
A prudent navigator should check if instrument forecasts match the realized outcomes. This short blog post examplifies this necessity by examining the difference between the St-Lawrence river’s currents forecasts in the application Navionics (Boating), in the St-Lawrence General Observatory Navigation Forecasts, and with the official forecasts from Canada’s Department of Fisheries and Oceans planning tool. The punchline is that the last two agree with realized outcomes, while Navionics does not.
The example is given near Tadoussac on November 11th 2024 (2200 TUC-5). This date is out of the sailing season. However, the region is a typical spot for a crossing from one bank to another.
The difference in forecasts was first noticed in August 2024. I was then planning a crossing from the Brandy Pot anchorage to the Tadoussac Marina. Around this place, currents can build up to 5 knots. If crossing the river is considered, then the current will be on the beam, and planning a course-to-steer properly is of the utmost importance.
Navionics (Boating)

The Navionics forecast is shown in the screenshot on the left. There is a one hour difference (TUC-4) as the screenshot was taken while being in Nova Scotia. Thus, the time 22:59 TUC-4 is in fact 21:59 TUC-5.
The important point is that the app predicts an Ebb current of 2.69 knots along the direction of the river flow (roughly NNE).
Observatoire Général du St-Laurent (OGSL)


The General Observatory of the St-Lawrence River (OGSL) monitors a variety of aspects related to the St-Lawrence, most of them being biology related. It however has a navigation page, which tracks winds and currents in Québec and in the Atlantic provinces (and I would not be surprised if the page is the current testbed for the next generation of Canada’s official forecasts).
A forecast for the Bay of Fundy’s current is shown at the top of the page, while the forecast on the St-Lawrence, near Tadoussac, is shown at the beginning of this section. The current is forecasted at 2.5 knots at -117 degrees (picture on the left). The « -117 degrees » may be unsettling, but it should be read as « the opposite direction as 117 degrees » which means 297° (117 + 180), or roughly WNW. In other words, this is a flood current.
Canada’s Official Forecast
The difference between the two forecasts should already raise an alarm.

For an official forecast, one must (as of 2024) resort to paper methods to figure out the currents. The official Atlas of currents reports currents by the hour in terms of the tide cycle in Pointe-au-Père (QC). As such, one must look at this tide forecast in the official Tide Tables. In particular, where does 2200 TUC-5 fits in the tide cycle, and how big is the tidal range?
The tide table (excerpt below) informs us that the high tide is at 2227 TUC-5 with 3.5 meters above datum. The low tide is of 1.3 meters above datum (at 1614 TUC-5). Thus, at 2200 we are within the one hour window before the high tide, and the tidal range is of 2.2 meters.

The one hour window before the high-tide is shown in the image at the beginning of this section. Each triplet of vectors represent a full hour current displacement for an average tide. The vectors are also colored, representing the same magnitude in a different way. The triplet at the spot of interest shows a current between 2 and 3 knots, roughly in the WNW direction.
These vectors are for average tides, and must be corrected by a weight indicated by the graph in the top left corner. Because a tidal range of 2.2 meters is slightly smaller than the average tidal range (about 2.5 meters), we must factor by a weight of (roughly) 0.8 to get the proper current magnitude. So the current should be between 1.6 knots and 2.4 knots WNW.
Conclusion
The OGSL forecasting tool and the official currents’ prediction agree on the direction and somewhat agree on the magnitude. The Boating forecast does not. The direction is incorrect (and most often, so is the magnitude). An experienced sailor will know that a 0.5 knot difference in forecasts is not unusual, nor something to be overly concerned about. However, a roughly 5.0 knots difference because of the incorrect direction will lead to an incorrect course-to-steer planning, and most likely a confusion during execution.
I doubt this example is general. I had no need (nor interest) to check for other innacuracies on Navionics. On the St-Lawrence, I however had to cross the river with Jean-du-Sud and since the currents are strong, a classic course to steer planning is required. In the St-Lawrence, the error is systematic. It was monitered and checked a few times from August to November of 2024 the error in this example was present everytime.
This type of error may be gone in the future. Similar errors may also be present in other places. The point is that even with a migration to electronic instruments, the enduring wisdom of double-checking instruments is as valid as it was when the compass was invented. Nothing is wrong with the app, nor the general information it provides, but it displays innacurate current information. That type of error comes with no warning and can only be corrected with human cross-verifications. In other words, a fancy display is not a substitute for a human appraisal.