Axe Bow concept

Discussion in 'Boat Design' started by jmercer, Oct 2, 2009.

  1. LyndonJ
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    LyndonJ Senior Member

    Is there any validity at all in putting such a bow on such a small boat?

    The down side would be lack of waetrplane centre shift in wave encounter, could be quite dangerous.

    Note that the ships need a lot of freeboard at the bow because of this, ie they don't pitch but they do penetrate.

    A lot of good ideas that work for ships don't scale to small vessels.
     
  2. apex1

    apex1 Guest

    Fully concur.

    And asking for details of this concept is senseless, Damen holds a patent on it. There are no drawings or data available on the net.

    Here a recent launch from Amels Holland (part of the Damen group).

    [​IMG]

    Regards
    Richard
     
  3. u4ea32
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    u4ea32 Senior Member

    One of the things not mentioned in the paper, but obvious in the photos is that the axe bow has a similar effect as a bulbous bow, but is more effective: Both bulbous and axe bows increase volume forward, increasing prismatic coefficient and therefore reducing the bow wave.

    But the real advantage of the axe bow is that the entry remains sharp -- and therefore fairly immune to wave pitching, and generating a smaller wave in the first place, but then the decreasing depth of the bow aft of the stem causes a pressure reduction right where most boats see a pressure increase.

    A bow wave goes up because of the high pressure caused by the bow forcing its way through the water. By causing the keel line to rise aft of the bow, a low pressure area is introduced. This cancels out the high pressure, resulting in almost no bow wave.

    Look at the picture above, and you'll see this to be the case: the vessel is going much faster than hull speed, but there is almost no wave train, because there is no bow wave!

    This is probably also why the axe bow does NOT bow steer in waves. Bow steering is due to the bow wave: lots of pressure on the bow. Steering is no problem if the bow wave is the same on both sides of the hull, but steering becomes a real big problem if the wave train results in a temporarily larger bow wave on one side -- huge side force far forward, and the little rudders aft can't compete, and a broach happens.

    The on-board observations of axe bow vessels is that they exhibit tremendous longitudinal stability even when surfing down large seas. This is to be expected because there is no bow wave, so no opportunity for unbalanced forces.
     
    Last edited: Apr 30, 2010
  4. rambat
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    rambat Member at large

    Axe bow

    Hmmmm, I have heard that in service Axe Bow boats do experience Bow Steer in rough water. So much that owners are looking for solutions. But that should not kill the general concept due to its good efficiency and less slamming loads. The first large Catamaran Ferry's were about to be scrapped due to ridership seasickness until ride control systems (RCS) were commercialized and made affordable. Without RCS there would no large Cat Ferry's, certain technologies go hand-in-hand.
     
  5. Boston

    Boston Previous Member

    well given that the article did not suggest that this concept wouldn't scale I felt if reasonable to ask the question. nor does it mention that there is a patent as this concept has been around for quite a while

    the vessel Im working on is roughly half the size of the vessel they considered in there studies so yes there may be issues with scaling which I had not considered, thus the question.

    all in all regardless of positive or negative response I have found it particularly interesting to challenge myself with this effort of designing my own craft.

    I understand the issues of shifting the center of lift in wave encounter and that that is the reason for the raised bow on this type of vessel and also that a vessel of less length than the fluid wave length is in danger of being drove under however with the addition of a bit of flair it seems that a limit on plunging might be introduced that reduces the issue to a manageable level

    seemed like a good question for this particular conversation

    anyway thanks for the info
    B
     
  6. Boston

    Boston Previous Member

    agreed
    cats and particularly trimarans can have horrible motion and can make there passengers seriously sick

    I dont personally have sea sickness issues but I have a lot of sympathy for those who do
     
  7. apex1

    apex1 Guest

    I think thats heresay.

    Ax bows do┬┤nt bowsteer.
    And the cats you mention are not built with a Ax Bow! But many have a terrible behaviour, thats right.

    Boston

    This Ax Bow concept is relatively new and it is patent protected.
    The lookalikes of the early 1900 ed years are not the same design.

    this picture shows the bow pretty good:

    [​IMG]

    Regards
    Richard
     
  8. Boston

    Boston Previous Member

    they may not have had the reverse curve of the keel line but they had a lot of the other features

    although a fair argument can be made for any elongated shape

    interesting stuff though
     
  9. LyndonJ
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    LyndonJ Senior Member

    There's two linked events with wave encounter, one is the shift of waterplane centroid the other is the shift of bouyancy.
    A vessel trims around the waterplane centroid but the restoring force is from the COB, there was a thread on this that was incorrectly answered initially.

    I'd really recommend running the model in seakeeper for an indication of the response amplitudes. As a vessel gets smaller, there's a point where semi sheltered water waves are representitive of heavy weather for a larger sister design shape offshore.

    Just a thought, but I know nothing of your application of course and you probably considered all this anyway.
     
  10. Tad
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    Tad Boat Designer

  11. Morro
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    Morro Junior Member


    If Damen holds a patent on this concept the details WILL be available. That is how a patent works.
     
    Last edited: May 1, 2010
  12. apex1

    apex1 Guest

    Right!..........
     
  13. Morro
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    Morro Junior Member

     
  14. apex1

    apex1 Guest

     

  15. daiquiri
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    daiquiri Engineering and Design

    If the axe bow's purpose is to minimize half-angle of entrance and the wave drag, is there some particular reason for not having a generous flare above the waterline? It does look like a case where a flare would be beneficial in order to prevent bow plunging, plus it could (just guessing) diminish the structural weight when compared to such high hull walls at bow.
    Since it is apparently not being done in practice I must be missing something here...?
     
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