Yachtmaster Theory - Have I been wrong all this time?

[Twister_Ken]

Don't get the tidal flow thing at all.

Say the time is 1230. You're at a tidal diamond, and want to know flow and direction. HW was 1200. Do you look at the HW numbers, or the +1 numbers?

I've always interpolated. If it's 1.3/200 at HW and 1.1/220 at +1, for me it's 1.2/210 at HW +0.5.

 

[Moodyjim]

Tide height to a point in time, tidal streams +/-30mins for calculations from diamonds and atlas. Eeeeeeezy peeeeeezy.

 

[oldharry]

Err sorry but 2 hours before HW where?? Surely the tide turns at the top of the tide ie.local HW. If it 'turns around 2 hours before HW' then that is only because the tide table was for somewhere else.
Am I missing something here?

Come and try the E end of the Solent: A couple of hours or so before HW the tide starts going out again even though the level is still rising! At neaps, it more or less stops running altogether for an hour or so up to HW. So to catch the 'top of the tide' going west from Chichester bar, you have to be crossing the bar a couple of hours before HW - which is a pain, because there is still a strong INflow into the harbour for another hour and a half or so as the tidal flow in and out of the harbour does conform to the HW/LW times more or less. Wait for a fair current out of the harbour, you will lose 1/3rd of the ebb tide time, as it turns east a couple of hours before LW. All this changes as you go on down the Solent.....

 

[Thistle]

Hi Brian,
as JCP says, it depends on the context. For the purposes of answering an exam question, if the question relates to calculating the set and rate of a tidal stream using either tidal diamonds or an atlas, then the half hour before to half hour after rule is applied. If on the other hand the question is asking for a height of tide at a given time then the exact time is used.

Chris

Exactly! The RYA exam (in common with many other exams) requires you to deal with a completely predictable theoretical world where the tide will always be at the height calculated from the tables and tidal curves and where the stream will swish merrily at the rate and direction given by the nearest diamond for exactly 60 minutes. It appears to make exact calculation possible ... and easily marked!

We all know that the real world is not so simple and that most of the time we can get along well with a series of rough estimates based on the tables and charts and our experience. We constantly update these estimates as we go along. We also know how to spot critical points where a better estimate or even calculation might give us a better base for further estimates (eg calculating the depth over a cill based on predicted arrival time and then applying estimated corrections for wind, pressure, wave and our preferred safety margin.)

 

[Phideaux]

As a diver I think of predicted slack water as being with no current (or very little). This certainly does not coincide with high or low water in some areas.

 

[LittleSister]

Remember that the tides have not read the tables.

They certainly haven't, as when we found ourselves doing 12.1 knots over the ground off Brittany (our max. hull speed is c5knots), when both the plotter and almanac were suggesting it was slack water.

That said, the discussion of the difference between predictions (which are based on models of the key variables) and actuality (which is influenced in ways more complex than the models, and also by weather and other 'unmodelable' factors) only serves to confuse the original question, which was about how you dealt with predictions - as others have said, it's really about the difference in format between predictions of tidal height and predictions of tidal flow.

Others have given examples to show that max tidal height doesn't necessarily equate to the turn in tidal current. This can be different for both coasts and rivers. In rivers the flow will often turn before high tide, as the freshwater coming downstream is initially 'filling up' the river valley faster than the tidal outflow recently turned to sea is emptying it.

On the coast the water isn't coming in and out, it only seems that way. The tide is actually more like a giant shallow wave proceeding slowly past the coast (for most of the UK). The analogy can't be taken too far, but as you'll know from paddling on the seashore, once the highest part of the incoming wave has passed your knees, the water is still coming in past your ankles!

 

[Danny Jo]

Badger and the many others who opine that Dad is right for height and son is right for flow get my vote, but with a qualification that is required to deal with Twister Ken's point:

Don't get the tidal flow thing at all.

Say the time is 1230. You're at a tidal diamond, and want to know flow and direction. HW was 1200. Do you look at the HW numbers, or the +1 numbers?

I've always interpolated. If it's 1.3/200 at HW and 1.1/220 at +1, for me it's 1.2/210 at HW +0.5.

Tidal flow diamonds are generally used (as I believe they are designed to be used) to estimate how far from your dead reckoning position the tidal stream has pushed you. The figures given for the different tidal diamonds represent the average flow for the hour that starts half an hour before the specified time and ends half an hour after it. If however you start and finish on the hour, the best you can do is to interpolate as Ken has done. But just because that body of water moves, to use Ken's figures again, roughly 1.3 nm over the hour between HW-0.5 and HW+0.5, it does not necessarily mean that it'll be running at 1.3 knots at HW. The effect is of course more important on the "shoulders" and "hips" of the (generally sinusoidal) tidal cycle, where the rate of change of the stream is greatest.

Small print: the writer does not claim any special knowledge or expertise, and his opinion, based partly on a poorly remembered Yachtmaster theory course undertaken sometime in the last millenium, is just that - an opinion.

 

[alant]

Err sorry but 2 hours before HW where?? Surely the tide turns at the top of the tide ie.local HW. If it 'turns around 2 hours before HW' then that is only because the tide table was for somewhere else.
Am I missing something here?

Tidal Diamonds/Tidal Stream Atlases, use a Standard Port as reference, not "top of the tide ie.local HW". With the hour of HW, being taken as +/- 1/2hour from the HW time & subsequent -1HW,-2HW, +1HW, +2HW, etc, likewise treated accordingly.

Other than Southampton Water, the Solent tidal information (mentioned as "2 hours before HW where"), is based upon Portsmouth as the Standard Port, so East or West Solent, use these predictions. With many from Poole East, including Southampton 'harbours' using Low water Tidal Curves for calculations (secondary ports etc), because HW times are difficult to predict. Just outside the Western Solent, the main reference port & tidal stream atlases, are based upon HW Dover.

Easy to use the incorrect reference port when referring to Tidal Stream Atlases, have been stuck at night with a foul tide rounding Ushant, because someone used Brest instead of Dover.

 

[alan_d]

I think the other poster's comments about context was right.For Tidal stream information you son is correct. For Tidal height information you are correct.I bet you son's exercise was based on either calculating an Estimated Position or working out a Course to Steer and nothing to do with a tidal height calculation. In that context , he is correct, The information for what is called the tidal hour or the reference hour is good for half an hour either side of the time shown in the reference source. Eg 1 Hour before HW means the information for tidal set and drift is good for 1.5 Hours before HW to to .5 hour before NOT 1 Hour before until HW. Fot tidal height information you use the actual times on the curve. Anyone agree ?

I agree with you 100%.

 

[Richard10002]

Was trying to help my son with his YM theory homework. He's been given a couple of old exam papers to try himself out with answers. We almost came to blows when he described HW spanned a whole hour and therefore the calculations he made were based on half hour before and half hour after the stated time of HW.

snip

Who is correct, me or my idiot son?

Cheers, Brian.

Quite a few tidal calculation depend upon the "hour" you are in, rather than the precise time. Others rely on the precise time.... so you are both right in parts, and both wrong in parts

IIRC, flows rely on the "hour", and heights rely on the exact time...

 

[peterb]

It's very easy to postulate a case where minimum flow does not correspond to maximum (or minimum) height.

Imagine a large area inland lake joined by a small canal to an outer tidal sea. If the canal is small enough and the lake large enough, then the water level in the lake will stay almost constant at the mean level of the sea. The biggest difference in height between the two ends of the canal (and hence the highest tidal flow in the canal) happens at high or low tide outside; slack water in the canal will happen at mid-tide when the tidal height is at mean sea level.

On the OP's question, I suspect that both the tidal height and the tidal flow are (or can be) calculated as an instantaneous figure. The tidal curve is a continuously variable function; both height and flow change smoothly, not in jumps. The height predictions are published as a smooth curve; in theory one could calculate a tidal height second by second, though such a calculation would be very spurious. As others have said, the tide can't read the tables, and frequently differs from them.

Tidal flow, though, is different. Firstly, because it requires two quantities to specify it (rate and set), rather than the single figure to specify height. That makes flow much more difficult to show graphically in any meaningful way. Secondly, and probably more importantly, it is extremely rare for anyone to need any instantaneous figure for tidal flow. What we need instead is a figure averaged over the period of our navigation. In general sufficiently accurate figures can be obtained by giving the figures hourly, with the proviso that each of these hourly figures can be considered to work for half an hour on either side (before or after) of the quoted time. Yes, we know it's not quite right, but it works well enough for all practical purposes.

 

[Elessar]

Was trying to help my son with his YM theory homework. He's been given a couple of old exam papers to try himself out with answers. We almost came to blows when he described HW spanned a whole hour and therefore the calculations he made were based on half hour before and half hour after the stated time of HW.

THE RYA booklet which came with his course certainly supported his understanding.

But my understanding has always been the time of HW stated in the tide tables is set in stone. The half hour periods (and they are variable periods not a fixed hour) can be referred to as slack water but that is a different thing. There may be very little movement of the tide over the period of slack water but HW is neverthless the instant in time when the tide will reach it's highest point in the full tidal cycle so if it it states 14:26 - then it's 14:26 and not the period between 13:56 amd 14:56.

Who is correct, me or my idiot son?

Cheers, Brian.

The confusion here is between tidal height and tidal flow.

Height is shown in tide tables and is as you said set in stone. Always remembering it is a prediction of course that can be affected by atmospheric pressure etc which is not part of the prediction.

Using the tide curves or a proprietary program you can therefore calculate the predicted height for any minute.

However tidal flow as shown in tidal tables and tidal diamond is an average over the hour.

So if HW dover is 1200, then the flow shown for HW is the average for the hour 1130-1230.

 

[Elessar]

Tide height to a point in time, tidal streams +/-30mins for calculations from diamonds and atlas. Eeeeeeezy peeeeeezy.

didn't see this 100% correct reply before I posted which was far more succinct than mine!

 

[Elessar]

Don't get the tidal flow thing at all.

Say the time is 1230. You're at a tidal diamond, and want to know flow and direction. HW was 1200. Do you look at the HW numbers, or the +1 numbers?

I've always interpolated. If it's 1.3/200 at HW and 1.1/220 at +1, for me it's 1.2/210 at HW +0.5.

1230 is the start of the +1 tidal hour, and the average for the next hour is likely to be what you need to know if calculating your course to steer. So +1.

But if you're calculating an EP without bearings or GPS available you want to know what's happened in the previous hour so in this case you need HW.

If you want to calculate your current SOG without GPS then your interpolation is the way to go.

 

[Robin]

I've not read all the replies so might be repeating but Moodyjim & Elessar are correct for me. Tide height times are spot on as given but with due allowance for local variation based on barometric pressure and wind strength and direction. Tide flow rates and directions are average rates over a one hour period, so logically from half hour before to half hour after the stated time, ie use the HW time for the average flow from half hour before then until half hour after then. In practice if you took it as starting from HW to plus one hour and so on throughout the cycle the error would be small and within the bounds of normal plotting error anyway.

We had instruments linked that could show the actual tide rate and direction, like GPS linked to compass speed and distance log with a display on a Navdata repeater that showed the tide. In the central English Channel where the tide flow is very strong and basically east-west, the tide certainly didn't reverse at the stated changeover times, but followed a circular route so that during the one hour period it might start as say east going and end as west going but in between for a while it could be south or north going or any points in between. The average total overall movement however, set and drift, is the figure in the tide book or tidal diamond.

Apologies if that repeats what others have said.

 

[moluag]

Actually, there is only a tidal current where the tides are restricted by sea-bed and coastline. In deep oceans, the sea still goes up and down but there is no detectable tidal current. All the tidal currents we see are artefacts of basin geometry and interconnection. You are, of course, quite correct that in many places the tidal streams in basins are rotatory around the basin.

Anyway, my basic point was that "slack water", if it exists, is effectively instantaneous. You, quite correctly, point out that there may be no "slack water" at all.

Tides can be rotatory on a very small scale! I recall a few minutes of thinking "What the...." while rounding the southern tip of Rathlin Island - and we had timed the tides pretty well!

Slack water does exist in my sailing neck of the woods you can actually see when the rise of tide stops and the ebb begins, i usually see this when ashore in one of the many anchorages.

 

[djk]

Slack water does exist in many areas off the south coast.
Divers need slack water to dive on wrecks and this usually done on neaps as the visibility is better and the slack period can be longer.
Slack water can last from as little as 10 minutes to over 40 minutes depending on a number of factors.

 

[Marsupial]

Err sorry but 2 hours before HW where?? Surely the tide turns at the top of the tide ie.local HW. If it 'turns around 2 hours before HW' then that is only because the tide table was for somewhere else.
Am I missing something here?

Sorry but yes you are. The point I was trying to make is that if you view tides as coming in or going out you only see half the problem. Flow and direction of flow is only loosely connected to the rise and fall of the tide, because of other factors including but not limited to the shape of the coastline, the surface pressure and the speed and direction of the wind.

The UK is influenced not only by the tide but also by the Gulf Stream and this really makes for a bad day at the office when you’re a rising tide around Dungeness/South Foreland (and many other places around the coast). The difference in direction of flow vs max tide height at Dover is 2 hours; that is the direction of the tide changes 180 deg two hours before high water but the tide keeps on rising until high water. To make matters worse there is a slack water around 2 hours before high. The actual time of high water is then variable dependant upon the speed and direction of the wind and the surface pressure.

Once you get used to this system you can leave Dover around 2.5 hrs before high and have favourable tide for 12 ish hours all the way to Ipswich. Ipswich to Dover however is a bit tricky and you will get to know all the houses at Deal as you wait out a tide swinging on a hook.

 

[tcm]

you're right, he's using rya-approved approximations, which aren't utterly "wrong" but for the purposes of your argument, he's wrong. Which makes you right, and not him. Hah!

Due to lots of factors (including sun, moon etc, several dozen affecting solent frinstance) there is a moment when the tide turns from rising to falling and that instant is called high tide, or high water. The tide doesn't "park up" at any stage. It definitely doesn't watch the clock and say oho, half an hour to HW, that's me done for the next hour. HW is an infinitessimally small instant when the local sea level goes from overall rising to overall falling.

The approximations make it easier to work fings out. Like these days they use the acceleration due to gravity as 10ms-2, instead of 9.81 which is harder to use, but more correct.

Tony Blair and George Bush, simultaneously the most stupid leaders of the UK and USA, are entirely to blame for most if not all government and quasi-governmental dumbing down of which this is but one example.

 

[Bosun Higgs]

You are wrong - slack water does not necessarily tie in with high water.

Sure high water is at a specific time give or take a bit allowing for the fact that tide tables are only forecasts anyway. And its weather dependant.

What your son is referring to is the convention that the data in tidal diamonds is for half an hour before HW and half an hour after HW ie its the average for an hour.

Strikes me that neither of you really understand what its about