design question (brett)

Discussion in 'Boat Yard Basics' started by marshman, Jan 27, 2011.

  1. marshman

    marshman just try to follow me....i dare ya...

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    question....right up your alley brett...

    side panels...im just piddling with a scale model right now..

    seems im coming up with a rocker in my bottom at the rear...cant have that for sure...  my memory is fuzzy...seems that we have discussed this before somewhere..do i need to concave my bottom edge to eliminate the rocker, or just strongback it??  i know on the fs 18 drawings the bottom edge is concave, but i thought that was because of the vee...

    maybe im crazy...maybe its just the crappy material im working with??  lets discuss this...i know youve been playing with shapes and whatnot very thoroughly....
     
  2. Brett

    Brett > PRO STAFF <

    Rocker can be caused by curvature in the chine.
    I keep the aft sections of the chine straight and parallel to each other.
    From about where the plane is aft, the bottom panels restrain the chine.
    And the sides restrain the bottom. That eliminates the rocker.

    [​IMG]
     

  3. Brett

    Brett > PRO STAFF <

    What looks like a curved chine in this image,
    is simply a reveal caused by allowing the panel to hang
    down past where the chine really is. Hold a straight edge
    to your monitor and line up the rear of the chine
    with the screw heads along and above the chine.
    All in the same line, not curved as the image would make you think.

    [​IMG]

    Here's the reveal, I figured to use this difference
    between the bottom of the hull and the side panel
    to create a splash rail to deflect spray by filling the gap
    with microballons to the shape of that small triangular area under the plane.

    [​IMG]
     
  4. marshman

    marshman just try to follow me....i dare ya...

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    ok...i kept everything straight for my 8 footer...didnt get any rocker...but this one is gonna be 18 feet....so i figured maybe thats where it came from...
    [smiley=1-mmm.gif] [smiley=1-mmm.gif] [smiley=1-mmm.gif]
     
  5. Brett

    Brett > PRO STAFF <

    Are you using laths to frame your shape, or just flat panels?

    This is what I started with, laths at the chine and shear.
    All curves, only the keelson and transom are straight.

    [​IMG]

    The bottom panel edges are parallel with the keelson
    and when the chine strips are fastened to the panel edges
    are forced to stay straight up to where i wanted the curve to begin.
    About the second clamp from the transom.

    [​IMG]
     
  6. marshman

    marshman just try to follow me....i dare ya...

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    huh??  just flat panels...when i build with ply, ill be using a frame near the front(deck support), one in the middle(tankseat like a jonboat) and a frame in the rear(deck support)...and 2 longitudinal stringers...and then a couple ribs/side supports to support the gunnels....
     
  7. Brett

    Brett > PRO STAFF <

    I'd like to see a pic to understand where the rocker is coming from...
     
  8. firecat1981

    firecat1981 BBA Counselor

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    I think I know what you are talking about, it's a issue with my design as well since it is mostly flat bottomed. The panels will have a natural tendency to not want to lay flat once any type of curve is made in them, usually the bow, for me it's a bulge right behind my second frame that I will need to work out.
    Putting on the side pannels makes it worse because you now have an unnatural force pushing down on the hull bottom. A strong back will help, and adding stringers if done well can correct the issue.
     
  9. marshman

    marshman just try to follow me....i dare ya...

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    ha.. too late..i tore it apart...my pieces (cardboard cut with scissors) were a little off from eachother...  like i said, crappy material...i feel sure that once i start building ill work it out...im just bored and getting antsy..

    just for some reason an old convo stuck out in my mind about concave....cant reember where i got that from...hell, may not have even been on here...

    as you can tell, im not building to any plans or anything...i have a picture in my brain of what i want...im just gonna start building and with a little trial and error, we will see what i come up with...the only constants i have are some dimensions that i want... long(18), skinny(36 bottom, 48 beam), low sides(14 inches), as light as i can possibly achieve( i will be dragging this boat over dams), high transom for a 25 on a jack.....NO vee...all flat...pointed bow, or jonboat style...havent decided..
     
  10. Brett

    Brett > PRO STAFF <

    Sounds like a cypress skiff.
    A kind of flat bottom dugout canoe...sort of...I think

                                         :D
     
  11. marshman

    marshman just try to follow me....i dare ya...

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    yeah, it would look similar to that if i do the pointed bow... a squareback pirogue on steroids...

    im working on uploading an album of pics ive saved over the years that pertain to what im wanting to build...while, none of the skiffs in the album are what im building, each one has something that i like...
     
  12. marshman

    marshman just try to follow me....i dare ya...

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    except without that rocker!! rocker is the devil!!
     
  13. Brett

    Brett > PRO STAFF <

    I think the original bow rocker was to help slide over logs
    when roaming around back in the bayou. An all flat bottom
    would impact the log instead of sliding up and over.
    Old deadfall cypress don't float do they?
     
  14. marshman

    marshman just try to follow me....i dare ya...

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    yes, bow rocker for sure...thats what its for...to slide over logs, dams...i was talking about the rocker in the rear...the skiff you posted could never plane with an outboard...

    if youve ever seen an aluminum crawfish skiff (in louisiana there are thousands of them) they have a really pronounced "upswept" bow...thats the reason, to slide over stuff...and the steering wheels are all the way forward to be able to see better while navigating flooded woods...
     
  15. marshman

    marshman just try to follow me....i dare ya...

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    and theres no such thing as "floating" cypress deadfall around here in these parts....if its floating, somebodys gonna grab it and bring it to the mill to sell, or bring it home...

    but yes old cypress logs are stuck down on the bottoms of the swamps, lakes....they call these sinkers...there are some people around manchac louisiana that make a killer living going to find these sinkers and raise them up with cranes on barges, and big float bags and bring them to the mill....thats some of the most beautiful wood ive ever seen in my life...always been my dream to have a room in my house done in sinker cypress...its very very very expensive...
     
  16. marshman

    marshman just try to follow me....i dare ya...

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    take a look thru here...they are in no particular order...like i said, none of them are what im building, but each one has some feature that i like or am considering trying...

    http://s294.photobucket.com/albums/mm116/mikebro72/my%20marshskiff%20ideas/?start=all
     
  17. Frank_Sebastian

    Frank_Sebastian Well-Known Member

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    In regard to the bottom edge of the FS18 being concave, what drawing are you looking at?

    Knowing Jacques, that sounds unusual.

    Best regards,
    Frank_S
     
  18. firecat1981

    firecat1981 BBA Counselor

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    I'm thinking he meant concave in relation to the bottom, like the upward curve or rocker in the front of most boats? :-/

    I dig your design meathod cause obviously it's the same one I use, and most of us do some sort of major adjusting in every build. I would encourage you to get most of the planning done before the build as it will save you a ton of time and money. My current build is moving along much faster then the last one even though I only have 1/8th the amount of time to work on it. Planning and figures on paper are easier to go back and reference when you need them too.
     
  19. marshman

    marshman just try to follow me....i dare ya...

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    page 4...side panel..theres a 1" to 1 1/4" to 1" concave....
     
  20. firecat1981

    firecat1981 BBA Counselor

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    Ok I got what you are saying. Are you asking how do you determine the full scale curvature of the side panels so they don't distort the bottom panles and follow the flair correctly?
     
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