Trying to work out depths for Secondary ports.

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CraigB

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27 May 2005
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145
Location
Brighton, UK
ladyayesha.blogspot.com
Hi folks, I'm having some problems understanding how to calculate LW heights for seconday ports.

Here's a sample to explain, actually taken from the RYA YM book, for which I can't find an explanation:

Cherbourg (Standard Port)
TIME HEIGHT
0633 1.0
1211 6.0
1856 1.5

Differences - GOURY

HEIGHT
MHWS 6.4 +1.7
MHWN 5.0 +1.6
MLWN 2.5 +1.0
MLWS 1.1 +0.3

No problem when the LW height at Cherbourg is within the secondary range of differences, for example the LW at 1856 is 1.5 which falls within the range 1.1 to 2.5 and can be interpolated easily using the crocodile jaws or linear method.

But what about the LW at 0633 which falls outside this range - do you interpolate further backwards, turning the jaws into a pair of crossed lines (if the axis is 1.1m/+0.3 as usually seen)?

I know that in real life, it might be easy to say that the difference is marginal so just pick the LW value+0.3, but the deviation of data which makes up the MLWS value may be high, with some LW heights being half a meter lower than the MLWS value.

In the RYA example they just take the difference at 0633 to be +0.3 with no further explanation, but if interpolated as suggested above would show a difference of about +0.25.

Yes it's a bit pedantic but I'm also concerned about how to answer the YM theory question correctly.

Cheers, Craig.
 
We were told to get to .1 of a metre so .3 would be acceptable. In theory you should interpolate backwards if it was significant.

Secondary ports caused more trouble than anything else but I was the only one in the class who used a calculator and always got the most accurate answer in a fraction of the time. Not because I'm clever (although I am very numerate) but because it is the easiest. When I bought my new boat there was a calculator in the nav table!
 
In answer to your question; yes. The MH and ML figures are just that, and enable you to construct the heights diagram. There's no reason why you can't go beyond the mean figures if thats the actual height for the day, extending the grid as you do so.
 
You can extrapolate if you wish, but the normal RYA convention is to take the upper and lower limits; less accurate, but good enough.

There are other methods if you do not like drawing triangles, personally I spend too much time getting the scales right!

see sec ports
 
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the normal RYA convention is to take the upper and lower limits; less accurate, but good enough.


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Sorry but you're wrong. the RYA requires correctly that you do extend beyond the low and high water limits if that days tides require it. And also , for example, in the calculation of HAT

Mind you , anyone who relies in a tide calc to within 0.1m is a plonker in my view.
 
The Belfield Tideplotter program gives tides for most ports in Western Europe for PC or PDA. It costs £10 for a year or about £20 for 3 years. I find it very useful as a quick check but you should use e.g. Reeds if the depth is critical.
 
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Mind you , anyone who relies in a tide calc to within 0.1m is a plonker in my view.

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You clearly don't sail from or to Christchurch ( Dorset)
That's nearly four inches!!!! Mind you we've got to look at the barometer now 'n again.(and the deptho)
Cheers /forums/images/graemlins/crazy.gifBob E.. /forums/images/graemlins/laugh.gif
 
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You clearly don't sail from or to Christchurch ( Dorset)
That's nearly four inches!!!! Mind you we've got to look at the barometer now 'n again.(and the deptho)
Cheers /forums/images/graemlins/crazy.gifBob E.. /forums/images/graemlins/laugh.gif

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Nope - Bristol channel. If we wait another 20 minutes, that can be another metre. /forums/images/graemlins/grin.gif
There's either not enough water or altogether sufficient.

With the effect of waves, the baro pressure and wind direction - not to mention the inaccuracies of the tide tables and prediction algorithms, you reall cant rely on 10cm in a book calculation - anywhere round the UK. IMHO.
 
You're right there.... Lots of peeps seem to forget that there are troughs as well as peaks in a wave pattern.....The Run at Mudeford is really scarey sometimes but we always have a scotch on Allegretto to celebrate a safe homecoming and it seems to make it all worthwhile..
A pivoting plate 3ft deeper than my 3 ft6ins draught is the bestest depth gauge I've got..
cheers Bob E....
 
I want to thank everyone for the answers you gave. I think that the extrapolation method beyond the ranges looks like the most accurate (and of course, without local knowledge we'd never sail somewhere with only 10cm calculated clearance!).

Oh, and we both passed the YM theory exam this morning too. Now for a few thousand sea miles of preparation for the practical exam!
 
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Mind you , anyone who relies in a tide calc to within 0.1m is a plonker in my view.

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Calculating an accurate tidal height doesn't mean that you can't leave a margin of error; it just means that your margin of error can be less.
 
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