need advice - comfortable 7m water taxi design

Discussion in 'Boat Design' started by nuwanda324, Apr 3, 2015.

  1. nuwanda324
    Joined: Mar 2015
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    nuwanda324 Junior Member

    edit: I realize that my question down below is unnecessarily complicated. So i will write a simple one.
    I am designin a high speed vessel which must be good at seakeeping. Which hull type i should use?
    My limitations
    Length: 7m (it can be longer but not shorter for interior design)
    beam: 2,5m (it canbe wider but not narrow for interior design)
    speed: 25-30 knot (But there is a speed limit some of the areas which is 15 knot. So ship must behave good in 15 knot also)

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    Hello I am a ship engineering student. I am designing a seataxi for finishing project. But i couldn't decide which hull i should choose. So i want to ask you.
    My priorty is seaakeeping. I want to design comfortable taxi. 7m long and 3m breath is enough for my interior design. There is a 15 knot speed limit for my taxi. But it must provide 25-30 knot speed when it is needed. I cannot design for one design speed. My taxi should have good seakeeping in both 15 knot and 25-30 knot. First i tought to design a submerged hydrofoil boat, because hydrofil provide best seakeeping behaviour. But becasue of speed limit i cannot use hydrofoil. So i thought about choosing monhull. But seakeeping of monohull just not good enough. And then i realize length of my boat is too low and that leads to pitching. So for better seakeeping i must increase length. But increasing length leeads ttoo much resistance. I can solve roll problem by using catamaran but i can't solve pitching without increasing length. So i decide to use trimaran for just increasing length.
    I talk about it with my friends about using trimaran for decreasing pitching but they said that would be unreasonable and cause unnecessary weight increasing.
    So i am asking advice from you. I want to design a boat which have good seakeeping behaviour. I need at least 7m length, 3m width and 15-30 knot speed. Which hull i should choose? And what do you think about trimaran.
    Wave height is about 0,8-1,2 m.
    Sorry for long post and bad english. I am uploading the drawing for the idea about trimaran.
    https://drive.google.com/folderview...QtRnI2LXZHNXpPbThnVTNNd1BQeWxHSnM&usp=sharing
    [​IMG]
    https://drive.google.com/folderview...QtRnI2LXZHNXpPbThnVTNNd1BQeWxHSnM&usp=sharing
     

    Attached Files:

  2. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    Welcome to the forum.

    Why do you think a 23' monohull wouldn't offer good "seakeeping"?
     
  3. nuwanda324
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    nuwanda324 Junior Member

    In same wave height, longer vessel is going to effect less vertical accelleration. Because time between when top of the wave is hitting to the front of vessel and aft of the vessel is increasing with length. So if we increase length, period of pitching will increase and accelleration will decrease. Am i wrong? I saw small vessels are affecting too much from waves.
     
  4. PAR
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    PAR Yacht Designer/Builder

    If you SOR is this flexible, so will the results and possible design direction choices.
     
  5. WindRaf
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    WindRaf Senior Member

  6. Mr Efficiency
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    Mr Efficiency Senior Member

    How many passengers ? 7 metres is small for a commercial water taxi. Presumably it has to meet stringent rules of the jurisdiction it would operate in, which would include passenger capacity limitations. But you are hard pressed if the 15 knots is mandatory, as many boats that would be comfortable at 25-30 knots in choppy conditions, are marginal at that speed (15), planing cats are a popular choice for water taxi service, but 15 knots is an awkward speed for them. A monohull that can happily cruise at 15 knots, will likely not ride well at the higher speeds, especially in disturbed water.
     
  7. nuwanda324
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    nuwanda324 Junior Member

    My taxi sould be able to take 12(maximum) passenger. Length must be below 15m. 7m length is enough for interior design. If i choose catamaran i think 8-9 m is enough for lwl. If i choose monohull 10-11 m will be enough.
    About the speed thats what i was trying to say. Vessel must have good seakeeping behaviour in 15 knot also 25-30 knot. If i choose a planning vessel or hydrofoil 15 knot is not enough and awkward. If i choose monohull like this one then it will not have good enough seakeeping behaviour in disturbed water in high speed. And using catamaran doesnt solve pitching problem in disturbed water.
    I need a design that will provide good seakeeping in 15 kn and also 30 kn.
     
  8. nuwanda324
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    nuwanda324 Junior Member

    That is what i dont want. It is pitching in 1m heigh waves. I care about seakeeping. All the other things is not important as seakeeping
     
  9. nuwanda324
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    nuwanda324 Junior Member

    Last night i tried to compare couple of high speed vessel type by using maxsurf. Using catamaran significantly decreasing rolling accelleration. But increasing resistance. And not doing any significant effect on pitching. So using cat doesnt solve pitching.
    If you want i can upload my models and results. I used maxsurf sample forms.
     
  10. Mr Efficiency
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    Mr Efficiency Senior Member

    What is a 7-metre boat expected to do, instead of pitching, traversing a one-metre chop ? I have never seen a boat that size that really handles the close-spaced nasty chop 100%, it is just a matter of what passengers will accept in degree of bumpiness. Here's a 7 metre water taxi catamaran, and they ride about as good as anything you'll find in the size range, but 15 knots is too slow, it will not plane properly at all at that speed........hopefully this link will work.

    http://www.yachtandboat.com.au/classifieds/ad/22997/shark-cat-noosa-cat-commercial-boats-nsw
     
  11. WindRaf
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    WindRaf Senior Member

    your ideas are very confused: there is a big conceptual difference,
    for your indicating speed, in the geometry* between a boat of 7 meters long and another 12 - 15 meters long.
     
  12. PAR
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    PAR Yacht Designer/Builder

    This is a SOR problem, define your needs more precisely, so you don't have to ask a full plane design to putter along at semi displacement speeds or attempt to ask a semi displacement hull, to max out at near full plane. You'll also need to refine the sea state conditions you'll be operating in. Wind strengths are helpful, but the general local conditions can be considerably different than what might be typical for a force 4 (as an example) condition suggest it should be.

    Sea keeping ability is more a function of the skipper and crew, not necessarily the hull form. Given sufficient freeboard and available power, you can take a less suited craft into areas others wouldn't consider. Passenger comfort is a difficult thing to quantify, particularly in small craft. It's subjective in nature and you're not going to get any great improvements, between hull forms in the lengths you assigned, in some sea state conditions. Simply put, you can develop one hull form type that does well in some conditions, yet truly sucks in others. In other words, you're not going to find a hull form that handles all conditions exceptionally well, so pick the general sea state you'll be in and work around this. Once this decision is made, adjust the hull form for the anticipated direction the sea state might get on occasion. This might mean adding some freeboard for those occasions she'll encounter short steep chop or possibly adding length, to help bridge typical wave train lengths for a smoother ride, etc.
     
  13. Mr Efficiency
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    Mr Efficiency Senior Member

    OK, so it has "stretched" from 7m, to 11 or 12m. I hope the budget has that kind of flexibility ! First thing to determine is what the minimum size vessel would be under that jurisdiction, to carry your desired number of passengers. But getting a boat that will perform well at 15 knots, but also at 30, is still a difficult problem.
     
  14. nuwanda324
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    nuwanda324 Junior Member

    I decide to work on wave-piercing catamaran. I will regularly upload my design progress on this thread. Thank you for your interest.
     

  15. WindRaf
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    WindRaf Senior Member

    Multihull 'Waves piercing' are very sensitive to changes in weight; to safely transport 7 people you will need to do it very big. And this is one reason that they do not have successfully.
     
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