Why we plan a voyage
A voyage plan (also called a passage plan) covers the whole trip from berth to berth. International safety rules ask every ship to have one. Good planning keeps the crew safe, saves fuel and makes sure the ship arrives on time and with enough water under the keel.
The four stages: appraise, plan, execute, monitor
- Appraisal: collect information. Charts, sailing directions, tide tables, weather forecasts, Notices to Mariners, port rules, the ship's draft and speed.
- Planning: draw the route on the chart. Mark waypoints, courses and distances of every leg, danger areas, safe depth limits, tides and currents, fuel and the time of arrival.
- Execution: the bridge team is briefed and the plan is followed.
- Monitoring: check the ship's position again and again against the plan and change the plan if weather, traffic or breakdown makes it necessary.
Route, waypoints and legs
A waypoint is a point on the chart where the ship changes course. The straight piece between two waypoints is a leg. For each leg write the course and the distance. The total distance is the sum of the legs. Keep the track away from shoals, wrecks and shore by a safe margin, and prefer deep, marked channels. Mark no-go areas and abort points (the last place where you can turn back safely).
On long ocean trips the shortest path on a globe is a great-circle, which looks curved on a flat chart. A rhumb line keeps the same course and is easier to steer but a little longer.
Time, speed, fuel and tides
Time = distance ÷ speed. Add the departure time to get the ETA (estimated time of arrival). Fuel is roughly distance × fuel per mile, plus a reserve (about 20 to 30 percent) for bad weather and delays. Check under-keel clearance along the whole route and plan to cross shallow places at the right tide. A fishing boat also plans the fishing ground, ice or cold storage, and the time to return with fresh catch.
Weather and re-planning
Read weather charts and forecasts before leaving and every day at sea. If a storm, fog or heavy traffic lies on the route, change it: weather routing means choosing the route that avoids the worst weather. A good plan always has a contingency plan, such as a nearby safe harbour. Write every change in the logbook.
Try it
At home: plan a walk to a friend's house with two turns. Measure each part in steps, add them, guess the time and check with a watch.
In 3D: in the last step, slide the detour to 5. Why does the line turn red? Slide it to the south side. Compare the total distance.
Key formulas and definitions
- Time = Distance ÷ Speed
- Total distance = leg 1 + leg 2 + leg 3 + …
- ETA = departure time + time taken
- Fuel needed = distance × fuel per mile × (1 + reserve)
- Under-keel clearance = charted depth + tide − draft
Worked examples
1. A route has three legs: 16 nm, 18 nm and 16 nm. What is the total, and the time at 10 knots?
Total = 16 + 18 + 16 = 50 nm. Time = 50 ÷ 10 = 5 hours.
2. A ship burns 0.1 tonne of fuel per nm. It sails 50 nm. How much fuel, and how much with a 20% reserve?
Fuel = 50 × 0.1 = 5 t. With reserve = 5 × 1.2 = 6 t.
3. A ship leaves at 09:00 and has to cover 60 nm at 12 knots. Find the ETA.
Time = 60 ÷ 12 = 5 h. ETA = 09:00 + 5 h = 14:00.
4. Draft 5.5 m. Charted depth over a bar is 7 m and the tide is 0.8 m. What is the under-keel clearance?
Real depth = 7 + 0.8 = 7.8 m. Clearance = 7.8 − 5.5 = 2.3 m.
5. A port is 84 nm away. The ship leaves at 08:00 and must arrive before 16:00 high tide. What minimum speed is needed?
Time available = 8 h. Speed = 84 ÷ 8 = 10.5 knots.
6. A round trip of 120 nm each way uses 0.1 t/nm. Add a 25% reserve. How much fuel to load?
Distance = 240 nm. Fuel = 24 t. With reserve = 24 × 1.25 = 30 t.
Common mistakes
- Planning only the distance and forgetting the tide. A route can be safe at high tide and unsafe at low tide.
- Taking exactly the fuel needed. Always carry a reserve for bad weather.
- Not changing the plan when the weather changes. The plan is a living document.
- Drawing the route very close to a danger. Always keep a safe margin.