shrouded propeller vs nonshrouded opeller

Discussion in 'Boat Design' started by jacklynfong, Oct 29, 2008.

  1. matty87
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    matty87 New Member

    not entirly sure how old this thread is but i noticed a couple of posts up that a few surf life saving links were up about the effectiveness of these guards. it may be so that these guards have no real safety advantage to offer on very high speed craft when someone is hit at full tilt but as a member of the surflifesaving in australia i can assure you having been hit by these prop guards a number of times in training in rough seas that i went home very sore but with no cuts which i would have had if no gaurd was in place. the gaurds also protect the propelor from constantly running into the ground as the craft is beached. just a different point of view.
     
  2. yipster
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    yipster designer


    was reading up on CFD and came acros this bladeless is more article
    checked http://www.dyson.com/fans/ and the video's
    nobody stuck such an expensive fan upside down under water
    couse we all know drag would be terrible rite ?
     
  3. jehardiman
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    jehardiman Senior Member

    Venturi Pump design, very lossy with water.
     
    1 person likes this.
  4. tom28571
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    tom28571 Senior Member

    Bladeless fan. Cute but not worth a second look. On a cost to performance ratio it fits in with the Amish Edenpure heater and airconditioners.
     
  5. yipster
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    yipster designer

    rite, apart from the impellor and all i recalled a waterjet only works well above above the surface, remains a nice expensive fan tho
     
  6. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    Tom, you're being overly kind to the latest invention from sir Dyson. I heard one running at a Lowe's not long ago. I went to the trouble of turning all the other running fans off and it was loud all by itself and the air movement wasn't particularly impressive. Yep, it had no blades (well visible anyway) and interestingly enough it sounded like an Electrolux with a bad beater bar . . .
     
  7. MooringProJeffL
    Joined: Nov 2010
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    MooringProJeffL MooringProJeffL

    MorringProJeffL

    I have built a work barge with twin 50hp outboards and was considering shrouded props for better direction of thrust. There were several interesting points brought up in the discussions.
    A shroud designed to improve thrust most likely will not provide any reasonable protection of body parts at slow speeds. At high speeds you are going to be injured with just the impact.
    Having owned one of the first jet skis made and worked on them a lot. The clearance between the impeller and the shroud is measured in a few thousands of an inch. Run this jet ski up on the beach, suck in some sand, open the clearance by .01" and you will lose the hole shot and 5mph.
    The shroud on an outboard propeller will create cavitation problems at higher speeds and the package is less efficient.
    For better info on propellers ask Larry at I have built a work barge with twin 50hp outboards and was considering shrouded props for better direction of thrust. There were several interesting points brought up in the discussions.
    A shroud designed to improve thrust most likely will not provide any reasonable protection of body parts at slow speeds. At high speeds you are going to be injured with just the impact.
    Having owned one of the first jet skis made and worked on them a lot. The clearance between the impeller and the shroud is measured in a few thousands of an inch. Run this jet ski up on the beach, suck in some sand, open the clearance by .01" and you will lose the hole shot and 5mph.
    The shroud on an outboard propeller will create cavitation problems at higher speeds and the package is less efficient.
    For better info on propellers ask Larry at http://www.accutechmarine.com/


    Jeff Lefebvre
    www.New-England-Marine.com
    New-England-Marine LLC
    1429 East Lakeshore Drive
    Colchester, Vermont 05446
     
  8. SamSam
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    SamSam Senior Member

    I'd like to hear a little about that.?
     
  9. MooringProJeffL
    Joined: Nov 2010
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    MooringProJeffL MooringProJeffL

    There is a lot of info and pictures of me work barge at this link.
    http://www.newenglandmooring.com/co...ft-transport-new-england-mooring-work-barge-m

    The props are more for thrust than speed. Top speed is 12mph forward and 5mph in reverse. Each motor has independant throttle and steering. Yes, two steering wheels and they are not so hard to operate. Handling is superb and it can turn quickly rotating on its center.
    Even with the low pitch props, idle speed is 2mph. Too fast for trolling.
    I will install four blade props next season for more thrust. The 30" diameter pontoons are rated for 115hp each.
     
  10. SamSam
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    SamSam Senior Member

    Thanks!
     
  11. Willallison
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    Willallison Senior Member

    I'm a little surprised that the findings against Brunswick haven't been given more attention around these and other parts. It would be interesting to hear what Brunswicks lawyers are telling the company as a result... there was talk that it could result in ALL new outboards and sterndrives being fitted with safety shrouds, which would likely have a very significant effect on the performance / efficiency of the boats to which they are fitted.
    And since it happened in the country where personal responsibility appears to have been banned, I can't imagine that they would consider that it's worth the risk of hoping that they won't get sued again....
     
  12. Cruachan69
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    Cruachan69 New Member

    I think it might be time to update this thread with some interesting new information;

    As the thread has already established, shrouded props are good for low-speed, high torque applications. In other words, shrouded props grant increased thrust at low rpms. This fact may be taken advantage of in another capcity; that of electric propulsion and regeneration.

    With the advent of electric propulsion as an increasingly viable option for cruising vessels, a shrouded prop's ability to move higher volumes at low revs has an advantageous inverse quality, in that it can be turned with significantly greater torque at low speeds (such as when a cruising sailboat is under sail). This torque can be utilised by turning the motor into an electric generator. The shrouded prop has the advantage of being able to turn the generator at lower hull speeds- perhaps as low as 3knots- while producing an insignificant amount of drag given the advatages of power regeneration.

    Also, the cruising sailboat would never be required to utilise high rpms to maintain an acceptable speed of six knots under power- the motor can be tuned to deliver maximum efficiency at the low rpms suitable for the shrouded prop, making the electric propulsion system not only highly-efficient under power, but capable of regenerating its own fuel between uses.

    From the safety perspective, I'd rather be hit by a shrouded prop than an unshrouded one! Even at 6 knots! To me the argument is bruising versus mincemeat...
     
  13. philSweet
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    philSweet Senior Member

    In fact, none of that is true. A shrouded prop doesn't move higher volumes at lower revs, it moves lower volumes at higher revs compared to an open prop that has the same diameter as the max diameter of the shroud. And it also has vastly more drag than an open prop, both as a prop and in regeneration mode. The nozzle can be designed either to match the flow regime to the prop and the hull for propulsion, or generation, but it can't be designed to do both properly. Either the prop or the nozzle has to be variable geometry.

    In order to attempt regeneration, which is patently stupid unless you are sailing around the world at 20 knots, you go the other way entirely. Either a completely separate generation system that shares no components with the propulsion system, or you go with a very large diameter, low disk loading, low area ratio open propeller. The problem with the latter is that the torque perturbations in a seaway are now an order of magnitude larger than what is normal for that power level, and none of the commercially available equipment will function properly unless you are on a mill pond.
     
  14. Cruachan69
    Joined: Sep 2019
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    Cruachan69 New Member

    For the students in the room...


    Note that drag is eliminated by the coanda effect. Also, since efficiency is increased in thrust mode, there is no major concern over balancing that efficiency with the efficiency of regeneration mode, especially where the re is an alternative (and renewable) means of providing thrust. There is no requirement to be crusing at 20 knots as the prop is more efficient and can provide the same torque at lower revs than a similarly-sized open prop.

    I know this because I've seen it action on a friend's crusing yacht where he swapped the original 14" open prop for a 20" shrouded prop and achieved 2Kw/h of regenerative energy while having no detrmimental effect on the thrust gained. Note: cruising yachts has a maximum hull speed- that is the speed at which the hull permits efficient movement through the water. There is no point in trying to overpower a hull as it just creates wasted eneregy. Therefore, in using a larger prop, the system requires considerably less power to provide the same hull speed through the water.

    The net result is lower rpm to maintain efficient hull speed and greater torque to provide regenerative power.

    It works. Learn it.
     

  15. DCockey
    Joined: Oct 2009
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    DCockey Senior Member

    I don't understand what is meant by this statement. What is meant by "thrust" - the force on the boat when the propeller is under power? Or does "thrust" in the quote mean the retarding force, ie drag?
     
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