Where to find scantling formulas for a long, flat-bottomed hull with an extreme length to beam ratio

Discussion in 'Boat Design' started by John McCrary, Oct 14, 2021.

  1. John McCrary
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    John McCrary Junior Member

    Hello,

    this is my first post-here. Last year I was going insane in medical school (training to be a PA) and so began to try to self-teach myself the rudiments of naval architecture as a diversion. I quickly found that I liked exploring the fringes of design rather than the main stream. towards this end, I've been playing around with multi-hulls with extremely narrow and light hull and amas. I was using the scantling number's from Gerr's "Elements of boat strength..." as a rough guide but my gut feeling was that I was in an edge case where they didn't fit exactly. Last night I essentially confirmed that while going through Colvin's "Steel Boatbuilding" section on scantlings and finding the mention the need for extra longitudinal reinforcement for boats that were extremely long and narrow.

    My question is where would I find those calculations? Also is there a simplified source for calculating the crossbeam size for amas? I've found the ABS multihull formulas but I do not have the mathematics knowledge to solve those equations.

    thanks,
     
  2. bajansailor
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    bajansailor Marine Surveyor

    Welcome to the Forum John.
    Re multihulls with extremely narrow hulls - remember that the main limiting factor quickly becomes the buoyancy required to float them.
    Re Colvin's steel boatbuilding, this is light years removed from thin narrow multihull hulls - or are you considering steel as a material for these hulls?
    If so, then the 'light' aspect of the hull goes out the window pretty quickly!
    With multihulls you also have to consider the effects of the hulls twisting / racking (or trying to) relative to one another - and the forces involved here can be significant.

    How big are the multihull designs that you are playing around with?
    Such as length x breadth?
    Are you hoping to build a mutlihull to your design at some stage?
     
  3. Ad Hoc
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    Ad Hoc Naval Architect

    Professional Boatbuilder Magazine, issue 182, Dec/Jan 2020
     
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  4. John McCrary
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    John McCrary Junior Member

    Thanks Ad Hoc

    Bajan:

    I'm actually looking at Aluminum, but Colvin's was recommended by Karsten (IIRC) since many of the same concepts apply.

    The biggest design I am playing with is a 125 long tonner whose main hull is 216 feet with a constant beam of 8 feet from stern to 144 feet forward. The foreward 72 foot of the main hull incorporates a hybrid wave-piercing axe-hull configuration with a constant taper from the 8 foot beam to very fine point that is approximately the width of the the side plating. The amas are 116 feet each with a beam of 4 feet. The forward 16 foot of the amas feature a constant taper where the outboard hull tapers in until it meets the inboard side of the hull. Think the USN's Independence class LCS's or the fast trimaran ferries designed by Austal. The design does differ from the Austal designs in that there designs are optimized for efficient assembly and thus minimal complexity and therefore the sides of the hulls are perfectly perpendicular to the hull bottom and primary deck..

    -John
     
  5. bajansailor
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    bajansailor Marine Surveyor

    Ummm ok - have you calculated your volume of displacement(s) for the hull(s), and also done a weight estimate?
    Your main hull has a length / beam ratio of 27 - that is very skinny!
    Re the 125 long tons, is this your displacement at the load draught? If so, how does this compare with your weight estimate?
    What will be above the main deck?
    Will this be a power or sailing trimaran?
    Will she be carrying people, or cargo, or....... ?
    Can you post any sketches or drawings please to help illustrate what you are describing?
     
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  6. John McCrary
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    John McCrary Junior Member

    the 125 long tons is a weight estimated for the entire boat based off published rules of thumb for cargo, furnishings, and crew/passengers as well as published gear weights and calculating the structural weight based off of hull and deck square footages as well as a pessimistic estimate of frame/longitudinal weight. Design is a motor yacht with supplemental parasail drive. I've attached a jpeg of the rough hull outlines, in order from top they are the main hull and amas (8 foot tall, not counting the axe bow which adds another 2 feet), profile view and view of the second deck and catwalks. I know the design is going to be wet in any significant sea, thus the front of the extended Portuguese bridge is contoured like a a snow plow to deflect the wash away from the pilot house. Like the main hull, the deckhouse is 8 foot tall. In fact, almost all measurements are divisible by 4 so that approximately 90% of the hull can be rapidly assembled from 8'x4' aluminum plates that have CNC-cut edge rabbets.

    -John
     

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  7. Mr Efficiency
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    Mr Efficiency Senior Member

    What is the projected hull draft ? It sound to me like daylight will be visible under the boat in some wave conditions.
     
  8. Ad Hoc
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    Ad Hoc Naval Architect

    That is just a simple longitudinal strength issue then.
     
  9. bajansailor
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    bajansailor Marine Surveyor

  10. rxcomposite
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    rxcomposite Senior Member

    There is plenty on the net, even on line calculator. This a simply supported beam with a distributed load. Attached a 3 point load for simplicity. You can search for more point loads and then compute for hogging and sagging condition.

    Because this a very long and narrow boat, you will need to find EI or section modulus. That is tedious and another topic.
     

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  11. rxcomposite
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    rxcomposite Senior Member

    Something like this
     

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  12. John McCrary
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    John McCrary Junior Member

    IIRC. estimated draft was 3-4 feet.

    Boat is intended as a yacht with extra space for conducting train-the-trainer type medical mission work. The hab deck does span the full beam, but interior layout is designed to place heavy items directly over the hull/amas rather than on the spans. Realistically, I am not sure if I would build this or a smaller version of it. One of the reasons I am trying to figure out the required structural support is to get a realistic fabrication estimate for the hull. If the internal structure is fairly straight forward, then the labor hours should be low enough for me to afford the construction of the bare hull. If I can do that then I am good as I have the skills for fitting out t6he interior.

    Thanks for the additional info Mr. E and RX.
     
  13. bajansailor
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    bajansailor Marine Surveyor

    Just some random thoughts - if your length is around 216', then your overall beam is going to be around 70'.
    Where will you build a vessel with this size 'footprint' - and where will you haul it out come the time when you need to paint the bottom and do maintenance on it?
    The berthing fees will be horrendous everywhere for a vessel this size - and you will probably have a surcharge applied re the 70' beam.

    If you decide to anchor the vessel instead of mooring alongside - how will you achieve this?
    Would you just have a stern anchoring arrangement with a windlass?

    Bear in mind that with some flag states the rules re manning and safety equipment etc become more onerous when you get over 24 metres loadline length.
    How many crew would a vessel like this have?

    I am still 'seat of pants' thinking that by the time you tally up all the weights of the vessel in the lightship condition, and then compare this against the buoyancy available at the load draught, you are not going to have much 'left over' for cargo and passenger carrying capacity.
     
  14. John McCrary
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    John McCrary Junior Member

    Bajan,

    I am not tracking on the buoyancy. Displacement is equal to volume multiplied by the density of water right? Main hull is 9365 and each ama is 3456 for a total of 16277 cubic feet. If I multiply that by the weight of 1 cubic foot of sea water I get 480 long tons of buoyancy versus fully loaded mass of 125 long tons. Am I doing that wrong?

    As for anchoring, she's got that canard-like shape at the bow. That platform is for a holding the ready anchors. Long hawse pipes run back to where the bow has a little more beam and that is where the windlasses, chain locker and spare mooring gear storage is. There are also aft anchors on the stern of each ama since she's designed to handle beaching of the bow in sand/mud.

    Yard-wise, her original beam was trimmed down in order to comfortably fit within the drydock and haul out facilities in my region. Given the shallow draft though, using tidal grids or blocking up during high tide are options as well. Finally, the hull uses sea chests with closeable inlets and the modern IPS pods that can have their lower unit fluids changed from onboard to minimize the amount of exterior work needed. Also, though while it would not be my first choice, the shallow draft and aluminum hull makes underwater application of anti-foul theoretically economically feasible.
     

  15. bajansailor
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    bajansailor Marine Surveyor

    Yes it is - but are you using the draft (I think you mentioned 3' - 4'?) in your calculation?
    I wonder if you used the overall hull depth?

    You get a total of 16,277 cubic feet - if I use 4' draft, and make some assumptions re fineness I get about half that (approx 8,524 cubic feet).

    As an example, re the parallel mid-body length of the main hull (144'), the volume displaced at 4' draft (assuming that it is a rectangular box) with a hull beam of 8' is 4,608 cubic feet. But in reality it will be less than this, as I am assuming that the keel rises towards the stern (?).

    Re the bow section of the main hull, it is very fine - if I assume a coefficient of fineness of 0.5 (it might be less) for this 72' length I get a volume of displacement of 576 cubic feet.

    Similarly re the outriggers - if one assumes that they are rectangular blocks, 116' long, 4' wide, and at 4' draft, with an estimated fineness coefficient of say 0.9 (but it will probably be less than this), then I get a volume of displacement of 1,670 cubic feet per hull.

    Hence the total is 4,608 + 576 + 1,670 + 1,670 = 8,524 cubic feet.

    At 64 lbs per cubic foot of seawater, this gives a displacement of approx 243 long tons.

    You mention a fully loaded displacement of 125 tons, but this sounds VERY optimistic to me!
     
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