DIY catamaran design?

Discussion in 'Software' started by sailor305, May 23, 2012.

  1. sailor305
    Joined: May 2012
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    sailor305 future cat builder

    Hi all,

    is there a software capable to design (I mean doing the math) your own multihull which doesn't need months of training and worth the price they are asking for?
    Thank you
  2. CatBuilder

    CatBuilder Previous Member

    A very short answer for you: no. :)
  3. CWTeebs
    Joined: Apr 2011
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    CWTeebs AnomalyGenerator

    Michlet does a good job of predicting the wave resistance interaction between hulls (see the offset hull design that cancels the wave creation on the lee side). Michlet is useful because even though the Michell integral is a closed form solution, sort of the e=mc^2 of naval architecture, actually implementing it numerically is tough to get right and reliable (the solutions don't always converge when doing the triple summation), and Leo's done a fantastic job of coding that up. Michlet is free and I found it intuitive to use.

    Ansys-AQWA can be used for doing traditional seakeeping analysis, including interaction between hulls, but it is not free, not easy to use, and the theory is different in many respects from Michlet (e.g. "forward speed" in AQWA means post processing the forward speed to change the encounter frequency, mathematically a doppler steady component from the sources/sinks is due to forward speed, and the sources are placed ON the hull, rather than the Havelock sources placed on the centerline, which has even more mathematical implications).


    See Leo's Swath thread using Michlet:
    Last edited: May 23, 2012
  4. Stumble
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    Stumble Senior Member


    The short answer is no

    The long answer is no. Even if the software can compute the numbers, it is up to the designer to determine what those numbers should be, and which numbers are important, and what trade offs are acceptable based on the boats intended purpose.

    A software program may tell you that the CG is at 3' but what does that mean? And what does it mean if it changes to 3'1" and how will that effect the rest of the boat, the rest of the design, ect... In short the learning you are trying to avoid by using a program can't be done. Because the software just spits out numbers, it is the understanding that goes behind those numbers that is important not the numbers themselves.
  5. masalai
    Joined: Oct 2007
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    masalai masalai

    Sailor, it is far easier to build your own car, right down to making the tyres and doing all your own metal smelting and casting, and get registered for the road... But then again, why reinvent the wheel....

    Far more sensible to approach a designer who has something (hull-wise) that meets your fancy and build that, then if something goes wrong you know who to sue and also, more importantly, when you sell your boat, it will be within the design specifications of another and he can be sued - so long as he "signs-off' on your build as being competently done by you and complies with his specifications...

    Be disinclined to believe the promotional BS of some "gin-palace" production-line manufacturers... Speed is not all it is cracked up to be except when the seas are slight - but then the wind is weak so speed is down... When the wind is up, the seas are too so speed is often not so comfortable either... If you are ADDICTED to racing then be prepared to suffer... If you seek to cruise, 12 to 15 knots is fast, but most of the time, (estimated at better than 80%), you will be swinging at anchor enjoying the cultural experience and learning how to enjoy a timeless lifestyle...

    A good catamaran has internal SPACE - not cluttered with furniture - and is LIGHT in weight (relatively) as weight destroys performance... I am content with 6 knots on one engine, and with both engines powering along - 10knots and burning less than 3 litres/hour/engine... I can step a mast (all necessary fittings are built in), and use 2 genoa, set up in the style developed by John Hitch (hitch-hiker-rig) for efficient and comfortable reaching and tacking downwind... My design, '39C', is from the board of Bob Oram - to my mind, a bloody genius . . . exhibiting no tendency to broach, tracks true, and is a comfortable live-aboard home that is 40ft LOA and 21.5 ft beam weighing 4000kg to 6500kg ready for an extended cruise...
  6. groper
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    groper Senior Member

    Here we go again, it IS possible to design your own multihull. However, you will only get out of it what you put into it, so if your not prepared to learn a bit more about what makes a good multihull - GOOD, then be prepared to design and build a worthless junker which has little resale value despite the thousands of hours youve poured into the entire project.

    You need to do heaps of reading, but there is ample info on the web to make it possible to design it well tho. You also should have a good idea of building things, whats your background? You need to have an idea of how you will build it, and with what materials, so you can design it to be easily built - theres no point drawing lots of compound curves when your building in sheet ply for example. - you get the idea... and you need to be able to work out exactly how much stuff you want on the boat and what everything weighs considering the build method. Freeship software will work this out roughly for the basic structure for you if you split the panels up into individual layers and supply it with accurate material densities. The fitout is upto you to work out and control the weights so you dont mess up the CoG too much. You can make an excel spread sheet to automate the math as you keep adding things. Its a simple moment sum calculation. The learning curve isnt quick by any means... so if you dont have the time to devote learning all this stuff, your better off buying plans from a good designer and going that way - the cost of plans is nothing compared to the value of the finished yacht...

    But if you do have the time and prepared to learn, then these spring to mind; DESIGN RATIOS.pdf

    You can use FREESHIP software to do your lines plans and make useful drawings to get your hull shapes and frames ready for lofting. it will take curved panels and develop them into flat panels for cutting before bending etc. Again, beware of the limitations of compound curvature.

    For the structural engineering, you best off reverse engineering a similar successful boat to what you want - you need to do this early on otherwise you have no idea what materials sizes and thicknesses (and therefore weights) your dealing with...

    Once you start trawling the web, you will find heaps more stuff to help you.
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  7. John Perry
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    John Perry Senior Member

    There is a lot of satisfaction to be gained from designing your own boat then building it, then sailing it, at least I think so. I would encourage anyone to do it if they can manage it and can spare the time. However, it can be a very time consuming hobbly.

    The software that you could use to design a yacht is not necessarily different to that you would use to design a car or an aeroplane or other piece of mechanical equipment. In recent years I have used Autodesk Inventor and Dassault Solidworks. Both are good and well capable of designing a yacht, including stability analysis and weight analysis. With the full program suites you can also do structural analysis by FEA, if you want to take it that far, I suspect that even professional yacht designers dont always bother, relying on 'it looks as strong as the last one we did, and thats still afloat'. If you want to do CFD analysis then you may want some spectialist software, but you would then definitely be going further than most professional yacht designers would go and you would also be going a little into unknown territory, it is however the future I think.

    To do the hydrostatic analysis using a general purpose CAD program such as Inventor I just take the hull as a solid, cut it off above the heeled and/or pitched waterplane and look at the position the software gives me for the CoG. It does take a bit longer than using the specialist naval architecture software, but it is certainly doable. I am not saying that you should not use specialist naval architecture software, just that it is not essential to do so and that if you want a single program to do it all then a good general purpose CAD program may well be the best way.

    The two programs that I mention above are moderately expensive, I have only have the use of them due to work not connected with my hobby. You may be eligible for an educational licence, thats one way. Also, I believe there are much cheaper alternative programs but I am not the best person to advise, you really need to have used a program yourself to be able to say how good it is.

    In some ways I think that designing a multihull is easier than designing a monohull. With a monohull there is a tricky compromise between good form stability and low drag. There seems to be no clear best solution to this, which is why we have such an interesting variety of monohull hull shapes. Did not someone say 'If it mattered what the shape of a yacht is then they would all be the same.'. With a multihull the hydrostatic roll stability is almost independant of hull shape, and the pitch stability is fairly independant of it, which simplifies the stability problem at least. On the other hand, the need to keep a big lightweight spidery stucture in one piece leads to interesting structural issues.

    Going back to your original question, I am asuming that you actually intend to draw a boat with software, if you only want to 'do the math' then a free spread sheet program may be all you need, or a pocket calculator.
  8. Alik
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    Alik Senior Member

    ... and good run for one's money to correct all problems after launching, and also no resale value once one wants to get rid of it!
  9. DavidJ
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    DavidJ Senior Member

    What I always find interesting about these type of threads is what does one mean by "design". Exterior styling? Interior arrangement? Interior decoration? Hull form development? Structural design? Advanced hydrostatic or structural design using software such as CFD or FEA?

    What does the OP mean by "doing the math". What math? The weight estimate? The hydrostatic calculations? The scantling calcs? All of it? CWTeebs assumed the OP meant software to design an efficient hydrostatic hull shape. When I first read it I assumed he meant software to calculate scantlings. The best software I know to do math is Microsoft Excel, but I wouldn't design a hull with it.
  10. smartbight
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    smartbight Naval Architect

    Freeship/Delftship + Michelet + Braincells.exe = ₤/$/E = 0.000

    Attached Files:

  11. CatBuilder

    CatBuilder Previous Member

    Ok, I'm going to agree with Groper.

    It is possible.

    However, the odds of coming out with a good boat (resale value) are quite low unless you have 10 years + of experience using a similar type of vessel.

    Otherwise, it's all guesses, many of which will be wrong. You can't predict what you like in a boat until you use one.

    That's why for most, it is better to buy off the shelf.

    Even building from plans is a bad idea without years of prior experience on similar vessels. You can really botched a systems installation, for example.

    Of course, I am assuming you are doing it for the end result and not the joy of designing and building. .
  12. CWTeebs
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    CWTeebs AnomalyGenerator

    Nice screens

    Can you share any of those as IGES files? I want to do a radiation/diffraction analysis in AQWA and render the free surface interaction between the hulls.
  13. DCockey
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    DCockey Senior Member

    Consistent with what others have said, software is just a tool and doesn't replace knowledge. It's possible to buy an electronic keyboard at a quite reasonable price, but buying one by itself won't enable you to make music which anyone would want to listen to.
  14. CWTeebs
    Joined: Apr 2011
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    CWTeebs AnomalyGenerator

    In all fairness to the OP, his original inquiry specifically asked for software capable of "doing the math." While that's an awkward way to phrase a question, he may be more familiar with the overall design process than you give him credit for, which would make most of the responses here quite far off the mark. Could be wrong, usually am.


    I'd be willing to throw results from AQWA (not free) and OpenFOAM (free but seriously tough to use) into the mix of Delftship + Michlet + Brancells.exe proposed by smartbight, for seakeeping and resistance calculations that go beyond empirical methods.

  15. masalai
    Joined: Oct 2007
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    masalai masalai

    A boat is a compromise of opposing criteria that must be considered and reconciled... It makes a massive hole in the water into which we must throw continuous effort, resources and commitment to remain afloat in a forever changing and potentially dangerous environment for the 'unwary'......

    Think carefully of the purpose to which you seek to address in the boat... Some styles and designs are better in some ways and worse in other ways... It is a matter of horses for courses and experience and knowledge are critical ingredients... Maths in its own will not resolve these issues but can be a useful tool when broad experience and knowledge form the foundation in addressing a particular challenge...

    Everyone seems equally concerned and prepared to contribute and share in good faith and in the true spirit of with well considered advice, - - to a somewhat vague question, according to their knowledge and understanding of the intent of the question posed... My hope is that the original OP gains and appreciates community efforts...
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