Boat project 14ft catamaran flats

Discussion in 'Boat Design' started by EirikNorway, Jun 2, 2012.

  1. EirikNorway
    Joined: May 2012
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    Location: Norway, Oslo

    EirikNorway Junior Member

    Hi,
    me and a friend is going to build a fast small fishing boat that has good seakeeping in normal weather conditions.

    Specs:power:40-90hp yamaha 2t engine mounted in the midle of the tunnel
    Hull type:Tri hull(stepped main hull)/catamaran with wave piercing front hull.
    Tunnel hull
    Hull dimensions:L4,2-4,5m. B2m. Sponson width35cm 110cm spacing. Height55-60cm. Weight 150kg (estimate)
    Construction:9mm gaboon aa/bb laminated in keel/stringers and transom.
    10*25mm Profiled pine strips epoxyglued and reinforced with G.F.
    double hull- the deck and hull act as a sandwitch construction
    together with the stringers and the crossections(spacing 0,5m.)

    The design is a litle out of the ordinary but still looks like a boat. I tried to minimise wetted surface at cruise and full speed, the three steps have spray rails both inside of sponson coresponding to strakes on the outside of the hull.
    The step intended for fast plane have a with of 15cm-cruise 20cm-slow plane 7cm. Total crossection at fast plane 2*15cm=30cm 3-5dgr. deadrise. These steps is not built into the model but i will post a crossectional drawing.

    1. Will the prop ventilate at cruise speed or when the nose brakes the surface in front of the propeller?
    2. Can anyone with experience say if the boat is going to behave strange in some unforseen way.
    3. My observation that catamarans are more slow and clumsy, will the proposed design allso not plane fully?
    4. will the boat float on air?

    Link to pictures of model: http://www.flickr.com/photos/7682263@N02/
    Link to testing of model in river: http://www.youtube.com/watch?v=X2XJlLBGQQc&list=UU5oRJy8HQvh9DTJb-yEp8Tg&index=1&feature=plcp
    Link to testing of model in river(Without weigth)http://www.youtube.com/watch?v=sXLFm351Sak&feature=plcp:
    Model scale 1:10, drawn in Rinocheros, built in balsa wood.

    I will revise the design a few times before building. Startbuilding in a few weeks. Hoping that someone can give some informing refrences or tips.

    Eirik Solheim Aakhus
    E-mail: eiriksolheimaakhus@yahoo.no
     
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  2. messabout
    Joined: Jan 2006
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    Location: Lakeland Fl USA

    messabout Senior Member

    Testing the model in all that turbulence is not the best way to do it. You seem to be pulling the bow of the model upward in both the videos. At the least, you should be pulling the model with a line parallel to the water surface. Finally, test models do not give you the information that you need, maybe a general idea but not much else, especially in that wild river.

    That said, the models are sexy looking. I like models for their own sake. I build a lot of them and mess around in the water with them. Fun, not research.

    My first impression is that your boat might trip pretty easily and the second impression is that 90 HP is a bit much for a boat that size. Third impression is that you might do well to keep a life insurance policy in force because the tunnel may flirt with a blowover at speed.
     
  3. EirikNorway
    Joined: May 2012
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    Location: Norway, Oslo

    EirikNorway Junior Member

    Will the bended sidehulls brake the the hull down, i try to keep them as straight as possible allong the planning surface(with a litle margine). The intention of the front hull is to stop vertical motion downwards preventing the bended sidehull to tuch the water.

    The hope is that the boat will float on air as the speed excide 20knts.
    No life insurance yet.

    Eirik
     
  4. messabout
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    Location: Lakeland Fl USA

    messabout Senior Member

    Eirick; It is difficult to see all the details of the bottom of the models. It appears that the runners (or pontoons) are curved inward near the front of the boat. They also seem to be somewhat wedge shaped.

    Can you post some sketches of the bottom view of the boat? A section view would also be important. (section view is as if the boat was sawn in half near the midle of the boat. Think of a bread slice. Put some dimensions on the section view.

    And yes if the runners are wide enough to hold the boat up when planeing, you will get air under the boat. Yes the air may create some lift. If the lift exceeds the weight of the front part of the boat the bow will lift and continue to lift untill the boat turns over on its back. That is called a blowover and it has been fatal to more than a few drivers and passengers of fast boats. For your safety this needs to be carefully considered before you begin building the boat.
     
  5. messabout
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    messabout Senior Member

    Eirick, post some more information, as requested, if you are serious about your project. We can not give you good council unless we are given sufficient information. I have given you first impression comments but that is all.
     
  6. EirikNorway
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    EirikNorway Junior Member

    Developing the design. will continue to post.
    Drawings with messurements, concept-drawings and rendering comming soon.

    Eirik
     
  7. EirikNorway
    Joined: May 2012
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    Location: Norway, Oslo

    EirikNorway Junior Member

    Design proposal

    Design proposal number 12 on this speciffic design.

    http://www.flickr.com/photos/7682263@N02/sets/72157630333189438/

    The design didnt come out of the computer like i wanted it to. Running pad is to extend all the way back to the transome, and catamarans run a litle bit further back.

    The design has three planning modes static or combined.

    Step 1: Full plan - up to strake one on catamaran + ground effect 50+knts
    Step 2: Fast cruise - full catamaran 30+knts
    Step 3: Slow cruise/full pay load - catamaran + running pad 20+knts

    The nose of the boat is shaped as a sharp vee typicaly 60 degrees.

    How high must the transome be with a jack ok but bouency wise with a big engine.

    Up for coments

    Eirik
     
  8. rwatson
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    rwatson Senior Member

    Once again, the strakes curve from the bow. They should be absolutely straight fore and aft.

    This is a major design flaw and will create all sorts of problems.

    Also, Until you get up into 1:5 or higher scale models, you may as well forget about them telling you anything useful. Even then, the results don't prove much.
     
  9. messabout
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    messabout Senior Member

    Ditto RWatson.

    You are drawing futuristic boats that look like they came out of a Star Wars movie. Please abandon all that impractical artwork and do some research about proven hulls. Do not stray far from what the professionals build. The thing you have drawn will get you killed. Even though your art is impressive, your design is technically, far off the mark.
     
  10. EirikNorway
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    EirikNorway Junior Member

    The ambitions is do something simple and beautiful, the design has developed successively. That is why the design looks like it does. Going to build a model 1:5. And then go for a new design.

    What kind of problems does curved strakes create?

    Eirik
     
  11. rwatson
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    rwatson Senior Member

    The 'beauty' of a boat design must always start with something that actually works, as your life depends on them, especially at speed with big motors.

    Any other object is just sculpture.

    Strakes that curve will cause problems when one side of the hull is out of the water by small or larger degree ( eg. crossing a wake, or a wave ) You will get a slewing effect, that could tip the boat over. That's totally beside the inefficiency of hull performance in a straight line. Its like driving a car with two front wheels 'toed-in' by a large amount.

    That crazy stern shape will also cause all sorts of squatting problems.

    The 'standard' boat hulls you see around have years of engineering, experimentation and experience behind them. You should only deviate from 'standard' designs only if you have the engineering and practical experience backed up by big development budgets.
     
  12. Manie B
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    Manie B Senior Member

    Erik I always encourage guys to go for it and sometimes think "out of the box" BUT DO RESEARCH FIRST - educate yourself.
    Your design is dangerous and very wrong - get experience and start reading.
    In a nutshell - scrap the design completely and first build something small and standard AND GROW from there.
     
  13. EirikNorway
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    EirikNorway Junior Member

  14. EirikNorway
    Joined: May 2012
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    Location: Norway, Oslo

    EirikNorway Junior Member


  15. messabout
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    messabout Senior Member

    That looks much better than the original. It is unneccesarily complex with all the concave sections but it will work in a manner of speaking. I suspect that you are leaning more toward style than to function. You will be better served if you lose all that sexy stuff and design a conventional boat.

    In addition, the vee section at the transom may cause you trouble with cavitation or peculiar handling in turns or possibly porpoising. De emphasize that vee at the transom a little bit. If the boat is to be used in shallow water, as in a flats boat, the deep vee will force you to have the prop farther below the surface and it wont be a flats capable boat.
     
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