Cat with 'drop hull' (like drop axle on semi) for extra weight handling?

Discussion in 'Boat Design' started by black_sails, May 10, 2016.

  1. black_sails
    Joined: May 2016
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    black_sails Junior Member

    Has anyone ever seen, or be capable of commenting on an idea I have swirling in my head... about a catamaran (2 hull) that might at times become a 4 hull or potentially even a 6 hull - where you have additional buoyancy that can be used on demand to handle more weight. Think just some simple manual jack that you can lower additional displacement for additional buoyancy.

    I assume this is less hydrodynamically efficient than just having one hull designed for the weight, but it's about exploring tradeoffs between payload, efficiency, and speed. This is one (of several) ideas I had in my head trying to figure out a way of doing it. The extra hulls could even be completely demounted when there's no expectation of needing the extra weight capacity so that there isn't much if any weight penalty.
  2. messabout
    Joined: Jan 2006
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    Location: Lakeland Fl USA

    messabout Senior Member

    The idea is of questionable practicality. Please Abandon that brainstorm. Making a boat into a raft is going in the wrong direction for most purposes.
  3. Mr Efficiency
    Joined: Oct 2010
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    Mr Efficiency Senior Member

    Maybe use demihulls that have more taper to the sides, that way extra buoyancy develops rapidly with immersion, though you will pay for extra weight no matter what you do. Either with greater wetted area or wave making effects, or both. But if you are looking to preserve tunnel clearance, might be worth it.
  4. black_sails
    Joined: May 2016
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    black_sails Junior Member

    I am looking for a way to travel a long distance efficiently (like with long thin tapered cat hulls) and be able to travel at nice speed when having fun, but then be able to move moderate cargo weights between some islands not too terribly far apart a number of times, which can then return to efficient movement when not burdened.

    Normally catamarans just sink deeper so the limits on weight is about how deep your draft can be in part. So I thought why not have it encounter a second pair of wide cats once it sinks in say 20 inches? And then I just realized why not just put those cats on jacks and lower them so you don't have to sink at all before they help buoyancy without losing 20 inches of draft first... I don't see how this is making it a raft.

    Option two is making the cats completely demountable and just replacing them with wider hulls if I expect to need the weight capacity. I fail to see how this is better though as it means i'm making unnecessary drag during the longer unladen part of the voyage to/from an area.
  5. Squidly-Diddly
    Joined: Sep 2007
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    Location: SF bay

    Squidly-Diddly Senior Member

    See my gallery for my modular Cat concept.

    Start with two 39' x 4' containerable and trailerable hulls but be able to add an 8ft wide section and make it a tri.

    Mix and match.
  6. gonzo
    Joined: Aug 2002
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    gonzo Senior Member

    The only way to travel long distances efficiently at a good speed is to leave all the heavy crap onshore. If you need to take the living room and dining room along, trawlers are the way to go.
  7. Deering
    Joined: Feb 2005
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    Location: Juneau, Alaska

    Deering Senior Member

    Make each hull with a substantial 'step' in the beam a few inches above the waterline. In lightly loaded 'fun' mode the hull beam is X. But in 'cargo' mode the draft increases a few inches and encounters a significantly wider hull with more volume. You might take on several more inches of draft, but this gives you a simple solution with no moving parts and nothing to modify between modes.

  8. black_sails
    Joined: May 2016
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    Location: Minnesota

    black_sails Junior Member

    That's one of several options I was considering yes, I just really prefer shallow draft for it's beachability.

    And to the previous it's not about bringing the living room and dining room with it's about bringing cargo. Like bringing thousands of pounds of construction equipment from one island where it's cheaper or i've obtained it below normal market rates, and bringing it to the place where I want to build my beachside retreat. Or even something as large as a 20 ft sealand container storage unit which tend to travel one way (importing everything from china) and aren't worth hauling back... I have this idea of being able to bring one to set up somewhere else (empty) for storage use, but not where it originally ended up cuz it doesn't always work out that way. Hence trying to figure out how to handle say another 6000lbs of added cargo for that scenario which is just moving stuff 'locally' not around the planet with me.

    I've no clue whether what i'm proposing is feasiable or even worth doing - i'm just exploring many ideas of usability and being able to repurpose things.
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