Since the very onset of electronic position finding (from LORAN on up to GPS), we have taught that these aids are wonderful tools for telling us where we are, but we must still master traditional navigation skills, because the electronics will not tell us the best, safe, efficient route to where we want to go.
Well... that is still true, but there is now a huge new caveat to the topic: namely, most modern navigation apps, called electronic charting systems (ECS), offer some form of automatic routing. In short, they do indeed now offer to tell us how to get to where we want to go.
When this first came out, we made jokes about their advertisements, because the results were terrible and dangerous. Some still are.
But now, more sophisticated ECS like qtVlm have worked on this exercise using official electronic navigational charts (ENC) plus a lot more respect for traditional navigation routing that can be implemented, because of their careful rendition of the ENCs, which include more information than third party proprietary charts used by some apps.
In all cases, the ECS must warn the navigator to carefully check the results before activating the route and blindly following it, which would be negligent. This is, of course, also true with qtVlm automatic routes, but qtVlm already had in place a top quality route checker function, which can be applied to the routes with a button click, plus they present multiple alerts in areas that should be double checked.
qtVlm calls such routes "S57 Motor Routes," because they are based on S-57 internationally standardized ENC and the assumption is you are under power, so you can steer any direction you choose.
Then, when planning ahead and not in the boat already, you move the boat to the place you want to start, and then go to your desired destination and place a mark at that location, then right click that mark, and choose S57 Motor Route. That will initiate the computation of a proposed route (in the form of a pathway), taking multiple factors into account, which we go over below.
The route depends on the dimensions of our boat to account for water depth, bridge clearance, transiting locks, narrow channels, and even entering and leaving marinas! Also, there is the option for larger vessels to follow traffic separation schemes (TSS, "shipping lanes"), or (the default) for smaller recreational boat who would like to avoid traffic and must interact with the TSS according to Rule 10 in the COLREGS.
Prior to computing a route we need to enter a few permanent settings:
• Draft (keel depth below waterline, plus your safety margin)
• Air draft (max height above waterline, plus your safety margin)
• Length overall (A+B in the GPS location diagram)
• Beam (C+D in the GPS location diagram)
• Intended start time and date
• Intended knotmeter speed (so you can export a route plan with times)
• If LOA (A+B) > 20m (66 ft) you can choose to participate in TSS
The time and speed inputs are important to qtVlm, but not a usual input for other ECS, because qtVlm can load tide and current forecasts from the Operational Forecast System (OFS) model, which provides tide height and tidal currents across most US waters by GRIB download. When those data are loaded, or coastal current data from coastal models such as NCOM or RTOFS, the routes will take that into account when finding the most efficient route.
Forecasted water depth can be accounted for because qtVlm has the unique capability to also extract from the OFS data the actual water depths at every point on the chart, so it will account for those when applying your draft and air draft plus safety factors.
If you do not have OFS water depth or current GRIB data loaded, then the motor route ignores both tide and current. The depths controlling the route will then be the depth areas between contours, which you can read with a cursor pick on the chart. If the depth area is 6 to 30 ft, meaning its shallow side is defined by the 6-ft depth contour, and you entered a draft plus safety factor of 8 ft, then the motor route will not cross the 6-ft contour.
On the other hand, with OFS tides loaded and the tide in the area is 5 ft during your selected time of the voyage, then that depth area is effectively 11 to 35 ft and the route could cross it. Clearly in cases like this, we have to be sure we are traveling at the time we said we were — but there are various alerts to warn of deviations.
(To be a bit more specific: in routes with tides, the computation does not use OFS tides + charted depths, but rather it uses the OFS water depth, which is computed based on its own digital bathymetry of the area.)
With OFS tidal currents loaded, you could have the option of passing either side of an island, with favorable current on one side and unfavorable on the other. The route would typically choose the favorable side. Without current data loaded, the choice of side could be different.
Likewise, taking a notable shortcut under a bridge can depend on the tide height, because the charted clearance is relative to MHW. At near-zero tide, you could pick up a lot of extra clearance in some areas.
If you wish to avoid a particular region, qtVlm lets you draw in a barrier line or area that the route will not cross.
Once the route is computed, you can view and export a detailed route log with times, distances, and headings on each leg. The export could be as CSV for printing or as a GPX file to load the actual route into another device.
The video below illustrates some of these features.
[ to be added ]
