Yachtmaster Theory - Have I been wrong all this time?

[lw395]

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

I learned the rule of 12ths in a book (aged 18), learned it didn't apply at Studland in a more 'practical demonstration' (aged 18 1/2).

Fair enough it works reasonably well in (much of) Cornwall.

 

[alant]

I really think your village has lost its Pedant!

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..




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

"Not in the English Channel, the flow is completely out of phase with the rate of rise and fall"

When I need to worry about tidal heights/secondary port calculations in the English Channel, I suspect that I might be Tsnami watching!


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

Tell us all why, why don't you, we are gagging to know.

 

[dt4134]

When I need to worry about tidal heights/secondary port calculations in the English Channel, I suspect that I might be Tsnami watching!

I don't understand that statement. Are you saying you don't need to worry about tidal heights/secondary port calculations in the English Channel? You mean I spent two hours needlessly outside Littlehampton waiting for enough height to get over the bar (and into the pub) when I needn't have worried about it?

Interestingly, you have to beware of the cross set from the ebb an hour or two before HW at Littlehampton.

 

[alant]

I don't understand that statement. Are you saying you don't need to worry about tidal heights/secondary port calculations in the English Channel? You mean I spent two hours needlessly outside Littlehampton waiting for enough height to get over the bar (and into the pub) when I needn't have worried about it?

Interestingly, you have to beware of the cross set from the ebb an hour or two before HW at Littlehampton.

"Are you saying you don't need to worry about tidal heights/secondary port calculations in the English Channel?"

Ah!, you were talking about the edge bits.

"Fair enough it works reasonably well in (much of) Cornwall."
But not the rest of the English Channel?

So that we also don't have to spend hours needlessly, please explain your reasons, perhaps with detailed calculations (to satisfy the pedantry), why Studland & Littlehampton are problems, with regard to 1/12ths rule.

 

[dt4134]

Well here's Bournemouth which is similar to Studland.

http://www.pol.ac.uk/ntslf/sadata_tgi_ntslf_v2.php?code=Bournemouth&span=1

lw395 mentioned Studland but could equally well have mentioned Bournemouth or Poole.

Littlehampton is a more sinusodial curve, but does have a significant phase difference between flow & height off the entrance. I was using it as an example in the English Channel where secondary port calculations are important. Yes it is on the edge, but that's where both primary & secondary ports tend to be.

Now I've got into the debate I'd say that whilst the rule of twelths can be a useful tool you do need the experience to know when it isn't suitable. Novices using it could find a little learning is a dangerous thing.

Anyway, I'm not online for a few days, so I'll leave the debate to run.

 

[lw395]

So that we also don't have to spend hours needlessly, please explain your reasons, perhaps with detailed calculations (to satisfy the pedantry), why Studland & Littlehampton are problems, with regard to 1/12ths rule.

Are you seriously a YM Instructor?

As an aside LA (River Arun) is particularly interesting because the current (at least as seen by a smallish boat) often continues to flow out while the tide is rising!
The fresh water flowing down river, a) backs up and fills the harbour from the top while still flowing out, and b) flows over the top of the saltwater that is presumably flowing in below it.

Studland and Bournemouth have a tidal height graph which varies very little apart from a deep dip quite close to low water. It is therefore hopelessly wrong to assume that anything like half the rise of tide will have gone by 3 hrs before LW. Please don't teach people to find this out the non-pedantic practical way, that is sort of the idea of YM theory? ;-)