TIDES: TIME / DISTANCE RATIO

[TC]

OK,
Don\'t know if I\'m gonna word this correctly, but hopefully, you\'ll get the gist.................

Does anybody know what the ratio is between distance offshore and the times of high & low water? (eg add / subract 2 mins for every mile offshore???)

Also (as a bit of an aside, but still related to the subject), is there a point offshore (distance-wise) where there is no (or minimal) tidal movement?


[Edited on 19/7/2005 by TC]

 

[DaveMason]

If you look on the Admiralty charts there is a legend on the bottom . These take the form of little pink triangles ( No , not that type ) , these correspond to the marks ( a-b-c-d-e ) on the chart and give the time \"Lag\" of the tide to the offshore position on the chart . It also tells the tide speed relative to the tides ( springs or neaps) . The whole North sea is influenced by the tide movement and as a rule of thumb , rising tide is North to south flow and falling tide is South to north flow . I\'ll show you next time your on board .

 

[Norman]

Know what you mean TC, Daves right all the north sea flows, but obviouslly different, areas outsid major rivers are difficult to predict because outflow goes both North and South very close in then tapers north half a mile or so out. As for lag, I work on about an hour or so if I`m more than 3 mile out, just a view mind , no real science behind it

 

[mark]

there are points with minimal differnce between high and low water, but they are a long long way away, mainly in the middle of big oceans where there is little disturbance to the tidal flow, I remember the guy I sailed with when I did the RYA offshaore navigator thing, dragged out the charts for St Helena - all of 3foot between high and low water at springs. 3ft in the south atlantic can be 37ft at the same time at bristol

the only place where there is an almost continuous flow but no difference in height is the southern ocean where it just sort of goes round and round the pole, its land masses and underwater geography that make it all a bit ad hoc.

the inner hebrides have completely mad tidal streams diue to the hundreds of islands and underwater reefs - you can get over 10 knots in some places, was in an orkney with a 30hp in the corran narrows a few years back, engine going flat out into the tide and we were slowly going backwards!

 

[Charlton]

Tony i used to work on three hours difference at the ten mile mark and pro rata before and after that. That just about fits in with what Norman wrote. Found that used to work quite well so i could get at wrecks when there was little tidal current.

 

[TC]

Thanks for the replies, lads......very interesting.
I\'ve often wondered what the time difference would be.