Yachtmaster Theory - Have I been wrong all this time?

Have I missed the point here?

If high water is quoted as 16:00 then the tide for practical purposes is considered to be high from 15:30 to 16:30.
 
I would have thought that Dad is correct.

Especially as regarding tidal height, but sometimes streams. The difference between HW -15 and HW +15 can be huge. Yes I know that tides can vary by a lot according to weather etc and it is very necessary to keep an eye out as to when the change happens but using a precise time, maybe adjusted, cannot be wrong whereas a whole hour can put you in serious poo. IMHO of course. Good question though and I am enjoying the replies.

Thinking of New Brunswick and their 60 foot tides or even just the approaches to the likes of Treguier. Reality seems to concentrate the mind there as the direction is like a switch......almost instant turn around.
 
Badger said:
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 ?
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Totally agree!

Sounds like the best outcome for a father/son argument too... you're both right!! :D
 
"The difference between HW -15 and HW +15 can be huge."

If so how do you cope with conflicting predictions? I have two sources of tidal info to hand. One says HW Portsmouth was 0759 this morning, the other 0818. Seventeen minutes. That could make a huge difference in your view.

As far as I can judge from the tide gauge info, actual HW may have been a few minutes before 0800, and the tide height about 0.4m higher than predicted.

The real world and predictions don't always match.
 
flobbergob said:
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.
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Neither: tide tables are nothing more than predictions. The actual timing and height, together with rate of rise and fall are governed by so many variables that the tables can only give an indication of what is likely to happen.

Dell Quay at the top of Chi harbour obviously will have a later HW time than that predicted at the Bar Beacon: but by how much? Anything from 10 - 30 minutes later dependent on tidal rate, height and atmospheric conditons. Taking 30 minutes before to 30 minutes after will not do here either, as the tide creeps in very slowly until it peaks, and then drops nearly a foot in about 20 minutes. It then stands for sometimes over 2 hours before suddenly dropping away, the 'stand' timing depending on weather and tidal height.

Even locals cannot predict to within about 20 minutes to half an hour just when the peak will occur - and because of that sudden drop straight after HW it is essential to get it right if getting out of a mudberth. Once the tide starts to drop back you have very little time indeed!
 
Twister_Ken said:
"The difference between HW -15 and HW +15 can be huge."

If so how do you cope with conflicting predictions? I have two sources of tidal info to hand. One says HW Portsmouth was 0759 this morning, the other 0818. Seventeen minutes. That could make a huge difference in your view.

As far as I can judge from the tide gauge info, actual HW may have been a few minutes before 0800, and the tide height about 0.4m higher than predicted.

The real world and predictions don't always match.
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It's more often that its on arrival that you are trying to get times right. eg. Aiming above or below Chebourg etc. Sods law usually means that you are arriving bang on the change. If you paid attention to the previous change of direction, before or after predicted, it is usually pretty much the same.
I presume that conflicting predictions such as Ken, mentions are because one is at the top of the harbour and one at the mouth. Any other suggestions? Most harbour masters are pretty good on helping about this, especially in the CIs, IMHO.

A difference in Pompey does not mean much but things are a bit different across La Manche.
 
Read the question !

Once again we are drifting away from the O.P.'s original question. For the purposes of answering an RYA theory paper question I stand firmly by my original post above. In practice of course, things are never that easy !:)

Chris
 
The forum would be pretty boring if there was only one right answer to any question.
One of the very few advantages of paternity is the short term right to infalliblity which exists only for a short time, sounds like the OPs time may be up now.
 
I'm quite enjoying reading all these replies and I'm only replying myself now because I don't want anyone to think flobbergob is too rude to acknowledge peoples thoughts! What I've found interesting is the correlation between what I perceive a posters age could be, and the replies. I think most of the crinkly forumites are agreeing with me (I'm a crinkly myself) and most of the "barely out of nappies" forumites have a different understanding, probably because they've received more formal training via the RYA and the likes which wasn't available to me in 1876. This reply is not to say either side are correct over the side - it's looking to me, as has already been pointed out, as though both my son and myself could both claim to be correct in our different understandings.

I must go and visit him in hospital and see if the bruises are healing.

Cheers, Brian.
 
If HW is said to span the half hour either side of the predicted time does that imply that HW+3 spans the half hour either side of that time? If it does, the height of tide could be VERY different from start to finish of that hour.
Also, things don't always change at HW. For example the tidal stream in the Eastern Solent turns around 2 hours before HW.
 
Side to side or up and down?

When I was much younger I thought the tide came in and went out, a change of direction sort of idea that corresponded to high and low tide. Then I discovered the Reaves Fawlks (sorry about spelling) system of tides that describes tide as rising or falling and this makes much more sense (to me). For example Dover tide changes direction 2 hrs before high but is still rising, so the confusion of height and direction are very valid and on top of that we have tidal predictions that are often over 1 hour out when compared to reality.

When you navigate a desk for the Yachmaster exams they want accuracy based on prediction, but in real life you take more notice of the tide guage and which way the wake around a buoy is pointing.
 
As some one who is also doing Yachtmaster Theory at the moment I agree with sky sail
That is to say that the RYA teaching is that you use a tidal graph to calculate tidal heights and as such if high water is 0900 then thats absolute. However if you are calculating tidal streams then you will either be using the chart diamonds or a tidal stream atlass - either way the direction and rate of flow are defined at a point every hour before or after high water as an average of the tidal direction and flow over the half hour before and after the stated time.
So tidal heights are specific to a monet in time off the tidal curve graph.
Tidal rates and directions are stated as an average over a one hour period.
 
Theory is all very well and you need to be able to show you can do it, but in practice regard with deepest suspicion any calculations which rely on tenths of metres or tenths of knots. Remember that the tides have not read the tables.
 
My admittedly dim recollection is that father and son are talking about two different things, which harks to the 'context' issue mentioned above.

The +/- 30 mins refers to tidal stream charts. Since they don't give HW +3hr 15min, one assumes that the HW +3 stream rate applies from HW +2.30 to HW +3.30. So you'd use this when, for example, working out cross-sets.

If, on the other hand, you were working out available depth in an anchorage, you'd use predicted HW or LW (modified, perhaps, for unusual barometric conditions). There's no reason to invoke +/-. That would apply whether plotting on a graph or using shorthand such as rule of twelfths.

At least, that's how I remember it and how logic tells me it should be (but the boat's now in Greece so who cares? :-) )
 
alahol2 said:
Also, things don't always change at HW. For example the tidal stream in the Eastern Solent turns around 2 hours before HW.
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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?
 
doris said:
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?
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I'm afraid so. For example at Plymouth the tide is Eastbound for the top half of the tide and westbound for the bottom half. The change of direction occurs around HW +/- 3

High water will coincide with the change of direction if flowing in and out of a river etc. but along a coast they don't necessarily agree at all.
 
AntarcticPilot said:
Slack-water need not coincide with high water (or low-water) if there are other, non-tidal, currents such as river flow to be accounted for.
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Although not central to this thread, I would like to amplify the point made by AntarcticPilot, particularly as it may underlie what Doris is saying.

There can be places where the tidal current is in full flow at high (or low) tide.
St Sampson's harbour here in Guernsey is one instance. Cargo boats with large draft need to pass through the entrance channel at high tide and, particularly when the tide is springing, it is very noticeable that they have to steer upstream at a large angle to avoid being swept sideways.

On the other hand there are odd places where the tide never has a slack. There is a location to the north of Alderney where the tide runs at a pretty steady 2 knots all round the clock: the direction changes, but not the speed.
 
AntarcticPilot said:
However, from a PRACTICAL point of view, the rate of change of the sea-level near high water is very slow, and assuming it will change very little over the hour around high-water is a reasonable assumption.
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It depends on your tidal range - in the Channel Islands you regularly get a 11+ meter range and the rise / fall 30min either side of high is still significant.

As regards to the OP I guess your son is referring to tidal flow not height!

Jonathan
 
Quote from SNOWLEOPARD "High water will coincide with the change of direction if flowing in and out of a river etc. but along a coast they don't necessarily agree at all."

Not always. Westerschelde inflow continues well after HW Breskens.
 
nickd said:
As some one who is also doing Yachtmaster Theory at the moment I agree with sky sail
That is to say that the RYA teaching is that you use a tidal graph to calculate tidal heights and as such if high water is 0900 then thats absolute. However if you are calculating tidal streams then you will either be using the chart diamonds or a tidal stream atlass - either way the direction and rate of flow are defined at a point every hour before or after high water as an average of the tidal direction and flow over the half hour before and after the stated time.
So tidal heights are specific to a monet in time off the tidal curve graph.
Tidal rates and directions are stated as an average over a one hour period.
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I quite agree with nickd, and since he is based at Barmouth he will certainly know what actually happens! Any part of the CI, Bristol Channel, Cardigan bay, Liverpool bay tidal HEIGHTS are specific and generally match the tidal prediction tables very accurately. The published information on tidal FLOW is based on +/- half an hour. How it actually performs varies according to local circumstances, but it will not necessarily be evenly balanced either side of HW even if the tidal curve is symetrical.

Now that I am based on the South Coast, it is all quite different. Tidal range is much less, flow rates are much less, it is all less precise. Even the accuracy of low water in Poole is imprecise. This may affect the views of those posters who are solely based in the south.

I still think nickd got it right.
 
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