The large steel container equipped with sonar instruments and towed deep below the Indian Ocean by the Fugro Equator in the search for missing flight MH370 has been dubbed "Spero" by the ship's crew.

It means "hope" in Latin. But there's much more than optimism driving survey leader Scott Miller to keep scouring the 120,000-square-kilometre search area, 17 months after the plane disappeared with 239 people on board.

"The only outcome of a successful mission for us is quite grim, but the families of all those people, they want to know the answer," Mr Miller said during a brief resupply stopover in Fremantle Port.

"We want to know the answer to make air travel safer in the future so that this kind of thing can't happen again, whatever the reason.

"Without finding out what happened we can't answer any of those questions, so it's just got to be done."

The Equator is one of two ships, along with the Fugro Discovery, looking for MH370.

Following a grid pattern, the Equator can cover about 230 square kilometres of ocean per day.

A 10-kilometre-long cable tows the container, known as a towfish, holding the various devices scanning the seabed as the ship makes it way up and down the long lines of the search grid.

Inside the towfish there is a side-scan sonar, which captures a black-and-white image of the sea floor, and a multi-beam echo sounder, a device emitting 240 sound beams from which it creates a detailed model of the ground beneath the ocean.

It can detect a one-metre-cubed object.

There is also a petro-chemical sniffer able to detect oils, hydraulic fluid and petrol.

The towfish has to dodge underwater mountain ranges and volcanoes.

"When we're flying the fish and come up to that feature, it basically climbs up 1,000 metres in a very short space of time, and the guys refer to it as 'The Wall'," Mr Miller said.

"Without knowing that was there, we would have lost the fish on the first line tow."

The Fugro Equator is on a brief resupply stopover at Fremantle Port. ( ABC News: Nicolas Perpitch )

Flaperon discovery boosts morale

The Equator has so far found 26 items categorised as not appearing natural.

Six of those have been confirmed as not being part of MH370, including a wooden coal-burning ship, probably from the 19th century, found in May.

The other 20 items are not believed to be linked to MH370 but are still being investigated.

The July 29 discovery of an aircraft wing part, known as a flaperon, at Reunion Island boosted morale among the Equator's 16 survey crew and 14 ship's crew.

Scott Miller looks at mapping of the Indian Ocean used to help the search for MH370. ( ABC News: Nicolas Perpitch )

"I think everyone was relieved that something had finally been found of this plane after 18 months, with no evidence that anything had happened, of course," Mr Miller said.

"And it works well to quell all the conspiracy theories and lets us focus on the job that we're trying to do."

Mr Miller has been involved in the search since June last year.

He and another survey leader take turns doing a 42-day shift at sea, along with the rest of the survey team.

On this swing the seas were no bigger than four metres high, but recently three cyclones and a deep Antarctic low converged on the Equator's location, creating massive 17-metre waves.

Mr Miller said it could be draining being so far away from land and from "other signs of life" other than the people on the Discovery.

But he said he would not give up.

"I'm determined to find it. I'm not going off this job until I've done what I came out to do," he said.

"And I think most guys feel the same. Most guys have dedicated themselves to this job now, in very rough conditions."