About inclined underwater hull form

Discussion in 'Hydrodynamics and Aerodynamics' started by fredschmidt, Feb 18, 2012.

  1. fredschmidt
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    fredschmidt Naval Architect

    I have some thoughts on the subject exposed in the blog post:
    http://iomdesign.wordpress.com/
    Although he intended to sailboats radio controlled it is identical to real sailboats.
    Could anyone comment on the matter?
     
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  2. messabout
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    messabout Senior Member

    A boat heeled may show a waterline that is asymetrical. That is not the real concern. When heeled the boat may, and probably does, cause a line connecting the sequential section centroids to become curved. Lee helm or weather helm can be the result. If so there must be excessive rudder input to maintain course. I believe that the seriousness of the effect is somewhat dependent on froude numbers.

    There have been some previous discussions of this matter on the forum. Try the search function.
     
  3. fredschmidt
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    fredschmidt Naval Architect

    I tried doing a search with chine and have not had much success. In fact, the current problem and very interesting and polemic on RC sailboats (IOM) is the chine above the waterline. I think my thread title was not very smart but I wanted to take advantage and discuss a little how is the immersed hull when heeled.
    I think it is very difficult to have water lines symmetrical with the boat heeled, unless it is a goal to be achieved since the beginning of the project.
    I think that the big problem, seeing the inclined water line as a section of a wing, is that the formed foil is not positioned properly creating a need for a large angle of attack. It is as if we were to fly a plane with the wing placed with the bottom up. With the advent of the chine above waterline, when immersed, the foil formed restores the wing right position generating a lateral force in the opposite direction toward the wind with a minimum angle of attack.
     
  4. daiquiri
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    daiquiri Engineering and Design

    The contribution of a canoe-body hull to the side force is extremely low, when compared to the effect of the keel. If you want to look at it as an wing ("airfoil" is a 2D shape with infinite span, whilst "wing" is 3D since it has a finite span), than a boat's hull is like a super-ultra-micro-low-aspect-ratio wing with large-radius rounded wingtips. As such, the lift force it will produce at 3-5° leeway can be neglected for any practical calculation.
    The only thing chines can change is the wing-tip curvature radius, which is just a small step towards a greater efficiency, but still a negligible contribution when compared to the influence of the keel.

    Take a look at this cfd image from Bruce Farr's site:

    [​IMG]

    It shows flowlines below a heeled (and hence asymmetrical) hull. As you can see, the flow doesn't go around the hull, it flows below it, the same way it would flow if there was no heel at all. So the shape of the waterline doesn't imho help in any significant way to the generation of the lateral lift force. Much more important are the effect of heel on the sail-induced yawing moments around the boats' CG (weather or lee helm, as Mesabout correctly pointed out) and the keel and rudder's efficiencies.

    Cheers
     
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  5. fredschmidt
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    fredschmidt Naval Architect

    Hi Daikiri

    See this figure:

    [​IMG]

    and this:

    [​IMG]

    The first figure is the same boat of the second figure, only that it has a chine above DWL . Both boats are heeled in the same angle (35).

    See the shape of waterlines off both thinking in foils shape.

    What is better thinking in a wing that need develop a force AGAINST THE WIND?

    The actual questions in RC boats is: Why the hulls with chine above DWL have better performance than the traditional ?

    I know that the hull have a little contribution to resist the sail side force but the minimum is the great difference between boats with similar performances.

    Logically we have the helmsman, design parameters, etc, contribution.

    After all, what is the thinking about the chine above DWL ?
     
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  6. daiquiri
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    daiquiri Engineering and Design

    Shaping the hull to get a lateral force is a bad way to go, because it has a very limited depth - hence it's efficiency as a (lateral) lifting body is very low. It means that a drag penalty for the generation of any significant lateral force via hull surface and chines is very high, much higher than going the traditional way - canoe body designed to give minimum resistance and leaving the task of producing the desired lateral force to the keel/rudder.

    I would suggest you to see this forum discussion about the effect of chines on a competition sailboats hull: http://www.boatdesign.net/forums/sailboats/aft-hull-chine-sailboat-afecting-performance-25932.html

    When it comes to the production boats, most of the benefits are imho related to slightly bigger and more usable interior volume due to less inclined cabin walls, to a higher initial stability, a slightly increased righting moment, and - a sexy look. :) Beneteau's Sense:

    [​IMG]

    Cheers!
     
  7. Eric Sponberg
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    Eric Sponberg Senior Member

    Fred,

    Another factor that plays here is the center of flotation of the waterline, because it affects how the boat heels and trims. The shape of the waterplane is only part of the problem, and you have to take into account its center and its relation to the center of gravity of the boat. See the discussion on Center of Flotation from two years ago. In that thread there is also some discussion of chines on sailboats.

    Eric
     
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  8. fredschmidt
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    fredschmidt Naval Architect

    Daiquiri

    Seeing this hull in the photo what I realize is that the chine is much hight and not intended to increase performance.

    Some professionals use the hull immersed area to calculate the COG of immersed lateral area that is used (joined with keel area) to suppose that is the center of application of the lateral hydrodynamic force that withstand the sail lateral force. Using so, thinking that the hull is not efficient to develop this force, we do a mistake here?

    Eric

    I need a time to read it. Is big. But I will learn much there.

    I put here some Freeship calculations for the traditional hull and for the hull chined.

    Remember that the traditional and chine hull has the same geometry at rest:

    At rest:

    [​IMG]

    Traditional heeled:

    [​IMG]

    Chine heeled:

    [​IMG]

    Note the strange data for LCF.
     
  9. Eric Sponberg
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    Eric Sponberg Senior Member

    Fred,

    I know that is a long thread, and when I wrote all the lectures there, at the end I combined them all in a pdf booklet called The Design Ratios. It is available for download at the end of that thread, also on my website, and, for convenience, I attach it below. This is free; you can save it to your computer and pass it around. It has been quoted before by model sailors such as yourself as general guidelines for hull design.

    I hope that helps.

    Eric
     

    Attached Files:

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  10. fredschmidt
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    fredschmidt Naval Architect

    Eric

    At the time I followed the thread, seeing quickly, I thought it was another. I've done at the time the download. Great work.

    The great question is, beyond the chine reduce the bow down due the decrease of the beam of the side aft, decreasing the change of the LCB and LCF to aft, what more is the benefit of the chine?
     
  11. Eric Sponberg
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    Eric Sponberg Senior Member

    Actually, I think, in the way they are designed these days, very little. Daiquiri was very close to the truth with his picture of the Beneteau, they look a bit sexy. It's style, and if you don't have style, then you don't have that certain je ne sais quoi.

    On my design Project Amazon (1996), I went for full proper lifting strake type chines such as you would see on a powerboat. The idea was that she would be able to plane easier and stay on plane in most conditions. They worked, although they had a down side: when at anchor, waves slapping on the hull would also slap on the chines and it made living on board rather noisy. I post some pictures below.

    I also designed a chine into Bagatelle with the same expectations--that she would plane more easily. This was more like the chines you see today where they just form a corner in the hull and they bleed away to nothing going forward to the stem. It did not really work--it takes A LOT of enegry to get a sailboat to plane, and Bagatelle does not really plane. Light as she is, she is still pretty heavy in relation to her sail area. Most of the time she is displacement sailing and does not get a chance to use her chines as intended. She does pick up stability by virtue of the chines, and generally, stability is always a speed enhancer. I post a picture of the hull on Bagatelle, too.

    One of my favorite designs that has full-length lifting strakes is Mary Falk's boat QII, a Michael Pocock design. Pictures uploaded. Mary is an attorney in England. This is a real winning boat, finishing not only at the head of her class in one of the Transatlantic races in the early 90s, she also finished second, I think it was, in the next larger class ahead of hers. Mary has also won the Round Britain race in QII.

    In my opinion, the chines we see on sailboat hulls right now are only paying lipservice to what is really possible. We've been there, done that, and done it better than what we are seeing now.

    Eric
     

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  12. fredschmidt
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    fredschmidt Naval Architect

    Eric

    Beautiful boats.

    Particularly I do not believe in sailboats planing. Planing really, with a considerable dynamic force acting up in the bottom surface caused by boat velocity.

    This effective force only appears in hight Fn if the buttocks run horizontally in the bottom from the bow to the cut stern.

    The curved buttock in traditional boats prevents to reach this Fn in consequence from the boat wave formed.

    Seeing the Amazon bottom I think that you try attempted to overcome this.

    But, we need great power to do a boat planning.

    Personally when I hear that the boat is planning I wonder if the boat was not surfing.

    Well, for now there is the question. Personally I think in the possibility of an improvement in lateral hull side force and a smaller resistance due to a lower angle of attack.

    But, may not be.

    Cheers
     
  13. daiquiri
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    daiquiri Engineering and Design

    Look, an approximate calculus is possible by the use of the wing analogy. If you have time to do it, you could try to model the boat as follows:
    1. hull is an extremely low-AR wing with well-rounded tips and the keel as a moderately-high-AR wing.
    2. hull is just a wetted surface of the low-AR wing in the case 1), and the keel is as in the case 1). In this way you eliminate the lift and the induced drag from the hull, leaving just the friction drag to the hull and all the lift to the keel.
    See what is the difference in drag between two cases, for the same lift. It will give you an indication of what is gained and what is lost by letting the hull produce a lateral force. I already know what results you will get. ;)

    Cheers!
     
  14. Eric Sponberg
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    Eric Sponberg Senior Member

    Thanks!

    Have you seen some of the video from the Volvo Ocean Race: Here is an example, this is so cool!: Pirates of the Caribbean.

    My buttocks on Project Amazon were slightly curved, yes, and I do this on my powerboats, too, such as the Cherubini Classic 20, the Saetta Classic 20, and the custom Chris Craft Cobra redux. There is very slight rocker in the buttocks, but basically a constant deadrise over the after half of the boat, which, I think, gives the boats a very forgiving ride, meaning they run well in a small window of trim angles which allows for a comfortable ride no matter the loading and movement of LCG, depending on the passengers carried. They are very comfortable, stable boats at speed.

    Eric
     

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  15. daiquiri
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    daiquiri Engineering and Design

    Eric, I just love the look of the boat in the first picture! :)
    Very nice hull shape.
     
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