designing for legal size/weight/crew/captaining limits

Discussion in 'Boat Design' started by big_dreamin, Sep 19, 2016.

  1. fcfc
    Joined: Feb 2005
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    Location: france,europe

    fcfc Senior Member

    Seems that you are trying to find a problem for which the solution is "a lightweight powercat of 80ft ..." :p

    You do not have to justify yourself. There is NO rational behind a leisure...
     
  2. big_dreamin
    Joined: Jan 2014
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    Location: Minnesota

    big_dreamin Junior Member

    Well not really. :^) But any "dual use" business possibility I can get out of it makes it a little more tolerable to the missus, whom i'm hoping wont read this. <_< >_> That I think it could also be successful not just for male fun (recreational fishing) but possibly for pay (commercial fishing and whatever else I can do with it) is what i'm trying to justify. Getting to the water for less up front money and less fuel costs is what makes the design a win.

    The only point i'm having difficulty with is communicating what seems to be that i'm talking of a working boat design (since there are versions on the water) with what i'm told are impossible numbers like 12-15 knots with 3-4gph in a 60-75ft size. Having seen the boats on the water I know they work, i'm trying to reverse engineer to WHY they work instead of just copying blindly because it made me want to build my own and the rationales all made sense. At the core it seems to just be incredibly light weight for the length and deck space for efficiency, and hull/beam ratios over 10:1 providing better efficiency than blunter boats at twice normal trawling speeds for that waterline length.

    The guy who had an 80 footer built his for under $300k when youre quoting me multimillion dollar boats, even though the boat in question has yet to get other desirable upgrades (still open deck/he hasn't yet enclosed things) the fact that it floats and has taken 6 foot seas easily for that price is of interest to me. I don't need the luxury just the deck space, and even after I enclose in parts of the deck I see it costing a quarter of what boats of comparable deck room cost. If I find a business use for it so much the better since i'd like to scale it up to something even fancier if the first makes money trawling for tuna or working as a dive boat. :) Or if business works it means I can stop hauling fracking chemicals earlier and be on the water where i've wanted to be since childhood.
     
  3. fcfc
    Joined: Feb 2005
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    Location: france,europe

    fcfc Senior Member

    Before loosing any time or money, I suggest you recalibrate your speed meter and fuel flow meter. And review the measuring process for any obvious error and dependence on external conditions (wind, current ...) .

    See this power catamaran review published on the manufacturer web site :
    http://www.leopardcatamarans.com/si...43PC - Motorboat and Yachting (ENGLISH)_0.pdf
    Compare to engine consumption http://mackboring.com/wp-content/uploads/2015/12/6by3-tech.pdf

    It is obvious that the took the fuel burn for ONE engine to compute the range with the speed for BOTH engines. This is how catamaran have exceptional fuel figures.
     

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  4. bit
    Joined: Feb 2008
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    Location: Trieste

    bit Student

    The engine does not consume, consume the propeller
     

  5. big_dreamin
    Joined: Jan 2014
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    Location: Minnesota

    big_dreamin Junior Member

    I'll share what I know. Dual 150hp Volvo Pentas instead of dual 260hp's, so the GPH doesn't seem off. Displacement figure very close - but twice the length of the catamarans - very long and narrow 4 feet wide and a very shallow 2 foot draft. The boat i'm talking about had ridiculously long cats for the deck size (which did not even cover the full length of the boat) but probably not much more deck space than the one you linked because it was a totally open deck not even to the end of the cats with nothing enclosed at all. That's the big reason why it was so light for the "size". If the cat hulls are half as wide, based on the rules of thumb I was told should mean about half the drag at lower speeds, which would make sense why it's propelling with half the engine rating for the same 12 ton displacement. Yes more skin drag than a short cat but that's more of an issue at higher speeds, not lower if my understanding is correct. He reported about 12 knots under half load of one engine because the 18:1 hull/beam ratio per hull is apparently quite a bit more efficient at that speed.

    Which of the reported information seems well out of line from what you expect? It was originally other studies of the efficiency of these extremely long hull to beam ratios that I was trying to find hydrodynamic studies on - the boat you linked is nowhere near 18:1 so i'm not surprised it's going much slower at a comparable GPH/HP going to the prop.
     
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