About inclined underwater hull form

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

  1. daiquiri
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    daiquiri Engineering and Design

    Thank you Mikko. You have done a nice job in that thread, but you have given only numbers relative to the hull lift, if I understand it well.

    Now, how about running simulations for two hulls - one with a round-chine and the other with a hard chine aft. For each hull you could run simulations with and without the keel and tell us what happens (according to the CFD software) in terms of total (hull+keel) lift and drag. This discussion is centered on difference in hydrodynamic characteristics between round-chine hulls and those with hard chines.

    Thanks!

    Cheers
     
  2. Mikko Brummer
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    Mikko Brummer Senior Member

    I also believe there is something to the chine and the hull lift, my example being the Star. It has a very pronounced chine, but a shallow and basically inefficient keel with a large skeg in front of the rudder. In the VPP, which ignores chines and is not very good at hull lift, the Star is a lousy upwind boat, with a very large leeway up to 10 degrees or more. In the real world, it's nothing of the kind, but a wonderful upwind machine beating at 70 degrees from tack to tack, while heeling close to a comfortable 30 degrees and the keel root intermittently in the air. So I would be surprised if its chined hull had nothing to do with it.
     
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  3. jpquattro
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    jpquattro Junior Member

    I have printed your booklet and stored it next to the Larsson book...
    May be that you can direct me to a metric version of The S# formula?
    Or I must do my homework?;)

    Paolo
     
  4. Eric Sponberg
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    Eric Sponberg Senior Member

    Hi Paolo,

    I have not had an inclination to figure out the metric version of the S#--it would require a completely new study to convert the exponents. The SA/Vol^2/3 ratio, actually, would convert directly because it is dimensionless, not so with the DLR ratio. The best thing to do is to download the spreadsheet (see the Articles page on my website) and insert the appropriate columns to enter the metric dimensions of the boats, and add conversion columns to convert them to imperial units to use in the spreadsheet formula.

    Eric
     
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  5. daiquiri
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    daiquiri Engineering and Design

    Eric, Paolo:

    The DLR calculation in SI units is really straightforward. You just have to apply the following unit conversions:
    D (lbs) = 2.2046 * D (kg)
    D (long ton) = D (lbs) / 2240
    LWL (ft) = 3.281 * LWL (m)​
    The formula for DLR in Imperial Units:
    DLR = D,long ton / (LWL,ft / 100)^3 = 1e6 * D,long ton / LWL,ft^3​
    becomes, in SI units:
    DLR = 27.87 * D,kg / LWL,m^3​

    After which the usual formula for S# from Eric's "Design Ratios" can be used.

    If you need to obtain the DLR from the Slenderness Ratio (SR), the convertion formulae can be found in this thread: http://www.boatdesign.net/forums/boat-design/dlr-vs-slenderness-38853.html#post473760

    Hope it helps. Cheers
     
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  6. jpquattro
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    jpquattro Junior Member

    Tanks all, I have made my home work and I have trasformed to metrics the Eric's excel sheet...

    Paolo
     
  7. Mikko Brummer
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    Mikko Brummer Senior Member

    To get back to the original thread topic: asymmetrical waterlines or underbody while heeled, and lift associated with it, is not exactly a new idea. Designers knew and exploited it a century ago. It is also well known to tank testing that some hulls, when heeled, need to be towed at a negative leeway angle of 1-2 degrees, for a zero sideforce. Maybe the chine accentuates this?
     
  8. fredschmidt
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    fredschmidt Naval Architect

    I think that a research about the heeled underwater form (lookink for the hull asimetry) with and without chine reveal good things. I think that the chine per se has improved, but the chine with some care about heeled hull form improved much more. But we will need a test tank
     
  9. rwatson
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    rwatson Senior Member

    This quote from the modelling world piqued my interest as a possible very omportant factor in the design decision

    "no doubt time will tell if there is a real performance gain to be had by moving to narrower hull forms. .... This has led in turn to a closer look at the way the VPP handles the added drag of a hull passing through surface waves, a possible explanation of why less stable but narrow hull forms seem to outperform more stable but wider designs."

    Real Life performance adds a whole new layer of complexity.
     
  10. fredschmidt
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    fredschmidt Naval Architect

    Mikko

    I would like see the Star inclined waterlines, have you the table of offsets or you know where I can obtain it?

    rwatson

    I think that we will see a changing in the hulls in future. :)
     
  11. Mikko Brummer
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    Mikko Brummer Senior Member

    Don't have offsets, but here's lines with measuremeants added
     

    Attached Files:

  12. Mikko Brummer
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    Mikko Brummer Senior Member

    Here's free surface simulation by Juan K... The brown boat is base line and the orange a variation (or vice versa?),
     

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  13. fredschmidt
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    fredschmidt Naval Architect

    Thanks Mikko, very interesting and I see there some interestings things.

    I'm trying to buy the plans in the Star class. When I get past the drawing to the computer and turn it I'll put the pictures here to see the waterlines inclined .

    Meanwhile, I'm enjoying myself a little with the drawings that you sent me
     
  14. fredschmidt
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    fredschmidt Naval Architect

    I do some drawings with the Mikko Star plan.

    Star heeled 15 degrees and 30 degrees: what I see there - a beautiful wing ready to develop a good lift to withstand the sail lateral side force :)
     

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  15. rwatson
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    rwatson Senior Member

    I dunno fredschmidt - the curves look sweet, but they are very shallow, and I would be surprised if you get any real 'lift' from it.

    The wave piercing performance and lateral resistance of the 'flat side' seem more likely to me.

    Is there anyone out there that can do these underwater illustrations for other boat designs ? I have a project that would be interesting to run through this process.
     
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