Yachtmaster Theory - Have I been wrong all this time?

[tcm]

I think you meant "dependent". You also blew lots of apostrophes. Not really good enough if you plan to make punchy "pah you know nothing" posts to people like flobbergob who's sailed the atlantic so often he's forgotten to keep count?...

 

[Bosun Higgs]

Fair comment TCM - I hiot so many wrong keys these days ( eyesight I guess) that iI cant be bothered to correct apostrophes as well.

Crossing the atlantic qualifies you on tides?

 

[PeterGibbs]

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.

This has to be the answer - the variation from predicted height due to air pressure and wind combinations affecting the actual height over CD, are surely more than the tidal influences a few minutes either way of the predicted event.

These sailing course efforts at precision are, in my view, an irritating distraction from what it's really like out there. Innumerable students get chased around the classroom after a degree or two of course when no one under sail can actually steer to such a precise heading! Been there and rejected it!

PWG

 

[guernseyman]

This has to be the answer - the variation from predicted height due to air pressure and wind combinations affecting the actual height over CD, are surely more than the tidal influences a few minutes either way of the predicted event.

These sailing course efforts at precision are, in my view, an irritating distraction from what it's really like out there. Innumerable students get chased around the classroom after a degree or two of course when no one under sail can actually steer to such a precise heading! Been there and rejected it!

PWG

That's a pretty despairing philosophy!

Just sticking to water height, after doing the textbook calculations you can then build in other factors like atmospheric pressure (1 cm. per millibar), wind, wave height, and safety margin to get a usable result.

Steering under sail may be a rough game, but you you still need a course to steer, and if there's no wind then you can get pinpoint accuracy under motor.

 

[tcm]

Fair comment TCM - I hiot so many wrong keys these days ( eyesight I guess) that iI cant be bothered to correct apostrophes as well.

Crossing the atlantic qualifies you on tides?

not vital, altho likely not warranting your comment that he didn't know what was going on? Note that using either his or his son's methd wil get him a correct (enough) calculations and the fact that they're arguing a fine point means that they do know what's going on.

 

[lw395]

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.

Sounds like neither of you have a clear understanding. YM desk instructors seem to make this more confusing than it should be?
Slack water does not necessarily coincide with high, it certainly does not in the Eastern Solent.
The tidal diamonds I have always taken to be (predicted) 'snapshots' at the time stated relative to the (predicted) HW. Which will be a standard port, not local. For our purposes, they are the best data for the hour centred on that time, so are assumed to be valid for plus/minus 30 minutes. The RYA seems to call this 'the tidal hour'.
Likewise atlases of streams you have have to either interpolate between snapshots or add up vectors as appropriate.
None of the data is accurate to 4 decimal places, it all varies with the weather etc, but its only ever meant to give a broad guide to what the tide will be doing. It works well enough!

 

[Marsupial]

Sounds like neither of you have a clear understanding. YM desk instructors seem to make this more confusing than it should be?
Slack water does not necessarily coincide with high, it certainly does not in the Eastern Solent.
The tidal diamonds I have always taken to be (predicted) 'snapshots' at the time stated relative to the (predicted) HW. Which will be a standard port, not local. For our purposes, they are the best data for the hour centred on that time, so are assumed to be valid for plus/minus 30 minutes. The RYA seems to call this 'the tidal hour'.
Likewise atlases of streams you have have to either interpolate between snapshots or add up vectors as appropriate.
None of the data is accurate to 4 decimal places, it all varies with the weather etc, but its only ever meant to give a broad guide to what the tide will be doing. It works well enough!

Yes Yacht master instructors often make this whole subject more confusing than it is, that’s because many of them make the mistake of lumping together three environmental affects, the direction of the current, the height of the tide and the time of day as a single topic, there is a linkage but its only a very loose one with no hard and fast rules, the rules such as they are only apply to specific locations (diamonds), you cannot deduce the direction of the current by looking at the tidal height, and vs. versa; as many above have pointed out.

Therefore, with the absence of a universal set of rules that can be employed to predict height AND flow for ANY location the topics must be treated separately. Then the instructor should emphasise that at a particular time of the day BOTH features of the marine environment must be addressed to be sure the intended passage is possible and safe.

And by the way the linkage between tidal diamonds and the tide height prediction is only the time of day, if the topics are taught with that in mind then in my humble experience students will find it easier to understand what’s going on

 

[lw395]

And by the way the linkage between tidal diamonds and the tide height prediction is only the time of day, if the topics are taught with that in mind then in my humble experience students will find it easier to understand what’s going on

Not forgetting that the height of HW correlates with the time of HW and whether it's springs or neaps.
I've met more than one YM instructor who did not appear to twig that HW springs is always roughly the same time of day in a given location- lunchtime/slinging out time in Portsmouth.
And hence why the corrections in secondary ports are referenced to different times of day.

 

[snowleopard]

Note that using either his or his son's methd wil get him a correct (enough) calculations and the fact that they're arguing a fine point means that they do know what's going on.

I'd argue that the son's stance indicates that he doesn't really understand but is relying on rules of thumb. OTOH the son will get a safe result with his method.

 

[guernseyman]

Not forgetting that the height of HW correlates with the time of HW and whether it's springs or neaps.
I've met more than one YM instructor who did not appear to twig that HW springs is always roughly the same time of day in a given location- lunchtime/slinging out time in Portsmouth.
And hence why the corrections in secondary ports are referenced to different times of day.

Well said.

In the ever-changing tidal environment constants, such as the ones above, are worth hanging onto. Another one of value is the fact that every day, come springs, neaps and in-betweens, half-tide is the same height (wind and pressure effects excepted).

Rocks awash at half-tide are particularly useful to remember and, locally at any rate, are named accordingly.

 

[sailwithray]

HW times+or -

Hi ,

I read your memo with interest ,and some of your many replies.

Firstly i must say how nice it is for a father to help his son with his homework and also have a common interest.

With regards to your problem you are both correct!!

Times of HW for accademic purposes are as the almanac predicts, or for secondary ports as calculated.

Your confussion arises when using either tidal diamonds (which the RYA shorebased course use a lot), or tidal flow diagrams wich are based on either HW Dover or a conviniant standard port.

Therfore to ascertain a rarte of flow and dirrection amean average over an hour period +or - half an hour either side of that time is used.

in practice Y men rarely use diamonds but use tidal flow diagrams on the same basis

 

[alant]

Sounds like neither of you have a clear understanding. YM desk instructors seem to make this more confusing than it should be?
Slack water does not necessarily coincide with high, it certainly does not in the Eastern Solent.
The tidal diamonds I have always taken to be (predicted) 'snapshots' at the time stated relative to the (predicted) HW. Which will be a standard port, not local. For our purposes, they are the best data for the hour centred on that time, so are assumed to be valid for plus/minus 30 minutes. The RYA seems to call this 'the tidal hour'.
Likewise atlases of streams you have have to either interpolate between snapshots or add up vectors as appropriate.
None of the data is accurate to 4 decimal places, it all varies with the weather etc, but its only ever meant to give a broad guide to what the tide will be doing. It works well enough!

YM Instructors, do not teach that slack water "necessarily coincide with high", other than to point out eg 1/12ths rule, where the 1st & last hours the flow is lower than the middle hours.

Otherwise. what we basically teach, is exactly what you stated

"The tidal diamonds I have always taken to be (predicted) 'snapshots' at the time stated relative to the (predicted) HW. Which will be a standard port, not local. For our purposes, they are the best data for the hour centred on that time, so are assumed to be valid for plus/minus 30 minutes. The RYA seems to call this 'the tidal hour'.
Likewise atlases of streams you have have to either interpolate between snapshots or add up vectors as appropriate.
None of the data is accurate to 4 decimal places, it all varies with the weather etc, but its only ever meant to give a broad guide to what the tide will be doing. It works well enough!"

Some of the passage planning exercises, use the tidal stream atlases, to allow pupils to find out when slack water occurs, by looking at each hourly 'snapshot', which may not be HW.

Mention will also be made regarding air pressure/weather/tidal surges/geographical features, which can affect tides. Some execises, even at DS level, take wind direction into account, when passage planning.

We try NOT to make it complicated.

 

[aquaplane]

In my experience rule 1 is, dad is right.

If dad is wrong, refer to rule 1.

It works with my kids, sometimes it doesn't with my dad.

 

[lw395]

YM Instructors, do not teach that slack water "necessarily coincide with high", other than to point out eg 1/12ths rule, where the 1st & last hours the flow is lower than the middle hours.

This is the kind of statement that confuses some students, although the Rule of twelfths has some relevance to streams, it's really a tidal height thing, so not usually applied to flow.
It would work for currents in a perfect sinusoidal system of course.
The 'rule' of twelfths is a bit of liability around the central south coast anyway.

 

[alant]

This is the kind of statement that confuses some students, although the Rule of twelfths has some relevance to streams, it's really a tidal height thing, so not usually applied to flow.
It would work for currents in a perfect sinusoidal system of course.
The 'rule' of twelfths is a bit of liability around the central south coast anyway.

Yes agreed a Tidal Height thing, but when the tide is going from LW to HW, because its 'filling' faster in the middle hours than to 1st/6th hr, this also relates to flow.
Shouldn't be confusing.

Have got students to use tidal curves & 1/12ths rule to work out/compare heights, 'testing' results with depth sounder/lead line. Sufficiently close to give them confidence.

 

[Marsupial]

YM Instructors, do not teach that slack water "necessarily coincide with high", other than to point out eg 1/12ths rule, where the 1st & last hours the flow is lower than the middle hours.

Sorry to be pendantic but no its not!

It's the rate of rise or fall that is lower in the first and last hours.

FLOW is something else.

 

[alant]

Sorry to be pendantic but no its not!

It's the rate of rise or fall that is lower in the first and last hours.

FLOW is something else.

Ok, so if you get a rate of rise 'n'fall, without flow, does this mean the tide height is thinner toward the bottom & forms some sort of column?


PS
OED "Pedant - one who overrates or parades book-learning or technical knowledge"

 

[Marsupial]

Ok, so if you get a rate of rise 'n'fall, without flow, does this mean the tide height is thinner toward the bottom & forms some sort of column?


PS
OED "Pedant - one who overrates or parades book-learning or technical knowledge"

Nice one!

 

[lw395]

Yes agreed a Tidal Height thing, but when the tide is going from LW to HW, because its 'filling' faster in the middle hours than to 1st/6th hr, this also relates to flow.
Shouldn't be confusing..

Not in the English Channel, the flow is completely out of phase with the rate of rise and fall
Moreover, in many places the flow does not stop so much as swing around.
The water does not 'go from LW to HW' or even HW to LW, the tide is a wave which travels at a different rate to the current..

Have got students to use tidal curves & 1/12ths rule to work out/compare heights, 'testing' results with depth sounder/lead line. Sufficiently close to give them confidence.

That would be false confidence in Studland, among a great many other places!

 

[Daedelus]

Don't know when I've enjoyed a thread more: everyone seems to be in furious agreement, like Alice in Wonderland's caucus race all have won and must have prizes, tcm has managed to blame it all on Bush and Blair, (Nice one), the only disappointment is that no-one has yet managed to compare anything or anyone to the Nazis or Hitler. Too gentlemanly a thread so far I suppose.


PS
(I try and keep my lot clear of rule of 1/12ths otherwise they never even look at a tidal curve).