Yachtmaster Theory - Have I been wrong all this time?

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Wandering Star

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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.
 
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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Perhaps it is a question of significance. For the purposes of working out safe water over shallow areas then assuming that the whole hour is the same may not be too far wrong. The principle is another matter.

As far as slack water is concerned - again it is a matter of significance. On some moorings it is quite possible to see some seaweed drift past one minute, and back again 10 minutes later.
 
Sorry Brian but in my humble opinion (and relatively limited experience) I tend to agree with your son. It's certainly how I was taught anyway. :o
 
What is the context?

Your son seems to be describing the use of tidal diamonds where the HW rate is used between HW-1/2 & HW+1/2. Not necessarily slack water of course depending upon the local tidal behaviour. Of course it is an approximation as I don't think any of us have ever seem the sea screech to a halt and head off in a different direction exactly 30 mins after HW.
 
The tide will reverse direction very quickly after high or low water and if depth is critical it can make a big difference.I was once trying to float off my boat off a wall and missed high water for less than 5mn and was stuck there until the next high tide.
The effect is much more pronunced in spring tides.
 
I seem to remember it is indeed taught as half hour before til half hour after.
Given much of this seemed very based around sailing, I m amazed yachts can time their arrival to within a few minutes :)
Anyway, much of the course is to explain the theory, rather than practicalities. Yes , yes, there is always that exact depth of cill to cross over, but you d have to be nuts to be timing your crossing it down to mintues with your IPS drive system underneath. I can see that once upon a time, these skills were all man had to navigate safely, but I m sure most people now use electronics.. or phone the harbour and ask ! You can argue that you should understand the concept and not blindly rely on a plotter etc, but I m not convinced the level of theory is really that necessary anymore.
 
HW is undoubtedly a precise time, but not the precise time printed in tables. Local conditions and meteo can modify HW times (and heights) considerably. I take the published HW time as an approximation but am happier being early rather than late - i.e at a tight spot I'd rather be there half an hour before with a few cms to go, than half an hour late with a few cms draining away. Incidentally, different tables often give different tide times.
 
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From an ABSOLUTE point of view, you are right and your son is wrong; High Water is DEFINED as the point in time when the water is highest, and can only be one instant. This instant is what tide tables give.

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.

In other words, this is an argument about terminology, and correct use of words. I suppose the RYA ought to cover themselves by wording like "The height of the tide at High Water may be regarded as constant for approximately half and hour either side of the instant of high water. In the rest of this discussion, "High Water" will be taken as referring to the hour centred on the actual instant of high water."

Slack water is also technically an instantaneous thing, except where there is a strong non-tidal current that can counteract the tidal current. However, once again, the current will be very little for a period around the actual instant of slack-water. 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.

PS, I feel for your situation! The worst argument I ever had with my Father was when we disagreed on a navigational matter.
 
flobbergob said:
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.
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It's certainly the way it's taught, and the way I have always calculated it. Best illustrated if you look at the tidal curves in your alamanac. The HW box at the bottom of the scale spans half an hour either side of the central line on the grid, and that continues for each hour +/-.
 
And furthermore.....

.....what about the Solent and Double HW?

When there are two HWs, the time given in the tables is the time of first HW and when there's a stand, if memory serves, it's the mid-point of the stand. Waddya do then? :D

PS: I don't know about you but in general, when I ask Mrs jhr the question "Have I been wrong all this time"? the answer is "yes", irrespective of the subject under discussion.
 
AntarcticPilot said:
Slack water is also technically an instantaneous thing, except where there is a strong non-tidal current that can counteract the tidal current. However, once again, the current will be very little for a period around the actual instant of slack-water. 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.

QUOTE]

I disagree with you on that one Antartic Pilot. Tides are usually rotary when they're not confined by the land and sea bed. However even when they appear generally rectilinear they tend to become rotary about the time of slack water, often turning inshore first. When racing around the time of so-called slack water this behaviour can be pretty significant.
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I'm surprised the RYA teach people to use the height of HW for a whole hour these days. When I did the YM theory they taught a graphical method to determine the height at a given time or time for a given height. I tend just to take percentages off the curve and use that but that method (at least IMHO) gives the same degree of accuracy.

Of course as most people know practical navigation is all about doing enough navigation for the situation. Most of the time just knowing it is HW +or- 3, for example, is sufficient, sometimes using the height of HW for a whole hour is sufficient and occasionally (in the Bristol Channel maybe) the predicted height 30 mins after matters, even allowing for it just being a prediction not a guarantee.
 
I suggest it depends on whether you are talking about tidal heights or tidal streams. For heights you can find a specific time on a tide curve ( in theory to 10 minutes).

For tidal streams the diamond or tide tables show streams for a period of an hour ie 30 minutes either side of a tide time some hours before or after high water.

So you are both right!
 
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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 ?
 
flobbergob said:
Who is correct, me or my idiot son?
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You are strictly correct but for most purposes one can get away with his approximation.

A quick scan of the tidal curves in the almanac shows that for some ports there is next to no rate of change over the highest hour but in other places it is significant. I opened the book at Sheerness - that shows a rise and fall of 4% of the tidal range during the last and first half hours. OTOH Falmouth has less than 1% over the same period.

As a practical example, I launch by floating off from a concrete apron on a spring tide. One year I did it on a marginal tide and we didn't float off until 5 min before HW!

So - while he is right that generally you can treat the top hour as constant for most navigational purposes, to believe that there is no change over that period displays a fundamental lack of understanding which would concern me.
 
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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
 
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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Yes I agree entirely with your comment Badger. In the context you describe you are correct.

As for double tidal areas, one works out tidal flow times in exactly the same way but use Low tidal curves for calculating tidal heights.. An excellent example of this is found on page 221 of the Reeds 2010 Almanac. Entitled special tidal curves from Christchurch to Selsey Bill.
 
JCP said:
I disagree with you on that one Antartic Pilot. Tides are usually rotary when they're not confined by the land and sea bed. However even when they appear generally rectilinear they tend to become rotary about the time of slack water, often turning inshore first. When racing around the time of so-called slack water this behaviour can be pretty significant.
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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!
 
It is many years since I took the exams but if I recall correctly tides are directly influenced by the positions of the sun and moon. As the clever ones on this planet can forecast the precise positions of the sun and moon and they know where both will be at any one time they are able to forecast the theoretical high and low tides to the nearest minute. This is presumably why the times given are to the nearest minute and not the nearest ten minutes.

However actual times can be influenced by other factors such as the wind, or even high or low pressure systems out at sea. To assume a margin of error of half an hour either side of the given times seems a sensible solution to cover these potential problems.

I suppose location can affect things. For example what do sailors do in the Bristol Channel which has a very high rise and fall of tide and very strong tidal currents? Do they follow the almanac times exactly?
 
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