Funnel around propeller what is this called?

I've seen this on planes I think and hover backpacks

Its like a circle that goes around the propeller almost touching it and the closer without touching the better.

It's supposed to increase the speed or the water/air I think

what is this called?

 

It is a nozzle or propeller shroud depending on what it does. A shroud generaly is to protect the prop or humans and is designed for minimum drag. Nozzles can either be accelerating or decelerating depending on what you need; i.e. high thrust/low speed or high thrust/high speed. Note that nozzles and shrouds never increase maximum efficiency, they only manipulate the thrust-torque-efficency curves and always at a loss.

 

a Kortz Nozzle

Steve

 

A shroud can easily increase efficiency over an open prop, at least for some point solutions. A quick modelling session with any competent prop modelling software will show how absolute prop efficiency very significantly improves (due to reduced tip losses from effective vortex control that result from reduced spanwise blade flow).

The downside of a shroud (of any type) is that is adds additional drag. As the power needed to overcome drag is proportional to the cube of speed, shrouds only produce an effective overall propulsive efficiency improvement at low speeds.

The main advantage of shrouds is that, for low speed craft, they offer greater efficiency from a given diameter of propeller, which can make them a useful option where there are constraints on prop size.

 

Note what I said carefully, nozzles never increase maximum efficency. While it is true, as both you and I point out, that they can manipulate point efficiencies by changing the torque-thrust ratios; the efficiency of a ducted propeller will never excede a open water prop designed for that same point.

That said, if there are limitations such a diameter, operational speed range, shaft torque limits, etc which will not allow a suitable open wheel solution, then a propeller in a nozzle is a viable choice...but they are not the first design of choice no matter how obligatory they seem on some hull types.

Edit: see these threads

http://www.boatdesign.net/forums/education/propulsion-kort-nozzle-4525.html
http://www.boatdesign.net/forums/education/just-question-5547.html

 

Some interesting real life experience with a prop shroud:

I had them on a previous catamaran with saildrives. The PO put them on in order to keep crab trap lines out of the props. Seemed reasonable.

I was very disappointed with the sailing and motoring hull speeds of this boat, so I decided to take them off.

Gained 2 knots under power and at least a knot under sail. They didn't work in my application.

 

We may be discussing semantics here, but if, for example, you take a 12" prop running at a hull speed of 5kts, then fitting a nozzle will always increase efficiency.

On the other hand, fitting a 12" prop with a nozzle on a hull that's running at 20kts will almost certainly reduce overall efficiency, because of the drag from the nozzle.

However, the prop efficiency (not overall propulsive efficiency) will still be higher even for the 20kt case, it's just the extra drag of the nozzle that reduces the overall performance at this speed.

I've worked on a lot of pump jet propulsion systems (chosen because of the noise reduction they give rather than their propulsive efficiency) and we spent years designing, testing and optimising them, something that Kort never seemed to do (as has been shown by the advantages that proper nozzles, like the Rice design, give over the older Kort profiles).

Generally, a prop in a nozzle at low hull speed will give an efficiency that is around the same as a prop that is maybe 30% larger (and slower running) running without a nozzle, so it depends how you choose to make the comparison.

Many of the commercially available add-on nozzles for outboards are total crap, mainly because they have lousy profiles, a very poor intake geometry and they don't use a prop blade profile that both matches the changed blade loading distribution and allow for tight tip clearances. Add in that they are often fitted to boats that run at more than 15kts or so and it's hardly surprising that they generally give poor results. I'm amazed the ********ters that sell them stay in business, to be honest.

 

FWIW, my "prop cages" (pseudo nozzles) were crap, as Jeremy describes. There were not properly engineered for drag.

 

Yes, we are discussing a rather esoteric point, but an important one. The point I wish to make is that if maximum thrust and maximum efficiency is the goal at 5 knots, why not use a properly designed slow turning 24" or 30" wheel? That would be far more efficient than a 12" wheel in a duct. And then again in low thrust situations, where a 12" open wheel can be "properly" designed, it will have more efficiency (i.e. ehp/shp at optimum J) than a ducted propeller. It is only when you apply some restriction to "proper" propeller design does inflow manipulation like nozzles become viable.

 

Here's a nice one. Can someone with experience comment please.

 

True Kortz Nozzles where they really shine, tugboats, pushboats and other low speed high torque applications.

Steve

 

Yep, 2500+ shp through a 6-8 foot wheel. And the free running speed is much much greater that the towing speed. Otherwise you would need a 12-15 foot wheel to absorb that much shp at slow speed. A larger wheel would have a better efficency though at the cost of increased draft.

 

That is not just a nozzle, it looks like a steerable nozzle...
In order to be able to turn the nozzle, a larger tip clearance is required. This decreases its efficiency compared to a 'normal' fixed nozzle.

I dont know how it pays out with regard to manoeuvrability. It is, I believe, often a solution where there is little to no room to add additional rudders.

 

Here are some real efficiency numbers for a low power 12" prop, figures that I obtained by calculation followed by some fairly rigorous testing (test measurement accuracy better than 5%). The application was one where propulsive efficiency was paramount, as the available power was low (a solar powered vessel).

Two blade 12" high aspect ratio open propeller running at 440rpm at 4kts = 81%

Two blade 12" high aspect ratio shrouded prop with square ends and modified blade loading also running at 440rpm at 4kts = 87%

As I needed to reduce the draft I decided to reduce the prop diameter and use a duct, so here are the figures for the final 8" diameter, medium aspect ratio, square tip, two blade prop running in a duct, same speed and thrust as above but running at 580rpm = 86%

Tip clearance for the 8" prop is 2mm and the blade twist and chord distribution was adjusted to account for the significant change in span wise blade loading caused by the duct reducing span wise flow (the tips of a ducted prop are much more highly loaded than those of an open prop). The mean blade chord for the 8" ducted prop is around double that of the 12" open prop.

These tests were all for a low speed, low power application, where duct drag was very low when compared with hull drag and the induced drag from the prop vortices. If the hull speed had been higher then I estimated that the break-even point in terms of overall efficiency would have been around 12 to 15kts. Above this speed the drag from the duct would have been greater than any advantage derived from recovering the energy normally lost to blade tip vortices with an open prop.

 

There called Kortz Nozzle's there placed around the prop(s) to improve efficiency, they work great we use them on all sorts of merchant vessels some large however the main use ive come across is on tugs and supply vessels, then again tugs now prefer things like Voith Schneider propeller, (also known as a cycloidal drive) setup or azipods tons of power and v. manipulatable to get the correct power and thrust angle. Kortz nozzles are useful if used on the correct ship design. (Jeremys numbers look spot on ) Hope this answers your questions

John
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Marine Engineering Officer 'BRITISH' Merchant Navy (Meng)