prop shrouds

Agreed. Except for me, we are not idiots here. These are in great part professionals with centuries of combined knowledge.

 

There is one possible reason a shroud might help the speed of the torpedo, but it's specific to the torpedo application.

 

The design used in subs and torpedoes is properly a pumpjet propulsor

http://en.wikipedia.org/wiki/Pumpjet

 

....in fact, several. But that does not mean they are all applicable in other disciplines of marine propulsion.

First a look at the dimensioning on the Mark 48 torpedo mensioned earlier. It has a shell diameter of 21" and an engine power of over 500 hp, giving a non-confirmed speed in excess of 55 knots. It is all a question of design limitations and trade-off. You must also understand that in todays torpedo-warfare, silence is essential.

An open propeller for this operation, would have a diameter that didn't fit into the tube, at the same time suffering from heavy cavitation, particularly in the acceleration mode; ie it would be noisy and difficult to control. Generally, with torpedoes, propulsion design follows two main routes: either open contrarotating props or pump jets, both having merits of their own. Both give complete cancellation of torque, preventing the shell to rotate. Which type you select depends on a number of design restraints. As an example, the M37 torpedo has been fitted with the M48 engine, but is using contrarotating props, while the M48 has a pump jet (note: NOT a "shroud"!). End performance is about the same. It is again a question of meeting conflicting design demands.

Now, the "shroud" you see on a pump jet is working on a different principle than the Kort nozzle. It is shaped to reduce the inlet velocity into the impeller in order to control static inlet pressure and avoid cavitation, just as we do on a conventional waterjet unit. The fact that the inlet is circular does not change the principle. With pj the engine load is more predictable, for instance at launch, than the open props. But the friction loss of the structure is still there, it is an unaviodable evil in order to get other functions right.

As for the submarine, silence is EVERYTHING!!!!! Again we have conflicting requirements; silence and efficiency demanding big, slowly rotating screws, maneuvering in confined waters, vulnerability and hiding ability et c. puts limits on diameter. Tip vortices on any open propeller are generating individual noise patterns for subs and are very difficult to avoid, in particular with the increasing power levels of the propulsion machinery.

So, "waterwar", there are different design restraints for all the varying types of vessels on and in the sea, resulting in differences in "fenotype". You can rest assured that the functioning of the various types of "shrouds" are well known and understood by the professional hydromechanical/hydrodynamic society!

 

Submarines and torpedoes use pump jets for specific and specialist reasons, neither of them related directly to propulsive efficiency in terms of power in to propulsive force out. New technology it ain't, as I worked on our first pump jet powered torpedo development in the late70s, early 80s.

In the case of a torpedo and fast attack sub, where burst speeds of 50kts plus are required at depth, the pump jet offers a means of increasing the allowable blade loading and getting more thrust from a fast turning, multi-blade, small diameter single, or more usually pair of, prop(s). It works by allowing the blade tip to be loaded (a normal prop will have near-zero tip blade loading due to span-wise flow) and reduces blade tip losses from excessive tip vortex shedding (by limiting mixing at the blade tips). Tip vortex shedding is also a major contributor to low frequency blade passing noise, a major noise source when the boat is silent running at low speed and a key reason for the adoption of pump jet propulsion on submarines and torpedoes, where passive acoustic sensors are used.

Pump jets work best with a large blade count, which increases the blade passing frequency. Increasing passing frequency shifts the noise spectra upwards, which aids overall noise signature reduction because high frequencies are easier to damp with sound absorbent coatings and low frequencies are easier to detect over long ranges than higher frequencies.

Torpedoes also use pump jets as a means of providing effective roll compensation from propulsive torque. By using two, well-matched, contra rotating props running in a duct, where flow conditions can be accurately controlled, the duct outlet can be arranged to have little or no swirl. This makes keeping the torpedo upright easier and reduces control surface losses from having to do torque-related roll correction.

Finally, the blade shroud on a pump jet powered submarine allows the use of acoustic damping material close to the blade tips that can further reduce the noise signature, particularly the all-important blade passing frequency.

The downside is poor efficiency at high speed - pump jets are not used on submarines because they are efficient - they are simply a point solution for a particularly demanding set of very specific requirements.

The additional drag from the duct is substantial when compared to a conventional prop and so reduces overall propulsive efficiency at speed. Because the power needed to overcome the additional drag of the shroud is proportional to the cube of the boat speed, this problem gets very significantly worse as speed increases.

Submarines live with this poor high speed efficiency, because they spend the vast majority of their time at low speed (to minimise the chance of detection and so their sensors will work to best effect) and only use high speed for relatively short periods. Torpedoes live with it because getting the thrust they need from conventional props would increase the prop diameter and noise from the tips to an unacceptable degree.

Tug and work boats use Kort and Rice nozzles for high thrust at low speed from a modest diameter prop, but you won't see these nozzles used on high speed civil craft because of the poor efficiency at speed. There are other issues with them, too. They are very effective at trapping weed and debris, which makes them inconvenient for use on smaller vessels operating in waters where such stuff presents a potential problem.

Jeremy

PS: I spent several years working on our first pump jet powered lightweight torpedo, Stingray, so do have an inkling as to how these things work.

 

so this jerk has got 51 post in a couple of day ...he must be laughing his duck (fannie) off

 

Yup, is that you having a new alias, pistnbroke?

 

Well you started it And anyway ,look what the rest of us have learned .

 

shrouds

I am so happy that you have had so much ego boosting I can imagine many heads puffed up .
The reality hasn't changed

I would still like to have some solution that works well. I have an old outboard OMC and light orlando clipper v hull and don't want to lose speed or performance.

http://www.propguardmarine.com/test.html


http://www.propguardmarine.com/index.html


http://www.ronhalemarine.co.uk/superbasket/images/products/151/b_propguard.jpg

I am curious if any of you Experts have any opinions on this product
I can only laugh at some of the more ridiculous posts piece of work...



a bemused Stew

 

professional hydromechanical/hydrodynamic society!

wow a mouthful
bet its counterpart in the middle ages pushed world is flat too and nothing changes...
'cause after all life is static'


Stew

 

so this jerk has got 51 post in a couple of day ...he must be laughing his duck (fannie) off

steering intelligent converse is not stupid only people who have to resort to trash talk are...

 

Not an expert, and I don't really appreciate the tone of your request much, but the principles are pretty straightforward.

Adding a duct around a prop will increase the prop blade loading (I've already mentioned this, but hey.....) so will increase the pressure ratio across the prop disc. This means a small prop will deliver a higher low speed thrust than it would without a duct - in effect it can shift a greater mass of water per unit time..

The downside (and I've already said this, too........) is that the overall power from the motor doesn't magically increase by fitting a prop duct and propulsive efficiency at medium to high boat speeds is significantly worse, primarily because of the extra drag from the duct.

If you want to build a tug boat, or have another need for increased low speed thrust, then ducts can be great, which is why Kort and Rice nozzles are so commonplace on tugs and work boats. If you want to make your boat go faster, then they aren't great at all.

The other downside (again, I've already mentioned it......) is that they are a pain on a small boat if you motor in areas with weed or debris, as the duct will hoover up stuff and it will wind around the prop far more readily than with a free prop with no duct.

Years ago I used to drive a small dive boat (an inflatable) that was fitted with one of these prop guard rings, for enhanced diver safety. Clearing weed and plastic bags from it was a pretty regular chore.

Jeremy

 

I'd say the end is near when repeative intelligent info starts to become repeative

 

Can you please back this claim with facts and examples other than torpedoes and submarines, which for sure do not make a trend?

 

The guy doesn't have any, I'm afraid, as they can't exist. He's been repeatedly informed of the facts relating to the efficiency, thrust and speed relationships relating to ducted props/pump jets and yet still seems to be sticking to his original (and wholly incorrect) assertions.

If he had the grace to just accept that his original assertions were wrong, then maybe people would cut him some slack, but somehow I get the feeling that it's not in his nature to accept that he jumped to an incorrect conclusion right at the start of this thread.

Jeremy