"Modular" boat design (to change future LWL/beam)?

Discussion in 'Boat Design' started by big_dreamin, Dec 10, 2016.

  1. big_dreamin
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    big_dreamin Junior Member

    So i'm playing around with a more specific idea (previous posts being feeling some things out, this being aimed at something less general) right now, and one idea on the bucket list is that at some point in the future i'd like to do the Great Loop. (the run from the Mississippi down to the Gulf of Mexico, up the Atlantic Coast, then inland through the Great Lakes until getting back to the Mississippi) With some possible side detours along the way. So this helped put some constraints on specific boat design.

    The dimensions for the max size to navigate the Great Loop itself are pretty ample - side trips not necessarily - so this is what is making me brainstorm this 'modular' idea. I asked once about demountable catamarans, and obviously others have done it, so having where it might be longer or replaced with shorter, or narrower for fuel efficiency vs wider for greater weight capacity, I know that's all possible if a good enough reason could be found to justify it.

    What I was next wondering about was having a design where the beam might change next. I was going to ask "is this a dumb or a good idea" but I was sure everyone would just post 'dumb!' so am revising my question. :) I like the idea of having a catamaran where you could vary the beam - going up the Atlantic wider would mean more stability vs certain waves. Going down some narrower inland rivers i'm less worried about the stability - for that matter having a design where it might get narrower might save money in docking fees, lock fees, or even outside of that be the only way to get inland at all.

    I've a few ideas how this might be accomplished like sliding metal poles with a form of screw drive inside which might doable 'live' at sea - others really wanting somewhere peaceful to mess around with moving metal frame units that are unbolted from one position and rebolted in another, just like some of the variable length trailers i've seen behind trucks on a few occasions. I see no reason such a design should be fundamentally unsound - just that one would have to be ultra sure it's engineered for the loads and stresses as a single monolithic member that would be there otherwise, and that there's no accidental way (locking cotter pins and such) things will come undone at sea.

    I thought of something similar for the cat hulls. Lets say I built a 40 foot long catamaran and in this case had fairly snubby front hulls for the moment. I have in my idea of adding another 20 feet - 10 to the back and 10 to the front - in some sort of bolt-on fashion. Gain a little more waterline length, a little more displacement, maybe sharpen the angles to try reduce higher speed displacement drag. When being docked or going through locks shrinking back down again from 'open ocean' mode. Again ignoring that doing such under any but stable port conditions (or even outside of drydock conditions) would probably be a bit of a pain in the rear, and that a way to do so would likely go through half a dozen revisions, is there something fundamentally unsound about the idea of "extend-a-hull"?

    I am aware there might be "unique legal considerations" as well so i'm wondering if such a boat has ever been done in the past...
  2. messabout
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    messabout Senior Member

    How many occupants will there be on the great loop circuit? How often do you intend to stop for fuel, water, provisions?

    Variable length and beam could be done with a multi but why would you need to do such a thing? It could be done if you are willing to spend a metric ton of money and go through a lot of careful engineering and careful construction for the couplings. There are some serious issues with the execution of such a plan. A 40 footer is sufficient to do a Transat so why would you need or want to add 10 feet to either or both ends.

    The trip could be done in a 16 foot Grumman runabout with outboard power if a solo trip. That one has been done before. The intercoastal has been done with a Lightning class sailboat and several other small boats. The Mississippi has been done countless times with a shanty boat or cargo raft. I think that it has also been done with a Jet ski. I would not recommend that sort of equipment but neither would I think that a 60 foot cat is either needed or practical.
  3. big_dreamin
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    big_dreamin Junior Member

    Maximum 4 occupants at a time. (the family that would like to come for a full journey will not be able to come, therefore 2 of them will switch out at varying times during the journey so everyone gets some time) Planning 500 mile minimum range as others who have done the Loop said that's adequate.

    I'm aware it's not trivial, and there is a bit of "because I can" there. Part of it being that if it works on a smaller scale reliably i'd like to see if I could scale it up. (put another way, this is more a test for the "real" project down the road, only a fool would set to open water without extensive testing) Even if a CAD program tells me that the loading should be safe, and comparable to what a fixed piece would experience, you always discover something new when theory meets reality. Then you revise, change, test further, and at some point consider it safe to scale up.

    I would like to add more than 10 feet i'm just trying to not get greedy to start. A literal doubling or 20 feet on both ends would be ideal. "But you still only have 40 feet of deck space" - well yes that would get longer too. :) Not the full 80 feet (was thinking more like stretching from 40ft with cats flush to 60ft of deck with 80ft cats) but like I said... dont get too greedy to start. Take smaller steps. Test.

    I know you can do transoceanic in a 40 foot trawler but big seas are ugly... yet they are only ugly because you didn't bring a bigger boat! 5 foot seas in a 40 footer is not too comfortable, in an 80 footer with a high bridgedeck catamaran it might be no big deal. (i know of someone else with a 78 foot cat for whom its no big deal whose design i'm wanting to copy)

    Big boat + ocean = yay. Small boat for inland = yay. instead of building two boats, can I do it with one? I started wondering why not just design it to be stretched later... is there any fundamental reason why thats a terrible idea... i'm aware it's an unorthodox idea, and i'm aware that it's an engineering challenge but that's not what i'm asking. I don't think its an insurmountable engineering challenge when I see stretchy trailers for hauling sealand containers behind semi trucks - they go from 20 feet to 40 feet depending whether youre hauling a shorty or a longy. Unbolt, slide, rebolt, and pull. Big boats dont fit everywhere so there's reason to not just use a big boat forever even if I had it.

    The trip is more than for the trip, it's about testing the boat. Yes I could do it in a 16 footer - but I need an excuse to test my design. Mostly i'm trying to see if the main problem would be the engineering/having an adjustable coupler design... or if there are other serious roadblocks OTHER than that. (and yes I am aware of the increasing global strength loads from longer catamarans/leverage too)

    Longer term would be some playing in rougher seastates than normal in the Great Lakes before i'd consider something transoceanic, but long term if the design was solid and I could stretch up to the length ideal, so there are still other reasons than "good enough for the Loop" as opposed to planning to do more with it long term. I see no reason not to prefer as big a boat as I can get even just from a comfort (not surviving a storm) perspective - the seas are never too big, your boat is always too small for the weather!
  4. upchurchmr
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    upchurchmr Senior Member

    No - it is practically impossible.


    You could buy 2 boats for the price of the one expando boat.

    OK, now you can take that as a challenge.
    Did you ever notice how Boeing doesn't make expandable aircraft? And they have the expertise to do it, if it was useful.
  5. gonzo
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    gonzo Senior Member

    What would you do with the sections. Have them shipped to the location where you will installed them? What do you do with them when you take them out, ship them home? Doesn't sound like a practical plan at all. The great loop has very large locks. Commercial barges and tugs make it through.
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    FAST FRED Senior Member

    Used boats are so cheap anything else is a hobby boat , not a looper.

    A 25 -28 ft "Bayliner style IO" can easily be found for under $5K and can just as easily be sold after the loop is finished.

    Do you want to run the loop? or build a boat????
  7. Rurudyne
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    Rurudyne Senior Member

    As far as varying width goes, there have been trimarans with amas that tuck in and open bridgedeck cats that fold.

    Gaining length seems likely to be more problematic though.

    I would guess a trimaran where the central hull could be extended might be easier to achieve than extending a cat because of the increased torsional loads on the bridgedeck of the latter. This would push it towards a stabalized monohull form.

    My own thoughts, for somewhat similar usage in fact, center around a (stabalized) monohull sidewheeler ... certainly not for blue water cruising but fine for the sorts of riverine and coastal use. Essentially the hull might built around the drive section with provision for swapping out or extending the stern and bow as the ability to afford more boat happens. In terms of scantlings that would mean taking into account the final intended maximum length, not just the initial length. Another reason I like the idea is that such a craft can be trailerable as separate parts ... there being two dams between myself and the Gulf of Mexico. To trailer the side wheels themselves would have to be dismountable. Ditto for small outriggers flanking the wheels if used.

    I figure that an attractive final sheer and roofline can be achieved with careful planning ... also, the sidewheel format will help hide any clunkiness of form when first built.

  8. black_sails
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    black_sails Junior Member

    I can't see how? If I build the simplest of catamarans with a totally open deck that's literally mostly frame, replacing one square tube member, with two square tube members (one fitting inside the other) and some kind of screw drive apparatus (similar to a screw drive garage door opener) is not a $50,000 job. Nor is a set of large pins to put Expand-O-Cats on either end with a little extra framing to handle the load.

    If you are talking about things more complex than that (fully enclosed whatever) then yes I agree, there is the potential for great expense, but cut me a little slack and let me tackle one problem at a time.

    Build a boat! The loop is part because I want to, part because if you want publicity for something you have to do something uncommon. Climbing the nearest mountain gets less attention than climbing Everest. What i'd do with the publicity I haven't decided yet. :p

    I have to journey somewhere representing real journeys, this is just multitasking. I want to go transoceanic in something too but it's as much about what I use to do that, as doing it to me.

    Thanks for taking my interest seriously. Really the idea started with thinking about demountable cat hulls, then realizing I could have a second set of hulls that didn't even need the same dimensions, then wondering whether it was possible to make a form of extendable hull (or shortenable hull if you'd rather think of it that way - say a 60ft hull built to be a 60ft hull that just happens to have pins to let you remove the front and back 10 feet of it, and a shape afterwards that at least is not hydrodynamic ruin) for a lower total cost than the two sets of demountable cat hulls and how that might let you do other cool things.

    If it's not feasible to make the hull shorten/extend without excessive cost and engineering, Plan B is still to have two sets of hulls.

    Separate from the hulls the "modular" idea still applies. Obviously a longer deck and such goes hand in hand with a longer set of hulls, it's more about designing for it even if it's something that would require being done in drydock. (And since I was planning on a way of beaching this for over-winter storage anyways, with the way lakes where I am freeze up, this is not impossible either) Again maybe it's too hard to widen/lengthen "on the high seas" despite the colness, or maybe its' too hard to widen/lengthen while it's on the water at all - having read about boats being lengthened and widened is no different than people putting an addition on their house. Engineering and reengineering. And I would think DESIGNING with the idea of "this will be longer and wider in the future" should make it much easier to do than starting with no such plan in mind?
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