SWATH vs Catamaran question

Discussion in 'Boat Design' started by Red Dwarf, Feb 13, 2013.

  1. Red Dwarf
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    Red Dwarf Senior Member

    I've been using the search function and reading about SWATH designs. Lots of good info but they seem like way too much trouble for an amateur. As far as I can tell the only thing they do well is seakeeping. In all other respects the SWATH seems to be difficult to design correctly, expensive, complicated and not much performance.

    I also have been looking at catamaran designs, they are well known and documented so I won't go into details.

    While browsing the web I came across the Stability 60 catamaran/SWATH vehicle. They claim to have the best of both worlds by using catamaran hulls on struts that can, when needed, be ballasted down to function as a SWATH. This seems way too complex for my taste.

    But while looking at the Stability 60 site I came across this picture which I think shows the boat in SWATH mode. Hard to tell but considering the amount of strut showing the hulls can only be inches below the surface. Note how the wave is passing under the boat.

    I also have attached a picture of the Stability 60 hull in catamaran mode. The design uses hulls that only stick above the water about a foot in catamaran mode.

    So my question is would you get some of the improved seakeeping of a SWATH, without making a SWATH, if you make a catamaran that has hulls that are very short vertically and mounted on tall struts as shown in the bottom diagram? The idea is waves under the bridgedeck height just pass over the hulls making it more like a SWATH for a short period of time as the wave passes. This would avoid the SWATH problems with excess surface area and sensitivity to weight. Basically, at rest and in smooth water it is a normal catamaran, only when a wave is large enough to engulf the pontoon will it act similar to a SWATH.

    I definitely see this concept using active control planes, just as a SWATH does, to control motion.
     

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  2. tomas
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    tomas Senior Member

    As our resident SWATH expert member AdHoc can tell you, the seakeeping benefit of the SWATH concept can only be realized by implementing the first three letters, Small Water-plane Area, with submerged hulls. The area that breaks the water-plane should be very small.

    There is no escaping the downside of water resistance for the hulls.
     
  3. Ad Hoc
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    Ad Hoc Naval Architect

    tomas has given you a very clear and succinct answer....it's the SWA part.

    But even ignoring that aspect....the structural aspects of swaths is another. Just taking that to one side (ignoring the seakeeping)...looking at the figure on the bottom, thin struts, wafer thin struts....seriously...you think that is LESS complex than a swath, the figure above. You wont find any material able to take the transverse load that thin. Unless of course you use HTS and the whole boats weighs way beyond what you expected and needs to be short otherwise the weight growth means she'll probably sink.

    It is horses for courses.....you want a fast boat...great...what else. Its the "what else" that determines the selection of hull form...and it is not a one liner either. No such thing as a free lunch.
     
  4. Red Dwarf
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    Red Dwarf Senior Member

    Yes I agree, the key is a small water plane area. In another thread Ad Hoc posted the following;

    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.



    In the case I am asking about the hulls have very little freeboard, let's say 6 inches. So a wave 2 foot tall will crest over the hull. Essentially placing the hull under the water. The crest of a wave large enough to submerge the hull will only see the water plane area of the struts. Obviously this is all very dynamic with the hull being exposed and submerged regularly depending on sea conditions.

    I am looking for an explanation of what is going on and if a better ride will result from the hull having very little freeboard. In the diagram below that is dimension "f". I know there will be some packaging issues but for now I am just interested in seakeeping.
     

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  5. Red Dwarf
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    Red Dwarf Senior Member

    Ad Hoc, we were posting at the same time. Thanks for your response. I am not interested in speed. I am interested in ride comfort. I have seen plenty of videos of catamarans that ride very poorly, the hobby horse thing or doing the rolling coupled "vomit comet" twist.

    If the freeboard doesn't matter at all could I get an improved ride just with active planes on a normal catamaran?

    Ad Hoc, one more question, What are the units in the expression, Ratio = WPA/(displacement)^2/3?
     
  6. Ad Hoc
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    Ad Hoc Naval Architect

    You’re missing the whole point. Here, this boat has minimal freeboard. Why not use this?

    BigShip.jpg

    As noted by tomas above..its the SWA part. When the boat is at rest in clam water…where is the waterline on the hull, what shape is it and what size is it. It is as simple as that.


    WPA = m^2
    Displacement = tonnes
     
  7. Red Dwarf
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    Red Dwarf Senior Member

    Thank you for clearing that up. Sometimes I have to ask dumb questions to see the obvious. That's what I love about this forum, plenty of experience to help us rookies.
     
  8. DCockey
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    DCockey Senior Member

    So the numerical value will be close to the non-dimensional ratio of WPA/(displacement volume)^2/3, with the exact relationship dependent on the density of water used for the ratio of displacement mass to displacement volume.
     
  9. Ad Hoc
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    Ad Hoc Naval Architect

    Ostensibly yes....but for "simplicity", the density is neglected, thus being non-dim ratio....as the density doesn't really affect the values, since its a mere guide, not an absolute.

    The supertanker above, has a very low value, not too surprisingly since it is on the extreme ends; has a very deep draft. It has a value circa 4.2. Whereas a similar sized cruise liner, like the QM2, is 5.9.

    In reference to the original Q, typical catamaran fast ferries like this:
    http://en.wikipedia.org/wiki/File:HK_Macau_Ferry_Piers_Sheung_Wan.JPG

    It has a ratio of 5.8...thus no real difference.
     
    Last edited: Feb 13, 2013
  10. tomas
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    tomas Senior Member

    I'm also interested in good seakeeping but alas, this part of our SOR means a compromise elsewhere, always.

    I'll probably end up going the catamaran route, with the longest hulls I can afford, which would at least lower the periodic frequency of hobby-horsing to something I could tolerate.


    Expert forum member BMcF, will remind that you can achieve good seakeeping via active stabilizing options. They can work with all types of vessels, large and small.

    Here's one example which you could purchase and install using gyro stabilizers:
    http://www.youtube.com/watch?feature=player_detailpage&v=-CpSL35Hxgs#t=22s


    If you're ambitious, you could attempt a variable-draft, dual-mode, catamaran/SWATH approach like the Susitna.
     

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  11. tspeer
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    tspeer Senior Member

    Isn't this the basic idea behind wave-piercing catamarans?
    [​IMG]
     
  12. Ad Hoc
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    Ad Hoc Naval Architect

  13. Ad Hoc
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    Ad Hoc Naval Architect

  14. tomas
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    tomas Senior Member

    Yes, I was just reading a different news article about it.
    (It's only free to governmental agencies for the time being.)

    Shame.
    It was about half the size and capacity that the local government wanted for use as a ferry, and it would take years to build the ferry ports so, it's being jettisoned due to the maintenance cost.
     

  15. BMcF
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    BMcF Senior Member

    What you've, inadvertently perhaps, touched on here is that there is actually a virtual continuum of designs bridging the gulf between the most radical of SWATH designs and the stiffest of catamarans. There are actually quite a few vessels that fall in the range of what many loosely refer to as "semi-SWATHS". Examples include some of the Austal cats, Stenna's HSS range, NG-BMT's "modcat" and the USN "sea Fighter, various designs that Ad Hoc has developed..and so forth.

    When it comes to optimizing the seakeeping performance using active control solutions, "the more SWATH-like, the better" is the basic rule of thumb. Remarkable results can be achieved with fairly small active appendages when the platform has very little initial static stability and low natural damping.

    Conversely, a big stiff catamarn (siad description including even some of the so-called "wavepiercing" types) can require a lot of control muscle to achieve even marginally acceptable results.

    Of course there is the important corollary...the more SWATH-like the vessel for a given length/displacement, generally the higher the resistance.
     
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