Built for the people who turn a route into money: charterers and owners estimating a voyage, operators planning it, analysts modelling TCE (Time Charter Equivalent — the voyage's net daily earnings). "How far is it?" has more than one answer — the straight-line great circle, the constant-course rhumb line, and the actual route a ship sails around land, ice, canals and emission zones. Getting distance wrong throws off bunkers, days and the whole TCE. We assembled how sea distance and routing really work — and why the shortest line is rarely the sailed one — into one reference.
The rynda voyage-economics calculator builds the real sea route — around land, through the canals you pay for, across the ECA zones that change your fuel — and turns the distance into TCE.
📖 See also: Canal transit limits · Freight rates & the market · ECA / SECA zones reference
Contents
- At a glance
- Great circle vs rhumb line
- The sailings — how distance is actually computed
- Why the shortest line is rarely the route
- ECA- and canal-aware routing
- Distance, speed and voyage days
- How distance feeds TCE
- Weather routing
- For developers and modellers
- FAQ
- References
At a glance
The short version, if you read nothing else:
- "Distance" has three meanings: the great circle (the shortest path over a sphere), the rhumb line (a constant-compass-course line, longer but simpler to steer), and the actual sea route a ship can physically sail.
- The great circle is shortest because Earth is (nearly) a sphere — which is why trans-ocean routes arc toward the poles rather than following a straight line on a flat map. The higher the latitude and the more east–west the route, the bigger the saving over the rhumb line.
- Sea distances are measured in nautical miles; one nautical mile ≈ one minute of latitude ≈ 1.852 km.
- The sailed route is almost never the great circle. It bends around land, through canals and straits, away from ice and traffic schemes, and is shaped by emission zones and weather.
- Distance is the master input to a voyage estimate: it sets days at sea, which sets bunkers, which — with port costs, canal tolls and carbon — sets TCE. A 5 % distance error is a 5 % bunker-and-days error.
- A longer route can be the cheaper one — routing around a canal to avoid a toll or a drought restriction, or through an ECA to shorten the passage, is a cost trade, not just a distance one.
This is a practical reference. Real route distances come from a sea-routing engine that respects land, canals and restrictions; the geometry here explains why the numbers differ.
Great circle vs rhumb line
Two classic ways to measure "distance" between two points on the globe:
| Great circle | Rhumb line (loxodrome) | |
|---|---|---|
| Definition | The shortest arc over the sphere's surface | A line of constant compass course |
| On a Mercator chart | A curve bending toward the pole | A straight line |
| Length | Shortest | Longer (except along the equator or a meridian) |
| To steer | Course changes continuously | One steady heading |
| Used for | Ocean passage planning (shortest) | Simplicity; short legs; the segments of a great-circle track |
The paradox that trips people up: on the flat Mercator chart every sailor uses, the shortest real route looks like a curve arcing toward the pole, while the straight line on the chart (the rhumb line) is actually longer. A great-circle track is sailed as a series of rhumb-line legs, re-checking the course every so often.
The saving is biggest on long, high-latitude, east–west routes (e.g. North Pacific or North Atlantic crossings), and near zero on north–south routes or along the equator.
The sailings — how distance is actually computed
"The sailings" are the navigational methods for computing distance and course:
- Plane sailing — treats a small area as flat; fine for short legs.
- Mercator sailing — computes the rhumb-line course and distance on the Mercator projection.
- Great-circle sailing — computes the shortest distance and the (changing) initial course over the sphere.
- Composite sailing — a great circle capped at a maximum latitude (a "limiting latitude") to keep the ship out of ice or bad weather near the poles: great-circle to the limit, along it, then great-circle again.
Distance is in nautical miles (1 nm ≈ 1 minute of latitude ≈ 1.852 km). A ship's speed in knots is nautical miles per hour, so distance ÷ speed gives hours at sea directly.
Why the shortest line is rarely the route
The great circle is a geometric ideal; the sailed route is constrained by the real world:
- Land. The obvious one — you sail around continents, not through them. A "distance" between two ports must follow navigable water.
- Canals and straits. Panama, Suez, Malacca, the Turkish Straits are the chokepoints that define whether a route is short or goes "the long way" around a Cape.
- Ice and seasonal limits. Composite sailing and routing services keep ships south (or north) of ice; the Northern Sea Route opens only seasonally.
- Traffic separation schemes and coastal rules shape approaches.
- Emission zones and drought. An ECA leg changes the fuel, not the distance; a Panama draft restriction can push a laden ship onto a longer Cape route entirely.
This is why a sea-routing engine — which knows the coastlines, canals and restrictions — gives a very different (and correct) distance from a naive great-circle calculator.
ECA- and canal-aware routing
For voyage economics, the route isn't just a distance — it's a set of segments with different costs:
- Canal vs Cape. A Suez or Panama transit shortens the distance but adds a toll (and, at Panama, a drought/booking risk). Routing around via the Cape adds sea-days but no toll — a direct cost trade. The canal reference has the limits and indicative tolls.
- ECA share. The number of miles inside an emission-control zone sets how much of the voyage burns the pricier low-sulphur fuel — so two routes of equal distance can have different bunker bills.
- EU-scope legs. Whether a leg touches an EU/EEA port decides the EU ETS/FuelEU coverage.
A good route optimiser therefore doesn't just minimise miles — it minimises total cost: bunkers + tolls + carbon + time.
Distance, speed and voyage days
Distance becomes time through speed:
Sea days = distance (nm) / (speed (kn) × 24)
Bunker at sea = sea days × daily consumption (t/day)
Voyage days = sea days + port days + waiting
Two levers hide here:
- Speed — cutting speed (slow-steaming) reduces daily consumption sharply (fuel burn rises roughly with the cube of speed), so a slower voyage burns far less fuel but takes more days. The optimum is an economic, not a physical, choice.
- Ballast legs — the empty repositioning leg to the load port is pure distance-cost with no cargo, which is why owners chase triangulation / backhaul.
How distance feeds TCE
Distance is the first domino in a voyage estimate:
distance → sea days → bunkers
↘ + port costs + canal tolls + EU carbon
↘ vs freight → TCE
Get the distance wrong and every downstream number is wrong: days (hence the daily-hire opportunity cost), bunkers (the biggest variable cost), and the canal/ECA segments. This is why a voyage estimate must start from a real sea-route distance, not a chart-ruler straight line — a few percent of distance error is a few percent of the whole voyage P&L. TCE and the full stack are covered in the freight rates reference.
Weather routing
Beyond the fixed geometry, weather routing services optimise the actual track voyage-by-voyage for wind, waves and currents — trading a slightly longer distance for less resistance, less fuel and a safer, more predictable ETA. On a long ocean crossing, riding a favourable current or dodging a storm system can save more fuel than the extra miles cost. It's the operational layer on top of the strategic canal-vs-Cape and speed decisions.
For developers and modellers
- Start from a real sea route: the voyage-economics calculator builds the navigable route around land, through the canals it charges, and across the ECA zones that change the fuel mix — then turns the distance into sea days, bunkers and TCE.
- Route + ECA distance API:
POST /api/ports-map/routereturns the route legs,eca_distance_nmand theeca_zonescrossed — the geometric input every downstream cost builds on:
curl -X POST https://rynda.io/api/ports-map/route \
-H 'Content-Type: application/json' \
-d '{"ports":["BRSSZ","CNSHA"]}'
# → { distance_nm: …, eca_distance_nm: …, eca_zones: [...], legs: [...] }
- Don't compute voyage distance as a bare great circle — it ignores land, canals and restrictions and will understate the real route, often badly. Use a routing engine; treat the great circle only as a theoretical floor.
FAQ and common confusions
Q: What's the difference between a great circle and a rhumb line?
A: A great circle is the shortest path over the sphere (it curves toward the pole on a flat chart); a rhumb line is a line of constant compass course (straight on a Mercator chart, but longer). Ships plan the great circle for distance and sail it as a series of rhumb-line legs.
Q: Why does the shortest route look curved on the map?
A: Because the Mercator chart flattens a sphere. The straight line on the chart is the longer rhumb line; the shortest real path (the great circle) shows up as a curve arcing toward the pole — most visibly on long east–west, high-latitude routes.
Q: What is a nautical mile?
A: The unit of sea distance — about one minute of latitude, ≈ 1.852 km. Speed in knots is nautical miles per hour, so distance ÷ speed gives hours at sea directly.
Q: Why is the actual route longer than the great-circle distance?
A: Because a ship must sail around land, through canals and straits, and away from ice, traffic schemes and weather. A sea-routing engine that respects those gives the real distance; a bare great-circle calculator understates it.
Q: Is a canal route always cheaper than going around the Cape?
A: No — it's a cost trade. The canal is shorter (fewer sea-days and less bunker) but adds a toll and, at Panama, drought/booking risk. On some ships and fuel prices the longer Cape route, with no toll, wins. Compare total cost, not just distance. See the canal reference.
Q: What is composite sailing?
A: A great-circle track capped at a maximum latitude to keep the ship clear of ice or heavy weather near the poles — great-circle to the limiting latitude, along it, then great-circle again. A safety/practicality compromise on the pure shortest path.
Q: How much does getting the distance wrong matter?
A: A lot. Distance sets sea days and therefore bunkers — the biggest voyage costs — plus the canal and ECA segments. A few percent of distance error is a few percent of the whole voyage P&L, which is why a TCE estimate must start from a real route.
Q: What is weather routing?
A: A voyage-by-voyage optimisation of the actual track for wind, waves and current — sometimes a slightly longer route that burns less fuel and gives a safer, more reliable ETA than the shortest line.
References
- Bowditch, The American Practical Navigator — the sailings (plane, Mercator, great-circle, composite).
- Admiralty / commercial distance tables and sea-routing engines — real port-to-port distances respecting land and canals.
- Canal authorities (Panama, Suez) and IMO routing measures — chokepoint limits and traffic schemes.
- rynda voyage-economics calculator and the
POST /api/ports-map/routeendpoint.
Document version
- 2026-07 — first public version. Great circle vs rhumb line, the sailings, why the sailed route differs, ECA/canal-aware routing and how distance drives TCE, in one frame. Real distances come from a sea-routing engine that respects land and restrictions.