Round full blunt bow on a cruising catamaran

Discussion in 'Multihulls' started by valery gaulin, Jan 20, 2017.

  1. oldsailor7
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    oldsailor7 Senior Member

    I had a 1970 Corvette which had a shape like that. :eek:
    At 110mph the nose started to lift. :rolleyes:
     
  2. semelis
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    semelis Junior Member

    There are waves.
    I've read a bit fast the thread, but it seems that the factor than the bow is sometimes out of the water and then it slams into it at the next wave is not beeing taken into consideration seriously enough.
    Inside a fluid , at a flat surface between 2 diferent fluids, and at wavish surface between 2 different fluids. These are 3 very different environements.
    A dish skimming at the surface (what the last minis are aiming at) has a very different behaviour than a long and narrow hull. In the second case the bow will try to cut/pierce thru waves, a blunt bow will not cut.
     
  3. bjn
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    bjn Junior Member

    It would have been fun to test it on a scale model, but I'm in Sweden, so too far away. I have made simulations though, which showed some interesting results. Not sure yet if we can trust the simulations to match reality.

    Results are here:
    http://www.boatdesign.net/forums/mu...ulls-have-pointy-bows-57133-2.html#post796262
     
  4. valery gaulin
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    valery gaulin Junior Member

    @bjn: I ask because who knows you could of been living in the next door!!! LOL It did happen to me couple years ago. On a blog a guy was actially living the city next door. It was not a boat blog but a project type type blog.

    Your results are really interesting and probably needs some further investigation. I totally understamd that there might ne some penalty to a round blunt bow but maybe not as bad as people would intiously think.

    If only I had more time on hands i would be already building some model. But i need to finish my current project and take care of the family, kids!!! But this is definatly to do list of project. Testing some blunt round bow vs pointy bow model.

    I tried to get Michlet simulation but can't find the download on their website! Is it still available?
     
  5. waikikin
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    waikikin Senior Member

    Cut backs..

    I think for the strategy of having a larger shorter boat a flat plumb bow transom coinciding with the usual fore beam intersection is the go... one could simply build the hulls with a normal pointyish stem profile somewhat in the vibe of the ghost frame , then cut off and back to the bow transom, this bow could be put aside once the spray settled or could also include a matching bulkhead for simply bolting on before putting to sea....
    In the same vein a nice mullet extension to the stern could be included in the build with boarding platform & steps offering the "stumpy cat" a beautiful and functional option of style and steps with low berthing costs once these aft segments where applied/removed with the same bolt up option.
    The clever marketer could offer these as performance options for this purpose of increasing profits and an easy payment plan on each additional corner, practical speed and performance tests could be represented for the benefit of the stern and bow options once applied and even tack to tack with bow and or stern option fitted to one hull only.
    Therefore the smart operator could have his cake and eat it thrice.
    Once the options where applied I'm sure there'd be no looking back.
    Jeff
     
  6. bjn
    Joined: Jul 2014
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    bjn Junior Member

    You can download the Michlet software from this forum, since it's written by a guy on this forum:
    http://www.boatdesign.net/forums/design-software/michlet-9-33-released-52865.html

    It's not that straight forward to use it though. Michlet has textfiles as inputs. I generated the coordinates of the hulls with some excel formulas, and some manual typing and copying.

    There is a menu system to generate hulls inside the software as well, but it has its limitations. Today I used it to generate a blunt hull similar to your 3D-model earlier in the thread. Take a look into my thread.
     
  7. bjn
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    bjn Junior Member

    Valery, did you see this idea of decreasing the length of the boat in the Harbour, without using blunt bows?
    http://harryproa.com/?p=1757

    The detachable ends are made from foam.
     
  8. Ilan Voyager
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    Ilan Voyager Senior Member

    That is rather high prismatic bows but far from round or blunt. I do not see anything hair rising, just the wish for very high prismatic hulls with cutting water bows, "Old" Lindahl knows its affair. He has been designing and building class A and 18m2 cats since before the birth of Buddha.

    Round bows on a sailing catamaran must be marvelous hydro brakes. A bit violent maybe. I would like to see the beautiful thick spray refreshing the faces of the poor crew at each wave.
    On boats having so big changes of attitude like multis in a formed sea, the first goal is to get a very hydrodynamic shape, a good repartition of volumes with progressive immersion and thus a sweet but efficient damping, and to not have the tramp of a submerged bow acting as brake because your diagonal stability will be highly compromised.

    After 50 years of hard work by hundreds of NA on thousands of multis of every size, use and made in every imaginable style, there is now a consensus, like with the cars which have now stupidly 2 front wheels, and not only one like the Reliant.

    Blunt bows work on slow boats, very heavy and in very calm waters, ie barges on a river. Spheric bows work underwater, torpedoes and submarines but not well on surface boats.

    Sailing ships based on the theory big head, slim tail have been built and used during a long time. Then, at the end of 18th cent. the American boatbuilders made exactly the contrary; fine entries fat stems. And their boats were surer and faster...The European corsairs adopted the design immediately at the very beginning of the 19th cent, that was indecently superior, no English warship twuce the size could pursuit them. The schooner America made a demonstration to the English yachtsmen around 1854, and all the fast ships including the clippers adopted it. End of the round bows which became the mark of the ancient mistakes in naval architecture.
     
  9. valery gaulin
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    valery gaulin Junior Member

    @bjn, Yes it is a good idea.
    But from my limited experience the less system you have on a boat the better it is in every aspect. Set ip time, reliabilty, etc.
     
  10. thiago1979
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    thiago1979 Junior Member

    What Software do you use to create this hull? I am looking to build a Cat, but so far I did not find a good more or less simple CAD program to work with. Thanks
     
  11. semelis
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    semelis Junior Member

    Joyon agrees with you :)
     
  12. Zulu40
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    Zulu40 Junior Member

    In the same energy stream the force of suction is stronger than that of pressure, and we have seen that occur in several design endeavours. So it goes then that if you were interested in reducing drag on the same power specifics, minimising suction rather than pressure should provide a better yield.

    Generally the suction side of an aerofoil provides more lift than the pressure side.

    Subsonic aircraft fuselages (also hulls) generally have their finer sections aft rather than forward. Think of aircraft such as the Douglas DC-3, well rounded forward and highly tapered aft.

    Aircraft without the complications of requiring beam for stability that sailboats require could have finer sections forward, but it would be compromising the hull volume on the same length and therefore the very things that make them commercially successful.
     
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  13. Rastapop
    Joined: Mar 2014
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    Rastapop Naval Architect

    All water borne ships are subsonic - but at the interface between fluids they all have to pay the price of making waves in water.

    Finer bows mean less wavemaking resistance.

    Round bows mean less viscous resistance, and some avoidance of flow separation when a vessel is subject to leeway, i.e. an angle of attack other than zero.

    From a resistance point of view, the difference between the two determines which you'll want. A large slow vessel (where viscous resistance dominates) might prefer a round bow, and a high speed vessel typically a fine bow.
     

  14. Konstanty
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    Konstanty Junior Member

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