ultra shallowdraft to seagoing multihull

Discussion in 'Boat Design' started by upstrider, Jan 2, 2014.

  1. upstrider
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    upstrider Junior Member

    This is to recognize I may never get to build or use the design I'm about to propose. It is also entwined with the philosophy or touring by boat on the water and a summary of what I think are the best parts of cruising. I've been over a lot of shallow water (less than 12"). My trimaran drew 26 inches (centerboard up) and that is not shallow water for me, since rowing to Alaska in a 12' boat. So how does one get comfortably to sea from a configuration designed for trailering that might draw as little as 6" in inland waters? It has to be by "transforming" the flat bottom craft to one that spreads over waves in a multihull configuration. Probably not just 2 or 3 hulls but maybe 8, similar to the design of a raft like KonTiki that made the trip from polynesia to north america decades ago. If you're enthused about the prospect, come on along. Maybe a friend from the old days will read this and relate what we accomplished with so little back in Piver's time :)
     
  2. Mr Efficiency
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    Mr Efficiency Senior Member

    How will 8 hulls joined together negotiate shallow water better than a flat bottom monohull with a large waterplane area for its displacement ? Do you envisage a variable-geometry design where a whole bunch of hulls concertina together, or separate apart, as required ?
     
  3. upstrider
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    upstrider Junior Member

    i see you see

    that a flat bottom boat has the shallowest draft. All hulls combine to be a flat bottom boat in the shallows, and of course multiple hulls must spread out to not pound a flat bottom boat to pieces in a seaway. Do you see the principle in KonTiki (a raft) in which the waves are damped in the mass of logs to make an ocean crossing possible? Envision a waterstrider at the pond as a seagoing multihull vessel.
     
  4. messabout
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    messabout Senior Member

    A flat bottomed boat, when heeled is no longer a flat bottommed boat. A decently designed one does not pound when heeled at 10 degrees or so.

    A boat with many individual hulls like a log raft has a disproportionate amont of wetted surface and in the case of the likes of Kon Tiki, or log raft boats, it was pretty likely that it would, indeed, pound in certain sea states. Those primitive boats were the result of having to utilize the existing technology and materials, nothing more.

    Sea kindliness and very shallow draft is not an agreeable combination whether log or flat bottomed. Some sort of compromise is achievable by fiddling length/width ratios for flat bottoms. Consider the banks dories for example.

    Micronesian dugout proas were shoal draft, beach launched, and reasonably sea worthy but they were long skinny boats that were modestly loaded and apparently well handled by the occupants. See James Michener books and his quasi fictional accounts such as his book; West Wind To Hawaii.
     
  5. Mr Efficiency
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    Mr Efficiency Senior Member

    I don't really understand the need for a boat that goes offshore, but also has ultra-low draft for venturing into shallows. The only thing that might be adaptable is a catamaran where the side hulls can fold under by rotating 90 degrees so the outsides becomes the bottom, broad and flat, compared to when offshore they are deep and narrow. That would entail plenty of structural difficulty.
     
  6. philSweet
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    philSweet Senior Member

  7. upstrider
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    upstrider Junior Member

    messabout/mrefficiency/philsweet

    messabout: good info as aspect ratio of a pounding hull in a seaway is important.
    mrefficiency: I thought catching a few fish outside and going for a midnight moonlit fish fry in the backbay would be great.
    philsweet: neat boat

    So for starters we have a collection of hulls in a group on a trailer that is a flat bottom boat, say 8.5' beam and 36' loa. They will have to be spread out in a seaway and survive. Many mechanical options have been tried but mostly with the final objective being speed. With seakindliness being most important the further out the akas (arms) and the more flexible and resilient the amas (hulls) survival might be accomplished but what is the elegant solution? Wharram added some flexibility. Brown tried welded crossarms. Current composites can probably do it best but I favor welded aluminum. I've seen aluminum fishing tenders in the 30' range get squeezed between the 150' mother ship and pilings. It's doesn't sound or look pretty but they pop back into position 'sortof' every time. I mention this because the liveable pod (main hull-vaka) must be as strong and safe as possible should an ama be lost or damaged. With 8 of them loosing one or two shouldn't be a catastrophe. .
     
  8. Squidly-Diddly
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    Squidly-Diddly Senior Member

    I posted a futuristic 'concept' boat that transformed from mono to tri a while back.

    IIRC it was a fast motor to sailing tri conversion.
     
  9. upstrider
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    upstrider Junior Member

    squig didly

    How about a pointer to this concept boat?
     
  10. Mr Efficiency
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    Mr Efficiency Senior Member

    OK, so you want a trailerable powerboat that will float in 6 inches of water (when needed), but run offshore in reasonable comfort. I assume you will need to also make headway under power in the 6 inches of water, and that could be your biggest problem. I see a cat with an inflatable 'airbed' in the tunnel that expands to fill the tunnel, and deflates and is drawn up tight against the tunnel top as required. The bottom surface of it could be a sheet of solid material like fibreglass, so it can take the bottom without too much damage, the rest of it a flexible calendared material. That will effectively mimic a flat bottomed punt when required, but when drawn up and deflated, a power cat. Add jet drives to each side and you are in business ! But I still don't see the need for such a boat.
     
  11. upstrider
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    upstrider Junior Member

    Mrefficiency: you're right with me so I can't understand your lack of need for such a vessel. You must not like visiting the shallows or gunkholing around islands or is it the lack of suitable cruising grounds for this kind of boat in Australia?
     
  12. Mr Efficiency
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    Mr Efficiency Senior Member

    Most people would only go into 12 inches of water to make the walk ashore shorter, no bonefish fisherman here, only bone-headed fishermen ! I can't see a pressing need for what you are proposing.
     
  13. pogo
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    pogo ingenious dilletante


    The other way !
    A single chined hull is a flat bottomed hull with less draft when heeled.
    My tri with 10m LWL and a nominal draft of 35cm comes in with 30cm.
    I guess that a draft of less than 3% of LWL is hardly possible with a LWL/BWL ratio of 8+

    http://www.boatdesign.net/forums/multihulls/planing-catamaran-30415-4.html

    By the way , the boat never slammed.

    For less draft one can built a catamaran with symmetric single chined hulls canted inwards---"pre-heeled"
    The boat then has vertical outer sides and diagonal inner hullsides for better usable space an better acces onto the bridgedeck.

    pogo
     
  14. upstrider
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    upstrider Junior Member

    first POGO, then Mr. Efficiency!

    Good work! I always wondered with what dimensions a flat bottom hull would not pound in a seaway and when the LWL to BWL would become excessive. The angled chine should be incorporated when continued immersion is not possible and shorter length to beam ratios are in the design. The Rocat is interesting if you haven't seen it. One can row comfortably in very rough water.

    And to Mr. Efficiency: I give a toast to all those people that have no interest in water of less than one foot! That's probably where you will find me. I'll have gathered a few oysters, clams and mussels. Done some beach combing and then slithered back into deep water or maybe stayed over night listening to the brandt croak and then left with the next tide. As the proposed vessel will probably so through the surf I'll have collected shells among the rocks and explored tide pools near cliffs which haven't been descended in decades. In British Columbia or Alaska where the tides might be 20' shallow draft is even more useful. Maybe take a nice nap in a little cove on a sloped beach, with an unnamed waterfall nearby, hearing an occasional call from a Swainson's Thrush, while the wind and waves whistle and slosh by in the strait,
     

  15. upstrider
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    upstrider Junior Member

    Pogo: An excellent tri (Nielsen one-off 35)...similar to the Crowther Zephyr 26 I'd built and modified with centerboard and side by side rudders for shallow draft work (and braking).

    Messabout: With the sailing Pogo and I have experienced I bet there are times when we just wanted to 'take it easy' so I for one have lost the quest for speed (less wetted surface, not very important).
     
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