Navionics iPad tidal data

[fastgrandad]

Not sure whether this should be posted here or in electronics section, but here goes.

I'm planning a bareboat charter in the Windward Islands in March and looking at cross-track tidal streams on passage from Grenada to Carriacou, having bought the Navionics charts for the area. Not a long passage but at its peak the stream could change set and drift depending on the state of the tide so it would be nice to do the RYA course plot.

When comparing the on-line tide table information with that presented on the Navionics chart, there is a huge difference in the times at high and low tides. I've compared several on-line sources and they agree to within a few minutes (although they probably use the same primary source) but the Navionics offset varies by between 1 1/2 and 3 hours, even after correcting for UT and AST, and over just a single day! So it's not a time offset problem.

Does anyone have an explanation or tell me what I'm doing wrong.

Thanks

Paul

[fastgrandad]

Hi hoghunter, thanks for coming back on this and apologies for the delay in replying.

I'm looking at tide tables, i.e. UTC or local times (AST), of high and low tide in St Georges Grenada using two sources, the Navionics iPad app and an on-line tide table on Windfinder. I appreciate that the tidal range is only about 30cm and therefore inconsequential in its own right, but the tidal streams change direction in sympathy with the tide times and the cross track streams are significant over an open water passage and I would like to know what the tidal stream is doing.

My preferred ready access source for tide and tidal stream data is the Navionics package on my iPad, however, when comparing the high/low tide time data from Navionics and that published on Windfinder, which I assume comes from NOAA or local sources, there are discrepancies of more than an hour in some cases and the times never agree although heights are close. Navionics times are UTC and local sources AST (=UTC -4) and I have allowed for the difference. Interestingly, sunrise and sunset times are identical. Incidentally, I've checked several on-line sources and they all agree although their prime source is probably the same and they all could be wrong

As things stand there's no way I would be sure which is correct although I'm inclined to believe the Windfinder data but won't trust either until I've sorted this out.

I probably need to contact Navionics but I'm interested to see if anyone on here has had a similar experience or can throw some light on the problem.

Paul

[Jammer Six]

Originally Posted by hogHunter

The wind data is from NOAA.

Speaking from experience, once Navionics get ahold of it, it ain't NOAA's data you see.

[tide_guy24]

WindFinder appears to be using an old XTIDE database to create their predictions. I say "old" because years ago XTIDE stopped including harmonics for stations outside the US. XTIDE got the data for St. George's Harbour from Jean-Pierre LaPointe, who got it from the French publication "Table des Marees des Grands Ports du Monde" which used to be the place to go for tide harmonics from around the world (but no longer, sad to say).

The predictions from the old database don't match the SHOM website for St. George's, but they're not out by a lot. I'd say you can still use them for your intended purposes.

I have no idea where Navionics gets their data from.

[fastgrandad]

thanks Tide_guy, that's very informative, you're clearly very close to this stuff

I think I'll wait until I get out there next month and see what the locals use and take it up with Navionics in the meantime. If I get any sense out of them I'll report back.

Paul

[simonpickard]

Did you get any more info on this?
We are also finding our tides are totally out here in French Poly.

[fastgrandad]

No, not really. I bought Imray paper charts for the trip and these show tidal diamonds at much the same locations as the Navionics tidal stream locations, and the set and drift of which are related to HW Georgetown, Guyana. These proved to be pretty reliable but haven't tried to reconcile them since.

My original Navionics purchase was UK and Holland, to which we added S. America and the Caribbean. I suspect part of the confusion is time zones but there's probably even more to it than that.

I plan to have a conversation with Navionics at the Southampton boat show in September to try to get to the bottom of it. If I get any clarity I'll post again.

Paul

[jtulls]

When I've had questions and contacted customer service in the past, the Navinoics reps were incredibly responsive and helpful -- if you don't want to wait until September, for a better answer, maybe just shoot them an email or call.