Ramform vs SWATH for seakeeping?

Discussion in 'Stability' started by crasch, Jun 8, 2017.

  1. crasch
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    crasch Junior Member

    I have! I'm a fan of the Triloboats, for example:

    Triloboats: Avast, Ye Curvy Dogs! http://www.triloboats.com/modeltours.html

    My aim is for a boat that is anchored out most of the time, even during storms, yet is still comfortable. A scow would not be that comfortable in a storm I don't think. SWATH's have been reported to be capable of going 35 knots in sea state 5 (waves up to 12 feet):

    Sea Slice to go to work as wind farm support vessel http://www.marinelog.com/index.php?option=com_k2&view=item&id=10065:sea-slice-to-go-to-work-as-wind-farm-support-vessel&Itemid=231

    So a variable draft SWATH seems like the best boat to fit the bill that I've encountered so far.
     
  2. TANSL
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    TANSL Senior Member

    Yes, of course, but with the proper draft.
     
  3. crasch
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    crasch Junior Member

    Right, hence the descriptor "variable draft". The idea is that it would be in deep draft SWATH mode during storms, then switch to shallow draft catamaran mode during transport or in the shallows. Like the M/V Susitna or the Stability Yachts:

    Infamous MV Susitna Finally Put to Use in the Philippines – gCaptain http://gcaptain.com/infamous-mv-susitna-finally-put-to-use-with-philippine-red-cross/
    Stability 60 Sail - Stability Yachts http://www.stabilityyachts.com/60-dual-sail.html
     

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  4. TANSL
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    TANSL Senior Member

    Of course, I had not understood the variable draft. But now we are not talking about low draft.
    Is not very complicated and expensive the construction and operation of what, after all, is nothing more than a house boat?
     
  5. Rurudyne
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    Rurudyne Senior Member

    Drat, his the wrong button at first! So if that like/unlike shows up in your stats please be understanding.

    Not just draft but bridgedeck clearance too. A cat will ride up over a waves because of reserve buoyancy. With so little reserve buoyancy under the deck a SWATH has to ride tall in the saddle too or face nature's wrath.
     
  6. TANSL
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    TANSL Senior Member

    @Rurudyne, I totally agree, I´m talking about reduced draft but the important thing is both the draft and the air gap.
     
    Last edited: Jun 18, 2017
  7. crasch
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    crasch Junior Member

    Complicated and expensive? Maybe? I haven't built one yet.

    As I'm envisioning it, there would be ballast tank that runs along the interior of each of the catamaran hulls. Each tank would be fitted with a Kingston valve:

    Kingston valve - Wikipedia https://en.wikipedia.org/wiki/Kingston_valve

    To fill the ballast tank, you'd open the valve. To empty the tank, you use a ballast pump. Is that especially expensive?

    With respect to bridgedeck clearance, the maximum wave height in the San Francisco Bay is about 8 feet. So in SWATH mode the bridge deck would need to be about 8 feet above the surface of the water.
     
  8. TANSL
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    TANSL Senior Member

    Crasch, do not bother me by my words, I intend to help. I also have never built a SWATH, and less of that size and the features you want. I'm afraid no one in this forum has ever had that experience.
    However, I think saying the following is not crazy:
    - to empty the ballast tanks you will need, in addition to the valve, to inject air under pressure to compensate the pressure of the outside water. If you simply open the flush valve, it will expose that some tank will fill even more.
    - I am sure, despite my lack of experience, that building two hulls is more expensive and complicated than with a double breadth hull.
    - The structure of the wet deck is more expensive and complex than the traditional deck of a monocoque. Although I have not built any, I do have designed some (less size) and I have acted as construction manager in its construction.
    - duplicate the engines and duplicate each and every one of the typical services of the hull is more expensive, I am sure, than in a monohull.
    - you must have a liquid ballast distribution service and a special system to monitor the ballasting / de-ballasting of the boat, to avoid uncomfortable situations or momentary loss of stability.
    There are sure to be many other points to consider for a two-hulled vessel with variable draft. But I have always thought that what does not occur to one can occur to another and that, sometimes, from a nonsense can arises a good idea.
     
    Last edited: Jun 18, 2017
  9. alan craig
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    alan craig Senior Member

    That Susitna vessel is mad - 35 tons payload on a total displacement of over 900 tons; one of the compromises mentioned I think by Ad Hoc.

    As this thread is heading towards a SWATH solution, can anyone explain to me how a SWATH gets its transverse stability? If a wind load or laterally displaced C.G. pushes the fat bit of a hull below the waterline it is no longer a SWATH.
     
  10. Ad Hoc
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    Ad Hoc Naval Architect

    Simple

    Think of an I-beam...what occurs when you separate the riders bars(flanges) of the I-beam further apart..it becomes stiffer. Now think of the two hulls....it's the same...separate the 2 hulls further apart...the WPA inertia becomes greater.
     
  11. Rurudyne
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    Rurudyne Senior Member

    Same with any cat or tri, or really beamy monohull, but if I understand how these thing function correctly (has to be said up front, not a NA) comfort comes from the way a SWATH, lacking reserve buoyancy (part of lack of capacity issue), has a slower roll (where cats are often said to be quite stiff) even though the range of the motion up in the accommodations may be larger.

    I'm guessing that as you try to boost cargo capacity you make SWATHs stiffer and less comfortable.

    A houseboat isn't a cargo boat ... so this may work. But when I see houseboat shows on TV the one thing they always seem to bring up is how great these things are at entertaining. Even drawings of the old Float-A-Home show having folks out. People (who aren't me) may not be heavy but we move about, mill about and lean over the railing and given what I know about SWATHs I would say this is a distinct advantage for the Ramform design as basis for a houseboat if this were my project. Yes, a ramform may be relatively stiff, being beamy, but accommodations are a lot closer to the water so the size of motions should be much less.
     
  12. Ad Hoc
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    Ad Hoc Naval Architect

    Nope. Barking up the wrong tree.

    What is the definition of swath? S.W.A.T.H.

    The clue is in the SWA part...Small Waterplane Area.

    The WPA is a function of the natural periods of motion of any vessel. With a lower WPA for the same displacement, the natural periods of motion are changed.

    Cats are stiff because they have a very high WPA inertia. A swath by contrast is very low, it is "tender".
     
  13. Rurudyne
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    Rurudyne Senior Member

    I'm sorry if I wasn't clear.

    Yes, small water plane relates to what is sticking through the water at the surface, not what is providing most of the buoyancy. That is the toon, however it is shaped, which is ideally underwater and out of the interactions at the surface.

    Having a slower roll relative to motions, not having a snap roll, comes from the fact that as it heels there is, as I said, not much in reserve buoyancy relative to the displacement of the boat. This last was where I was probably not clear.

    A catamaran is stiff because, also being wide, it has lots of reserve buoyancy relative to its displacement immediately at hand. At the water plane these motions aren't large. Up above the bridge deck they are larger no different than with any taller boat.

    This would affect a ramform hull too, since they are beamy; however, the ramform can have its central accommodations lower down nearer the water plane than any cat can. Even lower than a SWATH might.

    You might also consider stabilized monohulls for comparison, where the amas only contribute a small part of displacement. With these the small amas in a roll they are forced deeper before their reserve buoyancy adequately resist than if they were a large tri ama, so the motion of a stabilized boat is likewise tender relative to a tri and so you can have your accommodations higher than in a tri and still be comfortable.
     
  14. Ad Hoc
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    Ad Hoc Naval Architect

    Yup...you said that. But...

    So...you note right from the outset, you're not a NA..great.
    But then you shoot yourself in the foot by then attempting to sound like a NA and as if you know what you're talking about.

    So, let me ask you this question...what are the factors that determine the natural period of heave?
     

  15. Rurudyne
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    Rurudyne Senior Member

    We are essentially saying the same thing, just in different ways.

    Yet it is NOT water plane area (being a displacement-less mathematical surface on the the water where the hull penetrates it's surface) that resist being submerged as an actual craft heels, widely distributed water plane or not. It is the displacement that is associated with that water plane area. Having limited reserve displacement simply means that heeling will have little to resist it, so it will be relatively tender compared to a craft with a similar distribution of WPA, similar displacement, but both larger WPA and larger reserve buoyancy (those inherently go hand in hand, freakish hulls aside ... I'm thinking about particular type designed by, and I'm hoping I'm getting the spelling right, Andale roughly a century ago and briefly written up in The Rudder).

    And I do have an engineering background. Even if I didn't we aren't talking rocket science.
     
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