Amazing difference

Discussion in 'Boat Design' started by dskira, Feb 3, 2013.

  1. dskira

    dskira Previous Member

  2. tom28571
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    tom28571 Senior Member

    Considering the size of the ships, its impressive. Some of the pitching of the larger ship may be a result of its longer length though.
     
  3. Red Dwarf
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    Red Dwarf Senior Member

    It is a huge difference, too bad they are not more similar for an apples to apples comparison.

    Does anyone know what a "semiswath" hull is and have some links to images?

    BMT from the pentamaran thread claims they have a " state-of-the-art Extreme Semi-SWATH (XSS) hullform". BMT claims improved ride without the efficiency penalty of a swath. Or it it all just marketing BS? I have googled but I can not find a picture of the hull they are referring to.

    http://www.bmtyachts.com/News/?/411/0/1050
     
  4. tomas
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    tomas Senior Member

    Perhaps this is a better example from BMT of their XSS (Extreme Semi-SWATH):
    http://www.bmt.org/news/2012/10/bmts-xss-design-makes-a-splash/

    and:
    http://renews.biz/xss-support-vessel-in-the-water/

    They claim that it "offers significant improvements in motion levels over conventional platforms".

    IMO, it is at least partially a marketing term to distinguish the benefits of the design from other approaches. It tries to split the difference between the high resistance of a true SWATH submerged hulls design and a faster "wave-piercing" shape. I've never been on one to know what it's like.
     
  5. tomas
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    tomas Senior Member

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  6. Red Dwarf
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    Red Dwarf Senior Member

    Hmmm, that BMT design is a big fraud. Of course it works better, it has big T-foils under the hull. They don't mention that, they just say they have a "state of the art" semi-swath design.

    My guess is any hull with foils under it would ride completely different than the same hull without the foils. If those foils are actively controlled it will get even better.
     
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  7. Red Dwarf
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    Red Dwarf Senior Member

    By "big fraud" I am referring to the implication in all their press that they have a clever version of a swath, XSS semiswath, that does not have the drag penalty of a swath. In their press releases I saw no mention of a hull with T-foils. I think you could take a Malcolm Tennant canoe stern and do the same thing.

    In my opinion having those T-foils down deep like that seems to be asking for trouble. They will get easily fouled or worse broken off by impact. Not the kind of attributes you want on a working boat.

    Anyway, I guess I was let down by the fact that they introduced nothing new in hull shape. I'm always looking to learn something new.
     
  8. Ad Hoc
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    Ad Hoc Naval Architect

    Daniel

    I’ll explain in a very simplistic and generalised manner. Not in a patronising sense but because it is rather long winded and complex and leads into more questions than answers really, far far too many to answer simply. Thus for clarify and brevity a simplified explanation.

    Take a box, say 50x20x3m….floating stationary in the water. Its waterplane area is simply 50x20m^2….the length x breadth.

    If a wave of 1.0m passes by, what happens?...the box shall rise and fall. But, this is where the brevity comes into play. The amount the box shall rise and fall will be exactly the same as the amplitude of the wave. In other words, a 1.0m wave = 1.0m vertical movement of the box. The heave response of a 1.0m wave is 1.0m.

    If you now take that same box, but reduce the width of the beam, from 20m, down to say 1mm. It obviously wont float. So, you need to add buoyancy, so lets make a tube, circular or otherwise, that is under the water, that has the same amount of volume required as the box to float.

    So with a wave of 1.0m passing by what occurs?...well, there is no waterplane area to speak of, it is ostensibly infinitely small. So having no WPA there is no heave. In other words, the wave passes by and the box –with buoyant tubes under – does not react, it remains level. So we can say the heave response of a 1.0m wave = 0.0m. It does heave (excatly the same as before), but the very very long period of heave feels like it is not moving at all.

    We know that everything has a natural frequency, and floating objects are no exception.

    So, back to the box. If a wave with the same period as the natural period of heave for the box passes by what happens. Well we all know what resonance is, and the same occurs here. The box shall be “excited” by the wave which is at the same period. So the heave response of a 1.0m wave, at the same period as the box’s natural period of heave, will no longer be 1.0m. It could be significantly higher. Typically it would be around 2-3 times greater. So 1.0m wave = 3.0m vertical movement.

    What about the box –with buoyant tubes under?

    And this is the clever part. Having such a low WPA, the natural period of heave is so long instead of a few seconds, it could be measured in hours, again, simply. So the wave just passes on by again and has no effect on the box and buoyant tube.

    If we take the same two again, but now have them moving at some speed, what occurs? The only change is the period of encounter. By that I mean, when stationary if a wave passes by every 10seconds, the encounter period is 10seconds. If the box is now moving, the time for each wave to pass becomes less and less with increasing speed.

    Why is this important? Because the faster the box goes, the period of encounter at some point, will match the natural period of heave. When this occurs the heave response, as noted above, shall be larger….. the box will heave up and down in a more violent manner.

    So, back to the videos.
    The whole pint of a SWATH..is in its acronym. Small Waterplane Area Twin Hull. It is the Small Waterplane Area, the ‘SWA’ part that makes it work. By having a sufficiently small waterplane area, the response of waves is minimised. You can only do this with a small waterplane area configuration.

    In general, semi-swaths are nothing of the sort. They are “swath” by name only, but NOT by waterplane area. The WPA of a semi-swath will be very close to that of a standard catamaran.

    The simple rule of thumb is the ratio of WPA to its displacement, thus:-

    Ratio = WPA/(displacement)^2/3

    All boats have a ratio in the 5-ish range, all boats from the QE2 down to a simple canoe. Some are a tad higher and some a tad lower, but they are all in the same ratio range, in a nutshell.

    A swath is in the range 0.8-1.5. This is a huge difference.

    A standard slender catamaran hull form, like on fast ferries is in the 4.5-5.0 range. A ‘semi-swath’, as always shown by endless different companies has this ratio no better, or not much better. It is not a semi-swath in the strict technical definition. The WPA is no different from that of a conventional catamaran. It is semi-…in name only, i.e. marketing
     
  9. hoytedow
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    hoytedow Wood Butcher

    Cool video.
     
  10. dskira

    dskira Previous Member

    Always great to read you.

    Thank you
     
  11. hoytedow
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    hoytedow Wood Butcher

    Likewise and you are welcome.
     
  12. DCockey
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    DCockey Senior Member

    Ad Hoc, does a SWATH vessel have greater added mass in heave than a conventional hull of the same displacment? If so then I assume that would further lengthen the SWATH's natural period in heave - correct?
     
  13. Ad Hoc
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    Ad Hoc Naval Architect

    Yes that's correct. You can increase the added mass, by simple changes in geometry. For exaple a D/B ratio of 1.8 increases by 120%.

    The ability to "tune" a swath, owing to changes in geometry, to decouple its motions is the fun part...but not so easy in practice.
     
    Last edited: Feb 5, 2013
  14. MikeJohns
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    MikeJohns Senior Member

    Yes it looks like the larger vessel is very close to it's natural pitch frequency and would have quickly altered speed or heading unless it's a setup.

    Swath tends to be designed for a certain wave spectra and we could find a sea and a relative heading that would make the swath wallow uncomfortably while the ship slid past in comfort.

    Swath response isn't so good on some headings, such as a qaurtering stern excitation, and wave orbitals can add a bit to the equation on some heading/spectra. As AdHoc alluded to the transfer functions ( RAO's) can be quite different with some small changes in swath geometry.

    There is another avenue too, significant improvements to those RAO's are possible with active stabilisers. Some swaths are tuned with the roots pretty close to crossing into the unstable side of the plane for some heading-wave spectra responses and active stabilisers with computer control can help a lot and are completely transparent to the pilot.
     

  15. Ad Hoc
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    Ad Hoc Naval Architect

    What you want to do with a swath's motions is to make the natural periods of pitch, heave and roll to be significantly far away from those in which it shall encounter.

    The downside to this is that it can create some structural and stability issues with the final configurations that one finds suitable. Thus the compromise begins :)
     
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