wiggle drive propulsion application

Discussion in 'Propulsion' started by SudorracMechEng, May 7, 2012.

  1. swashdrive
    Joined: May 2012
    Posts: 57
    Likes: 1, Points: 0, Legacy Rep: 24
    Location: SEQ, Aus.

    swashdrive Junior Member

    I think you have missed read what i was trying to say
    The fin is swimming using LIFT.
    It is not using the SCULL motion or it would be more like a surface prop. which it is not.
    If you want to stick with ur KJELL effect thats fine, but i don't think it will sit well here and i don't think its whats producing thrust with the fin devise old mate !
     
  2. swashdrive
    Joined: May 2012
    Posts: 57
    Likes: 1, Points: 0, Legacy Rep: 24
    Location: SEQ, Aus.

    swashdrive Junior Member

    Kjell
    I can assure you they do cavitate and at high speed. It happens on any foil when the water surface speed exceeds it ability to maintain laminar flow over the foil surface, usually on the low pressure side.
     
  3. kjell
    Joined: Aug 2005
    Posts: 271
    Likes: 5, Points: 0, Legacy Rep: 31
    Location: mallorca

    kjell Senior Member

    If you don’t like my name Kjell, you can call it “Fan-Effect” it is the same. About the cavitations, I think you have to learn about why cavitations are produced. It is a combination of temperature and low-pressure and has nothing to do with speed.
    Can you define what in your opinion is swimming and how swimming produce lift.
     
  4. Jeremy Harris
    Joined: Jun 2009
    Posts: 978
    Likes: 60, Points: 28, Legacy Rep: 711
    Location: Salisbury, UK

    Jeremy Harris Senior Member

    Just for the record, and to correct an error about efficiency that seem to keep getting repeated in this thread as if it were fact, there are plenty of folk (me included) running props with a demonstrated and tested efficiency of better than 85%. Some, like Rick Willoughby, have tested props up at around 90% efficiency. The Human Powered Boat people have been using 85% plus efficiency props for years, it's really not rocket science to get a propeller up to this sort of efficiency.

    All the research I've read over the years suggests that fin drives can also get up to around 85% or so efficiency, but the challenge for them is reducing the additional parasitic losses from the fairly complex drive systems. The Hobie Mirage drive, for example, loses a fair bit in the mechanism and consequently only seems to have an efficiency in the 50 to 60% region.

    The propeller versus fin argument WRT efficiency is pointless, in my view, as both could be made to operate fairly efficiently. It is other aspects that will determine the practicality of drives like those discussed in this thread, such as the ability to operate in shallow water, or withstand floating object damage, or even deal with weed and debris entanglement.

    One reason that typical boat propellers are only around 60% or so efficient has to do with these issues. An 85% to 90% efficient boat prop looks rather like an aircraft prop (see photo of my 86% efficient prop below), and is prone to problems with weed and debris entanglement. It's also likely to be large in diameter (so increasing draft) and turn at low speed (needing reduction gearing with it's associated losses).

    If a fin drive can overcome some of the limitations that make high efficiency propellers somewhat impractical on many forms of boat, then that would be interesting. So far, every fin system I've seen has suffered from some flaws that make it less practical than it could be, be it the Hobie Mirage with its inability to go astern, it's large draft requirement and its relatively poor efficiency, or one of the other fin drives, like the Tailfin, that have never really been developed into practical products.

    Simple physics suggests that fin drives will suffer similar hydrodynamic losses to an efficient propeller with a similar blade loading (tip vortex shedding and viscous drag from high wetted area), plus fin drives have to find a way to reduce the drive train losses inherent in the more complex mechanism they use. As I have found over the past couple of years, drive train losses can be significant, even with just bearings and seals on a single rotating shaft. I lose about 4% efficiency just from bearing and seal drag on my electric boat drive, for example.
     

    Attached Files:

    • Prop.JPG
      Prop.JPG
      File size:
      211.1 KB
      Views:
      1,393
  5. DCockey
    Joined: Oct 2009
    Posts: 5,229
    Likes: 634, Points: 113, Legacy Rep: 1485
    Location: Midcoast Maine

    DCockey Senior Member

    From the perspective of sitting on a blade of a propeller or fin of an oscillating fin unit, the force on the blade or fin can be described as lift as well as drag. From the perspective of sitting on the boat the net effect of the forces from the propeller or oscillating fin unit is thrust and potentially a sideways force.
     
    Last edited: Jun 6, 2012
  6. DCockey
    Joined: Oct 2009
    Posts: 5,229
    Likes: 634, Points: 113, Legacy Rep: 1485
    Location: Midcoast Maine

    DCockey Senior Member

    Another way to say this is in a system with a reciprocating piston and rod, and a rotating crankshaft and flywheel kinetic energy flows back and forth between the reciprocating components and rotating components. As the reciprocating system slows kinetic energy from it flows into the rotating system, and as the reciprocating system accelerates kinetic energy flows into it. The rotational speed will oscillate as the kinetic energy in the rotational system varies.
     
  7. DCockey
    Joined: Oct 2009
    Posts: 5,229
    Likes: 634, Points: 113, Legacy Rep: 1485
    Location: Midcoast Maine

    DCockey Senior Member

    Laminar flow transitions to turbulent flow which is different then cavitation. One of the essential factors determing when the transition from laminar to turbulent flow will occur is the Reynolds number which is proportional to a speed multiplied by a length. So a higher speed will result in transition at a shorter distance with everything else the same.

    Cavitation is the formation of bubbles in a liquid due to low pressure. The bubbles can form from the liquid boiling or from disolved gas in the liquid. Higher speed of a fluid result in lower pressure so with sufficently high speeds cavitation will occur. Any system with liquid moving past solid objects or solid objects moving in a liquid can have cavitation if the speed is fast enough.
     
    Last edited: Jun 6, 2012
  8. Jeremy Harris
    Joined: Jun 2009
    Posts: 978
    Likes: 60, Points: 28, Legacy Rep: 711
    Location: Salisbury, UK

    Jeremy Harris Senior Member

    From the perspective of someone who's designed, built and flown aircraft and designed tested and used home made propellers, I can say with certainty that it's lift and drag that are the two primary forces involved with either a fin or a propeller. You can call them other things if you like, but they remain lift and drag in principle.

    When it comes to trying to calculate the performance of a propeller, wing or fin drive, you can use several different methods. For around a century people have been arguing over the nature of the forces that act on wings, but the bottom line is that using lift and drag gives consistently good results between modelling and the real thing.

    The same goes for propellers. I prefer to use a method that divides the propeller blade into thin slices, determines the inflow velocity and angle for each slice, calculates drag and lift based on local AoA and local blade velocity and then sums the results for all the slices. Some use the actuator disc model, which gives similar results. Other use momentum theory, and again get similar results.

    I'm certain you could apply different methods to analysing the lift and drag from propulsion fins and still get reasonably consistent results.

    When it comes to cavitation, then that is purely a function of the pressure ratio across the fin or propeller blade. Make this high enough and the shear stress limit of water will be exceeded on the low pressure side and cavitation will result. It matters not one iota whether it's a propeller or fin, or even a hydrofoil, if you get a high enough opressure ratio then you'll get cavitation.
     
  9. kjell
    Joined: Aug 2005
    Posts: 271
    Likes: 5, Points: 0, Legacy Rep: 31
    Location: mallorca

    kjell Senior Member

    I you have this kind of experience I hope you are agree with this picture showing in witch direction the different forces are acting.
     

    Attached Files:

  10. Jeremy Harris
    Joined: Jun 2009
    Posts: 978
    Likes: 60, Points: 28, Legacy Rep: 711
    Location: Salisbury, UK

    Jeremy Harris Senior Member

    That picture is roughly correct, but has the vector angles off slightly. Lift acts perpendicular to the wing mean chord, not vertically, so the vector should be inclined aft by the AoA. Thrust acts along the primary axis of the propeller, which is usually offset slightly in both the pitch and yaw axes, to correct for the cruise speed AoA and also for the prop P factor. Total drag and weight vectors are shown correctly.
     
  11. kjell
    Joined: Aug 2005
    Posts: 271
    Likes: 5, Points: 0, Legacy Rep: 31
    Location: mallorca

    kjell Senior Member

    I have been working as a propeller expert for 20 years with boats up to 30 meters LWL.
    Calculating, installing, repairing and resolving cavitations problems.
    I have been studding tails from Swordfishes, one of the faster swimmer, without finding any signs of cavitations.

    http://www.youtube.com/watch?v=B2kQF7eWkIs&feature=related
     

    Attached Files:

  12. Jeremy Harris
    Joined: Jun 2009
    Posts: 978
    Likes: 60, Points: 28, Legacy Rep: 711
    Location: Salisbury, UK

    Jeremy Harris Senior Member

    My guess is that you've always been operating at pressure ratios below the shear stress limit, that's all. TBH, all of my props also work well below this threshold, as best efficiency is at blade/fin loadings that are well below the cavitation onset threshold.

    Cavitation is a consequence of designers pushing the L/D, and hence blade/fin/wing loading to the limit. It's a sign of either poor, or perhaps limited application, design.

    I mentioned practical limits previously, including the desire to limit propeller drive reduction ratios and propeller diameter to give shoal draft. Both tend to result in propellers with smaller than optimum diameters, that operate at higher than optimum rpm, with blade loadings that are higher than optimum. This gives higher than optimum pressure ratios and takes some regions of the propeller surface closer to the cavitation point.

    If you've been working as a propeller expert for 20 years, you will be very well aware of the efficiency of high aspect ratio, slow turning propellers, as it is very well documented, both on this forum and elsewhere, with solid evidence to support the efficiency figures I quoted previously.
     
  13. kjell
    Joined: Aug 2005
    Posts: 271
    Likes: 5, Points: 0, Legacy Rep: 31
    Location: mallorca

    kjell Senior Member

    Nobody likes to have cavitations problems. It is important to calculate the right size of the propeller. I have a very good program with possibility to calculate needed engine power, boat speed and propeller sizes. This program is made by me. I would like to find a program that can calculate the same for tail propulsion. Up to now I have not find anybody with this program.
     
  14. Jeremy Harris
    Joined: Jun 2009
    Posts: 978
    Likes: 60, Points: 28, Legacy Rep: 711
    Location: Salisbury, UK

    Jeremy Harris Senior Member

    So why state that propeller efficiency cannot exceed 75% earlier in this thread? There is a lot of evidence to show that people have made propellers that exceed this figure, from amateur low power boat builders like me to well-respected MIT professors like Mark Drela (and a wide spectrum of other human powered boat builders/developers in between).

    I'm not disagreeing with the basic principles of fin propulsion, as there is a fair body of evidence to show that fin drive systems can get pretty close to the efficiency of an optimised propeller. I do disagree with the basic tenet that fin drive is inherently more efficient than a propeller. Both use the same basic principle to convert mechanical movement into useful propulsive force, and both have limitations; hydrodynamic, mechanical and practical.
     

  15. DCockey
    Joined: Oct 2009
    Posts: 5,229
    Likes: 634, Points: 113, Legacy Rep: 1485
    Location: Midcoast Maine

    DCockey Senior Member

    Standard definition of forces on an airfoil:

    Lif is the component of the aerodynamic force perpendicular to the oncoming flow. (Not perpendicular to the wing mean chord.)

    Drag is the component of the aerodynamic force in the direction of the oncoming flow.

    The total force will be inclined rearward from perpendicular to the oncoming flow, and may be close to perpendicular to the wing mean chord.

    Added:

    From Principles of Naval Architecture Vol II 1988, p135 Propulsion, 2.5 Blade Element Theory of Screw Propeller: "The force on a blade section set at an angle of incidence to the flow can be resolved into two components, the lift L and drag D, repsectively, normal to and along the line of the incident flow." Figure 4 then shows a blade section with the incident flow direction, pitch face and line of zero lift in different directions.
     
    Last edited: Jun 6, 2012
Loading...
Forum posts represent the experience, opinion, and view of individual users. Boat Design Net does not necessarily endorse nor share the view of each individual post.
When making potentially dangerous or financial decisions, always employ and consult appropriate professionals. Your circumstances or experience may be different.