Tide time differences....

[Way]

Sure there's a simple explanation...but why do NTSLF tides for eg show different times to my Almanac?

Appreciate there might be different ways of calculating but surely within reason with modern technology there can only be one answer?

 

[l'escargot]

They are only predictions. Different models come up with slightly differing results. Reality will often differ from all the predictions too. There is no totally accurate way of getting a tide time.

 

[Way]

They are only predictions. Different models come up with slightly differing results. Reality will often differ from all the predictions too. There is no totally accurate way of getting a tide time.

makes sense. I'm a product of a time where technology provides too many answers and sureties!

 

[nigelmercier]

There is also a lot of BS about tide calculations. When I did my YM theory the calculations had to be "accurate" so you get results like 07:06. Totally meaningless; "sometime between 07:00 and 07:15" is more than close enough. I can't even predict the length of a car journey closer than that.

 

[LadyInBed]

and as the curve is more an inverted U than a V you would need a lot of sample points or calculus to find the max, and it depends on how many harmonics are used, and it depends on the weather that day.
Oh hell, it's just a best guess, but a bit better than a weege board or consulting an astrologer!

 

[Kerenza]

Just working out some tide times.

Leaving Cardiff for Bristol on evening of 4th July.

Harbour Authority gives 1740 3.5m
Local marina LW 1754 2.9m
Neptune planner 1810 3m
Navionics 1815 3.07

Lock times outbound on the hour and half hour.

35mins and 0.6metres difference.

2.5m draft plus 2m siltation. total clearance needed 4.5m

1830 Ok by harbour authority
2000 OK by Navionics

1830 will hit my window for river entrance, 17.5Nm distance - (if I can believe that tide prediction)
2000 I'll be late!

Think I'll call the astrologer!

 

[Giblets]

Just working out some tide times.

Leaving Cardiff for Bristol on evening of 4th July.

Harbour Authority gives 1740 3.5m
Local marina LW 1754 2.9m
Neptune planner 1810 3m
Navionics 1815 3.07

Lock times outbound on the hour and half hour.

35mins and 0.6metres difference.

2.5m draft plus 2m siltation. total clearance needed 4.5m

1830 Ok by harbour authority
2000 OK by Navionics

1830 will hit my window for river entrance, 17.5Nm distance - (if I can believe that tide prediction)
2000 I'll be late!

Think I'll call the astrologer!

Probably best to go by car then!

 

[bitbaltic]

Just working out some tide times.

Leaving Cardiff for Bristol on evening of 4th July.

Harbour Authority gives 1740 3.5m
Local marina LW 1754 2.9m
Neptune planner 1810 3m
Navionics 1815 3.07

Lock times outbound on the hour and half hour.

35mins and 0.6metres difference.

2.5m draft plus 2m siltation. total clearance needed 4.5m

1830 Ok by harbour authority
2000 OK by Navionics

1830 will hit my window for river entrance, 17.5Nm distance - (if I can believe that tide prediction)
2000 I'll be late!

Think I'll call the astrologer!

We sail from Cardiff. Notice you don't mention reeds in your list of tide sources.

We always use reeds and always do the secondary port calculation. Results generally very accurate when we check height of tide with barrage control. Your sources are all local data where the secondary port calculation has been done by the source in ways unknown to you. Doing it yourself removes an obvious potential source of error and variation (that tide predictions are approximations notwithstanding.)

Main problem is the embarrassing degree of silting always in the approach channel regardless of design specs or recent dredging. Barrage usually very reluctant to be honest about it either, suggest calling them for a lock out at low water with your draft to get real numbers from them. However 2m (1m sill and 1m silt) sounds about right at the moment.

Cheers

 

[LadyInBed]

35mins and 0.6metres difference.

They could all be correct! Remember U not V
It just depends where the software takes the sample points.

 

[Kerenza]

They could all be correct! Remember U not V
It just depends where the software takes the sample points.

Point (sic) taken, but even a U must have an origin?

Bitbaltic
Reeds uses UKHO, accessible as easytide. Usually similar to Navionics, by manual calc or from Internet site. Neptune also uses UKHO !
HA use Proudman which should be the most accurate prediction. It's what the barrage uses.

No cil to worry about- 2m silt, due to be dredged next month.

Ploughed all the way in last Friday evening, we got clear over the entrance, then stuck alongside. Flows scour the middle of the lock. Back to a u shape!

 

[LadyInBed]

Back to a u shape!
More like pear shaped

 

[Mark-1]

and as the curve is more an inverted U than a V you would need a lot of sample points or calculus to find the max, and it depends on how many harmonics are used,

Seems most likely explanation. In terms of time the low/high points must all agree exactly on average otherwise they'd drift apart by an increasing amount.