Entry Angle vs Hollow Waterlines

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

  1. daiquiri
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    daiquiri Engineering and Design

    Exactly what logic wants them to look like. Increasing the righting arm (more power from sails) and decreasing the heel-induced bow-down trim (making both sails and keel work more efficiently) are both beneficial for the Vmg.
     
  2. idkfa
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    idkfa Senior Member

    Shouldn't there be a bulbous too, to make the hollow effective?
     

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  3. Mat-C
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    Mat-C Senior Member

    Hmm... sorry gents - should've been more specific...
    I was thinking about a displacement motorboat... not of any specific length as such, but one that doesn't ever run at beyond "hull speed". I quite often notice that on traditional looking hulls there is quite a bit of hollow in the waterlines up near the bow, whereas on more modern looking hulls this is absent.
     
  4. Leo Lazauskas
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    Leo Lazauskas Senior Member

    I agree that it would have to be a very gentle hollow. After all, the BL is
    very thin, and more so because of the favourable pressure gradient.

    And, yes, the BL plays a role in the aft portion of hulls. Michlet allows you
    to include the BL displacement thickness, but the effect is barely noticeable
    except at low Fr and on small towing tank models. As far as I have seen,
    though, it does improve correlation with experiments for those model hulls.
     
  5. Luc Vernet
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    Luc Vernet Senior N.A.

    Shouldn't the "champions" of hollow waterlines at the bows, F.L Herreshoff, and before him Steers, specially with his schooner America, or the old American and Canadian fishing schooners and pilots boats be cited here?

    Although reckoning the value of the "everything else being equal" or the "if one limits himslelf to"....for the sake analysis and the delight of the "number crunchers", I quite like the broader point of view, where of course the considered boat has to be defined more in its whole.

    So: my two cents...(if even of that value?): shouldn't one also consider a prime function of hollow waterlines in earlier vessels, which was to increase the efficiency of the lateral plane against leeway?

    If taking as what I think is a significant example: the American fishing schooners: before something like 1860, they had quite full bows, very inefficient against leeway, but they also had their bow piece protruding well forward the rabbet. The lines plans were actually drawn up to this rabbet, but the profile shows a very large portion of the bow (and the keel after that), forward of the "canoe" hull, then very efficient against leeway, combined in that with the very hollow aft sections below the counter, needed to balance their huge sail area aft.

    After approx. 1860, the waterline at the bows, or more exactly right below waterline, started to be very hollow while the fragile earlier stem did not protrude as much forward of the rabbet.

    Later, at the time of famous "Carrie E. Philips" (1887), when the main started to go from large to huge, and also in an attempt to improve maneuverability, the deep forefoot slowly disappeared in favor of deep aft sections constituting the essential of the lateral waterplane, and larger rudders, but the hollow "water-cutting" waterlines did not - or very slowly - until the late ones near the turn of the century. Waterlines then became straight, with very fine entries (sometimes less than 20 deg.). These were the fastest ever...but that is another discussion.

    Sorry for the (nearly) off the topic...
     
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  6. Doug Lord
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    Doug Lord Flight Ready

    Skiffs

    Interesting: Frank Bethwaite says that a wedge angle(entry angle) of 22 degrees(half angle of 11 degrees) is ideal for high speed sailing skiffs. Around 1960 the dinghy entry angle was on the order of 50 degrees, according to Bethwaite, and when the Finn came along it had a "revolutionary" "fine" entry of 30 degrees. By 1969 the Bethwaites had the entry angle down to 22 degrees with the primary benefit being upwind in chop. Its pretty hard to get 22 degrees w/o having hollow or ,at least, slightly hollow waterlines.
     
  7. rxcomposite
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    rxcomposite Senior Member

    I am attaching the hull form by Lindbladd & Todd, PNA. I have difficulty understanding it so I tabulated it.

    It seems the hollow needs at least two parameters, the Froude number and the entrance angle. The hollow at waterline trend start at Fn 0.19 and half entrance angle of 12 degrees and diminishes to half angle entrance of 6 degrees at Fn 0.30.

    Above Fn 0.30, the waterline is fuller or straight or above 9 degree half entrance angle, unless there is a bulb where the hollow comes back again.

    I am a little clouded by the Fast passenger Liners, Trawlers and Tugs column. Is the S shape not a hollow?
     

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  8. DCockey
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    DCockey Senior Member

    Interesting.

    For anyone interested the specific location of the table in PNA is Principles of Naval Architecture, Second Revision, Volume II Resistance Propulsion and Vibration, 1988, pp 70-71.

    Don't overlook the words in the text (p 69, 71) about the table:
    "The information given in Table 17 can only be used for general guidance in the preliminary design stages. In any particular ship design, more detailed analysis, based upon model and full-scale data for closely similar ships, must later be made to determine the most suitable form together with estimates of the probable effective and shaft powers."
    In other words a good place to start but may not be the final answer.

    The recent volume in the new PNA series, Ship Resistance and Flow by Larsson & Raven 2010, doesn't have a similar table. Rather Chapter 11 Hull Design constains an extensive discussion about hull shapes and the resulting waves. Emphasis is on Fn less than 0.3 and vessels with parallel mid-sections. The authors stress the importance of considering both the shape of the entrance and the bow "shoulder". "This bow fore/shoulder interference is a powerful mechanism that can have a large impact on a ship's wave making, and it is useful to attempt to optimally position the fore shoulder." p 194 An example is then given where reshaping the bow with a larger entrance angle and redesigned shoulder results in increased wavemaking from the entrance but a decrease in wavemaking from the shoulder.
     
  9. DCockey
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    DCockey Senior Member

    In another thread on bow shape Leo said:
    Isn't it a mistake to focus on the bow shape in isolation, i.e. without
    reference to the other parts of the hull geometry when talking about
    resistance or sea-keeping and not aesthetics?

    Ad Hoc agreed as do I.
     
  10. Leo Lazauskas
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    Leo Lazauskas Senior Member

    I disagreed with myself yesterday when I was thinking about the effect of
    bow shape on spray formation and splash drag. Then I realised that those
    changes, while they could eliminate spray, could also have consequences
    that might be worse, overall, on performance.
    So now I agree with myself, and you and Ad Hoc.
     
  11. Ad Hoc
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    Ad Hoc Naval Architect

    Forsooth....the 3 musketeers?? :p
     
  12. rxcomposite
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    rxcomposite Senior Member

    I ommited all other factors in the table and zeroed in on the hollows.

    Agree it is a general design guide as it is just a collection of data of several hundred ships and no particular calculation applies.

    I use the Lindblad & Todd in conjunction with Saunders, another design guide on the range of Cp, D/L ratio based on Froude/Taylor's number. Saunders table appears in PNA and "Design of Naval Ships". With the two tables, it gives me a very quick narrowing down on hull shape, ratios, and coefficients.
     
  13. rxcomposite
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    rxcomposite Senior Member

    The 4th one who will join will be named d'Artagnan.:p
     
  14. Leo Lazauskas
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    Leo Lazauskas Senior Member

    Three characters from the movie "Cocoon" would probably be closer to the truth. :)
     

  15. Ad Hoc
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    Ad Hoc Naval Architect

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