Catamaran hull freeboard minimum offshore?

Discussion in 'Multihulls' started by Autodafe, Aug 18, 2010.

  1. Autodafe
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    Autodafe Senior Member

    When is low hull freeboard too low?
    I am currently desiging a small sailing catamaran suitable for cruising, including offshore. Because it's a pod cat, and I'm not having any accomodation in the hulls, reducing hull freeboard seems like a great way to reduce weight and windage. Is there some rule for a sensible minimum?
    A couple of notes here:
    I'm not talking about strength. Obviously there is a structural component o the hulls, and they need to be big enough to take the bending and torsion loads applied.
    I'm not talking bow freeboard either. Most design rules have some guidance for bow freeboard to prevent bow immersion so I'm treating that as a separate issue, and aiming for at least 1m at the bows.
    While hull freeboard can affect underwing clearance I'm treating that as a separate issue also.

    To give a clearer picture of the boat, LOA=12m LWL=11.8m, BOA =6.3m, B hull = 0.6m, Disp to DWL=1750kg, Weight light = 1050kg, hull draft = 0.2m
    The hulls will be a constant section (and hence freeboard) over most of their length.

    I had been working to a minimum total hull buoyancy of 250% maximum vessel displacment per hull, but for no particular reason. When I started sketching the plans this looks very low (around 620mm amidships). On the theory that if something looks odd it may well be bad, I thought I'd ask for some other opinions.

    The kind of thing I'm worried about is vicious or unpredictable motion in large or breaking waves, or the deck being constantly washed by green water (white and occasional green is ok).

    There are plenty of seaworthy leadmines around with freeboard similar or even lower, but does the narrow beam of lightweight catamaran hulls produce
    different results?

    Any thoughts or experience appreciated.
    Thanks,
    George
     
  2. ThomD
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    ThomD Senior Member

    I don't know of any standards for this kind of thing. podcats while a great idea, are not all that common, so it is even more difficult find standards in that kind of area. Further, there are some wave piecing designs in this segment.

    For a conventional semi circular section hull, the draft is about 33% of hull height amidships. Which would give a height above the waterline of .4 M in your instance. :) Another way of looking at it would be that a powerful cat of this type could easily have 75% beam so that would take you to 9 m which would tend give you a underbeam clearance of about .9m conventionally, and even if you aren't connecting them it gives you some sense of what it might be here. At 66% you would be looking at .8m.

    Here are a few versions you can scale:

    http://www.multihulldesigns.com/images/38CATsailg.gif

    http://boboramdesign.wordpress.com/46-pod-cat-for-sale/

    You can import these into cad, and scale them, though obviously, without a broad understanding of the design there isn't much point in just copying them.

    Just eyeballing it, but the Hughes version looks pretty close to beingin the ballpark I predicted.
     
  3. Alex.A
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    Alex.A Senior Member

    10% of beam seems low.... 10% of loa ? On 9m - i am aiming for 90cm + but minimum cuddy above - with a pod only this may be difficult?
     
  4. Alik
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    Alik Senior Member

    For offshore cruising, per ISO12217 standard, downflooding height (i.e. freeboard for most of real-world boats) should be at least LH/17, where LH is length of hull.
     
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  5. Autodafe
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    Autodafe Senior Member

    Thanks for the reply. There is a bit of a dearth of pod cats out there.
    I'm aiming for a more modest beam - the very narrow hulls reduce my longitudinal stability compared to a typical cat of the same length, so there's not much point going wide.
    Hughes seems to have gone around 1/16th of length for freeboard. Oram's boats are nice, but he goes for a high line.

    A couple of pics of poddys attached:
    Machete from CSK and Goss' Blackbird
     

    Attached Files:

  6. Autodafe
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    Autodafe Senior Member

    The pod can accomodate taller hulls - I just want to avoid increasing freeboard for the sake of it.
     
  7. Autodafe
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    Autodafe Senior Member

    Thanks Alik. A solid reference is great.
    1/17 puts me a little on the low side. Upping to 0.7m sounds feasible.

    I don't think downflooding is a concern as I have 10 separate watertight compartments, but I presume the recommended height is likely to reflect frequent wave impact - which is important for me.
    George
     
  8. magwas
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    magwas Senior Member

    GL recommendation for freeboard:

    B. Freeboard
    Tables are to be viewed as a recommendation;
    deviation from it is therefore permitted where there
    are reasons for it.
    When calculating freeboard, the determined value
    shall be taken as minimum freeboard.

    Table H.2
    Minimum freeboard for open and partially decked craft 1
    Comment
    Operation Category Requirement
    V Fb = 0,15 + 0,15 B [ m]
    (1) (2) and (4)
    IV Fb = 0,15 + 0, 20 B [ m]

    Minimum freeboard for decked craft
    Comment
    Operation Category Requirement
    IV *- I Fb = 0,15 + 0, 25 B [m] (3) and (4)
    (1) In the case of outboard powered craft, the freeboard shall also be maintained when one person on board is
    near the outboard engine (attitude when starting the engine by hand)
    (2) The requirements when the craft is swamped in accordance with Section 5, A.11 shall be observed.
    (3) If operating exclusively in Operating Category V, decked craft may have freeboards as for open and partially decked craft.
    (4) Craft for operation in sheltered waters may on application be given a lower freeboard.
     
  9. Alik
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    Alik Senior Member

    That is for monohull craft, I think...

    There is another quick check of freeboard > 0.2*B, where B - beam of boat. This formula derives from downflooding at heel condition. But not really used for sailing multihulls.
     
  10. magwas
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    magwas Senior Member

    Yep, it is for monohull, sorry.
     
  11. ThomD
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    ThomD Senior Member

    "I'm aiming for a more modest beam - the very narrow hulls reduce my longitudinal stability compared to a typical cat of the same length, so there's not much point going wide."

    I don't get this. Couldn't one just as easily say this about trimarans? The less your displacement, the more you need beam. and in any case 250% is as much displacement as is usable. And it costs you very little structurally, marina fees, etc.. in part due to the podcats stiffening the beams, not relative to a bridgedeck, but relative to open deck plans.

    I'm not sure why Oram went so large on the hulls. We had a brief exchange on various podcats before he came out with his designs (though he had high capacity sketches at the time). He had just launched his 37 foot cat, and he went on a cruise with guy in a podcat, and the rest is history. He sent me pics of the friend's podcat, and it was very low freeboard, like a set of 150% amas, Or at least it seemed lower than the Hughes. So something about all that made him think of higher freeboard boats. Most other designs we had been discussion had low freeboard. I have the emails, butt not here, and I have been too stupid to figure out how to get into old DBX files.

    Minimum regs are afterall minumums, and really can't be taken as a rcomendation of anything other than a negative. At some point one needs to have a point of theory (assuming one is starting from a blank sheet of paper).

    10% was Nigel Irens in print somewhere speaking of cats in general. On cats the wave clearance is often from some point below the shear, on podcats the shear is often low, with the beams arched, and a footwell back down again. So it's in the ballpark for both.
     
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  12. Eralnd44
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    Eralnd44 Wanderer

    I am familar that 1.5" for 1' beam is a starting point
     
  13. Autodafe
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    Autodafe Senior Member

    I may not have been very clear on this - The narrow bows means that i have less pitchpole stability than a typical 12m cat, so my sail carrying ability is limited by fore and aft stability and adding more beam wouldn't make it any faster.
    But I'm open to suggestion if wider beam has other advantages?

    As you suggest, I'm planning to work out a sheer line for the hulls, and then use beam geometry to get to my target wave clearance.

    A lot of pod cats do seem to have very small clearance under the footwell. It doesn't seem to slow them down much, but I worry that it could be noisy inside in a chop. I've never sailed in one so any experience here would be helpful also.

    Cheers,
    George
     
  14. Autodafe
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    Autodafe Senior Member

    Thanks Eralnd44, do you know if this is based on bridgedeck clearance, or downflooding or something else?
     

  15. ThomD
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    ThomD Senior Member

    I noticed that Kurt's 38 foot podcat, and 38 foot tri have the same sail area in the main and jib. Not sure they have to be undercanvased. I also noticed Kurt has a new 38 foot podcat on his site, more high tech, and it has higher freeboard.
     
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