Sea Seesaw?

Discussion in 'Boat Design' started by Saqa, Dec 20, 2014.

  1. Saqa
    Joined: Oct 2013
    Posts: 489
    Likes: 9, Points: 18, Legacy Rep: 75
    Location: Hervey Bay

    Saqa Senior Member

    Hi fellas, I need some help with understanding a concept. I have tried searching online but either no one has been discussing this or I haven't been able to correctly phrase a query

    Take two 6m narrow hulls of identical outside dimension and identical total dry weight

    Hull 1 is 400kg + 80kg outboard on the back + 100kg of gear and fittings in the middle.. total dry weight = 580kg

    Hull 2 is 200kg + 80kg outboard on the back + 300kg of gear and fittings in the middle = 580kg​

    1 - Would I be correct that with 200kg of crew in the middle the draft or waterline be the same on both vessels?

    2 - What I want to learn about is if that 200kg of crew moved to the bow extremity, would one bow of one vessel dip more then the other if its crew did the same?

    Would really appreciate your help with some laymans terms types explains, thanks
     
  2. TANSL
    Joined: Sep 2011
    Posts: 6,196
    Likes: 291, Points: 93, Legacy Rep: 300
    Location: Spain

    TANSL Senior Member

    The two vessels have a different trim and therefore, in general, hydrostatic values for every draft will vary from one ship to the other. Imo that means that, in general, the draft at Lbp / 2 for 580 kg will not be the same.
     
  3. gonzo
    Joined: Aug 2002
    Posts: 14,507
    Likes: 659, Points: 123, Legacy Rep: 2031
    Location: Milwaukee, WI

    gonzo Senior Member

    1) You would only be correct if the center of gravity of the boat and the load were always the same.
    2) You can use calculus to figure that. However, more information is needed.
     
  4. Saqa
    Joined: Oct 2013
    Posts: 489
    Likes: 9, Points: 18, Legacy Rep: 75
    Location: Hervey Bay

    Saqa Senior Member

    What does this mean? LBP/2?
     
  5. Saqa
    Joined: Oct 2013
    Posts: 489
    Likes: 9, Points: 18, Legacy Rep: 75
    Location: Hervey Bay

    Saqa Senior Member

    Actually I cant use calculus, have no idea how to. What information can I add?
     
  6. gonzo
    Joined: Aug 2002
    Posts: 14,507
    Likes: 659, Points: 123, Legacy Rep: 2031
    Location: Milwaukee, WI

    gonzo Senior Member

    Basically, you would need to include the lines plan and weight distribution. For example, a hull with a lot of flare will have less trim change than one with vertical sides. What are you needing to accomplish?
     
  7. TANSL
    Joined: Sep 2011
    Posts: 6,196
    Likes: 291, Points: 93, Legacy Rep: 300
    Location: Spain

    TANSL Senior Member

    Saqa, Lbp means Length between perpendiculars.
    I see that Gonzo has taken the issue so I'll leave you in his hands. Surely he clearly explain you the concepts needed so you can make the calculation that he believes to be done. He will tell you what those calculations are, how to accomplish them and theoretical basis of the whole process.
    It is not necessary that there are two technical explaining the same.
    Good luck.
     
  8. Saqa
    Joined: Oct 2013
    Posts: 489
    Likes: 9, Points: 18, Legacy Rep: 75
    Location: Hervey Bay

    Saqa Senior Member

    It started with a discussion I had with someone about my light stich and glue 15' build vs his heavy ply on solid timber frame 15'er. His boat is almost twice as heavy as mine. Hulls have similar shape and vee. He reckons that his be easier to move around in, that mine would pitch and heel more when moving around the boat

    I am interested in making a pontoon cat about 6m with a pair of slender hulls about 50cm hull beam and 3m separation. Slight cosmetic bow rake (not reversed). My two options are stitch and glue ply or solid 8lb PU foam core. Building strong for bad weather mooring. The solid foam core will be around 3 times heavier then the ply pontoon

    If something like a Woods Scoota 20 hulls were foam filled vs regualr empty hulls
     
  9. gonzo
    Joined: Aug 2002
    Posts: 14,507
    Likes: 659, Points: 123, Legacy Rep: 2031
    Location: Milwaukee, WI

    gonzo Senior Member

    Do you have any photos?
     

  10. Saqa
    Joined: Oct 2013
    Posts: 489
    Likes: 9, Points: 18, Legacy Rep: 75
    Location: Hervey Bay

    Saqa Senior Member

Loading...
Similar Threads
  1. Velsia
    Replies:
    45
    Views:
    1,771
  2. rael dobkins
    Replies:
    3
    Views:
    405
  3. Squidly-Diddly
    Replies:
    1
    Views:
    379
  4. mustafaumu sarac
    Replies:
    23
    Views:
    845
  5. MJT
    Replies:
    16
    Views:
    959
  6. boatdesingloverer
    Replies:
    13
    Views:
    484
  7. thenavalarch
    Replies:
    0
    Views:
    313
  8. rael dobkins
    Replies:
    1
    Views:
    438
  9. Foxwedge
    Replies:
    16
    Views:
    2,217
  10. Mattis Maurin
    Replies:
    5
    Views:
    914
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.