PNW conflicting current & Tide data

[Bilgewater]

I've known this for quite a while but I thought I would post the data I have come up with for tomorrows currents at Sechelt Rapids and all the conflicting data. It happens that I have to take my work boat through there at full ebb and it's running pretty good at that time so I went through all my normal tides and current sources and here is what I come up with.

Sechelt Rapids - Current data - Saturday June 18th 2011

Nobeltec Tides and Currents Pro (laptop software)
Max 12:48 - 8.1 Kt ebb
Slack 16:24

CHS - Canadian Hydrographic Service (publication)
Max 12:53 - 11.6 Kt ebb
Slack 16:43

Aye Tides App (iPhone app)
Max 13:10 - 12.19 Kt ebb
Slack 16:41

Tide App (iPhone app)
Max 13:11 - 12.19 Kt ebb
Slack 16:41

Ports & Passes (publication)
Max 12:54 - 11.6 Kt ebb
Slack 16:43

Navionics (iPhone chartplotter app - embedded tides and currents data)
Max 12:50 8.12 Kt ebb
Slack 16:27

OpenCPN built in tide & current tables (Is not corrected for DST)
Max 12:11 + 1hr - 12.19 Kt ebb
Slack 15:39 + 1 hr

It's not just these rapids I get these wide variations, sometimes they're pretty close but in this case we have a variation of 4.09 kts. and I've seen a greater variation but this is quite substantial. Obviously if it's a critical crossing, I trust CHS or Ports and Passes but I use the others quite regularly for convenience purposes. In any case, on a large tide the numbers can be quite misleading in some cases.

[erps]

Wow. There is nearly twenty minutes difference in the listed times for slack and a 50% difference between 8 knts and 12 knts for the predicted ebbs. Must be the new fuzzy math they're teaching in schools?

[jrd22]

Well at least now I can blame bad data if I forget that all the Canadian tide books are listed in standard time, not daylight saving time:-))
Thanks for the heads up Steve!

[Bilgewater]

Ya, this is pretty crazy data for sure. I never really knew why CHS publishes the listings in Standard Time but Ports & Passes (a Canadian publication and my personal favorite) are corrected for DST and I also think they might get their data through CHS.

[blt2ski]

I wonder if some are going by old ave data, or some might be going be past years what they think will happen based on ave data. OR maybe they got together and had a beer and some one said "here hold my beer while I throw a dart at the target for an answer!"

I am sure some of this is a best guess estimate, but some has to be based on some kind of know science as to what has or is happening in the universe. I am sure some variations will occur due to larger than normal spring runoffs as we are having right now too. One would hope that those variations would not get into the future swags as to what is going to happen. South where I am, there are some pass's that one end will vary max wise vs the other by 15-20 min, and be greater on one end than the other, slack will be sooner vs later.

marty

[Bilgewater]

I updated the OP to include the OpenCPN PC & Mac chartplotting application's built in tide & current table data. OpenCPN was updated July 27 to V2.50 along with the tide/current data.

Steve

[jackdale]

Originally Posted by seayalatermoonglow

Ya, this is pretty crazy data for sure. I never really knew why CHS publishes the listings in Standard Time but Ports & Passes (a Canadian publication and my personal favorite) are corrected for DST and I also think they might get their data through CHS.

An oldy but goody.

Standard time is used with the Z difference noted because ocean going vessels use UTC.

The first time I used Ports and Passages there was a discrepancy between it and CHS numbers. I use CHS tables. I also have OpenCPN on the netbook and Navionics on the Android phone.

I also use a form to make calculations for secondary ports and stations. As I head for the Yucultas or Beazley Pass / Upper Rapids (or any other pass) these are posted on the walls near the nav station. I also use one for tides.

They are posted here

Swiftsure Sailing Academy

[Bilgewater]

Originally Posted by jackdale

An oldy but goody.

Standard time is used with the Z difference noted because ocean going vessels use UTC.

The first time I used Ports and Passages there was a discrepancy between it and CHS numbers. I use CHS tables. I also have OpenCPN on the netbook and Navionics on the Android phone.

I also use a form to make calculations for secondary ports and stations. As I head for the Yucultas or Beazley Pass / Upper Rapids (or any other pass) these are posted on the walls near the nav station. I also use one for tides.

They are posted here

Swiftsure Sailing Academy

Good work sheets Jack...Thanks

Speaking of secondaries, for those using "Ports and Passes" there are links/downloads for extra tables on their website which are not included in the printed publication: Downloads from Ports and Passes
They have some secondary tide and current tables, including Malibu Rapids "tides (not current)", Hole in the Wall "current", Nahwitti Bar "current", Second Narrows "current" and numerous others up and down the coast.

[zz4gta]

On the Ches Bay tides and current can vary from dead on to 2-4 hours off. I've been in an overnight race where the flood never showed up. Tides are from the moon, Currents are what you see on the water. They don't always match up.

[smurphny]

Seems to be a pretty common problem that published data is not what happens with tides and currents. I contacted Eldridge last summer about a discrepancy around Fishers Island here in the NE. I wasn't even cutting it close and wound up bucking 2 knots for 2 hours. They were interested in the info but indicated that a lot of the current data was compiled a long time ago and that the coastal survey was due to be reassessed soon. So I guess there is no good way to predict with any accuracy unless you've been nearby for a few days and become tuned to the tides. This is too bad because we sailors really depend on going with the current. I don't know about anyone else but I base most of my departure times on current information. The sunset picture in my page is close to where the discrepancy occurred:-)