Pump jet propulsor?
 

Can someone explain and perhaps post a few pictures, of the pump jet that submarines are using instead of propeller?
Why is it better than a conventional propeller?

I assume you are talking about a 'caterpillar system' like the one used on the Typhoon Red October?

If so I would think it would be for the lower noise signature presented ISO using a propellor for propulsion.

Before I get shot down I know Red October was a fictional system but there are research projects out there trying to use similar systems!! :)

A225HVY

No picture but Iam sure someone else will post one but here is a brief explanation. The slower a propeller rotates the less caviation (noise) it makes. This has led to large multiblade propellers like on the Ohio that move very slowly but still provide adequate propulsive force. As an anecdote amongst the improvements made to the 636 Improved Kilo to quieten it is a 50 percent reduction in rotation speed of the propeller to reduce cavitation and rotation noises. This gives you some idea how important a source of sound, propellers can be. Additionally propeller tonals are a major low frequency (long range) means of identifying and detecting submarines. The Pumpjet takes the idea of a large slow turning multiblade propeller one step further. Instead of a large slow propeller what if you could put the propller in a shroud so it could move slower and quieter still? By having a shroud you prevent water being pushed sideways by propeller so you can move propeller slower. Additionally the shroud acts as a side wall to noise emanating sideways from propeller. Sound is now in theory restricted to very specific directions of dead aft in a cone and just ahead of pumpjet but shrouded by the submarine hull itself. Now if you add multiple rotors (blades) and add stators (fixed blades like on jet engines to straighten out centrifuegal flow from rotors) you can have a very quiet propeller with many, many blades (up to 10, 15 on Trafalger class) that moves much slower than a conventional propeller and which is also quieter since shroud can mask noise of rotors turning as well. As an example of its effectiveness, the Trafalgar class can reportedly move from 5-15 knots with NO vibration.

Operationally this adds up to higher speeds whilst remaining covert and not cavitating. Additionally this means a higher tactical speed (speed at which you are covert but still able to use sonar). The only barrier to high tactical speeds once you have a pumpjet is adequate machinery quieting (double rafting like Akula / Seawolf), adequate flow noise coatings and good hydrodynamic design to reduce onset of flow noise from high speeds which result in washout (speed where flownoise blinds sonar), and to reduce large increases in broadband noise levels from flow noise.

Seawolf which was designed to fight it out in the bloodfest that would be a Barents Sea bastion battle (as part of the U.S navy's Maritime Strategy) was designed with a Pumpjet to confer it the ability to fight, detect and escape datums at high tactical speeds which individual Soviet units would not be able to match (a platform going faster than Seawolf or equal in speed would not be covert nor be able to use its sonar to detect Seawolf which itself would be covert and be fully able to see around itself with sonar maintaining a hopefully decisive advantahge in situational awareness). The concept is to fight faster than the enemy, similar to the supercruising concept of the ATF whereby soviet fighters could not be able to fight at the speeds ATF could and be robbed of the initiative since they could not engage or disengage at the time of their choosing. Thus pumpjets are invaluable for helping to raise tactical speeds of submarines.

The disadvantage is weight from the shroud, cost of design which can be challenging and until recently with the British, erosion of the pumpjet shroud which is now composite on the new Astutes to reduce this and its associated maintenance cost.

Wow, thanks for that in depth explanation. I knew it was a British invention but could find no description. I kept thinking it was some sort of water jet, like a jet ski. But it would seem to be more related to a Kort nozzle. I would have thought, like a Kort nozzle, that the associated shroud would cause increased drag and reduce the maximum speed. I have heard that, on ships anyway, there is a shoud called a "speed nozzle" that is pretty much a Kort nozzle but the shroud was more foil shaped and did not signifigantly increase drag and thus maximum speed was not affected.

Russian "Alrossa"

AbitNutz,
another "idea" behind the pumpjet propulsor is that specific number of blades. For a normal propellor, the aim is to have as many blades as you can, that gives a lower load on each blade and hence less cavitation too. For a normal propellor 7 blades is now the maximum due to the size of the blades. By shortening the blades and putting them in a nozzle (which is in fact a Kort Nozzle, doesn't really do that much to your top speed, only at low speeds the friction it causes is negative, once above that, it becomes more effecient, the smaller blades get an equal effect because of the higher amount of water. This is also the reason why you primarily see nukes with them as they generally use high speeds to transit (Alrosa is just a testbed)), you can add many more blades, as mentioned before. This gives a much lower load on the propellor and hence less noise again. Disadvantage being of course the added maintenance and inspections. Of course it is less vulnerable to collisions or snagging of wires compared to a normal propellor.

Has there ever been a move to fit these to surface ships ?

Probably yes, but not a bright idea. The underwater shape of a surface ship is not really good for it. Certainly not with multiple prop ships. The water flow is very different around a surface ship and the rudder is behind the propulsor.
As said before, the Kort Nozzle is used on some surface ships, but these are primarily tugs and coasters and these don't have the function of quietning, but more of making the propulsion more efficient. (which implies higher loads on the propellors, certainly for tugs).

Ok don't laugh at the analogy but I have a 26' sailboat with a Yamaha 9.9 HP outboard. When I turn the engine too much to port the prop would happily chew in to the rudder. (Don’t ask how this is possible...trust me, it happens to every owner of this boat)
Anyway, I fitted a "prop guard" which is essentially a Kort Nozzle. It's just a stainless steel ring that completely encircles the prop. Mine fits very tightly and there is very little clearance between the prop and the ring. It does its job, in that it completely protects the spinning prop from contacting something it shouldn't. It also provided an unreal increase in low speed thrust. This engine was never a dog and always had good low speed torque but with this "Kort nozzle" the power went up dramatically and fuel consumption dropped. I can attain hull speed, about 6 knots or so, at 1/3 throttle; before it was 2/3rds to attain the same speed.
When new you could order my sailboat with either an outboard engine or an inboard 9.9hp diesel. I chose the outboard model because it was faster under sail (certainly it couldn’t be because the diesel cost $8,000 more). There is a diesel model in my boat club (I guess he didn’t care about sailing speed or had an extra eight grand). When motoring the diesel owner always just walked away from me on acceleration to hull speed. That is no longer the case. I bury him and we’re both pretty sure that under cruise I’m using less fuel than his diesel. If this wasn’t on a hull that is pretty much limited to 6 or 7 knots, I’m sure as speed increased we’d see a reversal of this performance.
Ok, not sure anyone cares but thought I would share.

Adding to the contributions of Gepard and Neptune, pumpjets/Kort nozzles/ducted props exploit the venturi effect to realize higher mass flow and thus thrust. They can also offer a reduction in induced drag, especially when compared to the later-generation multi-blade props.

AbitNutz,
that is indeed strange, but nonetheless very interesting. Could you give some more information on the type of rudder you have and what type of propellor? I'll try to figure out how that is possible, or what causes it, it might be the shape of your underwater hull and the relation to the position of your propellor/nozzle.
(and how could your rudder possibly hit your propellor??? Do you work on your boat? Otherwise I have an idea to increase your manoeuvrebility if you wish so (and if it's not already in the design). You can also send picures if you wish, just send me a PM.

In many ways water or fluids act like air does, but in many other ways they are quite different. The main reasons for the differences are that water is much denser than air and that water doesn't have the compressability that air has.

One of the main problems with propellers is similar to the problems of swept wings. The air or water that goes over the wing and under the wing or through the propeller blades is good, it generates lift/thrust respectively. However some air/water travels along the wing/blade and generates a large vortex at the wing/blade tip. On aircraft the solution is for winglets, and on propellers in water and in air the shrouded propeller is a commonly used solution. Underwater this blade tip drag generates a hiss that is directed sideways and is detectible from very long range.

On the ducted tail rotor on the Eurocopter product line, the blades aren't axisymmetrically arranged- there is an unequal amount of space between some of the blades- it's balanced, but I was told by one of the company pilots that this was done to reduce cavitation and thereby reduce noise.

Could the same principle be applied to the pumpjet propulsors?

The latest car cooling fans have unequally spaced curved blades as well as stator vanes, no doubt for similar reasons....

Previous thread on Pumpjets here http://forum.keypublishing.co.uk/showthread.php?t=47978

Globally, the shrouded propelled (aka pumpjet) is an effective way to reduce cavitation, but is costing enormously in power.
That is the reason it is only suitable to nuke platforms.
Alosa is the only SSk, and is a trial platform only.
No other SSK has pumjet, not even the Collins or Oyashios, which are the most powerfull.

If you look at the tail rotor of the Apache you will see that the four rotor blades do not form a plus, but an X, where the gap between opposite sets of blades is equal but if you go around the blades the first gap is different from the next all the way around. A similar design is used on the Mi-28 helo. Its purpose is to reduce noise. (Tail rotors are notoriously noisy and are often second only to the engines... though except in helos with too few rotors like the two blade Huey.)