Quick Facts
| Free Tier | Windy.com, Passage Weather, zyGrib — adequate for planning, not offshore routing |
| Subscription Tier | PredictWind, LuckGrib, Weather4D — $15–50/month, ECMWF access, satellite integration |
| Professional Routing | $300–800+ per ocean passage from a qualified meteorologist |
| Isochrone Routing | Time-stepped optimal-path calculation using polar performance data |
| Expedition | Racing-grade, Windows only, complex — overkill for most cruisers |
| PredictWind Models | PWE and PWG — proprietary high-resolution models alongside GFS/ECMWF |
The proliferation of weather routing software has made it genuinely difficult to know what to buy, what to subscribe to, and when the software alone is insufficient. The category spans from Windy.com — a free, browser-based visualization tool used by millions of people who are not sailors — to Expedition, a Windows application used by professional offshore racing teams and so complex that most cruising sailors will never unlock half its features. Between those poles is where most offshore sailors need to land.
How Isochrone Routing Works
Before comparing tools, it is worth understanding what routing software is actually doing when it calculates an optimal route. The core algorithm is isochrone routing — a time-stepped, forward-propagating search for the fastest path from departure point to destination through a forecast wind field.
The software starts at your departure point and asks: given the forecast winds in all directions from here, how far can your boat reach in the next 3 (or 6) hours in each direction? Using your boat's polar performance diagram — a chart of boat speed at every combination of true wind speed and angle — it calculates a set of positions reachable in that time step. Those positions form an isochrone (equal time line). In the next time step, it repeats from every point on that isochrone, using the updated forecast wind for the new position and time. The process continues until an isochrone reaches the destination, and the path of minimum time is traced backward through the calculation.
The practical implications are several. Accurate polars matter — a poorly characterized polar produces inaccurate routing. Forecast quality matters — isochrone routing through inaccurate GRIB data produces confidently wrong routes. And the route it produces is the fastest route, not necessarily the most comfortable, safest, or most conservative route. Sailors routinely override isochrone suggestions when the fastest route passes through conditions they do not want.
PredictWind: The Offshore Standard
PredictWind has become the de facto standard for serious offshore cruising, and for good reasons. It is not the cheapest option, but it delivers a coherent package of tools that address the specific challenges of offshore passage planning in a way that no other platform fully matches.
The platform's most distinctive feature is its proprietary weather models — PWE (PredictWind European) and PWG (PredictWind Global) — which run at higher resolution than GFS and are calibrated against observed marine conditions. These models run alongside GFS and ECMWF access, so users can compare four models simultaneously. For departure planning, PredictWind's departure optimizer tool is particularly useful: it evaluates a range of departure times over a 7-day window and ranks them by passage time, giving a clear picture of which window is genuinely best for a given route.
The satellite integration — optimized GRIB downloads over Iridium, Iridium Go, and Iridium Go Exec — is among the best available, with file-size controls that make offshore data management practical. The offshore subscription (approximately $45–55/month depending on tier) includes ECMWF access, proprietary model data, satellite-optimized downloads, and the departure planner. It is the closest thing the offshore cruising world has to an all-in-one platform.
Expedition: Racing-Grade, Windows Only
Expedition is what professional offshore racing teams use. It is the most capable routing platform available in terms of raw functionality: advanced isochrone routing, polars management, layline calculations, instrument integration, chart display with GRIB overlay, and a depth of configuration that takes months to master. If you are racing offshore at a competitive level, Expedition is the industry tool.
For most cruising sailors, Expedition is overkill in the same way that a Formula 1 car is technically a valid way to commute. It runs only on Windows, which creates a hardware dependency that cruisers with Mac or iOS chartplotter setups cannot accommodate. The interface is functional but unforgiving — there is no handholding, and the learning curve is steep. The pricing model (perpetual license approximately $1,200–2,000 depending on configuration, plus ongoing updates) reflects its professional-user market. Recommend only if you are racing, have a Windows navigation station, and are prepared to invest significantly in learning it.
LuckGrib: Mac and iOS ECMWF Access
LuckGrib fills a gap that PredictWind and Expedition both leave: a well-designed Mac and iOS application with direct ECMWF access, excellent offline GRIB management, and clean visualization. For Mac-native navigation setups, LuckGrib is the most practical way to get ECMWF data in a usable format.
The application's offline caching system is particularly well-suited to offshore use: download GRIB data in port, view it at sea without connectivity, and reload when you next have access. The GRIB area selection and resolution controls make file-size management straightforward. It does not have isochrone routing — it is fundamentally a GRIB viewer rather than a routing tool — but used alongside a separate routing application or manual route analysis, it provides ECMWF access that has historically been difficult to get on Mac without a PredictWind subscription. Pricing is a one-time Mac App Store purchase with ECMWF data access via subscription (approximately $15–25/month depending on tier).
Weather4D and Browser-Based Options
Weather4D is an iOS-native GRIB viewer with clean marine-focused design and good integration with the Predictwind and ECMWF data sources. It lacks the routing depth of PredictWind Offshore or Expedition but handles GRIB display, route planning overlays, and satellite download integration well. For iPad-based navigation setups, it is among the cleaner options in the mid-tier.
Passage Weather (passageweather.com) is a free, browser-based tool that assembles GFS data into straightforward wind, wave, and pressure displays. It is useful for initial passage planning research and for sailors who need a quick overview without opening dedicated software. It provides no routing functionality and no ECMWF access, but for understanding the synoptic situation before committing to a detailed analysis, it is fast and free.
Windy.com is perhaps the most impressive free weather visualization platform available. Its interface is exceptionally good — smooth, customizable, capable of displaying dozens of weather variables in an animated format that makes synoptic pattern evolution genuinely intuitive to read. It supports GFS, ECMWF (via subscription), and several other model overlays. What it lacks is any sailor-specific functionality: no isochrone routing, no departure optimizer, no satellite download integration, no polar input. It is a visualization tool, not a passage planning tool. Many experienced offshore sailors use Windy as a supplement — for its superior synoptic visualization — alongside a more capable routing platform.
Routing Around High-Pressure Ridges
One of the most practically important routing challenges in the tropics and subtropics is navigating the trade wind belt and the high-pressure ridges that dominate it. The Pacific High, the Azores High, and the South Atlantic High are semi-permanent features that create both the reliable trade winds and the calms and light airs near their centres. Isochrone routing through a trade wind passage will often suggest sailing a longer course to stay in better wind rather than the rhumb line into lighter air near the high's centre.
This is where routing software earns its subscription cost on ocean passages: the optimal route from California to Hawaii in May is not a great circle — it is a curved path south into the stronger northeast trades before turning west, adding miles but shaving days off the passage time. An experienced navigator reading the isobars can approximate this intuitively, but isochrone routing through a current GRIB produces a quantified optimal path that accounts for the actual forecast, not the climatological average.
Professional Weather Routing Services
The alternative to self-routing an ocean passage is hiring a professional meteorologist who monitors your passage, provides daily weather briefings by email or satellite phone, and adjusts routing recommendations as the forecast evolves. This service was once the exclusive domain of offshore racing programs; it is now accessible to private cruising sailors and provides a level of expert analysis that no software alone replicates.
Professional routers combine model data, satellite imagery, surface analysis charts, and meteorological expertise to provide interpretation that accounts for model biases, regional weather patterns, and the specific requirements of your passage. When the ensemble spread is wide and the deterministic models disagree, a skilled meteorologist can add context that no software can. For ocean crossings where a 2-day timing error means the difference between a frontal passage in the open Atlantic versus a harbour anchorage, professional routing is often worth the cost.
Pricing varies from approximately $300 for a short passage with daily email briefings to $800 or more for a 2-3 week ocean crossing with satellite phone consultation. Multiple services offer routing for blue-water cruisers; recommendations vary by passage — Jimmy Cornell's network has historically served the transatlantic rally community, while services like Commander's Weather and Commanders of the Pacific specialize in North Atlantic and North Pacific passages respectively. For a first major ocean crossing, professional routing alongside your own GRIB analysis is among the highest-value investments available.
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Frequently Asked Questions
How does isochrone routing work and why do accurate polars matter? Isochrone routing starts at your departure point and asks: given the forecast winds in all directions from here, how far can your boat reach in the next 3 to 6 hours in each direction? Using the boat's polar performance diagram — a chart of boat speed at every combination of true wind speed and angle — it calculates positions reachable in each time step, forming a set of equal-time isochrones that propagate forward until one reaches the destination. Accurate polars matter because a poorly characterized polar produces inaccurate routing; forecast quality matters because isochrone routing through inaccurate GRIB data produces confidently wrong routes. The result is the fastest route, not necessarily the most comfortable or conservative.
What makes PredictWind the de facto standard for offshore cruising and what is its cost? PredictWind's most distinctive feature is its proprietary PWE (PredictWind European) and PWG (PredictWind Global) models — higher resolution than GFS and calibrated against observed marine conditions — running alongside GFS and ECMWF access, allowing comparison of four models simultaneously. The departure optimizer tool evaluates departure times over a 7-day window and ranks them by passage time. The offshore subscription costs approximately $45 to $55 per month depending on tier and includes ECMWF access, proprietary model data, satellite-optimized GRIB downloads via Iridium, and the departure planner.
Who should use Expedition routing software and what are its limitations? Expedition is what professional offshore racing teams use — the most capable routing platform in terms of raw functionality with advanced isochrone routing, polars management, layline calculations, instrument integration, and chart display with GRIB overlay. For most cruising sailors it is overkill comparable to a Formula 1 car for commuting. It runs only on Windows, which excludes Mac and iOS chartplotter setups. Pricing runs approximately $1,200 to $2,000 for a perpetual license depending on configuration, plus ongoing updates — reflecting its professional-user market.
What does LuckGrib provide that PredictWind does not and who should use it? LuckGrib fills the gap of a well-designed Mac and iOS application with direct ECMWF access, excellent offline GRIB management, and clean visualization for Mac-native navigation setups. Its offline caching system downloads GRIB data in port for viewing at sea without connectivity. It does not have isochrone routing — it is a GRIB viewer rather than a routing tool — but used alongside separate route analysis it provides ECMWF access historically difficult to get on Mac without a PredictWind subscription. Pricing is a one-time Mac App Store purchase with ECMWF data access via subscription at approximately $15 to $25 per month.
Why do ocean crossing routes often diverge from the rhumb line and how does routing software help? In trade wind passages, the optimal route is often a curved path that adds miles but stays in stronger wind rather than the rhumb line into lighter air near a high-pressure center's core. The optimal route from California to Hawaii in May is not a great circle — it curves south into the stronger northeast trades before turning west, adding miles but shaving days off passage time. Isochrone routing through a current GRIB produces a quantified optimal path accounting for the actual forecast, not the climatological average. An experienced navigator reading isobars can approximate this intuitively, but routing software provides the quantified result.
What does professional weather routing cost and what does it provide beyond software? Professional weather routing services cost approximately $300 for a short passage with daily email briefings to $800 or more for a 2 to 3 week ocean crossing with satellite phone consultation. Professional routers combine model data, satellite imagery, surface analysis charts, and meteorological expertise to provide interpretation accounting for model biases, regional weather patterns, and passage-specific requirements. When ensemble spread is wide and deterministic models disagree, a skilled meteorologist adds context that no software can replicate. For first major ocean crossings, professional routing alongside personal GRIB analysis is among the highest-value investments available.