best propolution method, normal Prop, Jet style or paddle wheel?

Discussion in 'Propulsion' started by Kalagan, Nov 5, 2009.

  1. Kalagan
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    Kalagan Junior Member

    Hi,

    I have been curious for a while which of those three propulsion styles is more efficient?

    For the most part I lean towards sailboats, so I am asking in reference to using one of them on a sailboat, but I would be interesting to see if using it for a Powerboat would impact people's decisions?

    One approach I have considered in the past is to place a vertical Paddle wheel on both sides of a Sailboats keel or one directly behind the keel.

    Any thoughts would be appreciated.

    Thanks
    Kalagan
     
  2. marshmat
    Joined: Apr 2005
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    marshmat Senior Member

    For sheer efficiency, it's hard to beat a large diameter, slow turning propeller with appropriately chosen blade shapes.

    Jet drives have some advantages in the 20 to 60 knot speed range, but will generally be less efficient than the best possible propeller for the application.

    Paddle wheels got the boot in the late 19th century when it became clear that even a simple propeller produced much more thrust than the best paddle wheels, given an identical engine. They never really came back except as decoration or nostalgia pieces.

    On a sailboat, of course, you must also consider the drag the drive system creates while under sail. It is for this reason that so many sailboats have small folding propellers, even though these are less efficient than a large fixed prop could be.
     
  3. yipster
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    yipster designer

    that said, when you put paddlewheel prop or jet for a search endless threads on the subjects appear
    here's an old jet vs belt drive test

    [​IMG]
     
  4. Kalagan
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    Kalagan Junior Member

    Hi,

    Thanks for the response. What is the major factor that creates the Thrust? Prop size is obviously one of the big ones, because it displaces or moves more water. but it seems like a wide paddle wheel could displace more water and the same rpm's (but would require more HP)? Is it the pattern of the rotating water after it has passed through the spinning prop? Does enclosing the prop, so that it could only grab water from in front make it more efficient (like a number of prop protectors claim)? If that is the case, wouldn't that bode well for Jet style systems, because there is no way for the prop to pull water in from the sides?

    I know I am probably asking very basic questions, but I am not sure where else to get these probably simple answers without reading am entire book about prop design.

    Thanks
    Kalagan
     
  5. Kalagan
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    Kalagan Junior Member

    Hi,

    Doesn't that bode well for the Belt drive (Paddle Wheel) system? Sounds like it to me, it just displaces more water.

    If the main object is to move more water, then even at slower RMP's, it would seem like a belt drive or paddle wheel system (especially belt drive) would move more water then a normal prop?

    Just think of a Cat (either power or Sail) with a dual belt drive system, one on each AMA or a tunnel hull power boat (where the tunnel is completely submerged) with a long belt drive system, that could move a lot of water and focus it within the tunnel. Sounds like that could be a pretty advantageous setup.

    Thanks Kalagan
     
  6. gonzo
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    gonzo Senior Member

    They are all different and better on particular applications. You got to be more specific to be able to compare them. For example, the snowmobile is faster as long as it keeps moving. Stopped, it sinks.
     
  7. Guest625101138

    Guest625101138 Previous Member

    The thrust from any of these propulsion units comes from imparting velocity to the water that passes the blades.

    The thrust is directly related to the velocity change on the volume of water affected by the blades while the power absorbed by the water is related to the square of the velocity change.

    If the blades act on a large area then the velocity change to get a certain thrust is small. As the area is reduced the velocity change needs to increase proportionally to get the same thrust but the power absorbed will increase more so the efficiency drops.

    There is a limit on the area because other secondary factors like weight of unit, drag on the blades and submerged supports come into play. At higher speed the optimum blade gets smaller in area.

    An efficient propeller will have very small losses due to induced drag - this is the situation where water flows around the tip of the blades from the high pressure region to the low pressure. However if the diameter is constrained then these losses can be significant and shrouding the blades in a duct can give an overall benefit. This is what is done with a jet. However there are now other losses because the duct will cause drag on the flow and this has to be taken into account. There are other losses as well such as lifting the water flow or nozzle entrainment if the discharge is below the surface.

    The size relationships are the same for paddlewheel blades but paddlewheels lose effectiveness in waves. They are really only suited to calm water operation.

    Rick W
     
  8. gonzo
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    gonzo Senior Member

    Paddle wheels are the system of choice for river casino-boats.
     
  9. Kalagan
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    Kalagan Junior Member

    Hi,

    Thanks for the info, that was helpful. I guess I would like to clarify one thing when I use the term Paddle Wheel, most people think of a river boat, which is totally understandable, but what I was thinking of was a completely submerged Paddle Wheel placed underneath a boat. To make it more efficient only one half of it would be directly exposed to the water, the other half would be shrouded or concealed within the hull of the boat, below the paddle wheel would be a wing to ensure that only water would be pulled into the paddle wheel from in front of the wheel not from below it. Similar in function to the circular Prop protectors (for a normal Prop) mentioned above.

    This could give you a fairly large surface area to move a high volume of water with a relatively low RPM, only expose the portion of the paddle wheel that moves water in the correct direction, limit the water being moved to the water that is in the direction you would like to go (whether that is forward or reverse) and allow it to spin freely while under sail (which could be used to generate power).

    Thanks
    Kalagan
     
  10. gonzo
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    gonzo Senior Member

    The problem is that the faster you go, the more water accumulates in the wheel and increases the weight. It takes time to drain the water of the paddles. Also, with increased speed, the difference in weight creates vibration because it is unbalanced.
     
  11. Guest625101138

    Guest625101138 Previous Member

    If you work through your idea for the submerged paddlewheel you will see it has many flaws relative to a propeller. The actual working portion is only a fraction of the swept area so there are high losses as the blades work around the rest of the arc. If you set the system out on a piece of paper with flow vectors you will start to get what I mean. The blades will definitely need to articulate. Once you do this there are better systems using foils.

    Have you seen the Voith-Schneider prop:
    http://www.voithturbo.com/vt_en_pua_marine_vspropeller.htm

    The Hobie mirage uses flapping foils:
    http://www.hobiecat.com.au/kayaking/sport.html

    I made a pedal boat with a large oscillating foil that worked well up to the point of the boat resonating vertically. Very high thrust and smooth operation. Good efficiency when not resonating. See attached.

    Rick W
     

    Attached Files:

  12. Kalagan
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    Kalagan Junior Member

    Hi,

    I have used one of the Hobie pedal drive systems and they are really fun. I have not seen the rotating drive system before, but obviously it is very effective. Your foil drive system is interesting and looks like it makes for pretty smooth motion.

    The rotating drive system brings up another question, why is a boats' propolution and steering systems always located at the stern end (for the most part) of the boat? It is certainly much easier driving a car with the turning wheels up front, try backing up and see how much harder it is. The same reasoning would seem to hold true for a boats propolution, why is it better to push a boat through the water as opposed to pulling it (by having the propolution in the front 1/2 or 1/3 of the boat)?

    At one point I was thinking of having a torpedo bow boat and placing the propolution (typically a normal Prop) just to the aft of where it ends, which I think would happen around the 1st 1/3 of the boat? (think of taking a torpedo and attaching it to the front of a sailboat, deep enough so that it wouldn't hit the hull of the boat)

    Is that only because it is less likely to come out of the water on a plane or over a wave? I guess to sum it up, is it netter to push a boat or pull it?

    Thanks
    Kalagan
     
  13. Yellowjacket
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    Yellowjacket Senior Member

    These guys appear to have made an improvement in terms of improving the efficiency of a tracked system. What they are calling efficiency is really slip, but as you go to higher speeds it might appear that the efficiency of a tracked system might be better than a prop. They have a patent and did a presentation at the Navy Small Craft Conference where I first saw it.

    Here is a link to the technical presentation https://macc.nswccd.navy.mil/presentations/2009/thur-Wernicke.pdf

    Here is a link to their homepage, they have some videos of this thing pulling a waterskier.... http://www.fasttrackamphibian.com/video.htm

    What I found more interesting was that at high speed they could lift the hull out of the water and run just on the tracks. At that point there is still wave making drag, but the drag from hull surface friction is gone and for really high speed operation it would seem to me that there is considerable advantage in the concept as compared to a conventional hull and propulsion system.
     

    Attached Files:

  14. Guest625101138

    Guest625101138 Previous Member

    Generally it is better to push a boat but not only because an aft mounted prop tends to stay submerged as the hull boat lifts over waves. Another benefit of an aft mounted prop is the protection that the rest of the hull offers.

    Interestingly a pushing propeller is inherently stable. It does not need any support and will always align with flow if unrestrained. A pulling prop is unstable and will dive if angled down initially.

    Most boats have the CofG higher than the centre of lift force on the rudder. Hence a bow mounted rudder will cause a boat to initially lean out of a turn. This is undesirable. It is preferable to lean in. Again a stern mounted rudder will tend to be in the water more than a bow mounted one when operating in waves.

    A boat is opposite to a bike from a steering perspective. It is almost impossible to ride a bike that has rear wheel steering. There are some articulated ones that work but the geometry is complex.

    Things are usually like they are for good reason but having a curious mind is a virtue. Sometime you might get an idea that has not been exploited. Many people actually believe what they read in text books without question. They do not truly understand and take the time to work things out for themselves.

    Rick W
     

  15. Guest625101138

    Guest625101138 Previous Member

    This has been proposed as a means of beating viscous drag. In fact the viscous drag becomes a virtue. You still need some means of supporting the weight either through dynamic lift or buoyancy.

    Rick W
     
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