I'm surprised PBCF isn't more common. It essentially reduces swirl and cavitation in the central vortex, converting it into additional thrust while also reducing shaft torque. It's an inexpensive retrofit (payoff time <6 mo) with broad compatibility, and the swap can be done while unloading with no downtime. And yes, it reduces rudder surface erosion which is caused by cavitation.
Seems like a no-brainer. Maybe folks just aren't aware of the technology?
Nuclear submarines feature very large, many-bladed, specially-shaped propellers often turning very slowly to reduce both noise and wear (though more the former: stealth is key to submarine operations).
Cavitation on hydraulic and steam turbines, as well as other hydraulic mechanisms, is a major source of wear and failure. Keep in mind that cavitation is equivalent to boiling, and involves both high (if localised) temperatures and extreme pressure fluctuations as cavitation bubbles form and collapse.
A couple of discussions from both perspectives:
<https://archive.navalsubleague.org/2012/a-century-of-america...>
<https://www.sciencedirect.com/science/article/abs/pii/S00298...>
For nuclear submarines, overall efficiency isn't termendously important, as when the sub needs to move quickly the powerplant can turn out a heck of a lot of power.
Checking just now to confirm powerplant output, I'm learning that the Virginia class of hunter-killer submarines, for which both quiet operation and speed for pursuit are concerns) use pump-jet propulsors rather than traditional bladed propellors, specifically to reduce cavitation:
<https://en.wikipedia.org/wiki/Virginia-class_submarine#Propu...>
Top speed is in excess of 25 knots. (I suspect considerably in excess, though
Powerplant is a 210 MW nuclear reactor, 280,000 HP equivalent. By contrast the Maersk E-class containership (largest in the world until 2012) have an 81 MW (fossil-fuel based) powerplant, 109,000 HP equivalent. The still-larger Triple-Es have two 31 MW engines (for lower total output), and the current size giant CSCL Globe 69.7 MW.
<https://en.wikipedia.org/wiki/E-class_container_ship>
<https://en.wikipedia.org/wiki/Triple_E-class_container_ship>
<https://en.wikipedia.org/wiki/CSCL_Globe>
And the fastest known military submarine seems to have been the K-222 Soviet Project 661 "Anchar", which could reach 41-45 knots (about 50 mph) submirged. The K-222 is slightly shorter than the Virginia class, and as hull-length has a strong effect on top speed (see hull speed: <https://en.wikipedia.org/wiki/Hull_speed>, though how this relates to submarines I'm uncertain, see also Froude number: <https://en.wikipedia.org/wiki/Froude_number>), I suspect Virginia class boats could likely achieve similar speeds should the need exist.