Advantages of hard chined, human powered, stabilized monohull, over round bottom?

Discussion in 'Hydrodynamics and Aerodynamics' started by W9GFO, Aug 10, 2018.

  1. Mr Efficiency
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    Mr Efficiency Senior Member

    If your amas can be made easily adjustable for height, you can fine tune them for least resistance, but still offering stability control.
     
  2. messabout
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    messabout Senior Member

    Stability is not a this or that proposition. The planform of the boat has an influence on the total initial stability numbers. A large prismatic coefficient of a round chine hull may actually have more initial stability than a hard chined boat, of the same length and displacement, with a smaller Cp. In general the hard chine has appreciably more initial stability than a round chine boat. But one must factor other things into the equation before it is fair to decide which is better.

    I favor hard chines and flat bottoms simply because they are easier to build. A worthwhile consideration is that hard chines will generate more eddies than a soft chine, thus have somewhat more residual drag. A flattie will have noticeably more wetted surface for a given displacement. That is not good but it may be a fair compromise with the advantages of increase in aspect ratio which mitigates entrance and exit angles and......................whew! So many variables, too little time.
     
  3. Mr Efficiency
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    Mr Efficiency Senior Member

    Certainly plenty of variables, but the "heave" factor of pedalling is a surprise to me ! I'm struggling to see how that works.
     
  4. W9GFO
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    W9GFO Senior Member

    Yes, lots of variables. If I decide to build one it will be 20ft or less, will have adjustable amas and the shape of the hull will not be determined by ease of construction. I have a bias against flat bottoms and hard chines but if it is the best shape for the purpose then that is what I will use.

    If a high l/b is paramount then why not increase draft slightly and avoid the sharp edges? Are the hard chines something that we really want or are they just a side effect of getting the most displacement for minimum breadth and draft, and least work?
     
  5. rwatson
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    rwatson Senior Member

    No, not the individual posts from that Thread, but a whole lot of other Threads have disappeared. A lot of my early ones have gone for example

    I wonder what the date and/or basis was of the deletions ?
     
  6. Boat Design Net Moderator
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    Boat Design Net Moderator Moderator

    When you "find all content" under your profile to show your posts, it returns 10 pages at a time. At the bottom of page 10 is a link to "Find older messages" which returns another 10 pages going back further, and so on. The URL format changed in 2017; most older thread links should redirect to the current format, but there may be some issues, e.g. if you saved links to older posts, there may be an issue if a different number of posts per page was previously selected. Will start a conversation with you to get more info to take a look.
     
  7. rwatson
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    rwatson Senior Member

    Thanks for that.
    I did find the "Find older messages", but it wasn't anywhere near all.

    Edit : Strange, when I go to my Profile, and do "Find all Threads", the all come back.
    I was using the Search with my user name, and it wasnt getting them all.

    There must be some disconnect. But anyway, now I can lookup lost info.

    Thanks
     
  8. Ad Hoc
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    Ad Hoc Naval Architect

    A high l/b ratios...no.
     

  9. BlueBell
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    BlueBell WOT

    By "round hull" I thought you meant cylindrical.
    Few "round hulls" are cylindrical, and their freeboard at midships, never is.
    By the sound of it, you're going to need to build both and let us know which is faster.
    OR
    Sniff out a used, 27' racing scull without any hardware for cheap-cheap, put a pedal prop and an ama or two on and have at 'er.
    If you're stuck at 20' then a surf ski may serve your purposes better.
    OR
    Rig a fixed centre board with an automatic canard (using a self levelling servo) at the bottom for roll stability (no amas)
    but you'll need a kayak paddle for stability at zero to low speeds until the canard and centreboard have enough water flow (speed) to make them effective.
    The WaveBike uses this principal along with a pair of spring loaded amas that fold in.
    The canard is controlled through the handle bars, brilliant.
     
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