3 ms·

I'm not a fluid mechanics guy, but all that cavitation from the screws seems like preventably wasted energy, not to mention a huge source of potential surface w

I'm not a fluid mechanics guy, but all that cavitation from the screws seems like preventably wasted energy, not to mention a huge source of potential surface wear.

schiffern 2y ago
Yes you're right. 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? https://www.youtube.com/watch?v=a8PjWSeLUzY https://www.youtube.com/watch?v=a8PjWSeLUzY
- _yb2s 2y ago
  Fascinating, I'm really into boating and sailing and have never heard of this- no consumer grade engines or boats that I know of have this.
- sevensor 2y ago
  Wow, that's awesome! Looks like a big win all-around. Save money, save fuel, save wear.
_yb2s 2y ago
A lot of the noise is from the combustion engines themselves- exhaust often releases underwater, and the engine vibration transmits through the hull.
dredmorbius 2y ago
Correct on both counts. 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-american-submarine-propeller-design https://archive.navalsubleague.org/2012/a-century-of-america...> <https://www.sciencedirect.com/science/article/abs/pii/S0029801820314190 https://www.sciencedirect.com/science/article/abs/pii/S00298...> For nuclear submarines, overall efficiency isn't tremendously 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 propellers, specifically to reduce cavitation: <https://en.wikipedia.org/wiki/Virginia-class_submarine#Propulsor 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 container-ship (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/E-class_container_ship> <https://en.wikipedia.org/wiki/Triple_E-class_container_ship https://en.wikipedia.org/wiki/Triple_E-class_container_ship> <https://en.wikipedia.org/wiki/CSCL_Globe 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) submerged. 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 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 https://en.wikipedia.org/wiki/Froude_number>), I suspect Virginia class boats could likely achieve similar speeds should the need exist.
- sevensor 2y ago
  Fascinating, thank you!

I'm not a fluid mechanics guy, but all that cavitation from the screws seems like preventably wasted energy, not to mention a huge source of potential surface wear.

schiffern 2y ago

Yes you're right. 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? https://www.youtube.com/watch?v=a8PjWSeLUzY https://www.youtube.com/watch?v=a8PjWSeLUzY

_yb2s 2y ago
Fascinating, I'm really into boating and sailing and have never heard of this- no consumer grade engines or boats that I know of have this.
sevensor 2y ago
Wow, that's awesome! Looks like a big win all-around. Save money, save fuel, save wear.

_yb2s 2y ago

A lot of the noise is from the combustion engines themselves- exhaust often releases underwater, and the engine vibration transmits through the hull.

dredmorbius 2y ago

Correct on both counts. 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-american-submarine-propeller-design https://archive.navalsubleague.org/2012/a-century-of-america...> <https://www.sciencedirect.com/science/article/abs/pii/S0029801820314190 https://www.sciencedirect.com/science/article/abs/pii/S00298...> For nuclear submarines, overall efficiency isn't tremendously 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 propellers, specifically to reduce cavitation: <https://en.wikipedia.org/wiki/Virginia-class_submarine#Propulsor 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 container-ship (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/E-class_container_ship> <https://en.wikipedia.org/wiki/Triple_E-class_container_ship https://en.wikipedia.org/wiki/Triple_E-class_container_ship> <https://en.wikipedia.org/wiki/CSCL_Globe 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) submerged. 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 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 https://en.wikipedia.org/wiki/Froude_number>), I suspect Virginia class boats could likely achieve similar speeds should the need exist.

sevensor 2y ago
Fascinating, thank you!