Pump jets vs.props - advantages?

[wayne frey]

Reading in the "subs in the news" I was looking at the triangle shaped hull sub in the link(weird !! But interesting,someone going to build one?).
Anyway, the concept of pumpjets vs props came back to me. Clearly, the american navy, and a few others sees enough promise in the pumpjets to put them on machinery that is quite expensive. The advantage is stealth, yes? As far as efficency though, I do not know. Is the prop more efficent than the pumpjet? The russians as a rule seem to be staying with propellors instead of propulsors,having tried it on at least on Kilo. I suspect they were not a successful with the design as we have been.
And what about shroud shape? Does the cone shape of the
pumpjet actually incease the flow to the impellor? Is it an effiecency thing of just a noise reduction thing?
And stators. Some designs have no aft stators behind the impellor. Is there an advantage to this? Also, some designs have curved stators.I would assume this is to reduce the torque effect of the drive. Yes?
Any other thoughts on the advantages of each?

[mike byers]

I suspect that it is

I suspect that it is a combination of stealth, effiecency, and speed.

A pumpjet propulsor has been used on the MK 48 torpedo for almost 20 years.

Regards

[wayne frey]

Yea,
Another question, is a pumpjet,all

Yea,
Another question, is a pumpjet,all things being equal,more efficent?
Who knows this stuff and where can on go to find Mr Wizard that knows this kind of stuff. Pump jets interest me.
It interest me that the brits and the russians do not use the aft fixed stators. Surely a lot of money goes into research.There must be reasons,good ones, why they are different. Steve R. gave me a brief synopsis about them a while back. I would like to learn more.

[koeze]

As you know the pitch

As you know the pitch and shape of the blades of a propellor create a pressure difference when they move. They work just like the wings of an aircraft. When a boats is moving forward the water is forced to go faster across the front side of the blade and slower across the backside of the blade. (When I refer to front and back I mean the

Bernoilli states that P times V (Pressure times Velocity) is a constant. When the Velocity increases the pressure drops.
On the front side of the prop a vaccuum is created, behind the blade pressure is built up. These combined pressures suck the boat forward.

As you know nature tries to equalize everything. Therefor water tries to flow from the high pressure area to the low pressure. The only place where that can take place is at the tip of the blade. This creates noise (not wanted) and loss of power. When a closely fitted shroud is fitted the water cannot flow around the tip of the blade neuralizing part of the pressure difference and the efficiency of the prop is increased.

This is also applied as Kort Nozzles in tugs

and more visible as winglets in aircraft


Hope this helps.

EJK

[koeze]

Can't seem to edit my

Can't seem to edit my post so I'll add this photo here


EJK

[Guest]

I discovered many years back,

I discovered many years back, that vectoring the thrust of a propeller through a shroud always resulted in better performance for low to medium speed applications.

For high speed applications, I would assume that drag from the nozzle can be a problem and outweigh any advantages.

I guess that is why you don't see them on speed boats!

As regards prop blades. Conventional wisdom seems to be that a three blade prop is the optimum for low to medium speed applications, with twin blade props, submerged or surface piercing (one blade prop?) being better for high speed use.

Too many blades on a prop can lead to the blads 'interferring' with one another.

The smaller the prop the less efficient they become, and model props with multiple blades, like a pump jet, will likely be very inefficient.

However, I don't honestly think this need be of any concern to scale model submariners, after all we have very capable electric motors and battery technology to power our models long enough for us to grow bored with sailing them.

Just so long as the propeller is capable of pushing the boat forward at a reasonable rate of knots without undue fuss, then I consider it's doing it's job well enough.

Andy

[carcharadon]

This video shows a sub

This video shows a sub fully operational using bilge pumps. No servos, no prop, no moving parts other than the internal bilge pumps themselves. This sub is 4ft. It also works on a 7ft version.
http://home.comcast.net/~tyourk/4foot.html

[adriaticsea]

Hi Wayne,

regarding pumpjets, I can

Hi Wayne,

regarding pumpjets, I can say that the after stators (the ones you see at the end of the shroud) have an important effect on the water flow set in motion by the propeller. Infact, when the props moves, it creates an helicoidal flow that can have bad effects on hull's stability, so the after stators re-direct the flow, forcing it to take a fully rectilinear component, thus maximizing efficency and reducing bad effects on the hull. This is what I read on some books I have here at home, mainly on one written by Camil Busquets on "Carriers, Cruisers and Submarines".
For the other set of stators, the ones ahead of the prop, I can only imagine that they better direct the water flow to the propeller, increasing prop's efficency.
I'd like to hear from Matt Thor about his Seawolf pumpjet propulsor, I read somewhere on this forum that the newly upgraded version of the kit features a very efficient and accurate pumpjet.
BEST REGARDS MAURO

[bob the builder]

I will be experimenting with

I will be experimenting with a shroud over my Nautilus' propellor in the next couple of days to see if it will help with the performance of the model.

My prop sucks air at anything over 1/3 throttle when surfaced, and I'm hoping to make the shroud tiltable on the vertical axis to help with maintaining depth and a level keel (tied into my dive plane linkages).

It will be made from transparent material so as not to severely affect the looks of this great submarine.

I'll let everyone know how it works out.

[wayne frey]

I have seen a picture

I have seen a picture of the Kilo pumpjet. But have only seen the one. Does anybody have any more pictures of this boat?

[feet wet]

Alot of interesting discussion going

Alot of interesting discussion going on here. Couple of points to consider, in 1958 an "experiment" pump-jet system was installed on DD 848, a Gearing class destroyer. Obviously the USN has been looking at this system for a while.Next to consider is the increase in prop efficiency relative to depth. The deeper the propellor the greater the H20 pressure on its surfaces, as such the tendency to cavitate is reduced. Can this be a factor in improving the efficiency of pump-jets?