Boatie versus Navionics versus Almanac tidal height.

[sfellows]

We left the Beaulieu River at 11.30 yesterday. At Stansore Point (the Bar) the Boatie App said the tidal height was 3.1m, Navionics said 2.4m, and my paper-based almanac calculation (based on the Secondary Port) said 2.2m.

Bearing in mind we had 1.1m at the shallowest point over the bar (we draw 2.2m and the sounder is calibrated to show 0.0 = aground), and at LAT the bar is 1.2m, I calculate that the tidal height to be = 2.1, so it looks like Navionics and the Almanac are accurate whilst Boatie is not, by nearly 1.0m - and that's a lot on a shallow bar when my margin of error is 1m!

Has anyone else experienced these discrepancies or have an explanation? I must admit tidal heights and depths is very often a brain freeze for me and I may have got the calculation wrong.

 

[AntarcticPilot]

First, as always, you need to be aware that EXCEPT at tide gauges, interpolation of tidal data is error-prone, especially in the vicinity of rivers or complex tidal areas like the Solent, where two tides meet. Second, in view of the gross error, it looks very much as if Boatie is using an over simple tidal algorithm that doesn't correctly account for some more complex situations such as that in the Solent.

 

[Bilgediver]

I have found Navionics to be remarkably accurate on the occasions I have checked it against an actual tidal height.

 

[Never Grumble]

'What time is high water?' - Yachting Monthly

 

[DFL1010]

Another thing to consider is that barometric pressure can have quite an impact on this. Granted, it's not massive (1hPa = 10mm), but 10hPa 'high', which is more or less what it is at the moment, takes 10% of your safety margin away.

 

[jwilson]

I've never heard of the Boatie app but it probably uses various non-UK Hydrographic methods of estimating height. And all of them can be a bit wrong from time to time due to winds and pressure. Unless it's a "serious" source I regard them all as good for general planning but not for proper navigation. Mind you I grew up sailing the upper Bristol Channel: wait 5 minutes or so and there would be another foot of tide, or less if you went aground. Funnily enough I never did much detailed tide calculations there, above half tide you were OK and that was it. Below half tide here be dragons, unless you were sailing with Bert who started life as the "boy" on a working sailing pilot cutter, in which case you could go through all sorts of interesting muddy channels.

 

[Never Grumble]

I've never heard of the Boatie app but it probably uses various non-UK Hydrographic methods of estimating height.

I have the Boatie App on my Ipad, I paid the extra to get a weeks worth of tidal curves. However they are just nicely shaped curves just like you could draw if you were to work out the height by the rule of twelfths. On that basis I would put limited reliance on them and prefer to look at and use those in Reeds, Admiralty Tide Tables or similar.

 

[cpedw]

Was allowance for BST made in every case?

 

[LadyInBed]

A big HP surrounding the UK at the moment, that will drop the hight a bit.
Tidal Height related to Pressure : Monty Mariner

 

[sfellows]

Thanks for all the responses. I'm going to contact "Boatie" as all the comments are helpful, but my major gripe is the difference in predicted height at that time between the Almanac/Navionics heights (accurate within a margin for error for variables like atmospheric pressure etc.) and Boatie height (not accurate IMO).

 

[Buck Turgidson]

There is probably a disclaimer in the app stating "not for navigation". Not sure what contacting them will achieve.