Controllable Pitch Propellers

[corsair7772]

Just a quick tech question. I know that Controllable pitch propellers are used on modern frigates and destroyers etc, but do any of the modern nuclear submarines have them? Most of them seem to use Fixed Pitch Propellers. The early November Class submarines used "Variable Pitch Propellers" and im assuming they are the same thing as CPPs. Could anyone enlighten me as to whether any of the modern submarines are using Controllable Pitch propellers. If you could tell me some of the features and advantages of CPPs vs FPPs, that would be appreciated as well. Thanks.

 

[Systems Adict]

Just a quick tech question. I know that Controllable pitch propellers are used on modern frigates and destroyers etc, but do any of the modern nuclear submarines have them? Most of them seem to use Fixed Pitch Propellers. The early November Class submarines used "Variable Pitch Propellers" and im assuming they are the same thing as CPPs. Could anyone enlighten me as to whether any of the modern submarines are using Controllable Pitch propellers. If you could tell me some of the features and advantages of CPPs vs FPPs, that would be appreciated as well. Thanks.

CPP is ideal for small to medium ships as it helps them go faster & stop on a dime.

VPP does have it's uses, in helping limited ability engines increase their speed by adjusting the prop accordingly, but most subs opt for Fixed pitch for one main reason. STEALTH.

Having VPP / CPP mean the props are made from parts that move. This movement causes cavitation (due to minor mis-alignment & other things such as gaps & mating surfaces). This creates noise & can be picked up at great distances.

The fixed prop is set up so that otimum performance is obatined at all speeds, bearing in mind that most subs while capable of reasonably high speed, tend to move at speeds below 20Kts.

Finally, with VPP / CPP the props need to have maintenance done at regular intervals in dry dock, fixed pitch props are usually more robust & can easily be checked by a diver.

While this isn't the be-all & end-all, it's a start of an answer which should hopefully bring in some discussion to either correct or expand what has been stated.

Hope this helps,

SA

 

[Bonza]

Very interesting post SA, thanks for the information.

 

[EXSSBN2005]

CPP is more complex than it needs to be when it comes to propellers, its ok on small combatants as if their screw malfunctions they are not going to the bottem of the ocean ( US subs are slightly positively bouyant but enough of that is due to forward momentum that we go slightly negative if we stop) so reliability is another concern. CPP also is another load on your hydrolic system and if hydrolics go your control surfaces pretty much go very mushy and the machinist's mates have to manually control heading and diving planes, to this we would have to add controlling the propeller also.

Cavitation is the formation of bubbles following the leading edge of the propeller that then collapse and the collapseing is what makes the noise and causes vibration. Parts that move cause machinery noise and thats something to be avoided on subs.

You get a pretty fair effieiency from the fixed blades that cancels out the small boost in efficiency that a CPP would give and my sub could produce about the same max shp as stated for a CPP per the wikipedia for controllable pitch propeller ( wiki not reliable for limits but this sounds about correct from what i know, willing to be corrected). one of the other things that I would worry about is poppet deformation comming from leaving the poppets in the same place at all speeds. My current job (powerplant with steam turbines) this is a problem and we have to cycle the valves to prevent cutting of the poppet seats and particulate build up from leaving them in the same place all the time. A rapid thrust reversal could also damage your shaft and thrust bearings that are whats actually pushing the ship.

Normal cruising speed was under 10kts for us, fuel efficiency was not really a practical concern with the nuclear power plant as much as it would be for say a smaller gas turbine, diesel, or other power supplies.

 

[corsair7772]

Thanks for info guys, it was pretty helpful and im gonna add it to my notes.
Just one more thing, has the CPP and VPP been completely eliminated from modern submarines as a propeller choice? Do all modern submarines have FPPs? If you can recall any modern submarine design that still uses CPPs and VPPs, lemme know.

 

[Systems Adict]

CPP is more complex than it needs to be when it comes to propellers, its ok on small combatants as if their screw malfunctions they are not going to the bottom of the ocean ( US subs are slightly positively bouyant but most of that is due to forward momentum that we go slightly negative if we stop) so reliability is another concern. CPP also is another load on your hydraulic system and if hydraulics go your control surfaces pretty much go very mushy and the machinist's mates have to manually control heading and diving planes, to this we would have to add controlling the propeller also.

This statement above pretty much explains why subs switched back to fixed props, Reliability. These systems work using Hydraulics to operate them, adding in a 'possible area of failure' in the drive train / powerplant.

The more complicated the system, the more that can go wrong with it. After all, if you were at 100 metres, doing 25 Kts & the captain calls full reverse & it goes pear-shaped, can you jump on the VHF & call for help ??

While, as the attached brochure' links' detail, things have taken advances in the last 10 - 20 years, It's obviously been decided at a higher level that to give the navy less to worry about on a sub, they've opted for fixed pitch props...

http://www.schottel.de/pdf_data/eng_SCP.pdf

http://www.wartsila-nsd.com/Wartsil.../marine_news/2004_2/improved_cpp_concepts.pdf

Anyways, hope this helps...

SA


:daz