Cruising cat design

Discussion in 'Multihulls' started by Moewaka, Jun 25, 2016.

  1. Mickdh550
    Joined: May 2018
    Posts: 7
    Likes: 4, Points: 3
    Location: Melbourne

    Mickdh550 Junior Member

    Hi Niclas
    Insulation is usually required for a major temp difference and to help in condensation control, agreed. But here is my question. Are you going to be cruising in extream temps? And re the condensation I strongly believe that air flow is super important on controlling moisture in any boat. I believe it’s much more important than insulation due to the pervasive moisture laden air that will get in regardless of insulation. My advise is to design your active and passive airflows carefully so that you manage air moisture via air movement /air changes per hour.
    Hope this helps.
    Niclas Vestman likes this.

  2. Niclas Vestman
    Joined: Aug 2017
    Posts: 97
    Likes: 10, Points: 8
    Location: Malmoe, Sweden

    Niclas Vestman Junior Member

    Hi Mick! I believe you are spot on regarding good ventilation. But to me it would be a little strange, not to use the best methods, if there are no extra actual hassles or drawbacks involved. Regarding insulation, you can't avoid condensation in anything but tropical climates, just because the temp difference between air and hull is to large. And for areas such as Europe, North America, Tasmania, NZ ao, insulation should be a given.
    My earlier remark also relates to the suggested beautiful Dix55 design. It doesn't have foam core, so some insulation would be logical. The ply skin, although probably quite thick, only has a lambda value of about 0,14 W/m2 dK. Meaning dissipation of 0,14 W per square meter, one meter thick wall, at one degree Kelvin (or Celsius) temp difference. if the boat has 3 layers of 1/4" ply skins with an area of about 200m2 (not including windows, hatches and doors, and cold storage (eg sail locker in bows), coz they are a constant), it would shed 0,14/0,01875*200*8 = roughly 12.000 Watts continuously in an 8 deg Celsius colder air/water environment. To counter that, you would need an industrial size diesel blower. And it is no wonder, since e g the Dix 55 has roughly 777,7 square feet och living space. About the same size as an average 3 bedroom apartment, or small semidetached house! It's Huge! The minimum requirement by code in insulation for a new built house in Sweden, would be about U=0,13 or 0,13W heat loss per m2 wall area and per degree temp difference. That would be equal to about 250mm or 10" XPS plus hard wall skins. Ok, maybe not entirely practical, and highly dependent on cruising grounds. Given the same ply thickness, but adding 4" of XPS (could easily double that), the heat loss now drops from 12kW to 0,55kW continuously.

    Also it was mentioned previously in this thread, that the boat already had been intended for "squiky" EPS insulation. So my thought was exactly for this type of case (or to be exact, mostly slightly smaller boats up to about 35' or 40'). Insulate with XPS 300kPa compressive- and shear strength, and 400kPa tensile. Just as shown in the Dix55 example. Only epoxy it in place and seal it with an inner skin of either laminate or 3 to 4mm ply. This way it becomes a structural part of the boat. Makes it float high if flooded. Gives a second watertight inner skin if outer is compromised. Prevents "squiking". Adds a huge amount of stiffness. Probably makes it possible to go for a thinner outer ply skin, and a slightly lighter and stronger construction. Especially since the now structural XPS helps transfer and spread the loads from outer to inner skin.

    Since the boat already is wood epoxy, it doesn't matter XPS is incompatible with poly- or vinylester. Since EPS already is recommended. The added weight is minimal. EPS 100 has 20kg/m3 density and XPS 300 has 32kg/m3. Also XPS has about 5% better insulation properties. And since XPS only takes up 0,7% water if immersed, compared to more than 2% for EPS, XPS helps keep out moisture and keep its insulation properties in the long run, whereas moisture laden EPS will do worse. Moisture will always exist on and inside a boat, so you might as well take it into account. Just my 2 Cents (x 10000) ;-)
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