Minimal offshore power proa

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

  1. Bob Oram
    Joined: Jun 2009
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    Location: oz

    Bob Oram Junior Member

    Hello Russell,
    Found this thread after posting on the other one.
    Nice, simple explanations. Well done.
    I think this may be a way to incorporate some retro style into a very SIMPLE effecient long distance remote area cruiser, say around 12/14mts. A single 40hp 4 stroke should give 10/12kts @ 8/9 lph @ 3600rpm and 8kts at 5/6 lph @ 3200rpm.
    The nice thing about this configuration over the traditional bridgedeck cat is using the main hull for most accom etc thereby reducing cabin height (windage) down by a considerable amount, also improving motion.
    A watermaker and clever lithium setup will be essential to keep weight down amoung other demonstrable advantages.
    A light composite tender that can simply be dragged over the low hull side means no davits, reducing weight, work, windage etc.
    If using an extra long shaft 4stroke outboard as the tender motor, then at a pinch it could be used as a back up motor. There could also be small lowerable mast with a full cut reacher type headsail to help with economy and/or alternative propulsion(probably never be used given the reliabilty of 4stroke outboards).
    Hmmm will have to do some more thinking.
    Bob
     
  2. Bob Oram
    Joined: Jun 2009
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    Location: oz

    Bob Oram Junior Member

    Gonzo,
    Regarding 'flare' on a fast multi.
    Buoyancy at the forefoot works much quicker and quieter with little spray than a flared topside.
    No flare (and no tripping moment) on the big racing tris etc.
     
  3. BrendanfromNZ
    Joined: Apr 2020
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    Location: New Zealand

    BrendanfromNZ Junior Member

    Thanks all, for your replies. its great to have such knowledgeable persons providing input.

    Outboards - I live in NZ home of $2.20 unleaded (and I guess the islands will be more) which really puts a damper on petrol power.
    Also concerned about keeping a transom mounted motor in the water in rough conditions, but I might be over thinking this, and it would be so easy to fit an outboard and go.

    Possibility of a tender outboard mount on the end of the Ama, for close quarter maneuvers i.e. fuel dock.
    Two person job but it wont be everyday with 750L fuel and 45hp

    I probably need to stress this is a coastal boat, with offshore capabilities, including speed to shorten passages (1000NM from NZ to the nearest Island)
    The difference between 7 knots and 10knots over 1000NM is nearly two days, from 6 down to 4, a big difference in a crappy stretch of water like this one.

    And 10 knots is a minimum speed to allow local fishing trips 30 - 50NM out over a couple of days without spending too long travelling.
    Not concerned about fuel usage at 7 knots when trolling, with a 10:1 ratio hull, its going to be economical no matter what

    Whatever the outcome - this project and plans will be assessed by a NA.

    I like the idea of low windage, and getting that living area down low, this is another reason keeping me from a cat.

    Probably better behaved on anchor than a 5m wide boxy wing deck cat, and with much less anchor load?

    What it boils down to is the pros and cons of a Single outrigger vs a Power Trimaran
    • Both single engine
    • Both similar Vaka beam and layout
    • Wave motion
    • Single bigger vs two smaller Ama's and mounting points
    • Haul out concerns
    Thoughts?
     
  4. tropostudio
    Joined: Dec 2014
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    Location: St. Paul, MN, USA

    tropostudio Junior Member

    Brendan -

    You mentioned early on in the thread re. building some Tennant hulls. Were they similar to this Bakewell-White power tri?
    https://autoinsiders.co.nz/autofair/sales-ad/21308/Blakewell-White-Power-Trimaran.

    I like the idea of a low DLR main hull with a long, low DLR outrigger for a power boat. Keep it all light and simple. Russell's contention that the outrigger should be 3/4-ish the length of the main hull makes sense to me. My 'seat-of=the pants' solar electric proa from '96 went that route. The small, aft-mounted amas sitting clear of the water in the Bakewell White tri, or even the as-built Kurt Hughes 38' tri-trawler seem like they'd allow for snap-roll in waves. Looks like the the latest version of the Hughes tri has longer amas, moved forward.

    Current sailing tri's are using longer amas, with LCB moved forward of the main hull, to mitigate pitch-poling. A single outrigger will always touch the water to an extant. Seems like with a power tri, you can hold course and have time to adjust ballast for wave conditions. A long, low DLR outrigger with that can pick up buoyancy forward when immersed, and sized to accommodate water ballast, seems reasonable. Otherwise stash some light cargo out there for lateral trim, or use the deck area for kayaks. The whole boat should be narrower than a cat, which makes it easier to pick with a Travelift. Slip access may still be a problem due to width - but , then you'd have shoal draft and a kayak, so maybe no problem?
     
  5. Mr Efficiency
    Joined: Oct 2010
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    Location: Australia

    Mr Efficiency Senior Member

    If you envisage long hops across the big blue, stability compromises are not acceptable, skinny boats with dinky outriggers frighten me, who wants an outrigger of the weather side, when the big one hits. I'd be looking elsewhere.
     

  6. BrendanfromNZ
    Joined: Apr 2020
    Posts: 49
    Likes: 5, Points: 8
    Location: New Zealand

    BrendanfromNZ Junior Member

    Yes the transoms were very similar except the Tennant had a straight keel right to the prop, with taper on the sides only
    Makes sense to me, surely there would be less squat aft at sensible speeds.

    Tennant displacement hull - Google Search https://www.google.com/search?q=Tennant+displacement+hull+&tbm=isch&ved=2ahUKEwij4cj21IbuAhV_IrcAHRiRDiYQ2-cCegQIABAA&oq=Tennant+displacement+hull+&gs_lcp=CgNpbWcQAzIECCMQJ1DB0gFYwdIBYIngAWgAcAB4AIABzQGIAc0BkgEDMi0xmAEAoAEBqgELZ3dzLXdpei1pbWfAAQE&sclient=img&ei=elb1X6O8LP_E3LUPmKK6sAI&bih=488&biw=909#imgrc=f0Z5sukSMcTfIM&imgdii=hMaVXbjaambCJM

    But then you have boats like Earthrace (Loomes I think) with the Ama's right at the back with a razor sharp wave piercing bow.
    I remember seeing an article about the 20ft test model they built, thinking that's never going to work....
    Blasting around the world at 30 knots.

    Perhaps we are all (even though I don't sail) thinking about Tri's and Outriggers Ama design through sailing tinted glasses?
     
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