Ramform vs SWATH for seakeeping?

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

  1. Ad Hoc
    Joined: Oct 2008
    Posts: 6,457
    Likes: 532, Points: 113, Legacy Rep: 2488
    Location: Japan

    Ad Hoc Naval Architect

    No..we are not.

    What does the reserve buoyancy have to do with natural period of motion? Reserve implies above the water line…ergo…absolutely nothing to do with whatever it is you are trying to discuss??

    So, I’ll ask the question again…what are the factors that influence the natural period of heave?
     
  2. TANSL
    Joined: Sep 2011
    Posts: 6,061
    Likes: 257, Points: 93, Legacy Rep: 300
    Location: Spain

    TANSL Senior Member

    As it seems you do not know it and you are interested in knowing it, I enclose a formula that many designers like to use. It's not the only one, but I think it will suit you.
    Rolling Period.jpg
     

    Attached Files:

    Last edited: Jun 24, 2017
  3. alan craig
    Joined: Jul 2012
    Posts: 250
    Likes: 38, Points: 38, Legacy Rep: 14
    Location: s.e. england

    alan craig Senior Member

    Ad Hoc, thanks for explanation - so a SWATH is much more tender than an equivalent size conventional cat., and would have to be much wider to have the same stiffness or stability. I am almost disappointed as I thought they might have active or dynamic means to keep them right way up.
     
  4. Ad Hoc
    Joined: Oct 2008
    Posts: 6,457
    Likes: 532, Points: 113, Legacy Rep: 2488
    Location: Japan

    Ad Hoc Naval Architect

    Correct.
     
  5. alan craig
    Joined: Jul 2012
    Posts: 250
    Likes: 38, Points: 38, Legacy Rep: 14
    Location: s.e. england

    alan craig Senior Member

    Youtube has been spying on me and offered this video showing SWATH development in San Diego in the 1970's. I am no longer disappointed as it shows models with active control surfaces and mentions automatic control as a possibility (12 mins).

     
    crasch likes this.
  6. TANSL
    Joined: Sep 2011
    Posts: 6,061
    Likes: 257, Points: 93, Legacy Rep: 300
    Location: Spain

    TANSL Senior Member

    A very interesting and instructive video. Thank you.
    It is necessary to look for a little more in Google because, although here it is spoken of automatic control I have not been able to know if it is automatic control of the fins or the variable liquid ballast.
    It is a great idea that, apparently, has almost 50 years. Let's look a little more to find out why it has been little used in real boats. It may be very useful for a particular type of vessel but not for any boat. And there are always the problems of price and ease of handling.
     
  7. Rurudyne
    Joined: Mar 2014
    Posts: 1,152
    Likes: 34, Points: 48, Legacy Rep: 155
    Location: North Texas

    Rurudyne Senior Member

    Well, there's always a pendulum, a bag of water hanging just below the bridge deck suspended from a point above it through a dry moon pool. With sailboats and very tall buildings they hoist weights that can move independently up high to counter movements, but with a boat which already has a high center of gravity I'm betting that may not be the best option.

    Incidentally, by lowering said bag into the water (or emptying it) and raising it out again you could, in effect, have part of your ballast system.
     
  8. gonzo
    Joined: Aug 2002
    Posts: 14,281
    Likes: 589, Points: 123, Legacy Rep: 2031
    Location: Milwaukee, WI

    gonzo Senior Member

    I think that since the SOR indicates the boat will rarely move, investing on a SWATH or any other design that has seakeeping capabilities as the main attributes is a waste of time and money. It would be much cheaper, simple and provide larger accommodations to build a barge and install a good stabilizer system. Otherwise, if the water will be really shallow (under 30 feet) a lift boat, as suggested earlier is the cheapest and simplest solution.
     
    crasch likes this.
  9. Ad Hoc
    Joined: Oct 2008
    Posts: 6,457
    Likes: 532, Points: 113, Legacy Rep: 2488
    Location: Japan

    Ad Hoc Naval Architect

    Who says they have a high KG??....and what has the KG got to do with it?

    Nope, that's not what the ballast system is for on a swath.
     
  10. Rurudyne
    Joined: Mar 2014
    Posts: 1,152
    Likes: 34, Points: 48, Legacy Rep: 155
    Location: North Texas

    Rurudyne Senior Member

    Thus the "in effect" aspect.

    You know, some people may not be able to afford a complex and therefore expensive system of pumps and internal ballast tanks that are in-the-box thinking for SWATHs. The irony, that there should be conventionality associated with their execution, is that SWATHs themselves were out-of-the-box ideas.

    A weight, with roughly the density of water, will work after a fashion for what I suggested. If suspended low enough to not cause center of gravity issues it will only give marginal pendulum advantages on motion, though being hung from higher than the underside will help for that function as it will permit a different quality of motion (slower, a bit wider). As for ballast, yep, when the bag (whatever) is in the water or emptied the boat will float higher than when it's full and out. This is for a craft to move as needed to not run afoul of limitations on anchorages, not for a crew hauler, so it may be good enough.
     
    Last edited: Jun 25, 2017
  11. gonzo
    Joined: Aug 2002
    Posts: 14,281
    Likes: 589, Points: 123, Legacy Rep: 2031
    Location: Milwaukee, WI

    gonzo Senior Member

    SWATH is a very sophisticated type of design. Something that "looks like a SWATH" will not perform properly. It is not a good choice for a low budget. If "good enough" is the requirement, there are many other designs that will do better. For example, a barge.
     
  12. Rurudyne
    Joined: Mar 2014
    Posts: 1,152
    Likes: 34, Points: 48, Legacy Rep: 155
    Location: North Texas

    Rurudyne Senior Member

    As I said in an earlier post: a hear that SWATHs are picky about loading and houseboats are simply things that invite strange loading situations as people mill about and congregate (makes me think of the old Python Free Mason skit where so long as the tenants are of light build and reasonably sedentary they have a winner). I think the ramform would have a clear advantage there and be simpler to design with fewer things to go wrong.

    I was joking about a ramform whose roof looked like an Imperial Star Destroyer.
     
  13. gonzo
    Joined: Aug 2002
    Posts: 14,281
    Likes: 589, Points: 123, Legacy Rep: 2031
    Location: Milwaukee, WI

    gonzo Senior Member

  14. Rurudyne
    Joined: Mar 2014
    Posts: 1,152
    Likes: 34, Points: 48, Legacy Rep: 155
    Location: North Texas

    Rurudyne Senior Member

    Yep.

    Me, I like the idea of a stabilized monohull of mere 6-to-1 or 7-to-1 LBR (depending on final length) and meant for modest speed and simple construction, a Wyoming with undersized amas, with the accommodations where they'd normally be in a monohull.

    For a movable object, rather than a properly powered one, a donut shaped platform on thickish stilts above a submerged ring seems like it would have all the advantages of a SWATH sought after, not need complex ballasting (you could have weights on a track under the deck that you'd move around till floating level and then lock in place), and be simple to design and build. As a bonus the inside facing balcony would offer some privacy.

    Edit: heh, a "Death Star" habitat sphere above the submerged substructure. Geodesic framing is light weight and well sorted out. Huge soaring ceiling and lofts inside, a low ceiling storage/shop deck below. That would really tempt me. ;)
     
    Last edited: Jun 25, 2017

  15. gonzo
    Joined: Aug 2002
    Posts: 14,281
    Likes: 589, Points: 123, Legacy Rep: 2031
    Location: Milwaukee, WI

    gonzo Senior Member

    You can make it look like a medieval castle or anything else.
     
Loading...
Forum posts represent the experience, opinion, and view of individual users. Boat Design Net does not necessarily endorse nor share the view of each individual post.
When making potentially dangerous or financial decisions, always employ and consult appropriate professionals. Your circumstances or experience may be different.