How OnWatch helped a crew choose the safer route, not just the shortest one ?

A 13,000 nautical mile crossing from Botany Bay to the US Gulf, avoiding the Panama Canal, can go two very different ways: eastward around Cape Horn, or westward around the Cape of Good Hope. When the request reached Onwatch, the operator had just flagged heavy weather building over the Tasman Sea ahead of departure. There was no time for a slow, one-shot analysis, only for a comparison built through back-and-forth with them before the vessel left port

Two oceans, three routes on the table

The vessel was a gas carrier. The operator's own experience favoured sailing east, but they wanted the numbers in hand, both to confirm their own read and to bring the master on board.

They also raised a third path worth checking on the eastern side: cutting through Cook Strait, between New Zealand's two islands, to shave off some distance. Onwatch modelled it properly alongside the other two, including a version with an earlier departure to see whether timing alone could clear the incoming swell.

What the table showed, and what it left out

Read as a table, Cook Strait looks like a free win: a little less distance, a little less fuel, an earlier arrival. But a comparison built only on distance and fuel misses the part that actually decides a voyage like this one: what the ship meets along the way, not how far it travels.

Cook Strait is sheltered by New Zealand's South Island on one side, but the digital twin's modelling showed no real drop in swell exposure on that path, if anything slightly more than the route that passes north of New Zealand. Onwatch also tested that same route with an earlier departure, to see if leaving sooner would let the vessel slip through before the swell arrived. It didn't: the weather still caught up with the strait before the ship did. Worse, if the swell arrived before the vessel cleared it, it risked having to stop and shelter rather than keep making way, a risk the distance and fuel figures alone don't show.

The Cape of Good Hope route was ruled out for a different reason. It added over two days at sea, and for most of the crossing it carried a high risk of a strong head swell. A lower wave height number doesn't tell the whole story either: what matters aboard a ship is the swell's direction relative to its heading. A quartering sea and a head sea at the same height are not the same voyage.

The route that won: not the shortest, the one with room to adapt

Onwatch's recommendation was the route north of New Zealand, on to Cape Horn. It wasn't the shortest option on the table. It was the one that kept a door open: if the weather shifted, the vessel could still divert further north and route around the worst of the swell front, something the Cook Strait passage, committed once inside it, could not offer.

Built with the crew, not delivered to them

The comparison took shape over a single afternoon of back-and-forth: the operator's own read of the incoming weather, their idea to test Cook Strait, a question about wave limits when sailing against the swell, all folded into the routes Onwatch modelled before the vessel had even left port. That's what Onwatch means in practice: not a report delivered the next day, but a working conversation while the clock to departure is still running.