Singha wrote:a pumpjet propulsor might need more power. someone has got to force this water into the funnel
It funnels water through a lampshade causing an increase in pressure at the locations where the blades slices through the fluid media. This increase in pressure delays the onset of cavitation. Cavitation happens when vaccuum "bubbles" form at the trailing edges of the blades when they travel through the liquid faster than the water can rush back in to fill the void they leave as they pass. As pressures increase, the propeller at which the onset of cavitation occurs also increases. This is why subs are "silent" up to a higher speeds as they dive deeper.
The lamp shade also shields the sound made by the propeller in most of the lateral directions. This makes the sub quieter laterally but may increase their sound signature longtidually -- especially dead astern.
The Ohios are very quiet with or without a pumpjet propulsor. They are quiet enough that the soviets are said to have never tracked one after it dives in deep water. One of the features of the submarine is that at slow speeds (low reactor output) the coolants are circulated through the core through natural convection without any pumps being used. The Ohios spent most their time at very slow speeds while on patrol. At 3~5 knots, it is doubtful if a pumpjet makes a difference.
Is it very difficult to get a thermal signature of the sub in such a case (discharging warm/hot water, used as coolant, into the sea)?