shrouded propeller v.s nonshrouded opeller

[jacklynfong]

Dear all, need help with regards to the difference between using a shrouded propeller and a nonshrouded propeller (of similar blade pitch and diameter and other things remaining the same) for a small seaboat which uses an Outboard motor of 40 HP. What would be the advantages and disadvantages of using a shrouded propeller instead?

[Rick Willoughby]

If someone falls in the water beside a turning propeller that is shrouded they are less likely to get injured than if it is unshrouded.

Rick W.

[daiquiri]

Wherever there is a need for high static thrust or near-static thrust, a shrouded prop is a valid option. Some studies indicates that a properly designed shroud (or duct) can double the static thrust of the prop, compared to the free-tip case.
A typical case for this are tug boats, for example, or bow and stern thrusters. In aeronautics, shrouded props (or ducted fans) are being used for some Unmanned Airborne Vehicles (UAVs) which need good hovering capabilities.
But the benefits disappear as the advance ratio J increases ( J=V/nD ) because a fixed shroud starts creating drag, thus diminishing the efficiency of the prop-duct system.

The two single most important design aspects for shrouded props appear to be the ratio of inlet and outlet areas, Ai/Ae, and the tip clearance.
The Ai/Ae ratio can be optimized only for a given advance ratio. In order to extend the useful range of forward velocities, a variable-geometry duct can be devised - though I'm not aware about any current design which adopts this solution, due to mechanical complexity and weight penalty.
The tip clearance has a big influence on static thrust. A static thrust decreases rapidly as the tip clearance increases. If the clearance is too big, you will basicaly have a conventional prop with a drag-creating duct around it.

So, all in all, it is a good system for designs which require a high static thrust and is much less justified in other cases. Remains the fact that it does provide a mechanical protection for the propeller blades, which is the reason why some designs use it in spite of propulsive efficiency losses.

[Rick Willoughby]

Originally Posted by daiquiri

Wherever there is a need for high static thrust or near-static thrust, a shrouded prop is a valid option. Some studies indicates that a properly designed shroud (or duct) can double the static thrust of the prop, compared to the free-tip case.
A typical case for this are tug boats, for example, or bow and stern thrusters. In aeronautics, shrouded props (or ducted fans) are being used for some Unmanned Airborne Vehicles (UAVs) which need good hovering capabilities.
But the benefits disappear as the advance ratio J increases ( J=V/nD ) because a fixed shroud starts creating drag, thus diminishing the efficiency of the prop-duct system.

The two single most important design aspects for shrouded props appear to be the ratio of inlet and outlet areas, Ai/Ae, and the tip clearance.
The Ai/Ae ratio can be optimized only for a given advance ratio. In order to extend the useful range of forward velocities, a variable-geometry duct can be devised - though I'm not aware about any current design which adopts this solution, due to mechanical complexity and weight penalty.
The tip clearance has a big influence on static thrust. A static thrust decreases rapidly as the tip clearance increases. If the clearance is too big, you will basicaly have a conventional prop with a drag-creating duct around it.

So, all in all, it is a good system for designs which require a high static thrust and is much less justified in other cases. Remains the fact that it does provide a mechanical protection for the propeller blades, which is the reason why some designs use it in spite of propulsive efficiency losses.

daiquiri
Took you a whole page to say what I said in one line!!!

Rick W

[jacklynfong]

Thanks for all replies.

May I know what is a static thrust?

Other than the increase in static thrust with the use of a shrouded propeller, are there any other advantages of a shrouded propeller as compared to a non shrouded one? I found out that shrouded propeller has these advantages: improved boat handling, greater steering control at higher speed, stopping ability of the boat is also dramatically increase as the shroud presents a greater blockage area, more power /forward thrust is created for the same input power and torque, produce greater dynamic lift which permit it to achieve faster acceleration and planing, decrease fuel consumption (as it's able to obtain a higher thrust and speed using a same input, reduce hull and engine stress and reduce torque effect of the rotating propeller.

The disadvantages can be the lost of it's advantage over propeller at about 10 knots (18.5km/h), additional load to the boat and increase in drag force, especially at high speed.

Pls advise and comment whether i have got it correct. Please add in if i miss out any stuffs. Thank you lots.

[Rick Willoughby]

Static thrust is the force measured with the boat secured to a fixed object and you give the motor near full throttle. You need to ensure the prop does not aerate. It is a meaningless test for an outboard. It does have meaning for a tug or trawler designed to tow heavy loads.

All the other stuff in your post is a good sales pitch that means nothing. For example how often do you find yourself needing to stop faster!!!! Also who goes around stopping and starting a boat every few seconds to get the benefit of better acceleration. Most outboard powered boats sit at constant high speed for long periods - this is where the shroud requires extra power. Like I implied in the beginning - For an outboard boat it is a safety device - and likely to be power sapping in most circumstances. Some jurisdictions are mandating them for safety reasons but they are likely to impair performance unless the outboard is poorly matched to the boat.

Rick W.

[PAR]

There are benefits to using a shroud in regard to the amount of damage it does to delicate marine vegetation and the washout effect seen at most launching ramps, caused by power loading incorrectly. In these cases the shrouded prop is far superior, doing considerably less damage in both instances. They also lay down a flatter wake, which is good in near shore situations or higher speed potential in minimum wake zones. Then of course the anti-fouling aspect when you run over a crab trap warp.

[daiquiri]

Originally Posted by jacklynfong

...
I found out that shrouded propeller has these advantages: improved boat handling, greater steering control at higher speed, stopping ability of the boat is also dramatically increased as the shroud presents a greater blockage area, more power /forward thrust is created for the same input power and torque, produce greater dynamic lift which permit it to achieve faster acceleration and planing, decrease fuel consumption (as it's able to obtain a higher thrust and speed using a same input, reduce hull and engine stress and reduce torque effect of the rotating propeller.

The disadvantages can be the lost of it's advantage over propeller at about 10 knots (18.5km/h), additional load to the boat and increase in drag force, especially at high speed.

You have to learn how to read between the lines when you have to do with commercial leaflets. Don't believe the hype.

As already pointed out, all the real propulsive advantages of the shrouded prop are limited to very-low to low speed boat operations, where the static thrust is an important factor.
At higher speeds it is a solution which penalizes efficiency because the shroud (or duct) produces an additional drag respect to a conventional, free-tip propeller.

The statements you have found (at some vendor's site, I guess) are just half-truths, because they claim things without saying under what conditions these benefits can be obtained.
For example, when they say "more power /forward thrust is created for the same input power" they don't tell you that, having a fixed-geometry duct, the claim is true only for a very limited speed range, because the duct shape can be optimized only for a given design speed.

Of course, remain the benefit of mechanical protection of the propeller blades, and (vice-versa) the protection of the external ambient from the propeller blades, as pointed out by Rick and PAR.

[Eric Sponberg]

Originally Posted by Rick Willoughby

If someone falls in the water beside a turning propeller that is shrouded they are less likely to get injured than if it is unshrouded.

Rick W.

I would like to add that the United States Coast Guard, after extended study and testing, has determined that shrouded or otherwise caged propellers DO NOT improve safety against being injured. This is an argument that comes up time and again in US court cases in which injured people seek compensation for open propeller strikes, particularly on outboard motors. The conclusions of the Coast Guard are, if memory serves correctly, that a shroud or cage around the propeller increases the dimensions of the lower unit, and being struck by such can still cause seriouis injury or death. The presence of shroud or cage can cause other problems related to fouling, maneuvering, and extra drag. To date, no one in the US, to my knowledge, has ever won an injury suit on such claims.

The argument that shrouds or cages around propellers are safer has never been established. They can, in fact, be just as dangerous as an open water propeller.

Eric

[Rick Willoughby]

Eric
You have shot down the only positive I could think of for a shroud or guard. It might not always be so though. I found this work:
http://www.rbbi.com/pgic/pregs/pregs.htm

I was certain it had been mandated in some applications. I found this:
http://www.progeng.com.au/prop.html
I guess they have not been reading the US Coast Guard conclusions.

These guys have not read the US Coast Guard conclusions either and quite possibly wasting their time:
http://www.slsa.com.au/site/_content...-docsource.pdf
It just seems a lot of effort for something that provides no benefit other than appearance.

I have personal knowledge of one case where I am confident a guard would have avoided injury. The fellow injured (badly) was a doctor, happened to be heavily inebriated and was trying to get into a boat over the outboard well as the motor was kicked into gear. Came close to losing his leg.

Rick W.

[daiquiri]

Rick, those links are not about shrouded props but about propeller safety guards. It's a completely different thing. A prop safety guards is a kind of "cage" which encloses the prop and impedes its contact with human hands, feet etc.

[Rick Willoughby]

Originally Posted by daiquiri

Rick, those links are not about shrouded props but about propeller safety guards. It's a completely different thing.

IS IT???

If a shroud provides no improvement on blade induced drag then its purpose is essentially reduced to a guard. That is what my original post was intended to convey. As I pointed out you took many more words to convey what I did in one line.

So the debate has now moved on to the real merit or otherwise of a shroud or ducting on a typical outboard. One of potential safety. Eric countered my opinion on the topic.

The other inference to take from the links provided is that few of the guard makers or regulators are trying to improve efficiency with proper ducts. If there was merit here that would be the carrot that would be used to sell the idea. To the contrary they offer data that shows reduced performance and the better guards all have minimal surface area.

Rick W

[Eric Sponberg]

Hi Rick,

Thank you for your update and the links. I will stand by my comment about the US Coast Guard with a modification: On reading your links and doing a bit of checking, I have to say that the USCG is UNDECIDED about improved safety with propeller guards.

The issue of propeller guards is quite complex, from machine safety, operator education, and legal standpoints. I was impressed to see that the idea of propeller guards is by no means dormant; people and companies around the world are continuing research and development on devices that may improve overall propeller safety, so thank you for that. One obvious solution would be the continuing development and improvement of jets which have totally enclosed propellers.

Eric

[daiquiri]

These are propeller safe guards:






And these are shrouded or ducted propellers:







I can see a very big difference.

Originally Posted by Rick Willoughby

As I pointed out you took many more words to convey what I did in one line.

Well, put yourself in Jacklynfong's skin and think about what could he constructively learn from your single line.
He wanted to know something more about pros and cons of shrouded props and that's what I gave him.

[Rick Willoughby]

Originally Posted by daiquiri

Well, put yourself in Jacklynfong's skin and think about what could he constructively learn from your single line.
He wanted to know something more about pros and cons of shrouded props and that's what I gave him.

He asked specifically for advantage and disadvantage of shroud or unshrouded propeller on a small sea boat with a 40HP outboard. I believe I gave him the correct answer although Eric has countered that.

It would seem he was reading a sales pitch that was primarily erroneous information regarding any performance advantage relative to his application.

I made the post to ensure the thread got a response which you did and now you have gone to a lot of trouble to collect some more information on it. None of which is relevant to the jacklynfong application other than as prop guard whether it takes the form of a pure guard or pure duct.

If you are really interested in ducts you should go over the linked thread from page 15 as it shows a potential application for ducts that I have not done the numbers on:
Windmill or Wind Turbine- powered boats: how many are out there, and are they viable?
Michael Praamsma is an advocate and I think there might be a potential advantage in this application. He was building a road machine to test his ideas.

With the turbine you are looking at very high velocity ratios similar to a tug application.


Rick W.