Catamaran composite beam design

Discussion in 'Multihulls' started by groper, Apr 29, 2012.

  1. Alik
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    Alik Senior Member

    2goper:
    Transverse strength of 'bridge' cat is usually by default satisfied if local strength is provided. If You want to check it, loads can be taken from ISO12215-5 - there were some in preliminary editions of standard. Those loads are applicable for sailing cats where one hull can by flying; rules of Classification societies do not account for that condition.
     
  2. cavalier mk2
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    cavalier mk2 Senior Member

    We obviously need better weaponry as observed by the ship sinking the monohull in the Newport Ensenada....It is a good idea to use all the tools available today when designing but if you can't think outside the box all you'll build is a box....The mentioned designers gave the box wings.....
     
  3. Ad Hoc
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    Ad Hoc Naval Architect

    Would you mind defining what this means please:

    I know what you’re going to write….everyone is blinkered and narrow minded and never considers something different. That is a red rag to a bull to any professional NA/Engineer. It is the client and their budget and safety aspects that are the limits, not the ingenuity of the NA.

    Thus, I would be grateful if you could define this term for me please, without being patronising and condescending to professional NAs & engineers whom do this “thinking out of the box” as you call it, daily, but covet no audience. (BTW..I detest this management term to define something “new” or exciting…blue sky thinking….over the horizon thinking..what bollocks!)
     
  4. Alik
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    Alik Senior Member

    To me, development of efficient rescue boat with easy-to-use overboard recovery system has more humanitarian value than 'breakthrough' speed increase for 0.01kts on another sailboat toy, or adding 'stunning' glass ladder on billionaire's 'floating palace'... Yes, it won't be on TV.
     
  5. catsketcher
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    catsketcher Senior Member

    I think I agree with everyone

    At the risk of sounding like Polyanna I can agree with everyone here. I would certainly love to know the basics that are the engineering behind the typical multis seen today as few were engineered from first principles (at least in Australia).

    That said it was very interesting to hear about John Levell engineering B and Q and working our how to incorporate crack stopping into laminates. I would love to be able to understand what someone like him does.

    I probably have a slight problem with the engineer only approach after two very well known designers in this part of the world did not accept any responsibility for their designs shortcomings when their high end engineering approach failed. In one instance the cat I sailed on had a permanent deflection of 15mm extra after we tried to race it in an inshore race. Like the other cat I mentioned it was rectified by a boatbuilder rather than the engineer. The main problem was that the designers let it be known in a public forum that they would sue anyone who dared to suggest their design was faulty. They claimed the builder had stuffed up. Strangely a sister deign suffered similarly from a different builder. In another instance the designer again said the boat had not been built properly. Not a lot of acceptance of blame from a few professionals which should not smear a whole profession.

    This was obviously bad engineering and should not be used to suggest that enineers are silly, just that they should be clever enough to work out proper load conditions and how to cope with them. Also that they should desgn structures that can be achieved by builders.

    Maybe as a science teacher I am a great advocate of natural selection. The Seawind 24, the Searunner 37, the Nimble, Kraken 40, the Winspeed series, Seawind 1000, Trailertris and F boats have proven themselves so well it seems as though they are great examples of how to design a multi properly. With apologies to Cav I would go stronger than the Nicol beam connectives and Piver's butt blocks and keep clear of the Searunner 31s alloy beams, Pennant and Double arrow cracked at the bridgedeck join so be careful at the join, ensure the hull to deck join is strong so it doesn't split like Rush (C10 late 80s). Reading that it seems like pretty hard work.

    So I can understand how a class society can be useful - you don't have to be a tragic to learn from the mistakes of the past. As Mike says there were some tragedies in the early days (22 dead on multis in the Pacific 1966-1968) and some of these boats were awful.

    But I think that if someone wants to design the old way that should be fine. It obviously can produce a great boat and considering the experiments have already been undertaken by others then it seems sensible to take advantage of the huge wealth of knowledge in builders heads when drawing a boat.

    Both the engineering and using the past approach can lead to disaster if applied incorrectly. I have stated with many examples why I am very hesitant to put any weight in a boat designed by a naval architect who doesn't implicitly understand building. I would think any engineer would agree and obviously the reverse is equally or even more applicable.

    cheers

    Phil
     
  6. Gary Baigent
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    Gary Baigent Senior Member

    catsketcher Phil or OS7, you two would know more than I do about the Crowther 45 Spindrift and how the beams and bridgedeck connections were designed. I remember when Ivan Cranch's Double Arrow broke above North Cape and was lost, crew all saved, there was criticism of the hull/deck connection, which, if I remember correctly, was a right angle junction, and that is where it snapped when a large beam sea came through. The talk, I think, was, if the area had been coved in a large radius and then glassed along the below deck junction length and feathered onto the hull sides, this would have reduced the explosive wave effect on the hard point and most likely would have saved the boat. A simple solution. Universal on cats today.
    I remember also Ivan pouring in untold months, years, constructing the huge boat (we were putting (little) Bamboo Bomber SJ together nearby just before he was ready to launch) - and I was more interested in open wing design so didn't take that much notice of the Crowther 45 construction. All that work and he only had it sailing for a couple of months. Sad, but no lives were lost, so not so bad. He went over to vintage cars after that.
     
  7. catsketcher
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    catsketcher Senior Member

    In the end Pennant stood up well but she did need reinforcement early on when cracks developed in her armpits. IIRC this was one of the beefs of the owner of Double arrow - he said he didn't get told about them. This is one of the early examples of where the ply did catastrophically fail. Crowther, as the best trained of any of his peers should have known about Griffiths and his law on cracks - increase the radius and the stress drops dramatically. Yet he did not radius that corner. Add to that the very thin hulls of the Spindrift which increase loads as well. Spindrifts are a pain to live on. Lock (as well as others) was blind to the potentials of a chamfer panel or steps.

    The radius idea was popularised here by Grainger with his chamfer panels but Jim Young did it before on his little cats.

    As an amateur builder I think that designers have to increase scantlings quite a bit to allow for a wide variation in building standards by the backyard builder. A Schionning was built with any glass on the coves because the owner thought they were dumb. My mate used 10mm aircraft nomex panels instead of the Duflex ones called for. He also moved the cabin 300mm wider so he could walk down into the hulls. On top of that he added a back deck and made his own version of walkthrough transoms which the designer copied. I couldn't get the 400 gm uni specified and the designer said 360 would be okay on my cats inside laminate. So many people add an extra metre onto the stern it is seen as a rite of passage in a cat's life.

    I think Ad Hoc and other professionals would be worried about the lack of quality control on backyard builds I have seen around. So the loads better go through something a mutt won't fiddle with. Cause the builders will fiddle. One of my friends put an extra 300mm freeboard on his 40ft cat and wondered why the designer didn't like it. It sailed like a dog. He built the boat out of construction ply to save $2000. He even put a walk through hole in the main beam - good thing the Hitchikers were way overdesigned.

    So what design standards should us amateurs buy and peruse? Is ISO12215-5 the only one for cats? Does it include beam construction?

    cheers

    Phil
     
  8. groper
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    groper Senior Member

    well after 4 pages, thank you alik... , finally gave the answer and reference I was looking for... I will have to have a read and compare of that ISo rule, be interesting to see what kind of load the class rule dictates... and compare to a few reverse engineered structures I see today...

    btw, there is free FEA software avail on the Web... difficult to learn, as garbage in = garbage out.

    I'm not sure if the aforementioned class rule deals with deflection and allowable limits? I'm away from home on mbl so can't do much till I return in a Weeks time, but I want to keep this thread going... once loads have been established, how does one decide on an acceptable amount of flex to control fatigue related failures?
     
  9. waikikin
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    waikikin Senior Member

    The foam sandwich cats( No 85/95) of Locks seemed to embrace bunk blisters & the 8 degree canted hulls effectively incorporated a champher panel to the inboard hull to underwing join, I'm not sure if that predated Graingers Champher panels but could well do. The popular adding of a meter or more at times seems to be a symptom of owners wanting too much stuff in their boats as designed, it's quite pertinent to the discussion on strength etc of connective beam structures, lucky there seems to be some fat factors of safety. Regards from Jeff.
     
  10. Ad Hoc
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    Ad Hoc Naval Architect

    Sorry, that is not quite correct. Griffith’s equation can be related to either plane stress , ie thin members, or plane strain for thick members, which requires Poisson’s ratio. In either case it is related to crack growth i.e. a crack that already exists of a unit value “a”. Which is related to the stress intensity factor and the stress field surrounding the crack tip. We could, if you wanted then drift into linear elastic fracture mechanics or plastic elastic fracture mechanics and fracture toughness, but it deviates far too much from the thread! However, nothing to do with a radius.

    A radius or chamfer is a simple way to reduce the bending moment, that’s all.

    For what its worth I have designed with and without a radius. If you design without, you need to understand how you’re going to attach the cross beams and what magnitude of load and then how you do the load transfer to minimise the stress concentrations, if any, and of course fatigue. Can be done, if designed from the outset. Here are some simple examples without a radius (not mine):

    CATAMARAN BEAMS.jpg CATAMARAN BEAMS-3.jpg CATAMARAN BEAMS-4.jpg

    Not at all. I’ve seen them, it is generally shocking. But it isn’t my concern. Only when a yard is building my own design I check their fabrication and quality personally, regardless whether they yard has an established QA system or not and whether it is being built to Class or not. If the yard is crap, in my opinion, I won’t use them or recommend them, period.

    Any standards you wish to use, Class or ISO, take your pick, it is your design. You develop your own preferences after a while. These are, generally, nothing to do with the rules per se, but the whole process. That is the ease of use of the rules, the plan approval and the staff at plan approval and the surveyor and his knowledge and approachability, oh and cost!

    Yes they do. And the stress you calculate is defined by your structural arrangement. For example, DNV has no limit on thicknesses, just minimum reinforcement. Thus if you infuse and obtain very thin laminates, great…but your “structural stiffness” is actually low. Thus if you are using a higher modulus material, then it’s ok, but if not, then you need to up the thickness (reinforcement) to ensure the stress is within the design allowable limits….the usual design spiral.
     
  11. cavalier mk2
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    cavalier mk2 Senior Member

    I appreciate the restraints a NA must work under......also the sad notion of designing warships for a species that is intent on self destruction.....bound to be frustrating as well as morally indefensible.....Seriously I apologize for the red rag, you are obviously looking beyond your restraints as the puzzle of sailing multihulls is interesting you on your free time. And that is the fun of working on something with the pure purpose of efficiently harnessing the energy of the wind . It is also more environmentally responsible....It is good to be a sailing snob!
     
  12. cavalier mk2
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    cavalier mk2 Senior Member

    " With apologies to Cav I would go stronger than the Nicol beam connectives and Piver's butt blocks and keep clear of the Searunner 31s alloy beams"
    It's all right Phil, they worked it out back in the day and mine have been reinforced. That is why I recommend books like "Pelinta" to Nicol owners. That said there are many stock boats that have survived many years without problems so most off it was probably sloppy builds. I agree that the Piver butt blocks need upgrading- I read my D.H.Clark, The Searunner A-frame design is something of a convoluted mess....
     
  13. Alik
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    Alik Senior Member

    Besides some 'sailing snobs' are dreaming about pure efficiency for fun, others are already packing to India to help designing new ferries/improve safety on river transportation... And this is real social mission of naval architect and designer, not just making 'edgy' and 'out of the box' designs of useless toys :)

    Sometimes it is fun and challenging to design performance sailboats and it is fun to sail them, but those who design only such toys are exactly in a box themselves.
     
  14. cavalier mk2
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    cavalier mk2 Senior Member

    Ah now for some they are tools as well as toys. Mine has taught sailing to school children, taken injured Army troops sailing. been a research vessel for killer whale studies as well as taken me cruising.....All socially responsible roles (I need the R+R) very much outside of your box!
     

  15. cavalier mk2
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    cavalier mk2 Senior Member

    I should mention that Jim Brown and Dick Newick worked on Sailing workboats for 3rd world areas starting back in the 1980's. Brown would go in and teach them to build the boats. Wharram has come out with a nice fishing/work catamaran too....
     
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