Offshore Passage Planning
Once the coast drops below the horizon, you're committed. Everything you need must already be aboard, and every decision must already be planned.
Great Circle vs Rhumb Line
On an ocean crossing, the route you draw on the chart depends on the type of chart — and the type of route matters more than most sailors realize.
Rhumb line: A line of constant bearing — it appears as a straight line on a Mercator chart. Easy to steer (constant compass heading), but it's not the shortest distance between two points on a sphere. Over short distances, the difference is negligible. Over ocean distances, it adds miles.
Great circle: The shortest distance between two points on a sphere — it appears as a curved line on a Mercator chart (arcing toward the nearer pole). A great circle route from New York to Lisbon passes significantly further north than the rhumb line. Over a 3,000-mile passage, the great circle can be 100+ miles shorter than the rhumb line.
The practical compromise: Most ocean passages use a composite route — a great circle track modified for weather, currents, and hazards. Pure great circle routes may take the boat into higher latitudes with worse weather. The optimal route balances distance (great circle advantage) against conditions (trade wind routes, current exploitation, storm avoidance). Routing software calculates this trade-off automatically.
When it doesn't matter: For passages under 500 miles, the difference between great circle and rhumb line is usually less than 10 miles — not worth the complexity. For trans-ocean passages, it matters.
Most chartplotters can display both rhumb line and great circle routes. For any passage over 500 miles, display the great circle and compare it to the rhumb line. If the difference is significant, consider a composite route that captures some of the great circle's distance savings without pushing into extreme latitudes.
Why is a great circle route shorter than a rhumb line over ocean distances?
Ocean Currents and Trade Wind Routes
Ocean currents can add or subtract 24–50+ miles per day from your progress. Planning a route that exploits favourable currents or avoids adverse ones is as important as planning for wind.
The Gulf Stream: Flowing north along the US East Coast at 2–4 knots, the Gulf Stream is a river in the ocean. Sailing south from New England to the Caribbean, you want to avoid it. Sailing north from the Caribbean to New England, you want to ride it. Crossing it requires planning — the Gulf Stream's boundary creates steep, confused seas when wind opposes current.
The trade wind belts: The Northeast and Southeast Trade Winds create the classic ocean sailing routes. The Atlantic trade wind route — Canaries to Caribbean — exploits the steady NE trades for a downwind passage of 2,700 miles. The Pacific 'milk run' — west from Central America toward the Marquesas — uses the SE trades. These routes exist because generations of sailors discovered that sailing with the wind and current systems is faster, safer, and more comfortable than fighting them.
Equatorial Counter Current: Between the trade wind belts, an eastward-flowing counter current can cost westbound boats 20–30 miles per day. Routing software helps identify where to cross it (where it's weakest or narrowest) or how to avoid it entirely.
Current data: Ocean current data is available in pilot charts (monthly average current vectors), GRIB files (modeled current data), and satellite-derived surface current maps. Pilot charts are the starting point — they show the statistical average for each month. GRIB data shows the forecast for the specific passage.
Pilot charts are free from NOAA and available for every ocean. Each chart shows wind roses, current arrows, storm frequencies, and fog/visibility data for one month. Before planning any ocean passage, check the pilot chart for your month — it summarizes what thousands of previous passages experienced.
How much can ocean currents add or subtract from daily progress?
Waypoint Selection for Ocean Crossings
Ocean waypoints serve different purposes than coastal waypoints. On a coast, waypoints mark hazards and course changes. In the open ocean, waypoints mark strategic decision points, current boundaries, and weather assessment positions.
Departure and landfall waypoints: The departure waypoint is typically the last point of safe water as you leave the coast — outside the harbour, clear of the continental shelf. The landfall waypoint is the first point where you transition from ocean sailing to coastal pilotage — usually 10–20 miles offshore of your destination, aligned with the approach.
Mid-ocean waypoints: On a trans-Atlantic crossing, you might have only 3–5 waypoints between departure and landfall. Each marks a strategic point: the edge of the Gulf Stream (where you assess conditions and adjust heading), the point where you should be south enough to pick up the trades, the point where you cross the mid-Atlantic ridge (relevant for weather patterns), and the approach to the destination archipelago.
Weather routing adjustments: Unlike coastal passages where waypoints are fixed by geography, ocean waypoints may be adjusted en route based on evolving weather. The route from the Canaries to the Caribbean might aim for Barbados at departure but adjust south toward Trinidad if the wind pattern shifts. The waypoints are starting references, not fixed commitments.
The '1,000-mile box': Some ocean sailors use a mental model of a box — 1,000 miles wide — centred on the direct route. As long as the boat stays within the box, it's on track. Daily routing decisions (sail angle, tacking, responding to wind shifts) happen within the box. Only if the boat drifts outside the box does the overall route need reassessment.
For ocean crossings, load the rhumb line route into the chartplotter but don't follow it blindly. Use it as a reference — the plotter shows your cross-track error (how far off the direct line you are). As long as you're making reasonable progress toward the destination, the exact track doesn't matter. Follow the wind, not the line.
How do mid-ocean waypoints differ from coastal waypoints?
Rallies, Communication, and Logistics
An ocean crossing is a logistical undertaking beyond any coastal passage. Whether you sail solo or in a rally, the preparation and communication requirements are substantial.
Rallies vs solo departures: Organized rallies (ARC — Atlantic Rally for Cruisers, Pacific Puddle Jump, World ARC) provide structured departures with weather briefings, communication schedules, safety equipment inspections, and a fleet of boats making the same passage. The social support and organized weather routing reduce risk for less experienced crews. Solo departures offer flexibility in timing (you choose your own window) but require more self-reliance in weather interpretation and communication.
Communication schedules: Offshore passages require a communication plan. At minimum: daily position reports to a shore contact (via satellite phone, SSB radio, or satellite tracker like Garmin inReach). Many cruisers join SSB radio nets — daily scheduled broadcasts where boats report positions, share weather observations, and relay messages. These nets provide community, weather data, and a safety network.
Pre-departure logistics: Crew visas for the destination country. Customs and immigration requirements at the arrival port. Ship's papers, insurance, and registration documents. Spare parts that won't be available at the destination. Charts (electronic and paper backup) for the destination area. Critically: if anything on this list requires internet access, resolve it before departure. There is no reliable internet in mid-ocean.
Crew selection: For an ocean crossing, crew capability matters more than crew count. Two experienced sailors are better than five novices. The crew must be able to stand watches for the entire passage duration (14–25 days for an Atlantic crossing), handle sail changes, navigate, cook, and make decisions under fatigue. Discuss expectations, watch systems, and seasickness management before departure — not on day three when someone is incapacitated.
If this is your first ocean crossing, consider joining a rally. The ARC (Las Palmas to St. Lucia) and the Pacific Puddle Jump (Central America to French Polynesia) are well-organized, provide weather routing, and surround you with experienced sailors who've done it before. The entry fee is far cheaper than the knowledge you gain.
What is the primary advantage of joining an organized rally for an ocean crossing?
Summary
Great circle routes are shorter than rhumb lines over ocean distances — but practical routes compromise between distance and conditions.
Ocean currents can add or subtract 24–50+ miles per day. Pilot charts show monthly average conditions; GRIB data shows specific forecasts.
Mid-ocean waypoints mark strategic decision points, not navigational hazards — they may be adjusted en route based on evolving weather.
Rallies provide structured support for ocean crossings — weather routing, safety inspections, and communication schedules reduce risk.
Crew capability matters more than crew count for offshore passages — discuss expectations, watches, and seasickness before departure.
Key Terms
- Great circle
- The shortest distance between two points on a sphere — appears as a curved line on a Mercator chart, arcing toward the nearer pole
- Rhumb line
- A line of constant bearing — appears straight on a Mercator chart but is not the shortest distance on a sphere over long distances
- Pilot chart
- Monthly chart showing statistical wind roses, current vectors, storm frequencies, and visibility data based on historical observations
- Trade winds
- The persistent easterly winds in the tropics (NE trades in the Northern Hemisphere, SE trades in the Southern) that define the classic ocean sailing routes
- Rally
- An organized group crossing — boats depart together with structured weather briefings, communication schedules, and safety oversight
- Cross-track error
- The perpendicular distance between the boat's current position and the planned direct route — a measure of how far off track you are
Offshore Passage Planning Quiz
Over a 3,000-mile passage, how much shorter can a great circle route be compared to a rhumb line?
Why is the Gulf Stream important in route planning for US East Coast passages?
What information do pilot charts provide?
Why might ocean waypoints be adjusted during a passage?
For a first ocean crossing, what is the recommended crew approach?
References & Resources
Related Links
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NOAA — Pilot Charts
Free pilot charts for all oceans — monthly wind, current, and weather statistics
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Atlantic Rally for Cruisers (ARC)
The world's largest trans-ocean rally — Las Palmas to St. Lucia