Morgana
regular
Reged: 28/08/2003
Posts: 12177
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
regular
Reged: 24/06/2004
Posts: 417
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: 663
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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: 2698
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
regular
Reged: 20/07/2001
Posts: 1142
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: 1856
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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: 610
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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: 1142
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: 2698
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: 111
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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