Tidal diamonds

mcanderson

mcanderson

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Doing my Coastal Skipper and I have a question about tidal diamonds and tidal flow charts. If it is not either neaps or springs do you interpolate the speed or just use either the spring or neap speed depending on which phase the tide is closest to?

Could wait until Monday and call, but awful weather and doing some study.
 
You interpolate using the computation of rates chart in your practice almanac, or just use ratios.

The speed at springs may even need an extrapolation beyong mean spring rate.
 
I am not sure what the answer will be for RYA exam, but in reality it is better to interpolate if there is a significant difference.
 
Yes, you interpolate the speed.

For dayskipper, most of your exercises will be near enough springs or neaps, so should be no need for interpolation. Otherwise just guess the difference.

At yachtmaster level you have to work it out properly using a "computation of rates" table.

Good luck with the course
 
It ought to be taught by Shorebased Course instructors, but often isn't, that 'an appropriate degree' of interpolation of Stream Rates/Direction is desirable, and that over-doing it can be counter-productive. The RYA/Reeds/Training Almanac graphic method is often unnecessary, and I used to teach aspirant Dazed Kippers that a mental 'rule of thumb' was often sufficient - 'On Spring Rate, Halfway Between, On Neap Rate'.

It was then pointed out, with valid examples, when it may be important to interpolate/extrapolate more accurately. Some of the limitations of tidal prediction were detailed, such as meteorological influences, and that it could be important to refine one's 'set and drift or 'height of tide' calculations as much as reasonably possible, for e.g. timing of grounding/refloating/crossing a sill. An arithmetic, a linear/graphical and a sinusoidal/graphical example for the same place and time should highlight the context in which a refined prediction is worth the effort.

The 'Yotmeister' course hopes to take such stuff somewhat further. Some students fail to see the point, but again some real-life examples that a Practical Yotmeister may be expected to resolve helps to put the issue in context - such as getting a 1.9m draught Moody all the way up the Exe to the boatyard above Topsham, where a strong counter-current or early change helps you cheat a foul tide, or how high up/low down on the ladder to tie the dinghy painter to avoid either getting your shirtsleeve wet or having to abseil down into your tender after the pub shuts.....

Some of the practical tips and tricks also need a mention, for the habit of 'observing' is best acquired early. It should be pointed out to studes that the tidal trails from sea marks, poles, withies, pot markers, etc. are absolute indicators of local tide stream and direction, and that one should habitually 'scope' these for situational awareness - especially around the time of change, and when hunting for counter-current eddies. It is also valuable, at times, to lift one's eyes from the 'spot' Tidal Diamond info to appreciate what's going on elsewhere in the vicinity - for example, at the time of change, tide streams at opposite ends of the 'Fillettes' shoal in the Goulet de Brest may be running at close to two knots, in completely opposite directions. Similar contrarian behaviour occurs among the tricky sandbanks on the north side of the Bristol Channel - and in the centre of a couple of 'em, the stream sets right across the shoal. Experienced Solent sailors will be aware that, at certain states of the ebb, the stream in the outer Needles Channel suddenly switches from SW-going to strongly W-going, right onto The Shingles bank. That comes as a surprise to quite a lot of wannabe Fastnet racers, finding they cannot lay a mark of the course - the Needles Fairway buoy - and need to start engines to avoid going aground on a falling tide..... /forums/images/graemlins/wink.gif
 
[ QUOTE ]
but in reality it is better to interpolate if there is a significant difference.


[/ QUOTE ] In reality forget all about tidal diamonds and use a tidal stream atlas and the "calculator" in it. Those by Michael Reeve-Fowkes for the areas they cover are the most useful.
 
[ QUOTE ]
I am not sure what the answer will be for the RYA exam . .quote]

Just a mo! Isn't this the guy who 'accidentally' let us know he passed his RYA Yachtmaster ten days ago???

Mark, you can't fool all the people all of the time.
 
not really, just one who has navigated safely over 100s of 1000s of nm and is happy with his own method, but not wanting to undermine the official RYA method for someone doing the course.
 
[ QUOTE ]
Ah, a 'rusty' rather then a 'safe' skipper then . . .

[/ QUOTE ]

Have you any idea how pompous that looks now you have posted it? /forums/images/graemlins/laugh.gif

As posted elsewhere RYA theory is great for the classroom or where you have a well manned boat with someone allocated solely to the "navigator" task.

When single-handed & doing everything yourself all at the same time, then "Safe" takes on a different emphasis. Accurate enough, with minimum effort means you get a quick and simple result that can be easily verified by observations.

I know the approx tidal currents in my home waters and use my actual track to confirm/ modify the course as necessary. You do not normally need better than 1/4kt accuracy as directions and rates vary from predictions anyway.

Excessive effort after "accuracy" can be counter-productive and lead to confusion & errors. Do not over-work your calcs when out on a passage. /forums/images/graemlins/smirk.gif
 
Bit of fred drift but I was explaning rule of 1/12s to a budding dayskipper over weekend and when i looked at the tidal curves in Reeds found most to be a small curve for 1st and last 1/2hr and the bit in between completely linear.

I have never questioned what I was taught but found this discrepancy slightly worrying.

Is the rule of 1/12s relevant only for certain areas or an I missing something?
 
Totally agree. Excessive theory is soon written off by a bit of a blow causing a different surface level current, heights altered by a change in atmospheric pressure etc.

Far better to allow your 'safety margin' to cover all these inaccuracies.

Look at the chart - when was is last SURVEYED - not updated. Several areas are still back in the 1800's so how accurate is the starting data anyway!
 
In YM chartwork one is expected to work to totally spurious levels of 'accuracy'. By taking figures from the 'correct' tidal diamond one can work out an EP to the nearest yard that agrees with the official answer. In reality that is a load of nonsense. The diamond gives rates some distance from where you are actually plotting and they may bear little resemblance to reality at your position.

An example of where slavish adherence to diamonds is found in some chart plotters where they can show tidal currents flowing across a river because the data is taken from a diamond some distance away.

Incidentally I came across a very good way of estimating set&drift for a YM blind pilotage test: get the helm to stop alongside a buoy and stem the tide, i.e. remain stationary. Read off the heading and log speed and that is a reciprocal of the true tide.
 
[ QUOTE ]
Bit of fred drift but I was explaning rule of 1/12s to a budding dayskipper over weekend and when i looked at the tidal curves in Reeds found most to be a small curve for 1st and last 1/2hr and the bit in between completely linear.

I have never questioned what I was taught but found this discrepancy slightly worrying.

Is the rule of 1/12s relevant only for certain areas or an I missing something?

[/ QUOTE ]

It's an APPROXIMATION for a PREDICTION, which is affected by the weather. How much "accuracy" do you expect?

The rule of 12ths works very well for those of us who learned to count before decimalisation. That was it's great benefit - it came naturally! A 12th of 30' is 30" or 2'6". What could be easier & quicker? /forums/images/graemlins/laugh.gif
 
[ QUOTE ]
A 12th of 30' is 30" or 2'6".

[/ QUOTE ]

.....Which is, er, about, ummm, maybe, around 850mm. Now what's that on my shiny, new still-unwrapped metric chart.....? /forums/images/graemlins/grin.gif

Blugger it, halfway up to me bum, not allowing for the mud! So maybe I can still push her off..... /forums/images/graemlins/grin.gif
 
Hmm, they must have changed things since I di my theory courses many years ago. Then we were shown how to do the calculations, properly, and then the things that could upset the theory were explained, so we understood why real life was different to theory.

As for expecting pin point accuracy in exams, did any ever see the marking overlay, each plotted position had a decent circle of error round it and you got the marks for being inside the circle.

After all one of the fundamentals of all navigation is understanding what the errors in your plotting/computaion may be. In reality your plotting/calculation is worthless without an understanding of the errors and inaccuracies that may exist.
 
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