ScowCat 470

Discussion in 'Multihulls' started by Manfred.pech, Aug 10, 2016.

  1. Manfred.pech
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    Manfred.pech Senior Member

  2. daiquiri
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    daiquiri Engineering and Design

    I am a bit perplexed.
    When it comes to monohulls, the purpose of the scow-type hull is essentially to improve the speed potential of the boat, through:
    1) increase of the righting moment (can carry more sails)
    2) increase of the slenderness ratio of the heeled hull (less hydrodynamic drag)
    Cruising monos can also benefit from an increased available internal volume, especially towards the bow.

    In the case of this catamaran, the only evident gains I can see are:
    - the increase in the available internal volume in the forward part of the demi-hulls;
    - better shoal-water characteristics.
    The speed potential seem to be decreased, due to higher wetted area of the hulls.

    This is a small boat, and the internal space is tight. So recovering some internal volume can indeed be the goal behind the chosen hull form - but, am I missing something else here?
     
  3. patzefran
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    patzefran patzefran

    Strange !
    It seems it has a lot of fans.............
     
  4. cavalier mk2
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    cavalier mk2 Senior Member

    I'm puzzles too, those bows look like brakes going into a chop. The flat bottoms I can understand but would expect some pounding being that wide.Maybe a skipjack style V bow to give some deadrise to soften the ride and pointy ends to cut through the waves? It would be longer of course.
     
  5. Doug Lord
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    Doug Lord Flight Ready

    -------------------
    Slavi, in the early Moths a lot of different hulls were tried-the one that worked best was squarish instead of the lower wetted surface semi-circular shape because the L/B ratio of the square shape was higher-in other words the waterline beam was narrower. The narrower waterline beam reduced wave making drag and trumped the higher wetted surface.....

    A rough, but accurate sketch from years ago. The Moth hulls were pointed bows of a conventional type:
    click--
     

    Attached Files:

  6. daiquiri
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    daiquiri Engineering and Design

    I am not sure it works the same in this case, Doug.
    This is not a high-slenderness ratio hull. This looks like a displacement-speed boat, where the wave drag has pretty much the same importance as the viscous drag.
    Hence, a decrease in one drag component implies an increase of other drag forms. Hence, I don't see a clear advantage of this hull form in terms of drag, at least in the displacement speed range.
     
  7. Doug Lord
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    Doug Lord Flight Ready

    Scow Cat 470

    I don't know, Slavi-I can't find dimensions to check the actual L/B but it looks to me to be near to or narrower than a Moth hull. The problem,if any, is the bow in any waves-at least to some extent. I think it is very likely that the L/B ratio will offset the greater wetted surface, as far as speed goes, depending on how much negative effect the bow has(if any).
     
  8. daiquiri
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    daiquiri Engineering and Design

    I have to partially correct myself.

    LWL = 4.2 m (from the 4th pic.)
    Displ. = 650-680 kg = 0.634 cu.m (an estimate based on smaller models), hence D=0.317 cu.m per each hull.
    hence:
    Slenderness ratio SR = LWL/D^1/3 = 6.1 (approxim.)
    With these data I estimate a B of 0.55-0.60 m.

    Not a particularly high SR value, but one cannot make miracles on such a small boat.

    However, it is an SR ratio where residual drag makes some 30-40% of the total resistance at max. speed, hence wave drag is still the dominant one.
    Hence, your note might be correct, though the improvement in resistance with this hull form is IMO barely noticeable.

    I still believe that the principal reason behind this hull form is the recovery of the internal volume, which is precious on such small boats - especially if it has go provide berthing for the crew.

    Cheers
     
    Last edited: Aug 11, 2016
  9. Mr Efficiency
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    Mr Efficiency Senior Member

    It should be less prone to hobby-horse pitching with the full ends.
     
  10. Doug Lord
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    Doug Lord Flight Ready

    ========================
    Thanks, Slavi but from the third picture, just eduguessing, the hull doesn't seem to me to come close to the beam you suggested-no where near .5-.6m. Now, there could be an error in the perspective of the render but if its accurate I'd say the beam of each hull is much less than .5m. I wish I could take more time to search for dimensions......
    Anyway, I like the room the boat has!
     
  11. redreuben
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    redreuben redreuben

    I'm no expert on hull dynamics but have to agree with Mr E the prismatic must be huge ! Making it easier to drive hard off the wind ? Although the scow bow can't help much.
    Interesting though !
    I'd like to see a bit more V on the bow.
    And I hate flat bottoms.
     
  12. daiquiri
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    daiquiri Engineering and Design

    Doug, take your time then and measure it. :)
    If the weight estimate is correct, then the beam is roughly 0.55 m. Otherwise, the DWL lines are wrong in the drawing.
    There is also a person shown laying in one of demi-hulls. You can't have both the room and very narrow hulls.
     
  13. Mr Efficiency
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    Mr Efficiency Senior Member

    The person (corpse ?) appears to be lying half over the demi-hull, the rest over the deck.
     
  14. daiquiri
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    daiquiri Engineering and Design

    Hey folks, how about taking those pics and making a few measures, instead of guessworking? :)
     

  15. Doug Lord
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    Doug Lord Flight Ready

    I wrote to the designer to ask about the waterline beam of the cat hulls......
     
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