Bilge Diagonal

Discussion in 'Boat Design' started by PAUL XAVIER, Apr 10, 2014.

  1. PAUL XAVIER
    Joined: Apr 2014
    Posts: 14
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: chennai

    PAUL XAVIER Junior Member

    In every drawing, in the half breadth view there is "bilge diagonal". Why it is actually drawn, what is the purpose of that?
    TY :)
     
  2. gonzo
    Joined: Aug 2002
    Posts: 15,408
    Likes: 999, Points: 123, Legacy Rep: 2031
    Location: Milwaukee, WI

    gonzo Senior Member

    It is to check for fairness. Diagonals are used in other sections too. For example, in the flare of the bow.
     
  3. daiquiri
    Joined: May 2004
    Posts: 5,373
    Likes: 252, Points: 93, Legacy Rep: 3380
    Location: Italy (Garda Lake) and Croatia (Istria)

    daiquiri Engineering and Design

    Bilge diagonals are actually not present in every drawing. In the past, when boats were made of wooden planks, they were a pretty common feature. Today they are much less so.

    Bilge diagonals have a double purpose:
    1. They approximate the direction of planking, hence making it easier for the lofter to evaluate the required length and shape of planks.
    2. They approximate the shape of streamlines of the water which flows in close proximity of the hull at low speeds, thus allowing a trained eye to make a quick qualitative evaluation of it's hydrodynamic worthiness.
    Cheers
     
  4. jehardiman
    Joined: Aug 2004
    Posts: 3,028
    Likes: 538, Points: 113, Legacy Rep: 2040
    Location: Port Orchard, Washington, USA

    jehardiman Senior Member

    As said it, a diagonal is for fairness, and is usually the primary line used to fair the run. Think of it this way...the waterlines and buttocks intersect the shell at fairly acute angles at the turn of the bilge, so a little error can greatly change the slope of the shell. Diagonals intersect the shell almost perpendicular, so they are much better at controlling the sweep of the run when lofting from a table of offsets.

    However, as daiquiri points out, modern digitial lines which go straight to CNC cutting of molds and frames have reduced the need for a plotted diagonal. If lofting by hand on a mould floor then diagonals really help get a fair hull.

    See the lines plan in this thread for an example of how diagonals better control the shape than the buttocks or waterlines.
     
  5. navalex
    Joined: Feb 2010
    Posts: 11
    Likes: 2, Points: 3, Legacy Rep: 35
    Location: Madrid, Spain

    navalex Junior Member

    In the case of sailing vessels you can use it to see the angle of the run in different angles of heel, as well as to see how flat or rounded is the run for those angles of heel.
    Obviously to achieve this effect you have to draw these lines perpendicular to the hull in the section view, as jehardiman says.
     
    Last edited: Apr 10, 2014
  6. daiquiri
    Joined: May 2004
    Posts: 5,373
    Likes: 252, Points: 93, Legacy Rep: 3380
    Location: Italy (Garda Lake) and Croatia (Istria)

    daiquiri Engineering and Design

    Sorry for my linguistic ignorance, but - what do the expressions "to fair the run" and "angle of the run" mean?
     
  7. navalex
    Joined: Feb 2010
    Posts: 11
    Likes: 2, Points: 3, Legacy Rep: 35
    Location: Madrid, Spain

    navalex Junior Member

    The run is the aft part of the hull, more or less 1/3 of the length, so the angle of the run is the angle of the aft part of the hull between the hull and the waterplane.
     
  8. daiquiri
    Joined: May 2004
    Posts: 5,373
    Likes: 252, Points: 93, Legacy Rep: 3380
    Location: Italy (Garda Lake) and Croatia (Istria)

    daiquiri Engineering and Design

    Ok, thanks. So is the expression "to fair the run" intended in the same sense as the point n.2 in my first post?
     

  9. jehardiman
    Joined: Aug 2004
    Posts: 3,028
    Likes: 538, Points: 113, Legacy Rep: 2040
    Location: Port Orchard, Washington, USA

    jehardiman Senior Member

    Yes, If you look at the lines I posted in the link, the waterlines and buttocks have flats, hollows, and kinks while the diagonals are long sweeping curves. The diagonals are much more suited to showing the flow on vessels that sail "on their ear" with significant heel.
     
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.