Catamaran Performance: Comfort

Discussion in 'Multihulls' started by APP, Jun 2, 2011.

  1. APP
    Joined: Nov 2010
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    APP Junior Member

    Hi,
    In this site http://cruisingresources.com/Nautitech_82
    they calculate Performance Indicators for available Cats. How to calculate CAT's Comfort?
    Ted Brewer’s Comfort ratio is only for Monohulls. Any idea?

    Regards
    APP
     
  2. catsketcher
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    catsketcher Senior Member

    I am not sure how you would do this for a cat. In a mono you would probably start by working out the accelerations involved. My guess is that the displacement would be a large plus for comfort in order to minimize all accelerations. Initial stability may be a minus as firm bilges give a quick response.

    In a cat its not that simple. A heavy cat may have low bridgedeck clearance and may slam. It also may have considerable weight at the ends which may exacerbate pitching - although that is not strictly true. Weight in the ends reduces pitch damping so the thing just keeps on rocking.

    I think that a comfortable cat would have
    -plenty of clearance
    -good rudders
    - a low helm position - or viewing position under autopilot (high position will increase accelerations)
    -seakeeping accommodation near the pitch gyration centre
    -weight out of the ends
    -galley down or near pitch centre
    -cockpit near pitch centre (or as near as possible)
    -watertanks down low
    -light diesels or put them under cabin sole floorboards (need a bigger cat for this) or just go an outboard

    cheers

    Phil
     
  3. rayaldridge
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    rayaldridge Senior Member

    I might add overall beam to this list, since, within reason, the greater the overall beam, the more comfortable.
     
  4. Alik
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    Alik Senior Member

    Comfort factor - do You mean this one?
    COMFORT FACTOR = disp/(.65*(.7*lwl+.3*loa)*beam^1.33)

    Frankly speaking, this formula has nothing to do with comfort.

    Comfort is assessed by:
    - accommodations (ergonomics, space planning)
    - accelerations (MSI, MIF, MII, vibration)
    - noise (general noise level, sound privacy)
    - lighting
    - climate (temperature and its distribution, airflow, ventilation)

    The formula given above is supposed to be an indicator of acceleration level, but actually it is not as few important parameters are missing.
     
  5. rattus
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    rattus Señor Member

    So, Alik, how would you improve it, especially for multihulls?

    Non-constructive criticism doen't help anyone.

    Mike
     
  6. catsketcher
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    catsketcher Senior Member

    On from what Alik says I would like to put down front cabin windows as being in the mix. Steeply sloping windows look racey but are oppressively hot in the tropics. It's a bit of a fashion rip off from modern cars that have air conditioning - a very powerful air conditioner and a very small volume to cool. In Australia many of the sloping catamaran windows have shades or covers on them in the sun. I have had a lunch at a dinette under a hot window and it wasn't nice. I felt like it wasn't just lunch that got cooked.

    The French style windows would have to be more comfortable as they can also easily have vents installed. You get airflow and far less solar catch.

    Considering cruisers mostly sail downwind when cruising it seems a bit silly to go racey on the cabin front anyway for wind resistance issues. I rather steeply sloping windows (45 degrees) or vertical fronts - more like car windscreen design before air conditioning changed the shape.

    cheers

    Phil
     
  7. Alik
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    Alik Senior Member

    Comfort is a complex set of qualities (listed above) that can not be expressed in single formula. Generally comfortable level of accelerations for passengers is 0.2g RMS. But it is also frequency dependant.

    There is dependence of acceleration on speed, heading, seatstate and particulars of boat. For high speed craft it mainly depends on deadrise, chine beam and displacement.

    P.S. I believe term 'non-constructive' can be better applied to invention of formulas that do not have any physical sense. And application of these formulas for craft they are not intended for.
     
  8. Alik
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    Alik Senior Member

    Phil, have You seen my paper on comfort factors in last issue of Australian Mulihull? This issue is mentioned there, actually it is part of 'climate' issues.
     
  9. catsketcher
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    catsketcher Senior Member

    Will have a look

    cheers

    Phil
     
  10. Mark Cat
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    Mark Cat Senior Member

    If we move into the Heating Ventilation and Air Conditioning (HVAC) world, we find a Comfort Chart and the study of Psychrometrics (the study of air and its properties).

    We can plot an area of comfort for most adults on the Psychrometric chart, which locates an area pertaining to relative humidity, dry bulb temperature, wet bulb temperature, saturation temperature and enthalpy.

    So when below decks or in a closed space or cabin, the HVAC system must achieve passenger/crew comfort for the boats operating climate.

    The trend for aluminum hulled charter boats between 65 and 100 feet in length, is to address the HVAC requirements per individual cabin, or space (Galley for example). So we have a mini HVAC system in each space. The systems I have been working with are going towards all DC operation at 24V. So the Air Conditioning and Heating coil are powered from 24VDC.

    The house battery banks are large with chargers running from the Genset or shore power.

    The key in extreme climates is providing enough effective insulation and the use of forced ventilation (controls support either fresh or cabin recirculation).

    Also, when a cabin is not in use, you can shut down the AC and Heat and just run the forced ventilation. If setup properly a small brushless DC fan can provide adequate forced ventilation. I like to power to these small vent loads through the use of solar cells. So I size the cells and battery banks so as to support forced ventilation over a set room temp delta (running max 18 hours out of 24).

    Mark
     

  11. Skint For Life
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    Skint For Life Junior Member

    How about instead of a formula, everyone collates their ideas on what they think makes a cruising multi comfortable or not. Make a test up with these issues, then a simple 1 to 10 score is given to each boat on each issue. The boat with the highest score wins. Easy :)
     
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