Designing a new type of fishing boat .

Discussion in 'Stability' started by helen07, Oct 28, 2009.

  1. baeckmo
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    baeckmo Hydrodynamics

    I have been using one yard in Latvia for a couple of smaller (30´) al hulls, but it does take a lot of supervision. This was when the lat economy was on the way up, and after some time they became a bit "cocky" and arrogant, so I quit. It is not recommended unless you have a local connection you really can thrust.

    On the other hand I have good experience from Russia, both with al hulls and with 15 m vessel "your" size. But again, it does take extra time for visa and travelling aso, plus the absolute need for professionalism and a thrustworthy local connection in all directions!!!!
     
  2. helen07
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    helen07 Junior Member

    Sorry to be persistant but can someone please explain to me why the dimensions of the boat in the link below cannot be increased to allow for a
    7.5m beam ?
    I can't do the math but if every measurement is expanded surely we would end up with a tried and tested design very similar in dimensions to the
    15m L - 7.5 W - and 2.5 draft i originally suggested ?

    http://www.findafishingboat.com/detail.php?aid=12200
     
  3. Ad Hoc
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    Ad Hoc Naval Architect

  4. apex1

    apex1 Guest

    You can forget about Latvia for boatbuilding. The low wages are history, and the quality was hardly at the Russian or Polish level. And how would it be? They did not have much Capacity in The three Baltic states, during soviet times.
     
  5. helen07
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    helen07 Junior Member

    Thanks Ad Hoc for trying to help but i only read the small boat link and unfortunately it convinced me of nothing at all .

    It seems to me a typical exercise in political hot air blowing . No consideration is given to ballast or weight carried above deck height - so should we assume both are unimportant factors ?

    It seems to me that the software used would get very different results from identical hulls if one had nothing but a sail added and the other had maybe 10tons sat on its keel . Like laws there is a general acceptance of what is usual but every law is tested on a case by case basis as the variables deny unquestionable calculation .

    Please except i am only trying to understand now and nothing on here or in all of what i've read on the internet seems to help , as far as i can tell boats have never been built half as wide as they are long because there was no need for them to be built this way .
     
  6. baeckmo
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    baeckmo Hydrodynamics

    Naah, I would rather say that just any proportion, weird or not, has been tried somewhere, sometimes in mankind's fiddling with floating objects! It has been a development of shape like "the fittest will survive". You may rest completely assured that the request for deck space and stability is not unique for you! One example that comes to my mind is the disc-shaped "Popoffka", L/B=1. It was not exactly a roaring succes..... . If there were a successful variant, we would have seen it somewhere!

    I'm quite certain that we could find both tank tests and de facto built boats with any extreme proportion you could think of. Please excuse me now if I become patronizing, but before you dig yourself deeper into that hole; listen to what experienced researchers, designers and sailors tell you here!! YOU CAN NOT SELECT BOAT PROPORTIONS AT RANDOM AND EXPECT TO ESCAPE BAD CONDITIONS ALIVE! Or are you in fact trying to get rid of the ol' man.........?
     
  7. helen07
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    helen07 Junior Member

    Hmmm ........ well he is insured !

    So am i to accept that an RNLI lifeboat will only float if it is the size it is designed to be and if every dimension of the original design were increased at all it would sink ?
     
  8. baeckmo
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    baeckmo Hydrodynamics

    Nope, but it would not make a profit as a fishing boat either.......

    Helen, for some reason you are not focussing on the issue where you and your husband are experts, namely what goes on on deck and in the loads departement, given a set of physically relevant restraints. If the restraint on beam is too sharp, the ONLY way to get around it is to go multihull!

    Before you have settled with at least a rudimentary specification (Ad Hoc's seven points), a discussion on hull shape is simply a waste of time. Tomorrow I will send you the pictures I promised, then I will leave your thread before the trampomobile brigade and perpeetuum mobile inventors crowd in your short, wide and shoal hole!

    Good Luck, and take care, it's mighty wet out there...
     
  9. apex1

    apex1 Guest

    Sorry,
    I could not resist to save that (in case Bodo will edit his post) it´s just so good, and so sad either, `cos it´s true................:D

    Richard
     
  10. helen07
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    helen07 Junior Member

    Who is designing anything ?
    The boat for sale on findafishingboat is perfectly suitable for the job at hand , however it is ideally suited to a 6ft tall man . If say for the sake of discussion a 9ft tall man wanted the same boat , does it not stand to reason that he could order it in proportion to his size without altering the boats seaworthiness in any way ?
     
  11. Tad
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    Tad Boat Designer

    helen....

    A search on the "Laws of Mechanical Similitude" will answer the question on scaling a given design.

    Scaling from 6' to 9' is 150%.

    If length, beam, and depth are increased by 50%......she'll have 3.4 times the displacement, 2.25 times the wetted surface, speed will increase by 22%, 3.4 times the resistance (at the higher speed), and 5 times the stability of the parent vessel.

    What this all means is that her handling (seaworthiness) will be completely different.....good, bad, or indifferent remains to be seen.
     
  12. helen07
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    helen07 Junior Member

    Thank you very much Tad for an informative answer . This let's me know that it is worth looking at increasing the all over proportions of hulls already in use . A pro would have the software and whatever else was needed to test the the boat long before anyone fired up a welder .
     
  13. gonzo
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    gonzo Senior Member

    Helen07: There are harbor tugs built on a 2 to 1 ratio. They have a limited scope of use, and never offshore. A beamy boat like that would handle poorly. At 3 to 1 they are already pigs. I wish the legislators spent a couple of weeks at sea on the boats they are help create with their regulations.
     
  14. helen07
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    helen07 Junior Member

    Yes Gonzo and in an ideal world all judges should spend a month in prison .
    I keep looking at tugs and wondering , put a deep v hull beneath it and drop all the weight below the water line with a much deeper prop , it would bob up and down like a yo-yo maybe but i just can't see why it would roll and worse than an old wooden trawler . I've got pics of my guy standing on the side rail up to his waist in water emptying dredgers , the boats rolled nearly 45 degrees and buried halfway up the deck in green water , and he couldn't even remember it happening a week later when i showed him the pic it was so run of the mill !
     

  15. gonzo
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    gonzo Senior Member

    He's a lucky guy to have someone like you worrying over him. :)
     
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