X-Bow Hull from Ulstein

Discussion in 'Hydrodynamics and Aerodynamics' started by Dr34m3r, Mar 2, 2020.

  1. Dr34m3r
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    Dr34m3r Senior Member



    I was watching the video above. Does anyone have paper or more opinion on this hull form ? How good are they hydro-dynamically ?
     
  2. KeithO
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    KeithO Senior Member

    There are many papers on the subject if you use google as well as videos on youtube about reviews of the subject and alternative options.

    The jist as I understand it is that the high bow concept really only works on large ships. For smaller ships visibility forward is an issue. Another way to get improved handling is to simply lengthen the boat while keeping the payload constant. This is generally considered one of the most effective options and is part of the concept with the Dashew FPB yachts.
     
  3. KeithO
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    KeithO Senior Member



    The naval architect review is above
     
  4. Alexanov
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    Alexanov Senior Member

    Some time ago I have spoken with company who have several X-Bow ships and classic bulbous bow ships. X-Bow works better in raw seas, but when weather more or less ok use more fuel, than classic bow.
     
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  5. Gunnar Sommerlund
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    Gunnar Sommerlund Marine Engineer

    These offshore supply vessels operate in shitty waters and are designed for to be versatile doing pretty much all jobs and should be able to be easy refitted for different tasks.
    Now, these vessels are not designed for cargo freighting where fuel efficiency due to wave drag is main cost factor of operation.
    Most of them are built to comply with dynamic positioning systems, hence all the thrusters. Older models used to run propellers at full speed and only adjust pitch for for and aft movement.
    Now imagine it doing ROV job or sitting 3 weeks next to an oil field.
    You need comfort and pitch reduction over fuel reduction at speed due to bulb reducing the wave making resistance.
     
  6. Ad Hoc
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    Ad Hoc Naval Architect

    As noted before, it is just marketing on an old existing theme.
     
  7. akashmenon
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    akashmenon Marine Engineer

    I think this is a good resource, a lot of the figures in it are as seen from a lecture I had at TU Delft. Our ship design lecturer developed the Axe Bow which Damen shipyards uses quite frequently for their designs. According to model tests both the Axe & X-bow designs are good in reducing pitch response. You do tend to have slightly more heading instability in beam seas (not quoting any numbers because I don't know how much). I could imagine that the Axe bow design is more prone to more green water on deck as opposed to the Ulstein designed X-bow (which looks more expensive to build). While pitch accelerations might be reduced with both designs, the bow of these vessels tend to 'dig deeper' into the water in pitch as they're finer at the bow with lower buoyancy. Really need good quantitative data to determine which bow design is fit for purpose- that's something we didn't have in our lecture slides (the data) due to commercial sensitivity. Nevertheless the video of the model tests seemed to show that the Axe bow is prone to more green water at higher speeds unlike the X-bow as the whole bow tended to 'dig' into the water- I think reduced buoyancy at the fore section means that with rough pitch motions, it will tend to 'axe' into the water more than you want it to. This really still depends on vessel length, wave encounter periods & wave heights, which again depends on vessel design & purpose.
     

  8. Ad Hoc
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    Ad Hoc Naval Architect

    All you need to know is... the performance in following and stern quartering seas.
    Then you'll know why the quotes of its impressive performance, are only in head seas, not a full 360degree polar plot. Around 60-80% of other headings it is inferior.
    It is why it is commonly labelled the "go home" boat.
     
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