail.

Hi Paul

That is a good explanation of Expedition. I have used it in Swiftsure and a Maui Vancouver return.

[jackdale]

Originally Posted by Seaworthy Lass

Has anyone played with Expedition? Does it tell you what tack to be on with variable cross current when the wind is directly on the nose?

I have expedition. It uses

1) grib files
2) current data
3 and boat polars

to show optimal routes. One of the beta testers for the software sent me a add-on to show the percentage of optimal VMG. As the tactician I got to either admonish or praise my crew.

[Seaworthy Lass]

Originally Posted by jackdale

Catching up after my tournament.

Most currents are sinusoidal. The drop in 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.

Hi Jackdale
Hope you had a good weekend!

I haven't given any tidal data before the start of the voyage, so how do you know the drop is less in the first hour than the second?

I have been in the Med full time for over five years so apart from one passage through Messina Strait (I was on watch at the tine and had an exhilerating down current ride on a broad reach, great fun until I needed to gybe towards Ithaka LOL), so I have no charts with tidal data. The gybe took me around 10 minutes to complete .

Anyway, don't worry about my example, try Andrew's. It is great fun and gets the grey matter working. It is apparently based on a few real life events he has experienced.

Which island would you choose and why?

[Dockhead]

Originally Posted by HappySeagull

http://www.cruisersforum.com/forums/...ml#post1140743
"As I've said before, I think there is a third, almost absolutely precise method lurking in our unformed thoughts, having to do with sine wave development of currents, and requiring even less information -- just limits and timing of limits of the streams."

Whether or not this is better than the "Piloting-Network-of-Wide-Canals-where-Waterflow-is-controlled" models I think I imagine here, is a moot point best left to Moots to figure out...

but Why not,

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

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


Example: MAX current is 6 knots and this happens 3.2 hours after slack.
What is current 1.3 hours after slack?

(sin((1.3/3.2)*90))*6 =3.574

so, current 1.3 hours after slack=3.574 knots
This can be plotted if you don't like calculators....
.
Minutes could be used instead of annoying decimal hours.
Creeping Errors beyond the ones in this simple concept..best used from max->slack or slack->max. Gets worse if it's slack->slack.......also, it's only applicable near the station you got MAX current from.Extrapolation up or down channel ,yada yada.
Might as well cast off into the stream with a calculation slightly "updrift" at "B" than spend another 15 minutes on exquisite accuracy?...include Windrift+/or Leeway if you must.

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 I'll have in a few days, or not. We'll see. I bet the stream data is derived, in fact, from limits and timing.

But in any case we can work it out pretty well from the 5 minute resolution grid.

The problem with one hour streams data is the partial hours. This data don't tell us when the the tide turns, so we have little idea what happens in parts. We increased precision in one problem by assuming that the stream at the moment between the hours is the average of the two hours, this still has a very large error - what if the peak rate occurs close to the border between the hours?

I think more information is contained in limits and timing, than in hourly averages. 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.

We have been accused by many of counting angels on the head of a pin - chasing unusable precision. I am guessing that SWL method is already within the limit of usable precision for a sailboat on a multi-hour passage, so we probably don't need this third method. But I am interested to see whether it works, so will play with it. It won' be plottable, so it's main value, if it has any, would be for an app.

[Seaworthy Lass]

Morning Dockhead
You are up even earlier that I am .
Don't you need much sleep as you get older? If not, I have something to look forward too LOL. I can add it to my list of advantages .

Which island would you choose in Andrew's example and why?
With your experience sailing in tidal waters your input would be really interesting. Plotting takes well less than 15 minutes. Probably only five min if you are set up.

Thinking about the solution takes longer, but it is an interesting and worthwhile exercise.

[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.