NATARAJAWeather routeing — the fastest track, not the shortest
Learning outcomes & permitted supports
- Explain why speed is a property of the ship AND the sea, and how added resistance grows with the square of the wave height
- Compare candidate tracks by TIME rather than distance, and read a least-time track off the comparison
- Weigh a deviation in three currencies: hours on hire, tonnes of fuel and risk to ship and cargo
- Name the weather products a bridge receives — NAVTEX, SafetyNET, weatherfax, GRIB, shore routeing — and what each is for
- State clearly that a routeing recommendation is ADVICE: the Master's authority over the ship's safety is not delegated
Supports in this lab: Calculator. The speed-loss law (0.18·Hs² times a heading factor) is a stated teaching model — a real routeing office uses the ship's own trial curves; the recommendation is always advice and the Master decides. Golden 1031/1031.
Glossary: weather routeing · least-time track · significant wave height · routeing chart · great circle
The shortest track is not the fastest track
A ship's speed is not a property of the ship; it is a property of the ship AND the sea she is in. Push her into a head sea and she loses knots to added resistance, to pitching, and to the master slowing down for slamming. Feed that into the passage arithmetic and the great circle — still the shortest distance — can be comfortably the slowest way across.
Right now the direct track takes 300.2 h and the best deviation 195.7 h — a saving of 104.5 hours for 57 nm of extra distance. That is the whole business of weather routeing in one line.
Why the heading matters as much as the wave height
Speed loss goes roughly with the SQUARE of the significant wave height — and then it is multiplied by where the sea is coming from. A head sea takes the full penalty; a beam sea about a third of it; a following sea almost none, though it brings its own problems of broaching and pooping. That is why a routeing recommendation is usually not "go round" but "cross the band on this heading, at this hour".
| sea relative to the ship | penalty factor | at 8.0 m, speed becomes |
|---|---|---|
| head (dead ahead) | 1.00 | 4.5 kn of 16.0 |
| bow | 0.70 | 7.9 kn of 16.0 |
| beam | 0.35 | 12.0 kn of 16.0 |
| quarter | 0.15 | 14.3 kn of 16.0 |
| following (astern) | 0.05 | 15.4 kn of 16.0 |
Hours, tonnes and the charter party
Routeing decisions are argued in three currencies at once. HOURS matter because the ship is on hire; TONNES matter because fuel is bunkered and paid for; and SAFETY matters most, because a ship damaged in heavy weather has an arrival date of "unknown".
Be careful with the fuel argument, because it is the one most often got backwards. The cube law — consumption ∝ speed³ — answers the slow-steaming question: what she burns if the master orders a lower speed in calm water. Heavy weather is not that. In a head sea the engine is still turning at the same setting and burning the same tonnes per day; the added resistance simply eats the speed. So the gale does not save fuel — she burns the same rate for more days. Applying the cube law to the speed she achieved in the gale would say a storm is economical, which is false arithmetic and dangerous to put in front of a cadet.
| Direct track | 300.2 h · about 375 t of fuel |
|---|---|
| Least-time track | 195.7 h · about 245 t |
| Difference | 104.5 h and 131 t — the deviation is cheaper in BOTH currencies, which is the usual result of avoiding a gale |
| The trap | Price the direct track by the speed she achieved (8–9 kn) and the cube law would say only 60 t — a "saving" that exists only on paper. She was never ordered to go slowly; the sea did that to her, at full consumption. |
| And the one that is not a number | less time in the worst of the sea: fewer green seas on deck, less risk to lashings, cargo and crew |
What the bridge actually receives
Routeing is only as good as the forecast behind it, and a ship gets that forecast through several channels — each with a different job. The recommendation from a shore service is advice; the master's authority is not delegated by subscribing to it.
| product | what it carries | how it arrives |
|---|---|---|
| NAVTEX | coastal warnings, gale and storm warnings, forecasts out to about 300 nm | 518 kHz, automatic, printed on the bridge whether anyone asks or not |
| SafetyNET (Inmarsat EGC) | high-seas warnings and forecasts for the METAREA, and shore-to-ship distress relays | satellite, the offshore half of the GMDSS promise |
| Weather fax / HF radiofacsimile | the surface analysis and prognosis charts — isobars you can plot your own track on | HF, still the picture most masters trust |
| GRIB files | gridded wind, pressure and wave forecasts a routeing program can compute with | email or satellite data, small enough for a ship's link |
| Shore routeing service | a recommended track updated daily, with the operator's liability attached to it | commercial subscription — advice, never an order: the master decides |
And the oldest routeing document of all still earns its place: the routeing chart for the ocean and the month, with its wind roses, mean currents, ice limits, and the percentage of gales you may expect in each square. It is climatology rather than forecast — the right tool when you are planning next month's voyage rather than tomorrow's watch.
| Direct (great circle) | 300.2 h at 8.66 kn average |
|---|---|
| Least-time track | 12° equatorward of the direct track — 195.7 h at 13.58 kn |
| Saving | 104.5 hours — 4.4 days of hire, for 57 nm of extra steaming |
| Worst leg on the direct track | 4.57 kn — she is barely making way through the band |