prop shrouds

[daiquiri]

I just want to hear thrust increase vs drag discussion to be scientific not chatteric

The way this product is described, it appears to be a device which induces an additional waterflow through the wanes and inside the aft duct, which has a considerably bigger diameter than that of a prop. Furthermore, the ducts (more than one, with increasing diameter) appear to be held in place by stator-like struts between them, which are shaped in such way to reduce the swirling motion behind the prop.
So, there is a hidden line in the test report: it compares (but doesn't explicitly tell you that) a small prop (call it's diameter "d") with no duct, to what effectively is a device which simulates a bigger prop (call this apparent diameter "D").

It is similar to what winglets do to a wing or to a hydrofoil. But, just as no winglet is as efficient as a straight wing of a same total wet area, this device cannot be as efficient as a non-ducted prop with diameter D. Or as a non-ducted prop with diameter D and a stator ahead of it. That's because, as Jeremy Harris has eloquently explained it, at high speeds you have that "duct rings" = "friction". And friction means lost power.

The point here is: as usual, you have to compare apples to apples and not to watermelons, because watermelons are bigger.

Cheers

 

[gonzo]

Eloquently put, a fruitful remark.

 

[DCockey]

I suspect there can be cases where the addition of a shroud may improve top speed of a boat which has a badly matched propellor to start with. But I haven't seen any independently tested examples. It such a case a properly matched propellor without a shroud would probably have even higher top speed.

 

[gonzo]

There isn't a single racing team running nozzles. That should be a strong indication of their lack of performance at top speeds.

 

[DCockey]

... It is similar to what winglets do to a wing or to a hydrofoil. But, just as no winglet is as efficient as a straight wing of a same total wet area, this device cannot be as efficient as a non-ducted prop with diameter D. Or as a non-ducted prop with diameter D and a stator ahead of it. That's because, as Jeremy Harris has eloquently explained it, at high speeds you have that "duct rings" = "friction". And friction means lost power.
.....

A wing and winglet combination optimized for drag reduction can have lower drag than a wing of equal wetted area. Such a combination is more than just the end of the wing bent up and is not trivial to design. The drag reduction mechanism is a reduction in the wake vorticity.

 

[Jeremy Harris]

A wing and winglet combination optimized for drag reduction can have lower drag than a wing of equal wetted area. Such a combination is more than just the end of the wing bent up and is not trivial to design. The drag reduction mechanism is a reduction in the wake vorticity.

True, but that's because the winglet on a wing gives an apparent increase in wing aspect ratio and a commensurate reduction in induced drag (the drag due to the creation of lift, rather than form drag due to projected frontal area and drag coefficient), which doesn't wholly apply to a ducted propeller or pump jet.

By allowing the prop(s) to operate at a higher blade loading, and by allowing the tips of the prop to do useful work, a duct can improve mass flow through the propeller disc for a given power input. This greater mass flow produces greater thrust, in much the same way that a larger diameter, slower turning, prop produces greater low speed thrust for a given input power..

The downside, as has been mentioned previously, is that the duct creates significant drag, so as speed increases the power needed to overcome the duct drag starts to exceed the efficiency gain from running the prop in the duct, and an open prop becomes more efficient.

Jeremy

 

[waterwar]

thank you for intelligent discourse

I note that Mr Shultz specifically addresses the the trim and balance issues .
The input you all most have provided has been very informative.
I have severe trim issues in my boat when returning with a full catch ( hundreds of gallons water in livewell) and appreciate the importance of trim.

 

[daiquiri]

A wing and winglet combination optimized for drag reduction can have lower drag than a wing of equal wetted area. Such a combination is more than just the end of the wing bent up and is not trivial to design. The drag reduction mechanism is a reduction in the wake vorticity.

DC, I know well that a winglet is more than just a bent wing. You are correct with that remark about the term "wetted area", I have tried to keep things simple for the thread starter, who appears not to have a sufficient knowledge in aero/hydrodynamics for more in-depth talk. The correct comparison considers the total of horizontal wingspan plus vertical winglet length, not the sum of the surfaces.

However, I agree that winglets can be beneficial if you are talking about wings with a structural (bending moment) constraint. And even in that case augmenting the span rather than using winglets is a better strategy. It was mathematically demonstrated by R. T. Jones (and another guy whose name I can't remember) that you need about 20% (of the span) high winglet to get the same drag reduction which you would obtain with just a 10% longer wing. Provided that you don't screw the boundary layer and induce the premature separation around the winglet root, in the former case.

Since this talk was about the difference in propeller diameters with no regards to structural issues, hence my observation about the wing/winglet analogy.

Cheers!

 

[DCockey]

My comment about winglets was intended only as a comment about winglets, not an endorsement of ducted propellors. I'll have to track down the RT Jones analysis.

 

[DCockey]

Found the RT Jones analysis: http://www.engbrasil.eng.br/index_arquivos/art60.pdf An integrated bending moment along the span was used as a constraint. The value of winglets for drag reduction of wings depends greatly on the constraints placed on the wing/winglet system, geometical, structural, and the operating envelope. But discussion of winglets is only marginally relevant to the current topic.

 

[Joakim]

If you note in the test link i sent Mr Shulltz claims a boost that overcomes the drag is this possible he claims a speed boost at high speed

Yes he does, but not at all scientifically. He doesn't tell the boat models, propeller sizes and models etc.

Just after a quick look the test boat #1 looks very suspicious. 18' skiboat with 230 hp stern drive and only 38 mph (33 knots). Most 18' boats are faster with 135 hp. So what is this boat and why is it so slow? Did it have a well chosen propeller.

The boat #2 is even slower and neither of these two gained any top speed.

 

[daiquiri]

I have found the patent claim for this device: http://www.freepatentsonline.com/pd.../2003/011/US20030114052/pdf/US20030114052.pdf
It confirms that it works primarily as a device which simulates a bigger prop, by waterflow induction through duct vanes, and also acts as a stator - somewhat reducing the swirl of the inflow to the prop.

So, as I said before, it should be compared to a bigger-diameter open propeller (either with or without stator) in order to give a fair comparison of the thrust and efficiency increase (or, more probably, decrease). Comparing it to a small prop (possibly non-optimal one, as Joakim pointed out) might be a common marketing practice, but is misleading from the scientific point of view.

Cheers.

 

[Jeremy Harris]

That patent is interesting, as it shows a design flaw that will significantly reduce efficiency.

The aft most ring is right behind the point on the prop radius where the outflow velocity will be around its fastest, so the effective drag on that ring will be the boat speed plus the prop outflow velocity, massively increasing the losses. The drag on the other annular rings will just be from boat speed, so will probably be tolerable at low speeds.

This is just basic physics, putting anything close behind a prop will cause additional loss, especially when it's as close to the blades as shown in the patent drawings (I'm referring to the annular ring annotated "38" on the drawing).

Jeremy

 

[waterwar]

thank you for the intelligent review of this product

I am grateful for the information that has been given freely. Particularly from the experienced professionals in the field.
Thank you

stew

If someone was to improve prop guards, to a level where they are efficient, it will only come from intelligent discourse on the subject.
The ability of this forum to do so is a credit to all.
This prop thread now has made a useful contribution to the state of the art.
Thank you all for your participation

 

[DCockey]

There was an earlier patent 4637801 http://www.freepatentsonline.com/4637801.pdf which shows a device with two rings, both larger than the propellor. This patent expired between 2004 and 2007.

The drawings in the second patent shows a device with two additional smaller rings, one in front of the propellor(s) and one behind. The claims seem be intended to cover configurations with both the four rings shown, and with three rings, a smaller one in front and two larger rings.

The device currently marketed is the latter configuration with only three rings, eliminating the aft ring which Jeremy identified as being a source of high drag. See slides 7 - 13 of http://propguard.net/slideshow/frame.htm for drawings of the device currently marketed. The website claims "Prop Guard® is the next patent generation of propeller protection from the noted engineer and inventor Wilderich Schultz." It appears that without the third, forward ring the patent protection has expired.