Passage Planning Workflows
Putting it all together — complete worked examples that show how qtVlm fits into the full passage planning process, from initial route research to real-time updates underway.
Workflow 1: Coastal Overnight Passage
A 24-hour coastal passage is the simplest complete weather routing workflow and a great place to practice the full process. Let's walk through routing a passage from Annapolis, Maryland to Norfolk, Virginia — about 150 nautical miles through the Chesapeake Bay and into Hampton Roads.
Step 1: Set up the workspace. Load a high-resolution regional GRIB (NAM at 3 km if available, or GFS at 0.25°) covering the Chesapeake. The small area means the file is tiny even at high resolution. Step through the entire forecast to understand the wind pattern — is there a sea breeze cycle? When does the wind build? Is there a front coming?
Step 2: Create the route. Create POIs at Annapolis and Norfolk. Since this is a bay passage with navigational constraints (channels, bridges, shoals), create a Pathway with waypoints at key navigation points — the Bay Bridge, the main shipping channel entrance, the Hampton Roads approach. Run the Routing with a departure time that puts the strongest wind during your waking hours.
Step 3: Analyze and refine. Check the logbook for the overnight hours. If the router puts you in 20+ knots at 0300 with a gybe at the York River, consider shifting departure by 6 hours. Compare 3-4 departure times using the statistics view. Pick the one that balances passage time with manageable nighttime conditions.
Step 4: Export and brief. Simplify to 8-10 waypoints, export GPX to the chartplotter, print the logbook, and brief the crew. For a bay passage like this, the weather routing adds the most value in departure timing — choosing the right 6-hour window can turn a slog into a pleasant sail.
For coastal passages under 48 hours, the routing value is primarily in departure timing rather than route optimization. The geographic options are limited by coastline and navigation constraints — but choosing to leave Tuesday evening instead of Wednesday morning might save you 4 hours and keep the nasty bit in daylight.
For a short coastal passage, where does weather routing add the most value?
Workflow 2: Multi-Day Offshore Crossing
An offshore crossing is where weather routing truly shines — the open ocean offers infinite route options, and the right path through the weather can save days and avoid dangerous conditions. Let's walk through routing a passage from Bermuda to the Azores — about 1,800 nautical miles across the open Atlantic.
Step 1: Big-picture weather analysis. Download a GFS 0.5° GRIB covering the full North Atlantic with a 10-day forecast. Step through the entire forecast and identify the major features: the Bermuda-Azores High position, any mid-latitude lows tracking across, the trade wind belt boundary, and the jet stream pattern. You're building a mental model of the weather evolution over your passage duration.
Step 2: Initial routing. Create the Routing with realistic parameters: polar efficiency at 90%, tacking penalty of 10 minutes, night efficiency at 85%, wind limit at 30 knots. Run the route and examine the result. The router might produce a surprising route — perhaps curving well north or south of the rhumb line to exploit a favorable pressure gradient.
Step 3: Multi-routing for departure. Run the same route for 4-6 departure times spaced 12 hours apart over the next 3 days. Compare the statistics: total duration, maximum wind, percentage of beating. Often one departure time is dramatically better because it catches the right phase of a passing weather system. This analysis is the single most valuable thing weather routing provides for ocean crossings.
Step 4: Iterative refinement. Once you've selected a departure time, add constraints if needed. If the route passes too close to a developing low on Day 5, add a Boundary around that area and re-route. If you need to cross the Gulf Stream in a specific area, add a Gateway. Simplify, optimize, export, and create a detailed passage plan with decision points.
For routes beyond 7 days, the GRIB forecast for the later portion is unreliable. The routing algorithm doesn't know this — it treats Day 8 data with the same confidence as Day 1. Plan to download fresh GRIB data every 2-3 days underway and re-route with updated forecasts. Never commit to the Day 7-10 portion of a route calculated before departure.
Why might the weather-optimal route be longer in distance than the rhumb line?
Updating Routes Underway
A route calculated before departure is based on the best available forecast at that moment. But forecasts evolve — the weather 4 days from now will be predicted differently tomorrow than it was today. Updating routes underway with fresh GRIB data is how professional sailors and ocean racers stay on the optimal path throughout a passage.
The update workflow is: download fresh GRIB data (via satellite phone, SSB/SailMail, or Iridium GO), load the new GRIB in qtVlm replacing the old one, create a new Routing from your current position to the destination, and compare the new route with your existing plan. If the new route is significantly different, update your plan. If it's similar, stay the course — small forecast changes don't warrant major course corrections.
How often to update depends on passage length and conditions. For a 5-day crossing, download fresh data every 24-36 hours. For a 2-week crossing, every 48-72 hours is sufficient except when approaching complex weather (fronts, developing lows). Near significant weather systems, update daily. Each update gives you a fresh routing that incorporates the latest forecast data.
Be cautious about chasing routing updates. If every new GRIB produces a slightly different optimal route and you change course each time, you'll sail a zigzag that's worse than any single route. Significant course changes should be driven by significant forecast changes — a new weather system, a faster-moving front, a shift in the high-pressure position. Small forecast adjustments rarely justify course changes.
When downloading GRIB data at sea via satellite, request a small area centered on your current position extending to the destination, at 1.0° resolution, wind and pressure only. This minimizes download time and satellite data costs while providing enough data for routing.
Why should you avoid changing course with every small GRIB update?
NMEA Integration and Live Navigation
qtVlm can connect to your boat's instrument system via NMEA (the standard protocol for marine electronics), turning it from a planning tool into a live navigation display. With NMEA input, qtVlm shows your real-time position, speed, heading, and wind data overlaid on the chart alongside your routed path.
To configure NMEA input, go to Connections → NMEA and set up a serial port (USB-to-serial adapter from your instruments) or network connection (if your instruments broadcast over Wi-Fi or Ethernet). qtVlm reads standard NMEA 0183 sentences and NMEA 2000 data via gateways. Once connected, your boat's position appears as a moving icon on the chart, tracking along your route in real time.
AIS data (Automatic Identification System) can also be displayed in qtVlm when received via NMEA. AIS targets appear on the chart as ship icons with course/speed vectors, giving you traffic awareness alongside your weather routing display. This is particularly valuable when routing through shipping lanes or approaching busy ports — you see both your optimal weather path and the commercial traffic pattern simultaneously.
The combination of weather routing and live NMEA navigation makes qtVlm a capable offshore navigation tool — not just a planning application. During a passage, you can monitor your progress against the routed plan, see whether actual conditions match the forecast, and re-route from your current position when needed. Many offshore sailors use qtVlm as their primary navigation display alongside a dedicated chartplotter as backup.
Even if you don't plan to use qtVlm for live navigation, test the NMEA connection at the dock before departure. Seeing your real-time position and instruments in the same software where you do weather routing is extremely valuable for comparing actual versus predicted conditions during the passage.
What does NMEA integration add to qtVlm during a passage?
Summary
Coastal passages benefit most from departure time optimization — compare 3-4 departure times to find the weather window that turns a difficult passage into a comfortable one.
Offshore crossings use multi-routing to find the best departure, then iterative constraint refinement — the optimal route may curve far from the rhumb line to exploit favorable wind.
Download fresh GRIB data every 24-72 hours underway and re-route from current position — but only change course for significant forecast changes, not every small update.
NMEA integration turns qtVlm into a live navigation display showing real-time position, instruments, and AIS traffic alongside your weather route.
Weather routing is part of passage planning, not a replacement — it works best combined with traditional planning, seamanship judgment, and crew management.
Key Terms
- Multi-routing
- Running the same route for multiple departure times to compare passage duration, conditions, and find the optimal weather window
- Route update
- Downloading fresh GRIB data underway and recalculating the route from current position — essential for passages longer than 3-4 days
- NMEA
- National Marine Electronics Association protocol — the standard for connecting marine instruments, enabling live data display in qtVlm
- AIS
- Automatic Identification System — displays commercial and recreational vessel traffic on the chart when received via NMEA
- Rhumb line
- The constant-bearing straight line between two points on the chart — often not the fastest route when weather is factored in
- Weather window
- A period of favorable conditions for a passage — weather routing's primary value for short passages is identifying the best window