Bulb vs Long WL Length

Discussion in 'Boat Design' started by Mat-C, Aug 23, 2010.

  1. Mat-C
    Joined: May 2007
    Posts: 255
    Likes: 12, Points: 0, Legacy Rep: 141
    Location: Australia

    Mat-C Senior Member

    I'm sure there's no single or simple answer, but that won't stop me from asking the question....:D

    Within the confines of a given length - say 20 metres for arguments sake - which is likely to have the higher displacement speed (or the lower powering requirement) - a vessel with a WL length of 20 metres (ie a vertical bow), or one with a more raked stem, but with a bulb extending to that same 20 metre mark?
    All other things being equal, of course.....
  2. apex1

    apex1 Guest

    The no bulb...........
  3. Mat-C
    Joined: May 2007
    Posts: 255
    Likes: 12, Points: 0, Legacy Rep: 141
    Location: Australia

    Mat-C Senior Member

    Hmm... so, it was a simple question afterall !:p
    Longer WL Length rules, then....
    Thanks Apex
  4. apex1

    apex1 Guest

    And WL is there in all conditions.
    A bulb can provide more drag than designed for, in some conditions.
    Last edited by a moderator: Aug 24, 2010

  5. Eric Sponberg
    Joined: Dec 2001
    Posts: 2,010
    Likes: 216, Points: 73, Legacy Rep: 2917
    Location: On board Corroboree

    Eric Sponberg Senior Member

    Actually, you will also have a pronounced effect on hull speed by changing the beam. A narrower boat has less drag than a wider boat. So, for a given length, a narrower boat is better for performance. A narrower boat will not be better for living accommodation, however.

    With bulbs, you can get them just right at some speeds, but they might be just wrong at other speeds. And whether a stem is vertical, or inclined with a bulb, or even reversed, may make little difference in efficiency in all weather conditions--it depends on the rest of the hull shape and what the sizes of the waves are. What is the combination of factors that results in the best efficiency? You can't limit the choice only to stem shape and waterline length.

    So you are right--there is no simple answer.

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.