Minimal offshore power proa

Discussion in 'Boat Design' started by BrendanfromNZ, Aug 11, 2020.

  1. bajansailor
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    bajansailor Marine Surveyor

    Yes, she is - and this is with the engines on the transoms, so about 7' away from the centreline.
    We did note though that she was / is perhaps too directionally stable when running down waves - if she decides to go down at an angle (rather than at 90 degrees to the wave), and you want to correct her, it needs a fair bit of helm, and perhaps an extra burst of power.

    She does have assymetric hulls though (as if a monohull is sliced in half down the centreline), which probably helps (?)
    Brendan, if you went down this route (re assymetric hulls on a cat) you would save a lot of welding re the inboard sides of the cat as it is just a vertical flat plate along the length.
    Our 50' cat has 2 chines on the outboard sides, so that is approx 200' of double continuous (inside and out, 400' in total) welding saved.

    I appreciate though that symmetrical hulls are more efficient at lower Froude Numbers like what we are operating at (10 - 12 knots typically), but it was much easier to build the hulls this way (re being assymetric).
     
    Last edited: Aug 13, 2020
  2. Mr Efficiency
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    Mr Efficiency Senior Member

    Apart from anything else, I wouldn't trust a skinny proa in a clutch situation, versus a catamaran of the same or even somewhat smaller length, the stability is simply not comparable. The advantages of a quicker, cheaper build, would not be redeemable in an extreme situation. Anything that keeps you out of Davey Jones's locker, merits attention. We are talking open sea far offshore here.
     
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  3. BrendanfromNZ
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    BrendanfromNZ Junior Member

    Thanks all for your replies.

    Agree a catamaran is the ultimate vessel for this.

    But as a self build, money, and most importantly time is what keeps me away from a cat.

    I've built and sea trialled power cats before (planing only) and I am well aware of the build time, and feel that about 45ft is the minimum for true accommodations in the hulls without it being too boxy.

    Problem is the build, two hulls, two drive systems, insulation of two hulls, twice the fitout all adds up.

    I still don't get the difference in a 5m wide catamaran, and a 5 meter wide proa (basically a cat with no accommodations in one hull) with ballasted Ama. Apart from the single engine......

    I could head back down the cat route, but I want Economy, speed and rough water handling. So its going to be narrow 15:1 hulls, short wing deck, low windage and small diesels. Gets to the point where the Trimaran hull starts looking more roomy....

    I'll have a sketch this afternoon.

    Bajan sailor, thats smart how you've done that.

    Whatever design I follow, will be making use of a lot of sheet folding as opposed to welding, much less distortion and far quicker. Dont want to go too ugly though. :)
     
  4. Mr Efficiency
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    Mr Efficiency Senior Member

    You don't need accommdation down inside the hulls with the cat, the wide beam gives you plenty of room on one level. This ballasted ama business is a bit outside my apprehension, are you talking about variable buoyancy in it ?
     
  5. BrendanfromNZ
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    BrendanfromNZ Junior Member

    Thats right, keeping everything on the wing deck. Done couple of very rough calcs just now, looks like I would need 2200 - 3200kg of gear, plus hull. So probably in the 4200 - 5200kg range loaded.
    12m x 0.75m hulls 16:1
    No weight in the ends
    Semi immersed transom
    2 x 30hp diesels
     
  6. bajansailor
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    bajansailor Marine Surveyor

    Please do post some more drawings when you have them ready - your new 12 metre cat version described above sounds interesting.
    A bit like Richard Woods' Skoota 28, which has all the accommodation on the bridge deck?
    Sailing Catamarans - Skoota 28 transportable minimum live aboard cruiser http://sailingcatamarans.com/index.php/designs-2/6-powercats/264-skoota-28
    Or her slightly bigger sister Jazz?
    Sailing Catamarans - Jazz 30 fishing, day charter or cruising http://sailingcatamarans.com/index.php/designs-2/6-powercats/438-jazz-30-fishing-power-catamaran
    Have you done a rough check on the buoyancy available with such skinny hulls? I presume the 0.75 m. is the beam at the hull waterline?
     
  7. BrendanfromNZ
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    BrendanfromNZ Junior Member

    Yes I like that Woods Jazz 30 charter, perfect fishing layout
    but NZ cat1 regs require a 12m minimum for offshore passage.
    0.75 beam works out to about 4100kg @ 0.4m, or 5100kg @ 0.5m
     
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  8. Mr Efficiency
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    Mr Efficiency Senior Member

    What is the scoop with these regulations, you mean for private, recreational use.....?
     
  9. BrendanfromNZ
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    BrendanfromNZ Junior Member

    CAT 1 is the rules and safety requirements around a passage from NZ to the tropics/Australia (1000nm +) mostly designed for sailboats
    But 12m minimum for power
     
  10. Mr Efficiency
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    Mr Efficiency Senior Member

    I don't understand, they can apply sanctions to people who sail off in what they consider inadequate boats ? Needless to say, not including COVID !
     
  11. Mr Efficiency
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    Mr Efficiency Senior Member

    In any event, a 12 metre (40 feet) catamaran is a serious (expensive) boat, and won't be within your budget.
     
  12. BrendanfromNZ
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    BrendanfromNZ Junior Member

    Thats right, even at minimal level its a big build.
    I can afford to build it but I can't afford to get someone else to build it.

    Mind you without labour costs its only
    • 50K alloy
    • 40K drive systems
    • 15K electrical
    • Minimal glasswork
    • Minimal woodwork / fitout
    • No paint other than antifoul
    And this sort of boat doesn't exist on the market here in NZ.
     
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  13. bajansailor
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    bajansailor Marine Surveyor

    Sounds good! Looking forward to seeing your new drawings.
     
  14. BrendanfromNZ
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    BrendanfromNZ Junior Member

    OK
    Minimal catamaran version 1
    12m x 4.8m
    4500kg medium load
    0.75m hull beam
    0.45m draught
    0.85 ac
    0.65 pc
    2 x 30 hp shaft
    Tennant style immersed transom
    0.75m Bridge clearance
    Helm stb, open side for fishing w/ roll down clears
    W/C to port 20200814_132723.jpg
    Benefit of not having any accommodations in the hulls
    • very easy to balance using fuel tankage.
    • Only requires spray foam insulation at W.L. level for sound deadening
    • Plenty of storage & reserve buoyancy
    Reverse window? Playing around.
    Flybridge would be epic, but not really in keeping with the minimal theme
     
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  15. philSweet
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    philSweet Senior Member

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