The Navy's new stealth submarines might not be so stealthy after all.

Key points: Ultra-quiet pump-jet propulsion technology was a key selling point in the subs' design

Ultra-quiet pump-jet propulsion technology was a key selling point in the subs' design It was supposed to help make the subs "the world's most advanced conventionally powered submarine"

It was supposed to help make the subs "the world's most advanced conventionally powered submarine" Executive director of Naval Group now says propeller technology might be used on the submarines

A top executive at the company building the subs has thrown doubt on whether cutting-edge pump-jet propulsion technology will be used as planned.

The ultra-quiet technology was a key selling point for the French design that won the Government's competitive evaluation process to build the new submarines at a cost of $50 billion.

Jean-Michel Billig, Executive Director of Naval Group, was asked at a naval conference in Sydney earlier this month whether pump-jet propulsion "remained viable" for the Australian submarine.

He said it "could" be viable, but also agreed that propeller technology might be used on the submarines.

The winning French bid to design and build the new submarines boasted of how the technology would help make them "the world's most advanced conventionally powered submarine".

"Cutting edge technology direct from France's nuclear submarine program pushes the submarines' stealth capabilities into a new realm," a narrator said in a marketing video for the bid.

"Pump-jet propulsion replaces obsolete propeller technology."

Experiments 'went nowhere'

Defence analyst Andrew Davies of the Australian Strategic Policy Institute said pump jets were designed to be used in nuclear submarines, not the conventionally powered boats Australia would use.

"There is a real question about how you marry a pump-jet with a conventional submarine," he told AM.

"The couple of experiments over the years that countries have done where they tried to put a pump jet on conventional submarines went nowhere."

"It's tempting to think this was example of an engineer thinking in public," he said of Mr Billig's comments.

Mr Billig is Executive Director of Naval Group, the company formed to build the new subs.

In a statement to AM, a Naval Group said it was "absolutely possible" to fit pump-jet technology to a conventional submarine.

"However decisions on forms of propulsion are ultimately a matter for the Australian Government."

A Defence Department statement said: "[The Government] intends to use the pump-jet propulsion system for the Future Submarine that was offered by Naval Group as part of their Competitive Evaluation Process proposal, and remains on offer to Australia."