Ramform vs SWATH for seakeeping?

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

  1. crasch
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    crasch Junior Member

    I'm planning to build an 'extreme houseboat' aka "bayboat" intended to be operated in the San Francisco Bay. The goal is to come up with a design that is as spacious and comfortable as a houseboat, but capable of anchoring out long-term in most regions of the bay under all weather conditions.

    Both SWATH and Ramform hulls are reportedly quite comfortable under heavy weather conditions. Has anyone done a comparison of their relative seakeeping ability relative to cost and cargo/space capacity? If you were to choose design for such a purpose which would you choose?

    Requirements:

    * Wave resistant - comfortable under heavy storm conditions, when wave height can reach 5' - 8'
    * Wind resistant - comfortable in heavy winds, which can reach 75 mph
    * Low draft (4' or less) - the average depth of the Bay is about 15'
    * Low initial cost - ideally, it would be a design that can be purchased incrementally
    * Easy re-anchoring - the SF Bay does not allow permanent anchoring, so it would need to be moved often.
    * Easy repair / maintenance / refueling in the field - should not need to go to shore except under unusual circumstances)
    * Easy crew/cargo transfer - since the boat is intended to be anchored out long-term, crew and supplies will be transferred by speed boat/rib

    The more the better:

    * spacious
    * high cargo carrying capacity
    * modular - it would be nice if each bayboat was capable of connecting to other bayboats
    * low fuel consumption - obviously, the lower the better, but it's mostly going to sit in one place.

    Thanks in advance for your thoughts!
     
  2. Rurudyne
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    Rurudyne Senior Member

    My two cents.

    You may have some issues getting a low draft with a SWATH unless you go to the trouble of a ballasting system. Likewise, to get well above the waves will push you towards being more affected by wind, not less.

    It seems to me that the Ramform, in a light or smaller boat, may also have issues with waves as they interact with abundant reserve buoyancy aft (that I'm betting become less of an issue as the boat grows relative to the waves). The pictures of the hull I remember suggest it should have no particular issue with either draft or wind.

    Have you considered the X-bow for dealing with waves underway? Possibly combined with a stabilized monohull (IIRC, where the amas about to maybe 10% of displacement) of less extreme length to beam ratio (merely 6:1 instead of much narrower that seem typical)?

    Oh heck, a ramform fore hull for stability with an X-bow nose and a narrower extended aft section on a submerged central toon. Just don't give it wings and claim it'll fly. ;)

    Edit: silliness aside, now that I thought about it some, a reduced water plane monotoon extension aft of a ramform, connected to the main hull underwater and sharing the keel, and adding maybe 10-15' to a 40' hull might add space for a flybridge, deck and a place to keep a tender without spoiling the benefits of the ramform hull.
     
    Last edited: Jun 8, 2017
  3. crasch
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    crasch Junior Member

    Thanks for the response! Yeah, I think for a SWATH design, variable ballast would be required to achieve the shallow draft requirement. Something like what the M/V Susitna does:

    E CRAFT http://www.gpai.com/e-craft/

    I hadn't thought about an X-Bow, thanks for the pointer!

    Combining the Ramform with a small water plane monotoon aft is interesting. I'm not sure I grasp what you have in mind though. How wide are you thinking of making the aft small water plane monotoon extension?
     
  4. Ad Hoc
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    Ad Hoc Naval Architect

    Your SOR, like all (to be fair) has conflicting requirements.

    If you want to use a hull form that has superior seakeeping abilities - such as a Swath - and a limited draft of around 1.20m (4') that limits you to a Swath of around 13-15m in LOA. A Swath of this size would have a payload of roughly 3-5tonne max; this must also include the ballasting, required to maintain level trim and draft. So this limits the "high cargo capacity" immediately. And that is just one of several conflicts.

    So, like everything in design. It is all about compromises. Thus, what are you willing to compromise in order to achieve your objective?
     
  5. Mr Efficiency
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    Mr Efficiency Senior Member

    This is an easy one, if the water depth is just 15 feet, have long adjustable stilts on each corner of your houseboat, and the ability to jack them up or down. Use a pad on the bottom of each stilt that has sufficient area to spread the load, if on a soft bottom. Just jack high enough to clear the waves. No worries !
     
  6. crasch
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    crasch Junior Member

    Yes, I think that might be a good solution! A jackup rig / catamaran combo like the little SSLV perhaps:

    OFFSHORE LIFT VESSELS http://emcsq.com/OFFSHORE%20LIFT%20VESSELS.htm
     
  7. Rurudyne
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    Rurudyne Senior Member

    I was thinking of an "ordinary" submerged pontoon where only the struts project above the water, kinda like an elongated bow bulb, just on the stern of the boat (may have to call the boat a "he"). So not very wide. The deck / bridge deck above wouldn't be full width either, though it might be wider fore and narrow to aft. From above the boat might look a bit like a skate or a ray.

    The trick might be the design of the toon where it joins to the hull. IIRC part of the ramform's design relates to where the props are placed, though this may have more to do with the assumption that thruster pods would be used for improved maneuvering rather than hydrodynamic efficiency.

    Rather than design it so it smoothly blends in it may be a good idea to intentionally create some turbulence at the juncture so that the bulk of the water flowing past will be moving past that turbulent boundary layer rather than the discontinuous form of the hull. In a higher speed boat this would be relatively easy like introducing one or two large steps like on a speed boat. Because the boat won't be moving fast, and also because the feature will wrap around, the size of the feature (whatever you use) will probably need to be smaller, keeping the scale of the turbulence inducing disruption proportional to the speed. So maybe a sequence of small protruding wedge ribs or one of these followed by some heat-sink like fins? It would be something you'd likely need to model to get right. For this you might use an appliqué of molded plastic, a removable skin that you can change out and experiment with without rebuilding the boat, and also replace when it gets fouled (which it will) by marine growth.
     
  8. crasch
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    crasch Junior Member

    Thanks! Why would you be limited to 13-15 meter in LOA? The M/V Susitna has a LOA of 59 meters and a shallow water draft of 4.5 feet. Of course, it has a variable draft system, so perhaps you're referring to SWATH's with a constant draft.

    High-Speed State-of-the-Art SWATH / Catamaran “Susitna” :: Towingline.com http://www.towingline.com/archives/1565
     
  9. crasch
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    crasch Junior Member

    Thanks for the explanation! It's clearer what you have in mind now. It would be an interesting vessel, that's for sure.
     
  10. Rurudyne
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    Rurudyne Senior Member

    Humorous aside: what do you think of making the roof of a ramform look like an imperial star destroyer, with a flying bridge perched up top in between the "shield generators"?
     
  11. Ad Hoc
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    Ad Hoc Naval Architect

    Well, look at your requirements:-

    So, you think this is possible on a LOA of 59m? o_O
     
  12. TANSL
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    TANSL Senior Member

    Why not?. Could we get some reason (just 1 or 2) to prevent it?. Thank you
     
  13. crasch
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    crasch Junior Member

    I didn't say a low initial cost was possible with a vessel of 59 meters. I was referring to this claim "If you want to use a hull form that has superior seakeeping abilities - such as a Swath - and a limited draft of around 1.20m (4') that limits you to a Swath of around 13-15m in LOA. "

    As I point out, SWATH vessels with low draft and good seakeeping ability and a LOA much longer than 13-15m already exist, so I was wondering how you arrived at your claim.
     
  14. Ad Hoc
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    Ad Hoc Naval Architect

    Apart from those 2 small but very important requirements you have listed your basic SOR too:

    So, how many house boats do you know that are 59m LOA?

    And how many 59m Swaths have a draft of just 1.2m?

    I would suggest you don't actually know what you want, or, unaware of the conflicting requirements you seek, in terms of naval architecture of Swaths and the actual costs of such vessels, make your objective unworkable, but, you either refuse to accept or understand this. As such it shall just remain in the fantasy world...like so many.
     
    Last edited: Jun 16, 2017

  15. TANSL
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    TANSL Senior Member

    Even if this is a dream, why should we despise the dreamer?
    Continue without losing the illusion, @crasch, many things start with a dream and we change them until they become reality.
    Some might be willing, without despising anyone, to explain your mistakes, if any, helping to transform a dream into a reality, or avoiding you going into a dead end. Good luck.
     
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