False bottom v on a flat bottom

Discussion in 'Boat Design' started by Juijitsufighter, May 4, 2015.

  1. Rastapop
    Joined: Mar 2014
    Posts: 278
    Likes: 5, Points: 18, Legacy Rep: 46
    Location: Australia

    Rastapop Naval Architect

    Haha, you don't think that will be enough to convince Tom do you? :D
     
  2. Mr Efficiency
    Joined: Oct 2010
    Posts: 10,386
    Likes: 1,042, Points: 113, Legacy Rep: 702
    Location: Australia

    Mr Efficiency Senior Member

    Ahh.......No. :rolleyes:
     
  3. latestarter
    Joined: Jul 2010
    Posts: 402
    Likes: 51, Points: 28, Legacy Rep: 233
    Location: N.W. England

    latestarter Senior Member

    I have been pondering this issue for a while and can see 2 ways of looking at it.

    The traditional Archimedes Principle, any floating object displaces its own weight of fluid.
    Gets the right answer, easyish to calculate however intuitively uncomfortable to me, my thought experiment is when you drop an object into water, how does it know when to stop sinking. ;)
    Is it calculating how much water it is displacing and stops when it reaches its own weight? :D

    The other method which can be a pain/impossible to calculate for curved surfaces but to me feels better, is the buoyancy force is the algebraic sum of the pressures on the submerged surfaces. The pressure on the hull is the only thing the object experiences.

    Regarding depth of flat versus vee hull of the same weight sketches A and B. The vertically hatched area gained is offset by the loss of the horizontal hatched area giving the same immersed volume.

    Any variation, of whatever shape, from a flat bottom must increase the draft.

    Focusing on comparing one half of the flat versus vee using the pressures on the submerged surfaces (sketches C and D).
    The inclined plate has the same average pressure but the length of the plate increases to z/cos c.
    However as the pressure is at right angles to the plate, the vertical component of the pressure will be p*cos c and the force will be (p*cos c) x (z/cos c) = p*z so that the buoyant force is the same whether flat or inclined. (Another way of expressing DCockey's point in post 59).
     

    Attached Files:

  4. Rurudyne
    Joined: Mar 2014
    Posts: 1,170
    Likes: 40, Points: 48, Legacy Rep: 155
    Location: North Texas

    Rurudyne Senior Member

    ... of course there's always the minuscule effects of surface tension to account for. :p

    Which brings up this question: if a boat could have a hydrophobic coating would there be a barely perceptible convex meniscus surrounding it where it sits in the water and would it float imperceptibly lower (it's displacement unchanged) as a consequence?
     
  5. DCockey
    Joined: Oct 2009
    Posts: 5,229
    Likes: 634, Points: 113, Legacy Rep: 1485
    Location: Midcoast Maine

    DCockey Senior Member

  6. yofish

    yofish Previous Member

    Two objects that have the same breadth and have the same volume. their total ship weight is 99.9999% of their displacement.
     

    Attached Files:

    • arch.jpg
      arch.jpg
      File size:
      30.1 KB
      Views:
      317
  7. Mr Efficiency
    Joined: Oct 2010
    Posts: 10,386
    Likes: 1,042, Points: 113, Legacy Rep: 702
    Location: Australia

    Mr Efficiency Senior Member

    Can anyone point me toward a view of the lines of this boat ( Bolger fast motorsailer) as pictured earlier in the thread ? I'd like a look at what is going on under there ! Can't track it down on the web, so far. :D
     

    Attached Files:

  8. yofish

    yofish Previous Member


    Misnamed, try 'Bolger Hawkeye' Then begins the magical mystery tour with Phil Bolger and friends!


    http://forum.woodenboat.com/showthread.php?139171-Bolger-quot-Hawkeye-quot
     
  9. yofish

    yofish Previous Member

  10. yofish

    yofish Previous Member

    In the BWAOM chapter on Clam Skiff, PCB says "The big solid shoe is
    derivered from the cutwater experiments I've discussed ... [in the
    chapters on Hawkeye and Microtrawler]". When describing Hawkeye, he
    says "... much of her weight is carried by the slim keel box."

    I don't think it matters much what it's called, and it's not easy to
    say where Bolger came up with the configuration seen in the two
    boats mentioned above. But it's certainly useful to think of these
    designs as having sponsons that provide sufficient stability and
    deck area, married to a hull that retains most of the virtues of a
    simple narrow planing sharpie.

    But you have to admit the bow of Miniature Steel Tug (non-planing)
    is very similar to that of, say, Fast Motorsailer (planing), even if
    yo say that the first one has a box keel and the second a narrow
    sharpie hull with sponsons.

    Howard

    --- In bolger@yahoogroups.com, "Bob Chamberland" <cha62759@t...>
    wrote:
    > I've long since been separated from my Bolger books however if I
    > recall correctly in the discussion of Hawkeye, Microtrawler and Bee
    > the center hull is not referred to as a box "keel" but as a sharpy
    > hull; the wider upper hull really functioning as sponsons. With
    power
    > the sharpy hull approaches a full plane. So it's is a case of
    > comparing apples and oranges. There is not a box keel on the
    Hawkeye,


    I think you're going to need to get hold of 'Boats With an Open Mind'.
     
  11. whitepointer23

    whitepointer23 Previous Member

    That thing looks like it would broach surfing down waves. The keel would overwhelm the rudder.
     
  12. Mr Efficiency
    Joined: Oct 2010
    Posts: 10,386
    Likes: 1,042, Points: 113, Legacy Rep: 702
    Location: Australia

    Mr Efficiency Senior Member

    Boat was outboard powered, I think, I am interested in what the rear underbody looks like, and whether the box keel shallows out, or tapers to a canoe-end, or something else entirely !
     
  13. yofish

    yofish Previous Member

    Buy the book. $14 on fleabay right now.......
     
  14. Mr Efficiency
    Joined: Oct 2010
    Posts: 10,386
    Likes: 1,042, Points: 113, Legacy Rep: 702
    Location: Australia

    Mr Efficiency Senior Member

    Thanks for the tip.
     

  15. SamSam
    Joined: Feb 2005
    Posts: 3,899
    Likes: 200, Points: 63, Legacy Rep: 971
    Location: Coastal Georgia

    SamSam Senior Member

    This sounds like he thinks an outboard jet drive is an accessory that slips on the prop shaft, easily removed and the propeller put back on in a few minutes.
     
Loading...
Forum posts represent the experience, opinion, and view of individual users. Boat Design Net does not necessarily endorse nor share the view of each individual post.
When making potentially dangerous or financial decisions, always employ and consult appropriate professionals. Your circumstances or experience may be different.