General comments

Discussion in 'Multihulls' started by bob the builder, Aug 2, 2009.

  1. bob the builder
    Joined: Jul 2009
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    bob the builder novice

  2. MikeJohns
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    MikeJohns Senior Member

    If you want qualified comment then you need to say what is you intended use of this vessel and it's vital statistics and some lines.

    I think the bridge deck is too low for anything other than sheltered waters.
     
  3. Ad Hoc
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    Ad Hoc Naval Architect

    I'd like to know if anyone has calculated the shear paths and deflections!

    "...i can change anything at this point in time..."

    Try keeping your posting just technical...drivel leaves just that and nothing sensible.
     
  4. bob the builder
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    bob the builder novice

    hi MikeJohns.
    clearance? - i wouldn't have a clue


    i'm a nerd (i live in a cave, have a pallor vampires envy, and my online name is Sword of Justice!)

    so i've lurked here for years, slowly digesting what i can.
    60cm seems not as low as 30cm, (quite a few cats have) n not as "!round the cape against weather, ARRR, " as 90cm

    i really dislike sailing against the weather, having had some (real) bad experiences.

    will be cruising, with all the time in the world to wait for good weather.


    cheers big ears,
    mal.




    (
    instead of the plans.zip then, i'll post all the pics here.

    see? now you don't even have to click on a single link!


    i now expect a comment frenzy. frenzy.
    )
     
  5. bob the builder
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    bob the builder novice

    South pacific island hopper.

    440cm wide
    31 feet long
    hulls 90cm wide at the aft waterline, flat, no rocker, 15cm deep
    60cm bridge deck clearance

    minimum sail plan;
    boom 3 feet from deck, 390cm feet long
    mast 10m high
    sail area 15m2 main, 15 jib (center of effort 4m above the water,

    minimum displacement 1100Kg
    average cruising 1300


    Live on. Lots of flat open usable space.

    Lots of aluminium beams - (120 Kg of ) Stop rig unloading, minimum flex around deck/hull joins means the glass will still be unbroken and not sloppy after a year on the water, to keep the deck stiff and unbuckling and in plane under load.

    Flat hulls (considering over standard minimum drag jobbies)

    Tiniest bit heavier than normal scantlings - want it still stiff in 30 years (i have a THING! about sloppy fiberglass EEW! etc)

    100 Kg/m2 weight/ water planing area

    fiona sinclairs for a single (700Kg) hull
    around
    30 kg drag at 10 knots
    70 kg drag at 15 knots


    i like the design so i'm about to have it engineered and softwared.


    [​IMG]


    [​IMG]


    [​IMG]


    [​IMG]


    [​IMG]
     
  6. bob the builder
    Joined: Jul 2009
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    bob the builder novice

    and to help start all you guys speaking!
    here's some ideas i'm thinking sticking on.




    the hull shape . . .

    fiona sinclairs applet generated fat cat hull shape
    optimised for surfing, planing.


    i can either build plain vanilla hulls (low drag optimised displacement) or flat ones.


    this question seems to have been asked over and over on these forums (was asked again just the other day)

    elsewhere on the forums . . .
    "
    .... Would the planing hull boat be faster ?

    At 20kts a 12m planing hull displacing 6 tonne would require twice the power of the best 12m cat of the same displacement.

    Rick W.
    "
    and he knows his maths.

    but then the question, why bother having a planing sailing boat at all? if displacement hulls do have lower drag than flat? something is not right here.



    so
    will i end up building it? i really can't say, waiting for this thread really.
    http://www.boatdesign.net/forums/sailboats/new-flat-hull-shapes-28676.html

    those aren't old designs, but expensive ultra new jobbies which gives me hope



    i might blend it with a michlet

    how much will a bit of rocker hurt planing?
    or
    how much will flat aft sections hurt normal hulls drag?

    my michlet progresses slowly, and rick has already done an example cat with full transom somewhere round here.


    why so much effort?
    why not go plain vanilla minimum drag?
    the weight to planing area is so good! 100 Kg/m2 (same as a LARGE surfboard. imagine the fun! in the open ocean . . .
    and the Sail Area /Weight ratio, as you can see from the pic above,


    makes me ponder . . .

    and ponder some more.





    A bridle on the nose
    to attach the front stay to.
    want the boat to have as long a life as possible.

    attaching to one point gives all the shock and load in the middle of the beam. sticking a bridle on reduces the length of beam to 1/3 of what it was, (which increases strength due to shorter beam) while halving the shock load, since there's now two points.

    might do it, might not





    More sail
    the conservatism of the minimum sail plan is high because i'm very lazy. n im not sure if i wanna get off my arse (or worry) for squals and drivel. (35 knots to lift a hull with full sail)

    but if the hulls will plane early, around 10 knots i reckon, then i'll maybe go with 45m2

    (
    i'm pondering
    aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa! etc
    )



    comments?
     
  7. apex1

    apex1 Guest

    You might be surprised Bob, but sometimes Mother Nature does´nt play to YOUR rules, so avoiding adverse weather means sitting at anchor or cruising round the approach buoy.

    Richard
     
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  8. MikeJohns
    Joined: Aug 2004
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    MikeJohns Senior Member

    Ah I see this has been discussed on another thread, I missed that.

    Although if this is the concept then the design spiral will unfold in due course.

    But right at the start.... the design is not suitable for offshore Pacific IMO.
    Poor bridge deck clearance and an exessive fore-n-aft extent. In offshore cat design and there are many poor designs that are way slower than a monohull and pound intolerably and dangerously at sea.

    Planing hulls ?
    Planing hulls are useless in this sized vessel in a seaway. A cruising vessel also carries a lot of weight. The Pacific trade wind belt is boisterous there's often a big underlying swell and 25 knots constant unless the southern convergence kicks in then it blows 50 knots for a few hours. It's not the smooth sailing people imagine. This vessel as you have designed it is not suitable for anything other than limited coastal, sorry to be blunt.
     
  9. Guest62110524

    Guest62110524 Previous Member

    yes well mike I may well be a romatic at heart

    I have also done a lot of SE trades sailing and yes it is not calm weather at all
    It is wonderful, BUT you need a strong boat with strong gear and strong people
    Paricularly light cats do not sit well at anchor, and often up in the Islands it is impossible to find an achorage where the swell does not roll in the motion is very very uncomfortable in a light cat .
    the boat that sits best is a chine mono
    BOB you must be bonkers, there are so many GOOD second hands cats out in the Carib, for 30% of what you will build for
    Buy me a ticket I,ll sail it back for you Real Rum there too
     
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  10. bob the builder
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    bob the builder novice

    thanks Mike.

    don't be sorry, i'd PAY to hear these sort of comments.

    how else am i gunna learn?



    good call.
    limited coastal is exactly what it started as.

    and i've thought for years making it a plain vanilla job, ie large tramps that go halfway etc,
    i've done several larger designs before this one, but decided to stick to day sailing/island hoping and the smallest boat i can reasonably get away with.

    basically i wanted a great barrier express with better long term accommodation.

    i didn't want something too large and heavy. and went the very light way instead.



    "Poor bridge deck clearance and an excessive fore-n-aft extent."
    yes, spotted perfectly. it would have pounding if you were on the wrong heading, and it was in weather.

    performance is not primary with this design, obviously accommodation is.
    it is not the best design for open ocean, but on the other hand, it is better than a LOT of production cats that only have 30cm clearance, and they call them open ocean. and it has much better accommodation than all the boats it compares to, ie Great Barrier Express etc.

    years ago all my designs were plain vanilla ie no full bridge deck. but i've been reading stories from owners of full bridge decks and they prefer the extra area over all weather ability. and thats exactly where i am.



    purely as a matter of interest,
    which of these would you call unsuitable for South Pacific Island hopping?
    http://www.sailingcatamarans.com/25_30.htm



    "a few degrees course correction can end it" (pounding) ??
    can anybody confirm this statement?
    anybody ever done it?



    (
    i've recently been on big cats, 35 to 40 and decided i wouldn't do it at all if i had to stick with these. just too big for me to feel comfortable handling all on me lonesome.
    so i'm not all that keen on building bigger though.
    i just don't want a big heavy cat. so i can either loose some front accommodation or change course, or not sail that day. i can build the hulls a few feet longer, but they are already 10 (10) feet longer than they started.
    )



    cheers,
    mal
     
  11. bob the builder
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    bob the builder novice

    whoosh . . .hmmmmmmm.

    "the boat that sits best"

    a yacht eh?

    hours spent trying to reach safety, and safety so near, and yet so f!ing far away, vomit everywhere, bashing into a southerly, engine on, all sail, and making no way. F that. been there. rather die trying to stick a yacht up my arse than actually sail one again.

    Strong people? an old lady once felt my hands and said i had the softest hands she had ever felt on a man, and had i ever done a days work in my life?

    Bonkers?! after they released me they even gave me a piece of paper which PROVES i'm not.

    i'll buy you a ticket, but only if it's one way.


    (see you monday)


    mal
     
  12. bob the builder
    Joined: Jul 2009
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    bob the builder novice

    Beams

    Beams

    i don't have a clue what's really needed or a good size. i have zero experience in the real world. truly, you the reader has a much better idea than me. i just chose because you gotta start somewhere.

    REALLY wanna shed some weight from these specs (120 Kg on the beams), but then, i don't really know. maybe this is really really good? (certainly is brilliant the amount of work they help avoid! no wooden beams or beam boxes to build, just some aluminium welding, then bolt them onto bulkheads)


    comment positive or negative if you have any real world experience with aluminium specs please.

    [​IMG]


    beam boy shows i've got twice the modulus needed on the mast beam (Ix 3 700 000 )
    (using 15000 Newtons moment in the middle gives 4mm deflection for two of 76mmx76mmx3mm, one on top of the other (one under the deck, one on top - trying to minimise the impact on living space.)


    here's that catalog link again, just in case you feel the urge (bored on a Sunday afternoon? ) to try and save me some weight.
    http://www.gjames.com.au/cgi-bin/priv?url=/marine/gjmarine-metric.pdf



    [​IMG][​IMG]


    http://www.boatdesign.net/gallery/showgallery.php/cat/500/page/2/ppuser/31994

    pics of all the beams here
     
  13. bob the builder
    Joined: Jul 2009
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    bob the builder novice

    Weight

    Weight
    (i'll change this photo as people think of stuff to add)

    do i really need such thick timber?
    bulkheads, hull frames etc. whats the minimum i should go?


    [​IMG]




    18mm ply @27Kg/2440/1200 0.3472 x 27 Kg = 9.3 Kg/M2

    kiri
    25mm 7.5 Kg/M2
    20mm 6
    15mm 4.5
    12mm 3.6
    10mm 3.0
     
  14. bob the builder
    Joined: Jul 2009
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    bob the builder novice

    General stuff

    Deck

    i was just about to make my first purchase on ebay, (excited!, first boat purchase!) some carbon fiber for the decks. cheap.
    but this stuff is cheaper,

    http://composites.jp/english_site/carbon_fiber.html

    minimum order 20m
    which is exactly how much i'm after
    (to do all the top deck, (7.5 x 2.6m wide)
    so i'll have that walking on steel feel for 30 years)

    600gsm stuff, 12k (cheapest, strangely. all the 3k 200gsm is twice the price. weird)
    $23 US/m2



    Centerboard. aluminium/kiri
    (flat aluminium strip 25mm wide x 2mm T6, kiri 25mm square) x 20
    = 54cm long x 2.5cm wide

    n say 90cm deep under the water

    ??





    already shed all the water, down to 25 L plastic fuel tank, 10mm strip plank on the cabin

    the quest . . .

    "
    building lighter than what is now considered normal, you will just have a better, cheaper boat, paid for by the saving in materials
    rob denny
    "



    Strip plank sizes
    anybody here done it before?
     
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  15. Guest62110524

    Guest62110524 Previous Member

    yes well the biggest majority of these PACKAGE monos, do not sail to weather
    , sailing to weather is not fun, but inna boat that can sail to weather it is very satifying
    the very same boats make so much leeway even cracked off, that they never go places
    I,ll be there are many cruisers who dont know where the block should be on the traveller in various condition upwind
    Reckon you would learn more on a crack racer in one weekend, that a decade fiddling:)) So Mr Oates, is looking for ballast , ciao
     
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