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Old 19-02-2013, 10:16   #766
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Re: Inaccurate RYA Teaching : CTS - Quest For a New Method

Quote:
Originally Posted by goboatingnow View Post
Rather then people doing huge quantities of CTS compution using "stylised" tide data from exercise books, People need to plot sceanarios from actual tide data extracted from say tidal diamonds.

......

Dave
well,then, your turn.You can put this argument to rest,finally, I hope. Offer what would satisfy you...
with a start->finish and a boat speed too.

...with one condition. You present it as: "Averaged current for the period"or "Drift in miles due to current for the period " a period being 1hour, half hour, even 15 minutes-and, please, with a short explanation as to how you arrived at the averages, Include the current data in knots on the hours or half-hours if you used it to arrive at these. Or, whatever, in way of data if you used .

(suggest )Decimal format for hours is easier. 1,2,3,4 or 1.25, 1.5, 1.75 etc. You needn't re-interpret it as a clock. It saves a step.
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Old 19-02-2013, 10:17   #767
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Re: Inaccurate RYA Teaching : CTS - Quest For a New Method

Happy ( big white bird)

Sorry no hurt to wild birds intended. I dont see what you are modelling , I see two currents and a CTS, but I dont follow what the ground track is really representing , ( i could just be dense today , it happens)

dave
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Old 19-02-2013, 10:20   #768
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Re: Inaccurate RYA Teaching : CTS - Quest For a New Method

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Apparently the RYA picks the nearest whole hour, but there is nothing preventing you from calculating with fractions of hours, too. There are no errors induced in this method - it gives you a precise result for the information you put in.

You have an issue with extrapolation from that result - and you are correct that if the tidal set varies widely in that last partial hour, an extrapolation won't give you a good result.
This is the kernel of the argument, but the answer cannot be demonstrated by SWL repeatedly using her own method and data assumptions to demonstrate her own answers. you have to model the results using some additional known good data , thats available at a significantly greater frequency than the proposed methods

dave
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Old 19-02-2013, 11:08   #769
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Re: Inaccurate RYA Teaching : CTS - Quest For a New Method

Hi lads
Back online here after a long, but satisfying day. A 30kg box of new halyards arrived in the local village today (from the UK) and is now safely stored on board. A 20 km trip to "the big smoke" saw both our big Greek gas bottles exchanged. The simplest of tasks seems to take ten times longer when a dinghy is the only form of regular transport .

I will try and answer all the recent posts tonight (the glass of wine at my elbow will slow me down), but let me first dig up one of the examples for Lodesman and Happy where looking at the proportion of D1B/D1B+BD2 doesn't work .
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Old 19-02-2013, 11:10   #770
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Re: Inaccurate RYA Teaching : CTS - Quest For a New Method

Quote:
Originally Posted by goboatingnow View Post
Happy ( big white bird)

Sorry no hurt to wild birds intended. I dont see what you are modelling , I see two currents and a CTS, but I dont follow what the ground track is really representing , ( i could just be dense today , it happens)

dave
the rowboat sketch is just a rowboat crossing ...say a river. Imagine it's heavy mist at water level No range marks available, but every "period" the oarsman twists his head and sees a church spire above the mist on the opposite shore.

the second one, is harder. I reused and cropped the earlier. it. It COMPARES a single CTS with say, an autopilot that can steer to a gps location.Better, as a titlewould have been a helmsman similar to the rower....constantly correcting for obvious drift but with no wit,time, or means to look it up.

The interesting thing might seem to be how little it matters. But several methods are here described at great length to avoid this very thing.
Mostly,the value in these methods, in my not-at-all humble* opinion, is so that sailors will take the current into account, and NOT haplessly arrive in danger, as you said.

*there are no humble seagulls
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Old 19-02-2013, 11:36   #771
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Re: Inaccurate RYA Teaching : CTS - Quest For a New Method

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Here, there are many currents. An hour will see you through several. I mean to say, a complicated vector-analysed "CTS" needn't be hours long....

PS: if anyone lacks for inspiration for a model, this book does it.

One page for every hour, Neaps,Springs included as serial pages.
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Old 19-02-2013, 12:40   #772
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Re: Inaccurate RYA Teaching : CTS - Quest For a New Method

Quote:
Originally Posted by Lodesman View Post
When I first considered how to resolve the multi-tide single CTS problem, I had considered constructing ACıDı and AC²D² and comparing DıB to BD², but rejected it for what I thought was the more intuitive method, as it was a refinement of what was presented as the RYA method. They are really just two ways of skinning the same cat.
I can't visualize where it would go wrong - can you point me to a scenario where this method will not work?
Quote:
Originally Posted by HappySeagull View Post
me too, please. It seems dandy to me. I've been trying it on various scenarios seems ok.
Wine and a comfy settee and a good movie have slowed me down tonight, but here is an example for you to try:

B is 3nm from A, bearing 30 degrees true
Boat speed is 2 knots, motoring in calm conditions
Current:
Average for first hour: 3.0 knots 77T
Average for second hour: 1.4 knots 77T

What is the CTS and time taken?
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Old 19-02-2013, 14:50   #773
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Re: Inaccurate RYA Teaching : CTS - Quest For a New Method

Quote:
Originally Posted by Seaworthy Lass View Post
Wine and a comfy settee and a good movie have slowed me down tonight, but here is an example for you to try:

B is 3nm from A, bearing 30 degrees true
Boat speed is 2 knots, motoring in calm conditions
Current:
Average for first hour: 3.0 knots 77T
Average for second hour: 1.4 knots 77T

What is the CTS and time taken?
I will be blunt. This scenario is ludicrous. Anyone who actually ventured out in such a situation is demonstrating incredibly poor seamanship.

SWL - I indicated to you earlier in a PM that this exercise had some value. I am beginning to get ready to retract that statement.

Some boats I use do not even idle at that speed.

Can we please get realistic scenarios, please?
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Old 19-02-2013, 14:54   #774
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Re: Inaccurate RYA Teaching : CTS - Quest For a New Method

Quote:
Originally Posted by HappySeagull View Post
Attachment 55234

Here, there are many currents. An hour will see you through several. I mean to say, a complicated vector-analysed "CTS" needn't be hours long....

PS: if anyone lacks for inspiration for a model, this book does it.

One page for every hour, Neaps,Springs included as serial pages.
The essential publication for the Gulf and San Juan Islands.

I am still bemused by the current in Navy Channel. For those who do not know the area Navy Channel is NW of Saturna Island. On a flood, it runs the "wrong" direction.
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Old 19-02-2013, 15:41   #775
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Re: Inaccurate RYA Teaching : CTS - Quest For a New Method

Quote:
Originally Posted by Seaworthy Lass View Post
Wine and a comfy settee and a good movie have slowed me down tonight, but here is an example for you to try:

B is 3nm from A, bearing 30 degrees true
Boat speed is 2 knots, motoring in calm conditions
Current:
Average for first hour: 3.0 knots 77T
Average for second hour: 1.4 knots 77T

What is the CTS and time taken?
Think I'd spend another hour reading my book and then set off steering roughly north, and save a few liters of diesel, save enough diesel and I could buy an engine which would make better than 2 kts.
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Old 19-02-2013, 16:03   #776
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Re: Inaccurate RYA Teaching : CTS - Quest For a New Method

...hold off on critique please,guys. This is Seaworthy's answer to our (lodesman+Happy) request....

Until then, consider it a trolling(-as-in-fishing) trip....
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Old 19-02-2013, 16:23   #777
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Re: Inaccurate RYA Teaching : CTS - Quest For a New Method

OK

I did the numbers right away

Nigel is correct - wait one hour. Steer 004 ETA 1 hour SMG 3.0 knots

Or leave immediately steer 317 ETA 2 hours SMG 1.5 knots

Do you want the plot?
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Old 19-02-2013, 17:00   #778
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Re: Inaccurate RYA Teaching : CTS - Quest For a New Method

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OK

I did the numbers right away

Nigel is correct - wait one hour. Steer 004 ETA 1 hour SMG 3.0 knots

Do you want the plot?
That is what I meant to post. Ignore the other.
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Old 19-02-2013, 17:08   #779
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Re: Inaccurate RYA Teaching : CTS - Quest For a New Method

sure- wait an hour and catch no fish!

given A-B 3 miles bears 30T
boatspeed 2 knots
drift hour 1 =3 miles@77T, hour 2= 1.4 miles@77T



Boat CTS= 316-317.
Boat sails about 2.5 miles 1 hour 15 minutes



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even given overpowering current,
lodesman method does work if you assume closest approach which is 90 degrees per A-.B line to 1 hour current....
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Old 19-02-2013, 17:33   #780
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Re: Inaccurate RYA Teaching : CTS - Quest For a New Method

SWL ...CTS 316 and 2.66 miles
2.66 versus 2.5 I found with lodesman method is not a point of argument, obviously.
Red line is "track" by the way, while the zig-zag dotted lines are sketched vectors ...boat steering CTS+current=track.
The other stuff (green) is the work sheet. You can't see it all

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