Page 24 - Australian Defence Magazine August 2018
P. 24

“The question of propeller versus pump jet really comes down to the acoustic signature; a well-designed pump jet is considerably quieter than an equivalent propeller.”
Appearing as a private citizen, Morrison spoke to and presented his paper to the Sen- ate Economics Reference Committee on 7 June while Brent Clark, now Senior Adviser to the Chairman, Naval Group Australia, was also in the committee room, waiting to give evidence.
There, Morrison stated that an improve- ment of 50 per cent or more in dive range or endurance could be achieved by revert- ing from a pumpjet system to conventional propellers.
Later offered the opportunity by the com- mittee to respond to Morrison’s remarks, Clark declined to do so.
“The Australian government and the Aus- tralian Department of Defence obviously have access to classified research from the US Navy, the Royal Navy and the French Navy on this matter, and I don’t intend to get into a debate on this topic,” he declared.
In an ironic twist, the committee request- ed a rapid assessment of the Morrison paper from Defence’s Capability and Sustainment Group (CASG) – the body responsible for
progressing the Future Submarine program as it stands - if necessary to be delivered at an in-camera briefing. This assessment did not appear to be included in the Committee’s report on the future of naval shipbuilding report released on 27 June.
Although requests to speak with subject matter experts from CASG, the RAN and Naval Group were all unsuccessful, ADM did have the opportunity to discuss propel- lers, propulsors and Sea 1000 with Dr David Kershaw, chief of the Defence Science and Technology Group’s (DSTG’s) Maritime Division and undoubtedly one of the coun- try’s best-informed experts on such matters.
Dr Kershaw’s recent CV speaks for it- self – head of the Torpedo Systems Group, head of the Submarine Combat Systems Group, Research Leader Submarine Sys- tems and Sea 1000 Science and Technol- ogy adviser, Research Leader Undersea Command and Control, and Chief Mari- time Division (in May 2016).
Selecting and designing a propulsor de-
pended on the specified application – ships tended to have big propellers, fast attack craft had water jets, and submarines tra- ditionally used open propellers although pumpjets were now common on large nucle- ar submarines, Dr Kershaw pointed out.
The criteria used in choosing between a pumpjet and a propeller involved thrust, ef- ficiency, vibration, and acoustic signature, all driven by how the surface ship or subma- rine was to be operated.
As explained by Dr Kershaw, there was a degree of misunderstanding concern- ing pumpjet design and performance. To propel a surface ship or submarine in- volved overcoming resistance to push the vessel through the water with a suitably- designed propulsor.
Surface ships need to overcome wave-mak- ing resistance, which varies with speed. Their propulsors therefore need to be optimised for the required operating speed; when operating at off-design speeds, efficiency drops off.
Since submerged submarines do not have
to overcome wave resistance, the efficiency of a propulsor can be considered indepen- dent of speed and the aim is to design an ef- ficient propulsor.
In other words, selecting the propulsor for a submarine is driven by the types of mission it is expected to undertake and where the boat will operate, Dr Kershaw stated.
“The question of propeller versus pump jet really comes down to the acoustic signa- ture; a well-designed pump jet is consider- ably quieter than an equivalent propeller,” he said. “One of the advantages of a well- designed pump jet is that it can smooth out the water flow that is coming off the back of a submarine before the flow hits the moving rotor and there’s less unsteady flow, and thus less noise.
“This smoothing-out also increases the propulsive efficiency; a well-de- signed pump-jet can have efficiencies equal to, or better than, an equivalent propeller. You just have to have the capa- bilities and facilities to be able to design an efficient pumpjet and we know that France can do that.”
Echoing RADM Sammut, Dr Kershaw stressed that the pumpjet on a submarine must be designed in conjunction with the hullform to which it was to be attached.
24 | August 2018 |

   22   23   24   25   26