monohydron or slight rocker?

Discussion in 'Powerboats' started by copenhagen, May 10, 2011.

  1. copenhagen
    Joined: Apr 2011
    Posts: 22
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    Location: copenhagen

    copenhagen Junior Member

    Ok guys.. here is my secont question (not as uninformed as my first thougt) ive been studying some planning craft hulls and it seems to me that quite a bit of them has some amount of rocker built into them..

    especially runabouts and larger craft.. the handfull fo planning hulls that were included at examples in delft ship in partucular..

    i even drew up a crackerbox in delft ship just for practice in importing chines and it also turned out to have a 1 inch rocker in the aft section..

    (even Tige boats new waterski hull boasts of haning ricker and trimtabs instead of a flat rear section ?!?!)

    what is the point of this, im asking since i got the idea of using the crackerbox running surface on my "homebrew" waterskiboat

    the .txt "boxlines" is to import the chinelines in delft ship and the picture is just an old crackerboxplan but it shows teh curved aft section nicely
     

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    Last edited: May 10, 2011
  2. PAR
    Joined: Nov 2003
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    Location: Eustis, FL

    PAR Yacht Designer/Builder

    The design you show is quite old and before the development of the '63 series hull form tests. Information from these tests dramatically changed the way we look at full plane hull shapes.

    If you look closely at the design shown, it hasn't any rocker at all, in fact the reverse is true, it has a hook, which it damn well needs given the speeds it's designed to blast along at. I've driven these shapes as a young man and I can tell you they are moderately fast by modern standards, but really unstable at the top end. Many have been hurt and killed in just this hull shape. Screaming along at 70 MPH when she decides to swap ends, barrel rolls or flips over is more then enough to literally break your neck.

    This hull has a dead straight bottom from station 6 aft, if you ignore the hook. It has a slight warp and very modest deadrise, but the centerline profile is straight, except for the hook.

    It's precisely these subtle details that makes the difference in design understanding Copenhagen. Over looking, or in this case not knowing what to look at, can mean the difference in a completely unsuited design being thought of as acceptable. A little understanding is much worse then none, which is what I think is occurring here.

    This is a high speed, very light racing hull and nothing you want to be out in when there's even the slightest chop. These things fly through the air, standing on their props, chine walking their way down the course.
     
  3. tunnels

    tunnels Previous Member

    I love the thing of chine walking ! By doing a slight mod on the bottom you could make it a lot more user friendly . Increase the vee (deadrise ) and intoduce a couple a strakes set a lttle way apart and will stop the chine walking and make i fly even more . :D
     
  4. Dean Smith

    Dean Smith Previous Member

    i dunno what COPE is going to use her for
    But I tell you chine walking is downright scary especially downwind at sea
    mono is where the chine runs parallel to keel and is very suitable for jet drives
    I have tried most degrees of vee and chine config the chine is close to 150mm wide aft and gets her up fast. I did 10 of these in various formats, , inboard diesel, twin and single outboards and 8 with much lesser vee, bang your teeth out , but this one can jump waves
    And this works, without strakes , which IMO make for a harder ride
     

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  5. tom28571
    Joined: Dec 2001
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    Location: Oriental, NC

    tom28571 Senior Member

    From looking at the offsets station 6 aft, I see some rocker in the keel and a wee bit of hook (or might be called warp) in the chines. I have no idea how it might handle at high speed but suspect it woould not be good. Having seen some of the crackerboxes doing what PAR describes, I'd think (hope) this design has become obsolete. A reason for rocker in this case might be that the down angle of the shaft and forward weight of the motor would tend to hold the bow down and some rocker would tend to pull it back up to a more optimum trim angle for high speed.

    Too many forces fighting each other in my view. That could be the source of some of the ill handling seen in videos of crackerboxes. One thing we need to keep in mind is that many such boats are not designed from scratch but evolved by following trends set by features found on recent winners. That could lead to conflicting elements of design on individual boats.
     

  6. tunnels

    tunnels Previous Member

    Older designed boat also had less hp and heavyer motors so think of today where we have high hp and lighter motors ! From te chine down need to be redesigned and bought up to date . There a place that done just that with the old Chris craft Capri and made a much better boat out of it . In the water still looks the same but performance and handling etc is much better .same thing need to happen with the Cracker box . The oldies didnt have computers and they knew what worked because they exsperimneted with differant things , most didnt know why they just new it worked .
    The stake thing is like that . I have had great success with fitting strakes on older boats and repowering with much bigger motors than they were ever designed to have and they fly . But thats another story ! Flying i mean !:D
     
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