Tides

[MikeB]

Most of us know the "rule of 12ths" I guess - anyone come across the "3rds rule" ?

As I understand it, "in the first hour, the rate of flow is a third, in the second its two thirds and full flow is in the third hour - then decreasing in reverse"

Thoughts ? Experiences ? Preferences?

Mike.

[sub5rider]

Yes, but as I understand it the "rules" are for two different things.

The "twelves" rule is for tide heights, and the "thirds" rule is for tidal stream flows. The latter is oft used, is it not, but this is the first mention of the former outside of a book...
Nigel, aka Sub5Rider, Onioneer

[Douglas Wilcox]

Hi Mike, yes its a rough reckoner for open water but it does not apply in many places in the West Coast of Scotland. eg in the Cuan sound; max flow is just after turning.

There are several accounts in club websites of Cuan Sound capsizes just after the turn of the tide. In one in particular, they chose to wait until "the second hour".

In the Sound of Jura most of the flow takes place in the first three hours after a change of tide, followed by a 3 hour stand. In the Dorus Mor the SE ebb flow starts up to 2 hours before high water(as you would see it on the beach.) but the NW flood starts up to 3.5 hours after HW so the flood lasts longer than the ebb then further north in the Sound of Mull the flow starts about an hour after HW and LW. When I used to sail the west coast we made use of this to get up to 9 hours of fair tide from the Crinan Canal to Tobermory.

To avoid nasty surprises the Tidal stream atlas and Martin Lawrence's Yachtsman's Pilots are very good.

Lastly, if you book mark todays tide times for your favourite port on the BBC web site be aware that it will still show todays time when you next visit in 6 months. You need to click the date in the "Tides for:" bar at the top to refresh the times and the graphic.

To avoid this, the UK hydrographic office makes it more difficult for you to bookmark an actual port.

"Nae man can tether time nor tide,
The 'oor approaches, Tam maun ride."

R. Burns

[Phil]

Nigel is quite right in saying the twelths rule is for height prediction, although I know that people use it as a ready reckoner for tidal stream flow. The thirds rule is just a simplified version realy. The problem with rules as Douglas points out is that they dont always apply, for instance the assumtion (for the rule) is that the tidal stream is rectilinear and not rotary, it further assumes that the tidal curve is regular, flows around islands in particular can cause a long Stand or even a double high water (as at Southampton). As Douglas points out, Tidal Atlas, Tidal diamonds, and pilots combined with whatever local knowlege you can glean are a good way of avoiding "surprises"
So you realy need a rule for using the rule,
1/ is the tidal stream rectilinear? (flows in two directions)
2/ is the flow equal (in time and rate) and regular in both directions? as is usually the case along an open coastline.
if both the above are true then its a good "guesstimate"
or in other words treat with caution.
Phil

[Douglas Wilcox]

Oh, and I forgot to mention Soay Sound where the ebb and flow both run in the same direction!! So off Rubh an Dunain, you usually get wind against tide chop in the prevailing westerlies. This is also mentioned in Lawrence's excellent pilot.
Douglas

[apage]

I remeber reading somewhere.. that in the sound of Jura, just off Islay there is a point where the tide remains constant (tidal anomaly). The spot moves around, but I guess its due to the complex shape of the sound and the added complexity of the Corrvreckan and Sound of Islay. Tony

[Douglas Wilcox]

Indeed there is; it is an amphidrome where the tide levels do not change it acts like a pivot point in a basin round which the water sloshes. But it moves about a bit, this rsults in it being spring tides in some places round it and neaps in others, very complex!
Douglas

[Jim]

Yep, as Phil points out the rule of thirds is just the rule of twelfths simplified to think about the flow and it assumes an even tide curve. Even on the Clyde we cannot really apply these rules if we want to keep our ships afloat because the tide curve is very definitely skewed towards high tide (perhaps river flow rather than the geometry of the firth?) so much so that the tide never really stands at all, once it maxes it starts to ebb immediately.

Unfortunately we have definitely come accross people who think these rules are gospel, and also refuse to beleive when the pilot indicates contraflows and circular tides because they can only grasp ebb one way flood the other.

I don't have a decent tide stream atlas (the yachting pilot has little stream information) so if the tides look strong, I would much prefer to have Phil doing the calcs (and cross checking with Dover) than me, and especially over someone I will not name here.

I've often tried to point out that the tide state at the falls of lora is probably not the same as at Oban although we use corrections based on Oban tide tables to work out when they are running based on experience. And all too often I hear comments like we're only 10 miles from such and such so the tide should be the same - how long is Islay????

JIM

JIM

[Phil]

Hi Jim
Nice to hear your back, have a good time with the colonials?
Ive managed to persuade K and the Admiral to do a level 3 Sea training course, its a kind of an attempt to address the lack of local training so its being held on the Wirral (Dee estuary). It doesn't stand still very long there either, in fact if you are not quick you have to walk home! (doesnt get very deep either). I think you are right about the river flow situation on the Clyde, it seems to be similar on many estuary's. It may also be partly explained by the "shallow water effect" where the trough of the tidal wave is retarded more than the crest, this distorts the timing such that the period of rise is shorter than the period of fall.
There is also the phenomenon of the amplitude (wave height) increasing as the tidal wave travels up a narrowing estuary resulting (in extreme cases) in a "bore" (theres probably a joke there somewhere!)try applying the rule of thirds to that!
Phil

[Jim]

He he, the Admiral doesn't need much persuading, tell him a course date and he'll usually be booked on it before you've finished telling him :)

Dee Estuary should be a good place for him to learn about tides as the bars have some great phenomena! If you can just teach him to drive to Hoylake it might be useful, I've started buggy racing down there and could do with him knowing the way to give me lifts :)

Pedestrian is down watching the buggies on a Saturday and starts to get worried:
"Excuse me, what time does the tide come in?"
"Tuesday!"

JIM

[philwinchlineonenet]

Yeah should be fun for the lads, well for the four hours or so the tides in at least! Len Hartley is the level 5 in charge of proceedings, The Dee estuary is his back yard so the arguments should be epic!
Your right about the bars, can get quite naughty.Not as bad as the pubs in Holyhead though!

Oh yes, this threads about tides isnt it?
The course has been timed for springs in June. If I asked the Admiral whether the tides were lagging or priming would he know?

Hoylake it is, Ill give K another little job, I know people who dont know its beside the sea at all!

Phil

Is a buggy one of those kite driven things? looks like great fun.

[Jim]

Very drole, you do see some wonderful phenomena in the bars as well as on them :) Hoylake lights is fast becoming a favourite!

Not sure if the Admiral would know about lagging and priming, I don't but I'm sure it has something to do with the patterns of tide cycles. The admiral has done his day skipper theory so he might know, if it's the sort of thing that gets covered.

Yep, the buggies (or parakarts) are powered by kites. I've been to a couple of races hosted by Wirral Sand Yacht Club, and I've applied to join the club so I can use the bar other than open race meetings as you have to be a member with the way the council license it. You won't see anyone there on springs mind! It's loads and loads of fun, even if you don't want to race them (my top speed is 37.5 mph and that wasn't in a race, 40mph is supposed to be a magic speed!)

JIM

[Phil]

Quite right Jim, basically describes whether high water occurrs before or after the moons transit of the meridian.
Between new and first quarter and full and last quarter=Prime, HW before transit.
Between first quarter and full and last quarter and new=Lag, HW after transit.
Its also got something to do with DIY, but the less said about that the better!

Hows about a go on your buggy then? I can fly a parafoil, never tried it on wheels though.

Phil

[Jim]

I see, and since springs are just after New (waxing crescent) and Full (waning gibbeous) they must always be priming.... So how do local tide anomolies affect this?

You can try the buggy anytime, erm, assuming I have it and there is space! Technically only members can use the bar at Hoylake (means the club can't do come and try it sessions so they are looking for a change to that rule) but I only do a bit of my buggying there - I'm normally on the Ayrshire coast or sometimes at St Andrews. If you can sail and fly parafoils, buggying is easy :)

JIM

[Phil]

Good question! answer is, not at all, however the overall picture may create anomalies, Sound of Harris possibly?.
Springs always priming?
Umm, well no again,if you imagine that the water on the earth bulges towards the moon and also bulges towards the sun also each has an opposite antipodal bulge. then think of the relationship between these two bulges you will quickly realise that with opposition and conjunction this will explain springs and neaps. eg at neaps the two bulges are 90 degrees out of phase, hence both lower with a reduced "range"
At springs the two bulges are in phase and work together hence higher tides and with a greater range.
However when priming, the solar bulge is behind the lunar bulge (as when moving away from conjunction at new moon for instance)
Similarly when lagging the antipodal solar bulge is ahead (as when moving towards a full moon)
So in the absence of a diagram, if New moon is on the right, same as the sun (at 3 oclock) and full moon on the left (at nine oclock) with earth in the middle, then working anticlockwise priming is between 3-12 and 9-6 and lagging is between 6-3 and 12-9.

Clear as mud eh?

Its really all about orbits and rotation,in particular the orbit of the moon around the earth and the rotation of the earth as it orbits the Sun. its also quite old-fashioned now (for which apologies). I remember when it was explained to me round a camp fire after I had revealed to my boss I had left the tide tables in the car. The explanation of tidal forces and astronomical relationships that I previously thought I knew, was a bit of a revelation.
Afterwards I understood why there are springs twice every lunar month, but equinoctial tides are only in March and September,and Solstitial tides in June and December. Why Springs and neaps will occur at approximately the same time of day at a particular place, why tides process about 55mins a day and why high waters are not a linear progression. Stuff that to my chagrin I learned was meat and drink to your average viking in the days before western civilisation had discovered the compass! let alone the clock,(springs and neaps are viking words by the way).

Short answer was leave when the water was twenty paces from the stick and it will be bigger than yesterday!

If your down at Whitesands in the Summer Im up for a buggy ride (they zoom about at low water, its about a kilometre long)Im up in Scotland in a few weeks to try the Minch again, smaller team this time, fancy a paddle?

Phil

[Jim]

I thought it would be too convenient and simple the way it read originally :)

My summer is pretty much wide open, except that I am probably going to spend a lot of time at machrihanish with kiting toys because a friend has a placement out that way. Drop me a line about Whitesands, I could possibly be persuaded to head down there, I think I have a couple of friends who still want to do the bitches (I didn't think it was that great). I could cheerfully just take kites and not worry about my boat :)

I'm pretty sure I won't be up for the Minch trip, I think I'm expected to do Bute at the end of the month but I'm not convinced my shoulder will be ready for that, whether it is ready for the minch or not I have a feeling I won't have enough holiday left. I'm sure either you or Nigel will tell me the dates anyway, if I don't already have them.

JIM

[Phil]

Will be starting on the hebrides trip next monday Jim, so its a bit short notice I realise!
Sorry to hear your shoulders crook, get well soon.
Pembroke dates are last week in July first weeks in August usually, will let you know nearer the time?
Phil

[MikeB]

Developing this further. what know we of the "50/90 Rule" ?

Mike.

[JW]

Flow rate at end of first hour = 50% of that tides max flow rate
90% at end of 2nd hour
100% at end of 3rd hour
90% 4th hour
50% 5th hour
0 6th hour

This an easy and useful ready reckoner, but is only a rough & ready 'generic' rule, obviously it takes no account of local anomalies. Even so it can be quite useful used in conjunction with the figures from tidal diamonds.

To find the tidal assistance given in a specific hour then interpolate the figures from the start and end of that hour i.e. for a 3kt tide then tidal assistance in the 2nd hour would be 1/2 way between 1.5 (50%) and 2.7 (90%) = 2.1nm

JW

[MikeB]

So, 12ths relates to vertical movement.

3rds and 50/90 relate to the horizontal (flow) movement. Which then poses the question as to which is the more accurate do we think?

(Forgetting local anolomies).

REgs - Mike.

[Jim]

So, 12ths relates to vertical movement.

3rds and 50/90 relate to the horizontal (flow) movement. Which then poses the question as to which is the more accurate do we think?

(Forgetting local anolomies).

REgs - Mike.

You can't forget local effects if you ask that question!

It depends on the shape of the tide graph for the area in question. Even if you don't oddities like double standing water, some places have steeper tide curves than others and some are skewed - I'll bet at some locations one method is more accurate on the flood, and the other on the ebb!

JIM

[CaileanMac]

Mike,

Personally have used the Rule of 3rds for many years without any major errors or mishaps in terms of tidal planning. Used on trips on W. Coast of Scotland ranging from multi day journeys & open water crossings (up 20m). However I have heard from one or two N.Wales paddlers that the 50/90 works better down there - any one care to shed light on that urban myth?

Jim is on the right track by saying look at the tidal graphs and their shape. Steady shape = 3rds or steeper bulge = 50/90 as big rule of thumb. However despite the best pub table or late night kitchen table planning there is no way of absolutely working out the tides pecisely (too many variables). So do your planning, get on the water and see what's happening and make judgement calls. Personally its part of the attraction of sea kayaking - not actually knowing exactly conditions / wildlife / etc are around the headland. Long live adventure!

Regards
Cailean

[MikeB]

Cheers - like you, I've used 3rds for a great many years - the 50/90 rule came into a debate on ISKA - seems the Irish lads like it, so maybe the Welsh connection holds true.

They (ISKA) seem to regards 3rds as a simplified version of 12ths (ie: vertical movement).

Mike.

[Matt O]

Tide Plotter 2005
www.belfieldsoftware.co.uk
A great bit of cheap software with a fairly comprehensive range of locations around the coast of the UK. Shows a 24hr tidal curve and also monthly tide tables for each location and for the three year version it costs £20. In addition you also get data for a range of West European locations from Norway to Gibraltar.

I know you can get a lot of this info for free from the likes of the BBC website but having it to hand is very convenient. Used in conjunction with the appropriate marine chart and yachtsmans pilot guide you can have great fun planning future trips...

[Jim]

They (ISKA) seem to regards 3rds as a simplified version of 12ths (ie: vertical movement).

Mike.

It is.
It also kind of assumes the flow increases in big steps at the specified times. The theory is that to gain 3 twelfths in hour 3, the flow must be 3 times what it was in hour one when you gained one twelfth.

It is just a rule of thumb and you could probably do a bit of maths using the equations of motion to come up with your own rule of thumb if you want, I just think it's easier to divide things by 3 and 4 (roughly) than to work out what 90% is. Of course those of you brought up on imperial you can probably work twelfths faster than most can do decimals! That after all is the goal, to have something rough and ready you can do in your head quickly.

JIM

[MikeB]

Ultimately the lesson is that most of the water is moving in the middle periods of the flow.

Useful - thanks all. I'll articalise it eventually - in the meantime this discussion is linked from the Almanac.

Mike.