When French submarine builder DCNS, now  Naval Group, bid to build Australia’s Future Submarines, a central part of the pitch was its pumpjet propulsion, billed as being orders of magnitude quieter than conventional propellers.

But what if it didn’t work as advertised?

That’s the central thesis of a well-publicised study by physicist Aidan Morrison who concluded that a pumpjet would be far less efficient than a conventional propeller at the low speeds at which a diesel-electric submarine mostly cruised.

Mr Morrison’s study, presented to the Senate committee inquiry into Australia’s naval shipbuilding industry in June, ran to 91 pages.

In response, Defence called on its own experts including the Defence Science and Technology Group plus unnamed external specialists in pumpjets. Its unclassified report, released by the committee this month, runs to six pages, with pictures.

Defence acknowledged that pumpjets weren’t common on smaller conventionally-powered subs as they were on larger and much more powerful nuclear boats.

That was because they were heavier than conventional propellers and therefore not compatible with the weight balance of smaller vessels.

“However, as the size of the submarine increases, a pumpjet can be accommodated, bringing its attendant advantages over conventional propellers,” it said.

Defence said propulsors were designed for differing purposes with the goal of optimising the performance of a vessel for its intended role.

“This is especially the case when it comes to pumpjets for submarines,” it said.

“The predicted performance of a submarine pumpjet needs to be assessed with regard for the hydrodynamic performance of the submerged submarine, having considered how the design of the pumpjet has been matched to the hull design to optimise the overall performance of the submarine for its intended roles.”

Defence rejected Mr Morrison’s thesis, concluding that performance of a submarine pumpjet could not be derived from a comparison with the performance of surface vessel propulsors.

But like much advanced technology, Defence said much would have to remain secret.

It said pumpjet design had advanced over many years with particular focus on the characteristics of all propulsor components.

“Notably many of these characteristics are classified and must remain so to protect all of the benefits that Australia will leverage to promote the regional superiority of the Future Submarine,” it said.

In his study, Mr Morrison said in comparison between two otherwise identical submarines, one with a pumpjet and the other with a conventional propeller, the pumpjet powered boat would have less range and endurance.

“The key conclusions that I arrived at were that pumpjets have a far lower efficiency than propellers at a low speed of travel in contrast to high speeds, where jets tend to become more efficient,” he said.

Mr Morrison’s study was sponsored by the group Submarine For Australia, run by businessman Gary Johnston, proprietor of the Jaycar electronics chain.

In his own submission to the inquiry, Mr Johnston said he had observed over many years the ongoing waste and incompetence exhibited in many Defence acquisitions.

“I have been concerned since the outset at the huge cost and immense risks around the FSM (Future Submarines) project,” he said.

Mr Johnston said only the Naval Group proposal allowed the RAN to effectively design their own submarine, as they were now.

“The result is a submarine that will be quite eye-wateringly expensive, with an inadequate capability and one that will be delivered far too late,” he claimed.