drag calculations for large catamaran

Discussion in 'Boat Design' started by fredscat, Oct 2, 2010.

  1. Ad Hoc
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    Ad Hoc Naval Architect

    Surely you must have a SOR or spec (specification) and GA for the vessel?
  2. latestarter
    Joined: Jul 2010
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    latestarter Senior Member

    Having just read the link on Wiki that I posted earlier on Finite Element Analysis, despite being aware of it for many years, I found it difficult to follow. Hopefully the following will be clearer if you have not come across it before.

    The method came into fashion in the 60s/70s when computers became powerful enough to do thousands of calculations quickly.

    Basically to analyze the structure it is divided into small areas(elements) usually triangles and the computer program works out the stresses in each element.

    The advantages are that you can see where the highest stresses are occurring and redesign or reinforce to deal with them. Also in areas of low stress you can save weight by having thinner material or smaller dimensions. Complex shapes can be analyzed that older methods could not do.

    When modern multihulls started, the crossbeams were of uniform cross section which was inefficient as the stresses change along their length but now with the structural analysis available you can tailor the cross beams to suit.

    Analyzing the connection of a crossbeam to a hull is complex with bending, shear and torsion all occurring at once, by considering small areas separately you get a good approximation of what is going on.

    FEA has been used for the large thin shell concrete domes over some sports grounds.
  3. mydauphin
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    mydauphin Senior Member

    Thanks, I actually know what they are, I used them in real life for marine construction projects but mention for others coming in. In large construction projects, we first have concept, design, engineering, compliance with legal, environmental, etc... and sometimes we spend a couple of years going around in circles. When the final engineer plans are finished and signed off by everyone, we create timeline, we buy material and get equipment for project. I would guess every 25% of a project we find changes that require some additional changes, sometimes minor- sometimes major. Of course customer change orders can be a big delay to whole process.

    We try to replicate designs and procedures, and do multiple jobs with one design, but it never fails everyone wants their own designs. So you see what you and I do are very similar, just my projects weight millions of tons and don't move.
  4. fburton
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    fburton Junior Member


    Hi Fred

    I love what youre doing. So very few make it as far as you have already.
    And you deserve respect for using the best safty factor there is. Size!

    Im not a designer so im not hobbled by fidiciary concerns so Im happy to bandy some figures around.

    Larssons "Principles of Yacht Design" discusses the simple formulea approach for wetted resistance.

    Frictional Resistance = 0.0021 (coefficent of friction) * 0.5 * 1025 (density of sea water) * 3.1^2 (speed squared, 3.1mps = 6 knots) * 48.13 (Wetted Area p hull)
    = 466 N p hull
    ( Im just guessing what your wetted surface (48.13m2) will be at 13.6 disp).

    Just for the hell of it I put some of your dimensions into Leo's michlet meat grinder.
    Using 26in draft and sections which are fuller than your shallow half circles it gave a BWL of only 60 in.
    But the figures will show the order of magnitude.

    Fn speed(mps) Knots Friction Wave Total
    0.18 3.1 6.0 510 60 570

    0.35 (upwind) 6.0 11.6 1868 640 2508

    0.4 (hull speed) 6.8 13.2 2303 1235 3538

    I hope this gives you a perspective on things.

    If you need real accuracy i would need your offsets.

    Best Fred
  5. naval ark
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    naval ark Member


    I too admire your bravery (perhaps stupidity, that's a fine line you know) to begin your catamaran build, based upon what would be considered 'back of cocktail napkin' sketches. I will try not to reiterate the very valid points that others have made - except that Leo's 'best' advice is just that.

    Although I admit to designing advanced marine vehicles for a living, I feel no fiduciary obligations to provide a professional opinion here as we're all just shooting the breeze on the interwebs.

    But before you get energized for another round of building, reinforced by the post above, remember all this advice (and 'help') is worth what you paid for it. That, and who has the most to lose if your project continues ahead on it's current course.

  6. fburton
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    fburton Junior Member

    free is not an argument

    naval ark,

    Are you really saying that free advice is worthless ?

    I read this free site because of the special knowledge not expressed anywhere else; though often you do have to dig for its treasure. My personal frustration is I have yet to see any catamaran sections that wern't for a plywood beach cat from the 50s. I dont know why sections have to be more confidential than draughts for nuclear bombing making.

    I come from the world of IT where giving is a culture. Im very proud that we as a group kick started the web; all based on shared knowledge. In fact giving is so rampant they even give what isnt theirs. Meaning 'pirates' is no longer chiefly a nautical slur.

    Thank you for reading my earlier post but if youre going to condemn it as without worth then im going to press you for an argument. Because its free is not an argument.
  7. naval ark
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    naval ark Member

    Ok, fine: Whilst I would agree that worthwhile advice has been offered in this thread, unfortunately I would not consider yours in that category.

    This chap obviously has no idea how to design a boat; as others have stated, proficiency in this field takes years of education and experience. Do you think we all just sat around wasting our time in uni and continue to do so at work every day?

    Why would sections (or other proprietary details) be 'confidential'? Well, they're not really, but you'd have to pay someone for the service to design your vessel's ideal form based upon the input specifications.

    Again, talk is cheap - although we all have no vested interest in this project, I would imagine the outcome to be of considerable concern for the OP. Hence the slew of similar responses.
  8. fburton
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    fburton Junior Member

    The point I make is this chap already had the hulls: so all this 'I wouldnt go to tipperary from here' is out of context.

    It takes guts to open up your dreams and work to the world and Im sure he did it because he needed some help. Replies were many but not one addressed his question. Lining up to suck air over your teeth is cringe-worthy but giving help doesnt deplete what you learned at uni.

    I challenged you because you derided my contribution. And yes talk is cheap when you cant defend what you say.
  9. rxcomposite
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    rxcomposite Senior Member


    To analyze what the boat will experience in water, take a long rectangular box, place some weights inside and put it on the floor. The box is in equilibrium and will lie flat on the floor. It is not moving.

    To analyze the maximum bending moment it will experience in waves, lift the box and pace a stool in the center. This is the hogging condition where a large wave is in the center of the boat.

    Now, support the box near the two ends. This is known as sagging when the waves are long enough to support only the ends of the boat. A simply supported beam theory.

    In both situations, the box will try to buckle. So you will add vertical structure to the box. Call this bulkheads. You need at least three, one for the collision, one for the engine room, one for the steering room. It must be watertight to isolate the compartment from each other. The idea is, if one compartment is punctured, the boat will not sink. BTW, did I forgot to say that in boats, no sections are the same?

    Bulkheads may not be enough to support the “skin” of the boat. You need other structures like web frames, transverses, girders, longitudinals, stiffeners, pillars, ect.

    Because you have a catamaran, tie the two boxes with a crossbeam. In still water, the crossbeam is a bridge, more like a simply supported beam as the box (hull) is not fixed and free to move. But in a crosswave with one hull in the crest and one hull is in the trough, the crossbeam is like a fishing rod with a fish trashing about in the other end. Cantilever theory?

    Now, if you lift only one end of (4 sides) the box (as in quartering waves) You will be twisting one crossbeam in addition to longitudinal bending exerted in the beam. The other cross will twist the opposite way. Similar to the wing of an airplane when acted upon by ailerons.

    Going back to the “skin”, The skin or panel is a fixed beam theory as its four sides are firmly fixed. Pressure on the bottom is much greater and becomes less as it moves nearer the surface. Archimedes principle? Yes, but as the boat moves, pressure is increased. Pressures on the panel are different from each other. In the bow, wave slamming will be constant. Wave height in the area is where you will be operating the boat (as stated in SOR) is critical in the calculations.

    Still up to it? If you can confidently say you can do this sort of calculations with your reference material on hand, maybe you will be conversant with the Naval Architect. NA’s eat this for lunch (and breakfast and dinner)

    Structural design is only part of the design spiral and cannot be written in a single page.

    To give you an idea of the complexity of calculations, It takes about 70 pages for an 80 footer composite boat and yet using a software. It is only a printout of the output.


  10. Tim B
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    Tim B Senior Member

    Just to clarify the above post, the bulkheads work very nicely for transferring load resulting in rolling moment (wherever that may come from), but you will also need significant structure running fore and aft to handle rig loads and wave bending. What many people forget is that the hull skin itself takes a significant amount of the torsoinal load on a monohull. On a multi-hull, this load is much larger, but must be taken through the bridge-deck, which almost certainly won't create a nice (almost) sealed tube as in a monohull.

    You should also remember, that looking at the static loads will not be adequate. You need to define loads which you (as a designer) feel happy are representative of the upper limit of sea-state which will be encountered.

    One thing that hasn't been mentioned yet is where you have long sections of hull which are laterally "unsupported" by cross-beams, which will have lateral force on them. I cite Team Philips as a good (if extreme) example of this. Don't forget that a 2m wave has a lot of energy if you try to stop it!

    Best of luck,

    Tim B.
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