dukes4monny
regular
Reged: 04/02/2008
Posts: 134
Loc: Devon, England
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Having recently completed my RYA YM / CS theory which included tidal stream calcs with computation of rates for periods between springs and neaps.........I've just realised that my recently acquired Reeds Channel Almanac does not (seem to) include a computation of rates chart.
Have I been short changed by REEDS or are the RYA teaching things that don't get used in the real world?
I could of course be being a complete numpty (highly likely) and just haven't found the right page yet 
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I came into this world with nothing..........so far I've managed to hang on to most of it
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Danny
regular
Reged: 23/10/2003
Posts: 666
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If you have any of the Admiralty tidal atlases have a look inside the front cover. Failing that get some linear graph paper or small squared paper and make up your own. It's fairly straight forward.
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Danny
TidePlan English Channel tidal passage planning software
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Searush
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Reged: 14/10/2006
Posts: 3956
Loc: k up if caught.
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I don't know/ use the publication, but tidal rates are fairly approximate anyway. I have just been having a discussion about practical navigation on another forum. What the RYA teach in a classroom is NOT wrong, but may be impractical on a small boat or a fast MoBo.
How long does it take to do an "accurate" RYA style DR plot? How much accuracy is necessary/relevant? How easy is it to use ruler, calculator, dividers, almanac and chart table?
When I got my first sailing cruiser I was given a lesson in practical nav that I found immensely useful. You do not need tidal streams to a tenth of a knot- they won't be that accurate anyway. Estimate to nearest 1/4 knot & assess impact on course, it may be negligible. You have Neap & spring rates on your chart. First use tide table to interpolate between spring & neap, then use 1/12ths rule to estimate rate at given times.
For quick small boat nav without a plotter, have a paper chart folded on your knee, align it to the coast (I'm assuming coastal pilotage) point your flat hand, held thumb upwards, at a known landmark then lay that angle off on the chart by moving your hand (keeping it parallel to the original bearing) so it is over the landmark. Note that line on the chart mentally, then repeat with a second landmark. That will give you a workable position in seconds with no tools or paraphenalia at all. You can do this without leaving the tiller & in any weather or sea conditions.
With an approx position, bearing in mind an egg cup sized area of uncertainty & any potential hazards, you can lay off a course in a similar manner, either using the chart compass or another landmark. These techniques are ideal when shorthanded, needing a sanity check on formal or electronic techniques or if you need an instant solution to a problem.
Don't forget it is difficult to hold a course within 5 degrees anyway, so even 3 degrees accuracy is generally pointless. The point I am trying to make is that you need to understand when close enough is good enough. Thes quick & dirty techniques allow you to check progress quickly, easily & often without spending loads of time below at a chart table making calculations and missing the sailing!
I've written another bloomin' essay again, DOH! Sorry about that! 
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dukes4monny
regular
Reged: 04/02/2008
Posts: 134
Loc: Devon, England
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Hehe.......thanks Searush. I guess that the RYA are taking things to the extreme with the classroom courses, I suppose this makes you stop and think about what is actually going on around you.......I think that we were all wondering how all of this theory works in practice........especially when bouncing around in a short chop with a chart table that is a third of the size of the table we used in the classroom.............
I just love these 'rule of thumb' methods like the one you describe about taking rough bearings, I'm collecting as many of these as I can for use in the 'real world'.
I'm off to Gib next week to take my DS Practical.......I hope this will open my eye's a bit to the realities of navigation / pilotage.
As I said at the end of my theory course "I can now navigate a desk anywhere in the world".........lets hope I can safely navigate a boat around Gib next week.
Thanks again for the reality check 
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I came into this world with nothing..........so far I've managed to hang on to most of it
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VicS
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Reged: 13/07/2002
Posts: 9249
Loc: Home: Kent. Boat: Chichester
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I don't think I have ever used that diagram.
It is only a method of interpolating between, or extrapolating from, the spring and neap rates that correspond to ranges at Dover of 5.9m and 3.3 m.
In the real world most of us would guesstimate but if you really want precise figures (note I said precise not accurate) I would suggest you use the Tidal Atlases by Michael Reeve-Fowkes for both rates and heights and relegate the classroom methods to the mental archives.
Danny is correct in saying that the diagram you are after is in the Admiralty Tidal atlases.
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Old Chemists never die, they just fail to react
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Damo
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Reged: 22/02/2005
Posts: 2626
Loc: k keeper,Portishead
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Same thing with tidal height curves - I used to do it religiously for anchorages, crossing bars etc, and felt very pleased to know the depth to a decimal. Trouble is it is nearly always "wrong". For example tonight at 2100 the depth outside the lock was predicted to be 6.0m, falling. It was actually 4.5m, so a yacht may not have made it in time for the last lock (1.5m max on the cill)
Now I tend to use twelfths and leave a good margin, even though the Avonmouth curve is quite asymmetric. The streams here are also high, and directions and rates vary a lot over short distances, so it is pointless trying to do accurate calcs IMO. I work on a rough calc for spring or neap rates for passage planning, and directions to compass points.
On saying that, I think it is good to know the theory and be able to "do it by the book", but you need to know when it is appropriate to use - and that is something that learning by rote won't help with.
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Never be at a loose end with the Yosemite bowline 
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SAWDOC
regular
Reged: 24/02/2008
Posts: 148
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Quote:
For quick small boat nav without a plotter, have a paper chart folded on your knee, align it to the coast (I'm assuming coastal pilotage) point your flat hand, held thumb upwards, at a known landmark then lay that angle off on the chart by moving your hand (keeping it parallel to the original bearing) so it is over the landmark. Note that line on the chart mentally, then repeat with a second landmark. That will give you a workable position in seconds with no tools or paraphenalia at all. You can do this without leaving the tiller & in any weather or sea conditions.
brilliant !! 
definitely going to try that out over the weekend. If it works I'll chuck the laptop!
Any more rule of thumbs of a similar nature?
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roly_voya
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Reged: 05/02/2004
Posts: 1049
Loc: Pembrokeshire Wales
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The shorebased courses equipe you with a basic set of skills. These will enable you to navigate anywhere but not necessarily in the most efficient of effective manor. Where you go from here depends on the sort of sailing you do and your interest but you should never stop learning. I reckon the diference between agood sailor and a bad one is that the go one has 20 years of experience and the bad one has the same experience for 20 years. I other words experience is only useful if you learn from it.
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CBT
regular
Reged: 16/02/2005
Posts: 260
Loc: UK
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The point of the computation table is that it gives a more accurate tidal vector on the day than can a cursory interpolation between the rates published. The latter are mean rates and do not show the actuals occuring say at the time of the equinoxes when the drift of the tidal stream might well be significantly greater than the mean. Of course this is only relevant in certain circumstances e.g. if you were navigating in a sea area where the rates are strong and you are in the equinoctial season. Much of the time you can safely interpolate 'by eye'. But it is right to know that sometimes it is not. Also worth remembering that this is not 'RYA' data; but Admiralty with a deep rooting in maritime navigational techniques.
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dukes4monny
regular
Reged: 04/02/2008
Posts: 134
Loc: Devon, England
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Thanks for all replies.........I use several online fora, and this one is my favourite. There's always many people willing to share advice and experiences, very refreshing.
My ultimate goal is to 'live the dream' of retiring aboard and cruise the oceans, and I'm enjoying the process of gradually 'learning the ropes'.
I spend a lot of my time tutoring people in my own specialty which is CNC engineering, and I start with the theory, and then we put it into practice. The theory and practice are in essence the same.
I don't spend hours trying to cram as much information into my students brains that they ultimately will never use, this actually impedes the learning process.
It seems to me that a lot of the RYA theory course is taught to an accuracy that is in the real world unachievable.
This is probably not a surprise to those of you that are already putting the theory into practice, but it is making me wonder whether all of the hours which was spent learning these accurate calculations was time well spent?
I'm sure the fog will start lifting next week when I do my (RYA) DS practical course..........
Thanks again for all your helpful replies.
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I came into this world with nothing..........so far I've managed to hang on to most of it
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Morgana
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Reged: 28/08/2003
Posts: 12434
Loc: East Coast
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It was definitely time well spent...
Over time you'll learn when to approximate, and when to be more accurate.... but knowing how to be accurate is a good start!
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Bored?.... why not read my blog .... its the developing story of the trials and tribulations of boat ownership!
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AliM
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Reged: 24/06/2004
Posts: 435
Loc: UK, Herts/Essex
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Agreed.
Having seen how meterological conditions play havoc with my careful calculations of tidal heights, I seldom bother to calculate to the nearest 5 cm any more, but I think a few years of experience after passing the YM theory have shown me when it's safe to approximate.
I guess that is tempting fate - wave cheerfully next time you pass us sitting uncomfortably on the mud!
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Danny
regular
Reged: 23/10/2003
Posts: 666
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Hey, c'mon everone! The marinas and anchorages are getting crowded these days. The last thing we want to do is show that boating is EASY!
Follow the RYA's lead and make everything as computational and obscure as possible. Be prepared to demonstrate that, in the event of thick fog with no radar and the GPS broke, you need to be able to compute exactly where you are using nothing but your last fix, the log, chronometer and your course steered and, of course, the computed tidal vectors.
Yeh! I know: rather than the GPS being broke it's far more likely that the log's jammed with weed, the compass hasn't been checked for years and the last fix written in the log was when we left the mooring. Never mind - at least let's pretend it's all scientific and accurate and not a load of approximations and guesswork.
Just take a lesson from the medical profession, for example...
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Danny
TidePlan English Channel tidal passage planning software
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Sailfree
regular
Reged: 18/01/2003
Posts: 2820
Loc: Solent
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Reading your posts you remind me of how I was 10yrs ago and we did our DS in Gib.
You do need to worry a bit more about navigating around Gib as with good weather you should manage an amounyt of Spain and N Africa possibly Ceuta.
Enjoy your DS - you seem to have the right attitude to do well.
Where will I find this computation graph for tidal streams as I have obviously forgot it and never used it!! On a X channel I look at the spring and neap currents, look at the tidal range for both and guestimate what it will be for the tidal range of the day I am sailing. Bigger the tidal range the more water has to rush in and out or am I being to simplistic?
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Roberto
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Reged: 20/07/2001
Posts: 1162
Loc: miquélic - Lorient
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Should you want to make your own you may base yourself upon this graph
it is based on the French system of "coéfficients" (a number proportional to tidal amplitude, 45 being neaps and 95 being springs) but you may disregard this, just replace "VE" with springs and "ME" with neaps.
In this example the tidal stream arrow read 3518, mark 3.5 knots along the VE line, mark 1.8kt along the ME line, connect the two.
Calculate the ratio between your relevant amplitude and the Spring/neaps ones, "coefficients" make this easier, in the example it was 72, then read the tidal stream speed.
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wotayottie
regular
Reged: 01/07/2007
Posts: 1892
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The answer is to do the simple mental arithmetic involved in approximating the rate to the nearest half knot or whatever. You dont want to be messing around with bits of paper in the middle of a wet and windy night.
As for the RYA teaching you a particular method - they dont. Your teacher might have been daft enough to do so but I hope not. The RYA advise you to find a method that works for you. Find your own route to salvation - use your noddle.
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lw395
regular
Reged: 16/05/2007
Posts: 737
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The graph is a nice way of doing it, but logically the line you plot should always extrapolate to zero tidal stream for zero coefficient. Therefore the sum reduces to rate=rate at springs x coeff /.95, gives 2.65 in your example.
Just remember that the coeffs are only predictions so will be wrong due to weather effects.
I would have just said that it was halfway between neaps and springs!
Also there is no guarantee that the direction will be quite the same in neaps and springs.
There can be a bit of a reality gap with the exam, but you have to show you understand the data as it is presented in the almaniac, and you're able to process it effectively.
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Roberto
regular
Reged: 20/07/2001
Posts: 1162
Loc: miquélic - Lorient
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Quote:
The graph is a nice way of doing it, but logically the line you plot should always extrapolate to zero tidal stream for zero coefficient.
I am not sure a zero coefficient exists, at least the SHOM does not contemplate it... saying that the highest and lowest coefficients are 120 and 20 means that the highest amplitude can be 6 times higher than the lowest amplitude.
I don't know if this is the case but it seems to be the assumption on shom tidal calculations.
Maybe it is derived from their harmonic model ?
Agree absolutely that it is much of a theoretical matter 
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Sailfree
regular
Reged: 18/01/2003
Posts: 2820
Loc: Solent
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Looking at it am I right in believing its a linear relationship so if tidal range is say 10m at springs (Current 4kts)and 5m at Neaps (current 2kts) if the range on day required is 7.5m then current by proportion is 3kts on that day.
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sticky
regular
Reged: 10/01/2004
Posts: 112
Loc: Cheltenham Glos
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The point of all tidal data being approximations has been well made, and this is even before you have taken into account what the barometer is reading. High pressure will reduce your tidal heights as well as lessen the tidal rate. If you cannot go to the chore of drawing up your own graph, if you have not Reeve- F excellent publications, try using these formulae to get a rate of tide.
INTERPOLATION OF TIDAL CURRENTS
When range of tide for the day is closer to mean neaps range:
Range of tide for the day x Neaps tidal rate divided by
Mean Neaps Range
When range of tide for the day is closer to mean springs range:
Range of tide for the day x springs tidal rate divided by
Mean springs Range
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Danny
regular
Reged: 23/10/2003
Posts: 666
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Sticky's formula is nice and simple and a good approximation in most cases. For anyone who doesn't like pencil and paper and can't be doing with mental arithmetic then try this. If it doesn't work or you want to install the whole thing then download this.
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Danny
TidePlan English Channel tidal passage planning software
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