Minimal offshore power proa

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

  1. BrendanfromNZ
    Joined: Apr 2020
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    Location: New Zealand

    BrendanfromNZ Junior Member

    Hi all.
    I find myself in quarantine in a foreign country, 14 days with nothing to do but draw boats and drink beer.

    So I have revisited a concept thats been bouncing around my brain for a few years
    I've been around the design spiral a fair few times with this, but keep coming back to a power tri, or proa. Factors that influence this decision include;

    Rough water capability (NZ to pacific islands)
    Economy of operation (1L/nm max)
    Economy of build
    Single diesel on shaft
    Stability at rest
    Fast cruise speed (10knots) to reduce passage time
    Basic accoms for 1 - 3 + persons
    1000 nm range +
    Fast build time
    Trailerable components (nice to have)
    12m length to gain cat 1 for passage

    Plan is to take the boat north to the pacific islands for 4 months, with a couple of mates every winter. Fishing, kite surfing, surfing and spearfishing (in that order). Month of just the boys, then a month each with family flying up.

    Must be aluminum, as thats my trade. Good around sharp coral and very fast to work in. No paint, simple outfit, sensible electronics,

    Dimensions I've worked to are;
    Length w.l. 12m
    Beam w.l. 1.2m (10:1)
    Beam o/a 5m
    Draught 0.4m approx
    immersed transom (Tennant style)
    Design weight around 3500 - 4000kg
    40 - 50 hp shaft single diesel
    Ama 5m x 0.3m beam ( adjustable ballast)
    Fuel 750L 20200811_160459.jpg

    I'll post up a pic, before this post gets too long, then come back with some questions for you all.

    Cheers
    Brendan
     
  2. Mr Efficiency
    Joined: Oct 2010
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    Mr Efficiency Senior Member

    Your drawing isn't definitive, but why do you want am immersed transom ? I didn't think Tennant had such a thing ?
     
  3. BrendanfromNZ
    Joined: Apr 2020
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    Location: New Zealand

    BrendanfromNZ Junior Member

    Hi Mr E.
    Tennant had a cool transom design that worked to reduce squat. I remember building a couple of his hulls years ago. Top half was a full immersed hard edged transom. Below that was canoe body tapering away to where the horizontal shaft exited. Wish I could remember the term for it.


    I didn't get to drawing up the underwater side, but had a good starting point;
    Beam 1.2m
    Draught 0.4m
    Area coef 0.85
    Pris coef 0.65
     
  4. BrendanfromNZ
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    Location: New Zealand

    BrendanfromNZ Junior Member

    My biggest concern is the single drive unit in the main hull. Will this pull to one side? or can the drive line be slightly offset or angled to compensate.
     
  5. Mr Efficiency
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    Mr Efficiency Senior Member

    OK, the Tennant canoe stern that has a flat transom a little above the waterline, I wouldn't call that submerged, but carry on, that answers my query.
     
  6. Alik
    Joined: Jul 2003
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    Location: Thailand

    Alik Senior Member

    Proa has very unpleasant roll motions... I can probably understand sailing proa (sail is damping the rmotions), but power proa??
     
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  7. Alik
    Joined: Jul 2003
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    Alik Senior Member

    As a matter of similar idea, but a stabilized mono... Not a proa.
     

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  8. BrendanfromNZ
    Joined: Apr 2020
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    Location: New Zealand

    BrendanfromNZ Junior Member

    Thanks Alik,
    that power tri looks epic!
    My design started as a tri, but I felt by going to a single Ama, increases the stability, by increasing the Ama to roll centre distance.

    I.E.
    Proa 5m roll center to Ama
    Tri 3m roll center to Ama

    Therefore could reduce the Ama volume compared to the volume of two Amas
     
  9. philSweet
    Joined: May 2008
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    Location: Beaufort, SC and H'ville, NC

    philSweet Senior Member

    If you want an example of a seaworthy simple hull, look at Hogfish Maximus. It's a modified sharpie design and it has done what you want to do. It displaces about 44,000 pounds. You need that, not some 4000 pound proa with motions that will kill you. The boat you show is not fit for 1 person, let alone 3 or 4. Make a stores list for your trip, go buy the stuff, and pile it up in your living room - you'll see what I mean. You could easily run 6 tons of gear and stores with the toys you mention. One liter/mile is fine at about 4 knots, I do better than that with a 31' wl, 13' beam sailboat with 8000# of lead.

    I carry 600# of ground tackle in the Caribbean. 200# of tools. 100# of safety equipment. A dinghy and outboard, 6 months food at about 3 pounds per person per day. Electrical system will be something like 800# for the boat and about 150# per person (solar panels and batteries). You'll want a month's worth of water tankage figured at 3 liters per person per day, plus a water maker. You'll need fuel for the genset, for cooking, and for propulsion. Say 400 gallons. I'd want at least 300 of it in built-in tanks. The above scales in at something like 10,000 pounds of people, provisions and basic support equipment for such a trip.

    This is what a minimal aluminum ocean motorboat looks like. It almost meets your requirements. SetSail FPB » Blog Archive » FPB 70 – A New Baby Sister! https://setsail.com/fpb-70-a-new-baby-sister/
     
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  10. BrendanfromNZ
    Joined: Apr 2020
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    Location: New Zealand

    BrendanfromNZ Junior Member

    Hi Phil
    Thanks for your input, but carrying all that weight slowly around is exactly what I don't want to do.
    Yes the accommodations are very tight.

    Maybe I should explain what I want from a boat.

    I spend a lot of time at sea, fishing all the crappy places with current and rough water chasing big fish. Working around the weather windows.

    Last week we were 30nm offshore, chasing bluefin, 150nm from my home port, a two day weather window, that still blew up. Slow boats aren't an option. We often camp out, then shore fish from on very remote islands.

    Currently we use planing hulls from 4 to 7.5 meters, which is fine for coastal work, but lack decent sleeping arrangements. Therefore we camp.

    This design would be used in two ways,
    • Short 2 - 7 day local trips from 20 - 150nm from home port.
    • Long 4 month tropical excursions ( I work an oil & gas long roster, so would come and go monthly)
    In the tropics its a place to sleep, and store a few toys. Ill be out fishing at dawn for a couple of hours, in for coffee, then kitesurfing / snorkeling the rest of the day, followed by beers and more fishing during evening bite.

    Genset haha, whats it going to run, I don't even have a TV at home let alone on a boat. Basic solar and alternator running basic electrical. USB charging for a few gadgets, stereo, navigation and safety equipment. No 110 / 240v required.
    40hp diesel doesn't need a dozen heavy batteries.

    I'm a boatbuilder by trade, well aware of build times, also of how stuff grows, end up piling expensive **** on that never gets used, and then breaks when in contact with saltwater. If I can't fix it, I don't want it, except for stuff like GPS which would have redundancy.

    Exponential increase more stuff, then bigger engines, then more fuel, then bigger boat, that has room for more stuff.....

    My very quick calcs suggested a combined load of 2250kg
    Fuel requirements should be 750 - 1000L only.
     
  11. BrendanfromNZ
    Joined: Apr 2020
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    BrendanfromNZ Junior Member

    Hogfish Maximus is a cool boat, much too big for me though. No interest in sail either, I'm not trying to sail the Atlantic on a shoestring, I want to cross 1000nm of rough water quickly and safely, then explore the island chains in more of a coastal cruising sense.
     
  12. BlueBell
    Joined: May 2017
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    Location: Victoria BC Canada

    BlueBell Ahhhhh...

    I think you're nuts, a stabilized monohull would be infinitely better.
    But to answer your question: offset propeller shaft aligned with an offset diesel.
    You'll need to tow test the finished hulls or calculate where the centre of effort will be for it to track straight without rudder correction.
    The offset will aid in roll stabilizing this beast.
    That's a lot of aluminum cost for a prototype.
    But it sounds like you're all set to go.
    Any other questions?
     
  13. Mr Efficiency
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    Mr Efficiency Senior Member

    Why has a power proa got especially "uncomfortable motions" ?
     
  14. BrendanfromNZ
    Joined: Apr 2020
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    Location: New Zealand

    BrendanfromNZ Junior Member

    Stabilized monohull or trimaran is the same thing I guess, where is the cut off point? If the main hull can stay upright on its own?

    Stabized mono only has 3m each side of centerline/ roll centre, requiring more reserve buoyancy in each ama

    Proa starts heading towards a catamaran, most likely with some ballast in the ama

    Please explain the 'uncomfortable motions' surely its somewhere between a cat and a tri?
     

  15. BrendanfromNZ
    Joined: Apr 2020
    Posts: 42
    Likes: 5, Points: 8
    Location: New Zealand

    BrendanfromNZ Junior Member

    Bluebell
    Thanks for the input.
    The main hull would be very similar in shape if it had one, or two Amas.
    Apart from the lopsided engine issue...
     
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