Entry Angle vs Hollow Waterlines

Discussion in 'Hydrodynamics and Aerodynamics' started by Mat-C, Apr 16, 2012.

  1. Mat-C
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    Mat-C Senior Member

    So... it's my understanding that hollow waterlines lead to an increase in wetted surface.... bad
    But, wouldn't they also lead to reduced entry angle... good....

    Which is of more importance?
     
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  2. Leo Lazauskas
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    Leo Lazauskas Senior Member

    It is impossible to answer that question in general. The answer depends on
    the size and type of vessel.
     
  3. CT 249
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    CT 249 Senior Member

    I'm no expert, but surely the entry angle doesn't just "go away" simply because there's a hollow just aft of the stem?

    If you put a hollow in the waterline, you get a finer entry angle where the hollow is, but a wider entry angle further aft....the overall angle from the stem to BMAX is just the same whether there is a hollow in the first bit, or not.

    Does (broadly speaking) the resistance of a waterline rise in linear proportion of the entry of the bow angle, or is it one of those things that rises at a higher rate?

    I noticed on a couple of older IRC maxis with bow extensions that the waterlines were so hollow that there was effectively no wavemaking going on. I asked a designer who agreed that if there was no wavemaking in the first couple of metres, then effectively the waterline length was shortened by that distance. Neither boat seemed to go anywhere near as well as a boat with straight waterlines, but of course many other factors were involved.
     
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  4. PAR
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    PAR Yacht Designer/Builder

    Hollow can be useful, so can a fine entry angle. Both carried to extremes can rob preformance considerably. As to how much of what and when to use it, well this is dependent on the preformance envelop you envision for the yacht.

    Hollow is a bit of black magic, used by designers and also a way to beat measurement rules. The target speed also has a good bit to do with where and how much hollow. A cruising boat will do well with some hollow at the LWL forward, but this should convex out as the boat heels, which picks up displacement so the boat doesn't nose dive on some points of sail. Higher preformance craft generally take a different tack with their entry, preferring to get the bow up and with the ability to pierce the wave train. The hull shape in the forward sections of these types of hulls is quite a different shape, generally being flatter athwartship, but finer in plan, compared to a typically fuller cruiser.

    Naturally, these comments are very broad strokes at a complex subject and a very unrefined question. In short it depends on what you want to do and the preformance SOR.
     
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  5. idkfa
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    idkfa Senior Member

    Anyone have pics/seen hollows on multihulls??
     
  6. DCockey
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    DCockey Senior Member

    Good comment. There is a natural tendency to try to simplify complex subjects to a few simple concepts, and it usually works. But then the assumptions needed for the simple concept to be valid get lost. It's not waterline length by itself which is important. Rather the length and distribution of the immersed volume is.
     
  7. daiquiri
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    daiquiri Engineering and Design

    That's how I see it too. The waterline shape is a minor player and the curve of volumes is the primary, because the immersed volume moving through the fluid is the culprit for the wave train and for the relative drag.
     
  8. DCockey
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    DCockey Senior Member

    Though not all volume has equal effect. The influence of volume on surface waves in deep water decays exponentially with depth with a coefficient of gz/U^2 where g is the depth under the surface. At slower speeds the decay is more rapid. So the volume of a bulb keel has less effect then the same amount of volume just below the surface.
     
  9. daiquiri
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    daiquiri Engineering and Design

    The bulb keel is usually one of those things which fall into "everything else being equal" category when the influence of the hull shape (waterline shape in this case) is being studied. So my above reasoning was for the canoe body hull of a limited depth, like (in particular, here) the CatBuilder's case.
     
  10. Leo Lazauskas
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    Leo Lazauskas Senior Member

    A case can be made that the boundary layer displacement thickness "fills in" the
    hollow at the bow returning the effective shape to a convex one, rather than concave.
    I have tried this sort of thing with laminar layers, fully-turbulent and even with
    transition, but it was just an exercise. In reality, boundary layer growth and stability
    is very sensitive to external forces and ambient conditions.
     
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  11. daiquiri
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    daiquiri Engineering and Design

    Mmmh... How much can the BL displacement thickness fill the hollow placed immediately after the bow, when the BL is still in it's infant stage in that area of the hull? I could see it play some role in the aft sections, where it is grown and "beefy" enough to make some influence on the flowlines...
     
  12. michael pierzga
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    michael pierzga Senior Member

    New 60 footer ACCIONA hauled at the shipyard... bow profile
     

    Attached Files:

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  13. DCockey
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    DCockey Senior Member

    I'm familar with boundary layer behavior on automotive type shapes. On the front portion of an automobile like shape I've never seen a boundary layer cause a concave are to become effectively convex, ie the streamlines outside the boundary layer reversing curvature. What does happen is the boundary layer may thicken faster than the usual "flat plate" growth rate due to an adverse pressure gradiant. If the concave area has a combination of sufficiently tight radius and large enough angle change then there may be a separation bubble form. While the separation bubble may add very little to the drag the flow immediately outside the separation bubble will still have concave streamlines. Note that these comments do not include free surface effects.
     
  14. DCockey
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    DCockey Senior Member

    Thanks! I don't see any indication of hollow in the bow surface.
     

  15. michael pierzga
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    michael pierzga Senior Member

    The bow is full...no hollow....bouyancy. Most of the high performance offshore boats, optimized for reaching , I see are the same.
     
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