Lift onset, planing, and lift coefficient

Discussion in 'Hydrodynamics and Aerodynamics' started by sandhammaren05, Aug 13, 2018.

  1. CT249
    Joined: May 2003
    Posts: 1,331
    Likes: 129, Points: 63, Legacy Rep: 215
    Location: Sydney Australia

    CT249 Senior Member

    What is "it" that you are referring too in this context? The point is that your paper claims "Sailboats with the appearance of a transom generally have a strongly rockered bottom so that the transom is always above the waterline."

    This does not appear to be true in any way. To repeat -the most popular sailboats with the appearance of a transom are the Optimist, Sunfish, Snark and Laser, followed by the 420, Finn,470, Topper (World Sailing figures plus an estimate for the Snark). There have been some 900,000 of these craft built. None of these have the transom always above the waterline.

    The most popular larger sailboats with the appearance of a transom would probably be the J/24, Catalina 22 and Folkboat. None of these has a transom always above the waterline.

    Using World Sailing figures, we see that of the International classes, only some 7% of sailboats currently built have "the transom always above the waterline" - and that is dramatically overstating it since some of those boats (ie Star, Flying 15) have their bottom of their transom at the waterline at speed.

    Your claim ""Sailboats with the appearance of a transom generally have a strongly rockered bottom so that the transom is always above the waterline" is not supported by data. You have not provided a shred of evidence for the claim, and it goes against the available data. If a student of mine did that in an undergrad paper they'd lose marks.
     
  2. CT249
    Joined: May 2003
    Posts: 1,331
    Likes: 129, Points: 63, Legacy Rep: 215
    Location: Sydney Australia

    CT249 Senior Member

    Two flaws have been pointed out, as noted in the previous two posts.It would be good to understand sailboat design before making false claims.
     
  3. sandhammaren05
    Joined: May 2009
    Posts: 410
    Likes: 19, Points: 18, Legacy Rep: 138
    Location: Texas & Austria

    sandhammaren05 Senior Member

    That's as much a fact as the fact that the moon is made of green cheese. Produce the proof for your wild claim.
     
  4. CT249
    Joined: May 2003
    Posts: 1,331
    Likes: 129, Points: 63, Legacy Rep: 215
    Location: Sydney Australia

    CT249 Senior Member

    Even you should understand that this is planing....check the beginning and the overhead shot at 1:15 onwards to show that there is no transom.

     
  5. CT249
    Joined: May 2003
    Posts: 1,331
    Likes: 129, Points: 63, Legacy Rep: 215
    Location: Sydney Australia

    CT249 Senior Member

    Sorry, but are you a child playing at being an adult?

    You didn't deal with the point under discussion at all. In the face of facts that can be backed by reality and official documents, all you could come up with is some childish babble. Imagine if one of your students was childish enough to respond in that way.
     
  6. sandhammaren05
    Joined: May 2009
    Posts: 410
    Likes: 19, Points: 18, Legacy Rep: 138
    Location: Texas & Austria

    sandhammaren05 Senior Member

    Even you should be able to understand that I have no interest until you show me how that bottom design performs with a motor. Scale
    it up, build the boat, run it, and make a report.
    QED
     
  7. CT249
    Joined: May 2003
    Posts: 1,331
    Likes: 129, Points: 63, Legacy Rep: 215
    Location: Sydney Australia

    CT249 Senior Member

    Even someone with your obvious cognitive issues should be able to understand that a craft that planes without a transom is proof that your claim that a boat without a transom cannot plane is incorrect. The motive power is irrelevant.

    C-. See me after class.
     
  8. sandhammaren05
    Joined: May 2009
    Posts: 410
    Likes: 19, Points: 18, Legacy Rep: 138
    Location: Texas & Austria

    sandhammaren05 Senior Member

    Sharp trailing edge. Hmmmm.
     
  9. sandhammaren05
    Joined: May 2009
    Posts: 410
    Likes: 19, Points: 18, Legacy Rep: 138
    Location: Texas & Austria

    sandhammaren05 Senior Member

    After a good pm conversation with Peaky I offer the following comments and then a summary. The important thing is the sharp trailing edge where lift develops because the flow separates sharply. For an airplane wing the lateral shape of the trailing edge may be rounded and often is. The maximum lift/drag occurs for an uncambered wing with elliptic shape (British spitfire), because that is what produces the elliptic distribution of circulation (my calculation, not in Newman). One might want a rounded 'stern' on a surfboard or wind surfer for reasons of better control. One does not want a rounded transom on a powerboat, exactly for reasons of control plus the extra drag that would reduce the top speed. With a powerboat one wants the bottom and transom to meet along a straight line.

    Here's my summary of planing for a powerboat:
    1. The boat plows through the water at zero trim angle and backflow up the transom from the bottom and sides.
    2. As the speed is increased the boat tries to ride up the bow wave. The transom is still submerged.
    3. At a critical speed (or depth Froude Nr.) the flow suddenly separates sharply from the bottom (the transom is suddenly dry), the water leaves the bottom as a sheet. That is a vortex sheet, a velocity discontinuity. The velocity sheet does not end at the transom but comtinues along the bottom. The sheet is necessary for lift. When the transom is suddenly dry, that is lift onset. Buoyancy is still carrying most of the weight.
    4. As the speed increases a planing hull rides up the bow wave with large trim angle but the location of the bow wave has moved aft.
    5. At a critical speed, planing speed, the hull is on top of the bow wave. Plaing speed can be determined as follows: with the boat on a plane reduce the speed until the stern sinks into the water and the boat plows.
    6. As the speed is increased beyond planing speed the trim angle is reduced until at a certain speed (or depth Froude Nr.) further increases in speed do not change the trim angle. This sequence is described with photos in my paper.

    Putting a pintail as sharp extension of tiny beam of the rear of a windsurfer will only increase the drag.
     
  10. DCockey
    Joined: Oct 2009
    Posts: 5,055
    Likes: 531, Points: 113, Legacy Rep: 1485
    Location: Midcoast Maine

    DCockey Senior Member

    The planform shape does not have to be "elliptical". A wing with a straight trailing edge and an elliptical distribution of chord will have the same circulation. The elliptical distribution of circulation resulting in minimum induced drag is covered in most intro to aerodynamics university courses. My recollection is it has been known since the 1920's.
    As you point out when the transom is fully ventilated the majority of vertical force on the boat is due to buoyancy (hydrostatic pressure) not hydrodynamic effects. There is not sudden change in the vertical force on the boat when the transom is fully ventilated.

    An airfoil, wing or submerged hydrofoil in a physical fluid does not have to have a sharp trailing edge to produce lift. https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19930088084.pdf https://apps.dtic.mil/dtic/tr/fulltext/u2/a101540.pdf (In the second reference not the results without blowing. Lift is produced by a wing with a large radius trailing edge.) Many real world wings, rudders, centerboards, keels etc have a squared off trailing edge with a finite thickness. The sharp trailing edge is used by the usual potential flow theory for lift by airfoils and wings to determine the circulation. However a sharp trailing edge is not required for lift by airfoils and wings in real viscous fluids.
     
  11. DCockey
    Joined: Oct 2009
    Posts: 5,055
    Likes: 531, Points: 113, Legacy Rep: 1485
    Location: Midcoast Maine

    DCockey Senior Member

    A boat does need a transom to have a sharp edge at the aft end of the bottom. A boat with a stern which is pointed or rounded in planview can have a sharp edge at the aft end of the bottom.
     
  12. sandhammaren05
    Joined: May 2009
    Posts: 410
    Likes: 19, Points: 18, Legacy Rep: 138
    Location: Texas & Austria

    sandhammaren05 Senior Member

    You're talking about half an ellipse. The calculation requires an entire ellipse for optimal lift/drag.

    Let me ask if you've actually worked out what you claim or if you're only giving your opinion. I expect that you have not worked it out, because if you had you would have learned that the only two wing shapes that produce an elliptic distribution of circulation are (i) a logarithmic mean camber surface of elliptic shape (the Kutta condition is not satisfied) at zero angle of attack and (ii) an uncambered (i.e., flat) mean camber surface with an elliptic shape. If you are going to claim otherwise then you need to produce the calculation to prove it. Furthermore, as soon as you add parabolic camber (or any camber at all) to a flat mean camber surface then the lift/drag is no longer max. because the circulation distribution is no longer elliptic, it's the sum of two terms, only one of which is an elliptic distribution. Should there be any reader interested in understanding this then I can attach the calculations.

    The onset of lift when the vortex sheet comes off the bottom reduces the amount of weight carried by bouyancy because the transom is dry. 'Ventilation' is an inappropriate term here, and the language matters. A surface piercing propeller is ventilated because the tip vortex ends on the water's surface. A boat transom after lift onset is not merely 'ventilatied' that transom is completely dry.

    You can design inefficient wings with round edges if you like but I am talking about best performance. You can build a boat with an anchor dragging the bottom if you want but I am not interested in it. 'Sharp edge' always means sharp beyond a minimum scale of resolution in measurement. A think trailing edge on a wing (like the squared end of the deck of a tunnel boat) can easily produce lift if both edges are sharp. However, the wake and drag will be large and the lift will be reduced, as with the rear of a tunnel boat.The rear of a tunnel boat is squared for construction reasons, and because tunnel boat builders generally are not aware of aerodynamic theory (Tim Butts, who built the Aerowing hydros that ran over 100 mph with alky-burning K├Ânig outboards, was an exception).

    Viscosity is irrelevant, it's necessary for generating vorticity but after that you can forget it.
     
    Last edited: Feb 15, 2019
  13. sandhammaren05
    Joined: May 2009
    Posts: 410
    Likes: 19, Points: 18, Legacy Rep: 138
    Location: Texas & Austria

    sandhammaren05 Senior Member

    Listen. If you want to produce a boat where the transom sinks when planing then by my guest and round the edge. I'm talking about high performance, I have no interest whatsoever in low performance.
     
  14. gonzo
    Joined: Aug 2002
    Posts: 15,592
    Likes: 1,094, Points: 123, Legacy Rep: 2031
    Location: Milwaukee, WI

    gonzo Senior Member

    Are you saying that the dynamics of planing change depending of where the input power or force comes from? That does not seem to be supported by facts.
     

  15. sandhammaren05
    Joined: May 2009
    Posts: 410
    Likes: 19, Points: 18, Legacy Rep: 138
    Location: Texas & Austria

    sandhammaren05 Senior Member

    No, but I prefer to stick to powerboats because that's what I've worked on in practice and with the equations.
     
Loading...
Similar Threads
  1. Tristan perry
    Replies:
    10
    Views:
    751
  2. DogCavalry
    Replies:
    20
    Views:
    1,312
  3. container
    Replies:
    29
    Views:
    3,324
  4. Yull
    Replies:
    10
    Views:
    3,006
  5. InetRoadkill
    Replies:
    19
    Views:
    2,722
  6. vejas
    Replies:
    34
    Views:
    7,692
  7. revintage
    Replies:
    14
    Views:
    11,446
  8. Ittiandro
    Replies:
    2
    Views:
    4,841
  9. DrawnOnward
    Replies:
    11
    Views:
    3,680
  10. vkstratis
    Replies:
    13
    Views:
    3,701
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.