multihull scantlings

Discussion in 'Multihulls' started by rapscallion, Sep 23, 2007.

  1. rapscallion
    Joined: Oct 2006
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    rapscallion Senior Member

    what is a good source for multihull scantling info?
     
  2. nero
    Joined: Aug 2003
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    nero Senior Member

    I do not think it exists. I used MacNaughton's strip planking scantlings for my boat hulls. Treated each hull as a bit less than half the total displacement for the basis of the calculations. They are real simple. And the cost was only $15.
     
  3. rapscallion
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    rapscallion Senior Member

    I can see that for a catamaran, what about a tri?

    and most importantly, what about the connectives?
     
  4. Alik
    Joined: Jul 2003
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    Alik Senior Member

    First, forget about scantling methods invented by the artists.

    Try ISO12215-5. They do not have separate part for multihulls now, but have a method for tunnel loads, etc. as an addition to monohull craft design method.
     
  5. Raggi_Thor
    Joined: Jan 2004
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    Location: Trondheim, NORWAY

    Raggi_Thor Nav.arch/Designer/Builder

    For the cennectives, what about calculation the load when all weight is on on ehull and the rest is in the air, and then add some for accelerations?
     
  6. Chris Ostlind

    Chris Ostlind Previous Member

    What boats are near you?

    And from the common sense school of boat design...

    Look at other successful multihulls of the same general size, displacement and sail area and see what they use for all the important connecting bits. Unless you are going to be producing something that has not been done before, there is a well-traveled road here for you to follow.

    Just cruise a boat yard known for multihulls and take lots of notes and photos. Soon enough, you'll have a large collection of data based on real working examples.

    Chris
     
  7. nero
    Joined: Aug 2003
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    nero Senior Member

    When I did my calculations, I looked at other cats of the same size. The results from the scantlings and from other boats were about the same.

    For my 14.5 meter catamaran, it worked out to 18 mm planking and 2 layers of 10oz unidirectional glass across the planking. I added a third layer of 10 oz bidirectional for other reasons.
     
  8. rapscallion
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    rapscallion Senior Member

    I was hoping to track down ISO12215-7 but it isn't finished yet.... I spent most of yesterday looking for a "working copy" on the internet.

    I really like your pluto design by the way, I would love to get some info on the "cruising" version if possible.


    Chris... there is only one trimran in the area... a catry 27 hull number 1.. and that one is experiencing some structral problems...
     
  9. catsketcher
    Joined: Mar 2006
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    catsketcher Senior Member

    Look at prior art

    I would go with Chris and look at well proven designs. It sounds like you have a great excuse to go down to the water and stick your head in lots of boats. get dirty with tape measures - micrometers and a well worn pad and work out factors based on what you work out as the ultimate loads. Then you reverse engineer what you want.

    I would be very cautious in picking up a non multi scantling rule for beams. I feel that since so many multis have been built and designed by non engineers standing on the shoulders of the giants that the inherited wisdom is huge and somewhat based on build methods rather than just design numbers. Trying to get this data to fit into a computer program may not be doable.

    A case in point - aluminum beams have a chequered history in tris. Aluminum is a great material fab for masts and booms where its light weight and high strength are used to the fore. But in beams the rapid cycling seems to make aluminum beams susceptible to failure at lower than ultimate loads. So although the numbers would suggest them as a good bet most, but not all, designers now don't use aluminium beams and go composite instead.

    Also do not neglect the incredible effect that design evolution has. Basically only the structurally sound boats survive a decade or two unmodified. So although some may say that the empirical method is old fashioned it is very well proven. For tris I would reverse engineer models like the Crowther Twiggy (simple box beams with wire tension members, a Piver Nimble (2 box beams in a plywood sandwich deck/underwing, Jim Brown's Searunner 31 aluminum beams and anything else that has lasted and easy to work out (Not a Newick wing aka - the load paths in one of these is very hard to work out - I built one and didn't really understand it). If someone gives you the data all well and good but really most of the best designers seem to have spent huge amounts of time looking into the boats of others. I would suggest it is a well proven route for a reason - the boats have a story to tell rather than a computer program that tries to correlate all that they consist of.

    cheers and have fun

    Phil Thompson
     
  10. rapscallion
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    rapscallion Senior Member

    thanks all for some great advice/insight. I'll look around for examples of the boats you mentioned, but multihulls are hard to find in around here... and when they are on the hard they are locked up in a boatyard.... trying to keep guys like me from touching them...
     
  11. rob denney
    Joined: Feb 2005
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    rob denney Senior Member

    G'day,

    What are you building? In lieu of boats to look at, I daresay the accumulated wisdom here could get you started.

    All the above is good advice for cruising boats similar to what you are copying. However, if you are contemplating a race boat built to the limits, or something different to the norm, then it is money well spent to get a composites engineer to look at it, particularly for relatively uncomplicated, well understood structures like the beams and rig. Ours works for $AUD75/hour which we usually recoup in savings from the seat of the pants approach we would otherwise use.

    I spent a few years selling materials to boat builders and designers. Almost all of them knew very little about the loads their boats would see and would beef up anything that broke without assessing whether it was a design, builder or materials fault. Consequently they all tend to end up overbuilt, exacerbated by materials suppliers acting as structural advisors and erring on the side of overbuilt for safety (and sales) reasons. One of, if not the first large cedar strip/epoxy/glass multi in the world was a 35' Malcolm Tennant tri. It had 6mm hulls with 200 gsm uni wrapped around them. The boat is still sailing. The norm for 35' cruising multis 5 years ago (when I left the industry) was 15mm core with 600 double bias each side.


    regards,

    Rob
     
  12. raw
    Joined: Jul 2006
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    raw Senior Member

    The Cedar Panel above - approx 3.5 kg/sqm (cedar 450 kg/m3, glass 1:1
    The foam Panel Above - approx 3.6 kg/sqm (80 kg foam 1:1 resin)

    And the foam would be far stiffer from thickness alone (and 3x the glass) - Are you sure this isn't progress?
     
  13. rob denney
    Joined: Feb 2005
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    rob denney Senior Member

    G'day,

    I was referring to cedar in both cases, but even if it was foam it would be a lot more expensive, so may or may not be progress.

    Incidentally, with your weights, you are are not allowing for the approx 500 grammes required to wet out the foam, the added weight of the overlaps in heavier glass or the lower impact resistance of the foam. In my experience, cedar boats usually come out pretty close to their expected weights, hand laid foam boats invariably more.
    regards,

    Rob
     
  14. raw
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    raw Senior Member

    We could debate this all day, but I I was just trying to point out your statement isn't cut and dry.
    Yes, I forgot the glue, in both cases :)

    One could argue that todays cruising boats, need greater hull laminates, since displacements are increasing with todays increasing complexity and increased systems added to what would have been reasonable simple affairs......

    Cheers
    Raw

    (designs plenty of laminates for a living....)
     

  15. rob denney
    Joined: Feb 2005
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    rob denney Senior Member

    G'day,
    My statement was cut and dried. 30 years ago 6mm cedar with 200 gsm was an acceptable laminate on a boat which is still sailing. Nowadays, 15mm with 600 each side is considered necessary for a similar boat, with similar payload.

    My point was that copying what everyone else does may cost Raps more money than necessary, weigh more than necessary and require more work than necessary.

    I have recently been convinced that strip planking is less good than infused foam in some situations. Happy to discuss the merits of each if you wish, but suggest we do so on another thread.

    Regards,

    Rob
    (designs, builds and weighs laminates for a living......;-)
     
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