prop shrouds

Discussion in 'Boat Design' started by waterwar, Jan 31, 2011.

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  1. DCockey
    Joined: Oct 2009
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    DCockey Senior Member

    Keep in mind that descriptions in a patent of how a device works are the claims of the inventor, and are not confirmed by the Patent Office. While the descriptions need to be plausible, they may or may not be accurate.
  2. waterwar

    waterwar Previous Member

    no endorsement is asked for

    Thank you for your expert analysis all insights have been informative.
    I certainly think that the state of the art may hopefully be pushed.Your astute observation of the patent rights expiring hopefully will promulgate even more innovation. We need one system that works.and guarding a prop from damages and or causing them to others.

    .once again thanks for your views
  3. waterwar

    waterwar Previous Member

    Mr. Shultz

    I feel that some more mention should be documented as the 'test' was flawed as pointed out but in defense of fairness, I found these disclaimers on the a vendor website;

    ''Prop Guard is the ultimate in prop protection, providing both protection and enhancements that every boater is sure to desire. Made up of two joined polypropylene rings (duct type guard) it provides protection for the propeller and a measure of safety for people and animals in the water.

    This model fits most outboards from 25 hp through 30 hp. Usually you will see an increase in performance at low speeds and little or no decrease in performance at top speed.

    This model fits most outboards from 9.9 hp through 20 hp. Usually you will see an increase in performance at low speeds and little or no decrease in performance at top speed

    This model fits most outboards from 4 hp through 9.8 hp. Usually you will see an increase in performance at low speeds and little or no decrease in performance at top speed.''

    I think we all and myself finally that there will be a loss of speed at high speed using anything that cause drag like a guard.
    The point being that some will still want to use one.and want the best .

    just for fairness .
    thanks stew
  4. gonzo
    Joined: Aug 2002
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    gonzo Senior Member

    Some people will want the weirdest things. It doesn't make them efficient or better. I still haven't found any answers to my questions about your claims.
  5. cthippo
    Joined: Sep 2010
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    cthippo Senior Member

    So does that mean that a prop guard with a flat interior profile (as opposed to a airfoil like a Kort nozzle) will generate more thrust by trapping the spanwise flow? Obviously there is a tradeoff in speed due to drag

    I know that Kort nozzles derive some of their effectiveness by recirculation of the flow, which probably would not be the case with a flat duct.
  6. Jeremy Harris
    Joined: Jun 2009
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    Jeremy Harris Senior Member

    In general, anything that reduces the magnitude of tip vortices will increase efficiency, so producing more output power for a given input power. A simple ring around the tips of the prop will do this to some extent.

    Adding a convergent duct around the prop (like the Kort and Rice nozzles) tends to increase the velocity ratio across the prop disc and duct combination, so increasing the mass flow rate and generating more thrust for a given diameter (with a commensurate increase in the power absorbed by the prop). In effect, a prop in a convergent duct behaves somewhat like a larger diameter open prop.

    The tradeoff comes when comparing the efficiency improvement that comes from the reduction in vortex magnitude with the power needed to overcome the additional drag created by the duct. As has already been mentioned, the break-even point tends to be just a few kts; a ducted prop is unlikely to be more efficient than a properly sized and pitched open prop above about 10 to 15kts or so.
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  7. SamSam
    Joined: Feb 2005
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    SamSam Senior Member

    Looking at the patent I noticed first in figure 2 that the artist failed miserably in drawing the two anything but workable counter-rotating props (but might have inadvertantly made a unique and improved Lawrence Welk bubble machine), and secondly that there is no claim anywhere that the device improves speed.
    The increased speed claim seems to be an example of advertising poetic license, which is not too uncommon, but with all the bulk of the thing in the drawings, doesn't seem likely. Extra lower speed thrust and torque sounds reasonable.
  8. waterwar

    waterwar Previous Member

    I do want to thank the 'Real' contributors again

    Thank you for participating and exchanging with your peers, sir.

    I am not an engineeer or even close to one. I note that there is 'something new' that has happened in the field of ducted props. I understand that the Mr.Shultz's original design has been 'tweaked' and improved. The fact that you gentlemen can exchange true technical expertise in a meaningful thread is commendable.

    You all are advancing the State of the Art. The real experts who who have so intelligently adressed the real issue. Only when the envelope is pushed can real progress be achieved. Some one has to sit down and want to logically attempt to improve or analyze how something works.

    Safety issues and optimizing performance. People always want things that are improved and opinions from those who are experts..... Wow, what a concept. Weird?

    I withdraw all my claims and concede(ok? Gonzo), I only intended to find out why the ducted propeller was not chosen by civilians; if it was so fast.IMHO the speed is in excess of 90 knots..

    I wish not to stop the exchange of meaningful information among the designers and engineers and other fine members of this Board by posting this.
    Stewart Loeblich
  9. portacruise
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    portacruise Senior Member

    What about minimizing the right angle drive motor appendage profile that lies directly in front of the motor? Seems like that blocks or disturbs flow to the prop and would have a greater impact on efficiency than ducts or nozzles. Maybe one reason most prop aircraft are pullers rather than pushers? McDenney in the electric boat list on this forum showed how using a fairing on the appendage and prop spinner can have quite dramatic effects at least on a low speed troll motor....

  10. viking north
    Joined: Dec 2010
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    viking north VINLAND

    Waterwar,I too have to commend you on your ability to re trace your steps not all have that gift of reflection as i hoped would happen re my post #44 and i have to admit some selfishness on my part because i wanted a good flow of info. to gain more insight on this topic for a little underwater robot i am planning. Tnx. to all Geo.
  11. waterwar

    waterwar Previous Member

    Submarine Tom
    I am no newbee to life. This not my first rodeo...

    Forums are generally extremely difficult to deal with particularly if the subject is a difficult or controversial one. Many participate for various reasons; I'm not going into all of them ,that's common knowledge

    I wanted to some real uncommon knowledge from a very closed clique so to speak.
    The truth is far more complex than just causing or arousing interest sometimes... you gotta break a few eggs to make an omelot
    That provides a true insight in behavior.
    For instance if someone is waiting to use a telephone the person on the phone will spend a longer time on the phone.
    Myself being a new person in a group of professionals and other experts in a fieldI know very little of ... I let myself be targeted as someone who should be taught that he is totally wrong an outsider so to speak.

    I didn't mean to offend the members- knowing that the experts were well used to complex problems (and generally speaking professional) but sometimes need to be 'drawn' out of the woodwork so to speak .
    I wanted to get the Truth... so I kicked the Door a little...

    If I wasn't so stubborn or combative. I don't think this thread would have happened it would've just petered out like the 'find another beach' thread from the annals of this site

    I think that level that this thread is achieving is unprecedented in pushing the envelope so to speak

    I know that propeller shrouds or guards or whatever you choose to call them are unpopular... how many time have i cursed as i cleared weeds from unshielded props I can only imagine with a shroud...Here in Florida fresh water lakes rivers Hydrilla ,water hyacinth you name it it grows down here.

    Propeller ducting devices are operating all over the world in various applications .... The members of this Board should be very proud of their meaningful contributions to the effort of improvement.
    Sometimes the application is lifesaving emergency craft.

    Who wouldn't be proud of helping to improve the efficiency if not just for the aid it would give them.

    *The source of my unpopular handle is related to a battle won many years ago against an unnamed bottled water cartel. Who wanted to increase their withdrawal by many fold from a great spring near me. I won, I celebrate the victory everyday when I write.


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    Last edited: Feb 7, 2011
  12. waterwar

    waterwar Previous Member

    Viking North

    I am so very glad that your pet project helped inspire your participation.
    Your post #44 was very useful for everyone and rallied the good professionals of this board.
    tip of the hat
  13. erik818
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    erik818 Senior Member

    Most of the outboards amongst my friends and relatives are small, typically around 10 hp. None of them have shrouds. If the outboard is more than a few years old the propeller is usually more or less dented. To my experience, all small boats and all small outboards eventually hit a rock (at low speed because you know you're in shallow water). It's more fair to compare the efficiency of a dented unshrouded prop with an undented shrouded prop.
  14. SamSam
    Joined: Feb 2005
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    SamSam Senior Member

    waterwar, you're new here, but when you have negative points you're supposed to send everyone else $20.

  15. Alfonso
    Joined: Sep 2008
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    Alfonso Junior Member

    Oh come on man..... When armies start building nuclear submarine, first they ask about the amount of money???

    Nuclear reactor is cheap comparing with huge cowling???

    HA-HA :)
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