Design Testing and models

Discussion in 'Boat Design' started by Kessica, Oct 15, 2006.

  1. Kessica
    Joined: Oct 2006
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    Kessica Junior Member

    I am wondering if many of you build a scale model of your design and do tests before committing to a build ?

    This is something that I am considering. Is it actually worth the effort or are you usually confident that your design wont fail ?

    Powerboats:
    What scale would you use ? If you wanted to test out things like speed and handling would you also try to build to a scale that allowed to to test out various props ? From the modelling perspective is it possible to use scaled power units to simulate various sizes of engines with different HP's ?

    Saliboats: Much easier to work things out but I was wondering if you can scale the wind down easily ? If your test model was 10th scale, and a 5 knot breeze was blowing, scaling that up would equate to ? I feel 50 knots is rather high, but does it work like that ?

    Interior design when trying to rearrange things is hard at the best of times, and trying to imagine, even when using good 3D modelling can often be misleading. It often gives a good view of what something might look like but space is often misjudged when youre trying to pack things in. I know this from designing my kitchen and then getting it built.

    I look forward to your comments and ideas.
     
  2. tgwhite
    Joined: Oct 2006
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    tgwhite tom white

    Build techniques

    Kessica:

    Your question is a good one. The real answer depends upon your budget and time frame. I must qualify my answer though by saying I've built many big and small things from scratch, but never a boat.

    Today, versus a decade or two ago, CAD programs rule the roost as far as boat design, in all aspects. CAD programs, when used in the right hands, will successfully allow the design and build for hull design, engine and drive train selection, sail and mast configuration, interior layout and estimated build cost. Always though, throw in the experience factor.

    Tank testing for a new hull design is an expensive and time consuming endeavor, but if you are racing or building a supertanker with a radical new concept or two, tank testng allows you to attract the right financing. Tank testing for a proven hull may be warranted if the conditions you envisage are "new" conditions such as ice on the rigging or in the water, heavy following seas etc.

    All said though, a proven production boat "basic" design for a new build, when combined with a good CAD desginer/naval architect, is the most successful route. Plans abound. A few issues ago in PMM, there was a great article about a new build on a good sized trawler using plans and steel from Europe, construction in a farm outbuilding in the San Francisco delta region and a gorgeous and safe yacht resulting. A GOOD do-it yourself job is possible if you are set up with facilites, knowledge and money.

    Interior layups with cardborad or plywood, once the hull and joists are in place is usually a good idea as everybody wants a different stove, fridge, settee, bunks, lockers or control panel arrnagement. Some new power boats have no wheel, only a joystick connected through the AP.

    Good luck and open your wallet.
     
  3. Kessica
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    Kessica Junior Member

    Good comments :)

    However rather than concidering the larger tankers, tank testing and all those ideas which the serious companies use, I was more thinking of the smaller designer who perhaps does this as a hobby or interest.

    A scale radio control boat can often show up design flaws and the boat might look on paper and the CAD shows a great design, but can the CAD be trusted that much ? The full blown CAD's can be very expensive and its a huge outlay for the amateurs.

    Is a model good enough for fine tuning ? Re-arranging the interior can change the CG, even just loading up storage compartments can have some adverse effect.

    The reason why I started the thread was this, (perhaps I shuld have mentioned this first) If I purchased some plans for a boat (sail/powered) but changed the interior or redesigned the helm position or interior, the modelling way would perhaps be the only way for me to test for handling. The hull is the important part; as far as I am aware one cannot simply purchase build plans for a hull. Im concerned that should I make changes, they might have a drastic adverse effect, and a scale model might be the way forwards.
     
  4. tom28571
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    tom28571 Senior Member

    I have done some testing of towed powerboat models in different configurations. The best advice I found was that you need at least a 48" model to get useable results, no matter what scale ratio that amounts to. I did not power the test model but did learn some things about behavior between different hull configurations. This was a planing boat. I don't think an amateur can get a whole lot out of sailboat testing except perhaps relative drag between models having very similar basic characteristics.

    You can have a lot of fun but will often wind up with more questions than when you started.

    Very difficult to determine cause/effect between surface friction and wavemaking because changing wetted surface requires a change in hull shape which affects wavemaking, etc., etc. This is why the old boys (before today's analitical tools) rarely made more than incremental changes from existing hulls to new designs. Too much risk with big bucks at stake.

    Testing of models, including rig and sail, with scaled wind conditions has rarely been attempted to my knowlege.

    A powered model would probably be best for looking at maneuvering qualities but is harder to evaluate than towing tests.

    Scaling effects often elude the experts even in expensive $$$$$ test tanks.

    In your case of changing the interor or deck layout, I don't think you have any chance whatever to get an answer by model testing in the water unless your changes affect the basic parameters significantly. Significantly, in this case, means a lot. You would probably get better results with a calculator and good guidelines from hull design texts.
     
  5. moTthediesel
    Joined: Apr 2006
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    moTthediesel Junior Member

    Get a copy of Weston Farmer's "From My Old Boatshop", no one who gives a hoot about boats should be without that book anyway. (I defy anyone to read the first chapter "Getting a Blast out of Boatbuilding" without laughing right out loud.)
    Old Westy devotes two chapters of the book to building and testing floatation models, and as always, he not afraid to give his opinion as to their (he swears by them) worth.
    moT
     
  6. Crashimir
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    Crashimir Junior Member

    Model testing

    I agree with you Kessica. I built a model of all my designs. Firstly because I build RC model boats as a hobby. But then I realised that it is much safer to make a lousy model than to make a lousy boat. I don't have a test tank, but things like behaviour of boat on waves, or steering can be very precisely tested on lokal pond or river with RC model of a design.
    The rule is bigger the model, more realistic it behaves.
    I use models wich are arround 1 - 1.5 m lenght over all.
    They behave satisfactory and are still small enough to transport them in a trunk of a car.
     
  7. tom28571
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    tom28571 Senior Member

    One thing bothers me about powered models, Crashimir. I can see the value of watching the model behavior but, how do you determine the scale speed of the model while it is doing maneuvers?
     
  8. rayaldridge
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    rayaldridge Senior Member

    Sam Devlin's book on stitch and glue boatbuilding has some good stuff on modeling-- it's particularly good if you're building a chined design.
     
  9. RANCHI OTTO
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    RANCHI OTTO Naval Architect

    I use model tests in Model Basin in order to measure resistance with and without appendages and power requirements.

    I design practically only military high speed boats and if the boat doesn't reach the contract speed the shipyard has to pay penalties very high until the refuse of the boats.....(2 knots less)

    Attached you will find the Classe Bagliani high speed craft, the data are as follows:

    Model Basin > Vienna , Austria

    Boat data:
    Length > 23 / 26 / 32 m
    Speed > + 45 knots
    Propulsion > sumerged cupped propeller

    Model scale > 13.5


    I have tested 3 models having lengths of 23 / 26 /32 m.

    The shipyard has used the same moulds (fixed breadth for the 3 versions) with after part open in way to added the other 2 blocks.
    So doing you save a lot af money for the manufacturing of the moulds..

    The power is measured with the torque of an electric motor placed inside the model, the propeller is a stock propeller tested in the cavitation tunnel.

    For each model length I tested 3 different l.c.g. position and 3 different displacements. Totally 9 resistance tests for each length...

    Using the resistance results, the open water curves of the stock propeller you have only to calculate your final propellerand the speed vs power curves.

    I send 2 pics, of the model (2.37 m for the 32 m version) and the craft once built.....

    The speed was reached during the sea trials without marge....45 estimated and 45 recorded :)
     

    Attached Files:

  10. RANCHI OTTO
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    RANCHI OTTO Naval Architect

    Sorry! the pic of the model is very small...you can find it larger in my photos gallery....
     
  11. tom28571
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    tom28571 Senior Member

    Ranchi,

    Your work is well beyond the availability and expertise of most of us amateurs here. We have to be satisfied with much simpler forays into model testing. Very impressive though that you hit the projection on the mark.

    One thing I do wonder about. When the US military lets a contract for a vehicle on land or sea, they often wind up adding all kinds of weight in the form of armor and armament or other facilities along the way. As a result, performance in the final product is often disappointing. Do you have similar problems?
     
  12. jamesflett
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    jamesflett Mech Engineer

    Kessica,

    I have just built 2 model test hulls out of plywood that may be beneficial to you. I used a program called ProChine3, which is free to download and allows you 20 odd saves. Its a pretty basic program but might need to look at the tutorials which I think come with the download:

    http://www.newavesys.com/products.htm

    Once you've drawn up your hull you can view the hull plates which give the actual shapes you need and the program produces all the co-ordinates. From there I just drew up an x and y axis on the ply - we used 1.5mm marine plywood. Then cut it out and assuming you have used a thin ply then everything will fit together. Our powerboats were 1m overall length, 1.25m along the waterline and only needed 1 sheet of 1200 x 1200mm per boat. We tied the gunwales and the hulls together with some fishing nlyon and then used a rather expensive Epoxy/Resin/Filler concoction to seel it all but a marine grade epiglue of some sorts would be fine.

    We started off with a station at the transom just to keep the shape and everything else fell into place.

    In terms of testing we just used an aluminium boom with a couple struts coming off it and hung it off our 5.5m powerboat. Ive got a heap of pics if you need any more help
     
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  13. jamesflett
    Joined: Jun 2006
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    Location: New Zealand

    jamesflett Mech Engineer

    testing
     

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  14. Willallison
    Joined: Oct 2001
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    Willallison Senior Member

    I'm with Tom on this one.
    Building a model and then getting meaningful results is not for the inexperienced designer. But more than that - in your case you don't need to do it.
    I'm assuming that the plans you have will show the boat's designed centre of gravity. All you need to do is make a simple weights & moments table - subtracting those things that you are removing and adding those that you are putting in. There are plenty of books (and probably web sites) that will show you hoe to do this if you don't know yourself.
    Failing that, contact the designer and show him / her what you want to do. For a small fee he / she might do it for you
     

  15. MikeJohns
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    MikeJohns Senior Member

    Other arguments aside and out of interest...

    The power from the sails is going to be retarded by the resistance of the hull. When the skin resistance plus the wave making resistance equals the drive then v max is reached. (yes also air drag)
    Your model can be produced with the same ratios (SA/D and SA/WSA) as the full size craft.
    So when you have scaled displacement (approx proportional to the cube of the sacle factor) and wetted surface (approx proportional to the square) you calculate your new sail area accordingly. Using a PC you just spit out the numbers.

    Build a 1.5m model for usefull results and loft your model sails well.

    By using a dam you can control your "scale weather" by varying the amount of fetch .
     
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