Is rigidity a religion? Amas on the move....

Discussion in 'Multihulls' started by Qmaran, Aug 31, 2021.

  1. Qmaran
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    Qmaran Junior Member

    Hadn't thought of that yet, may well be.

    Because I am working on soft wing sails I am going unstayed anyway so the shroud attachment is not an issue for me.

    I am not going for ultimate speed. Just relatively fast and easy to handle singlehanded so I'll stay away from foils.
     
  2. Skyak
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    Skyak Senior Member

    Designing flexible things is inherently more complex than designing within rigid ideals. Yes it can expand possibilities for reduced weight and stress, but it can also introduce new failure modes you never even considered. Impact stress, frequency response, fatigue, unstable fluid dynamics....all of these things you could ignore in rigid design are valid failure modes in flexible. Your trimaran can no longer be considered a single solid body, it must be three or four nodes with different motion.
    In high end racing tris there is no ignoring the flex of the light boat. Still, they don't design less stiff, they design light and deal with the flex. You don't have the engineering resources and your risk/return profile for a non-race boat should prohibit this experiment.

    Gougeon Brothers were not just engineers, they were geniuses with great experience. By all means study what they have done and learn from their successes. Adrenalin has a very specific design to improve 'hobby-horse' motion. I think today T-foil rudders would be a preferred solution but that is a frequency response question...
     
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  3. Michael Farmer
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    Michael Farmer Junior Member

    Wharram built flexible hull connections on his cats. They don't work either.

    Could you expand on the above quote

    Not being able to attach rigging is a trivial nuisance easily overcome.
     
  4. guzzis3
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    guzzis3 Senior Member

    Stiffness and strength are 2 different things. You can make a structure strong enough not to break, and to have a reasonable fatigue life. To make it stiff enough so it barely flexes is a whole other thing.

    Wharrams if properly built and maintained won't break, but they do flex. It makes it difficult to maintain stiff rigging, which might not matter. Wharram claims it makes the boats more seaworthy applying similar logic to that above. He makes similar claims about his V hulls.

    The REALITY is that plenty of cats of similar waterline complete similar voyages surviving equivalent conditions. We know from experience that it is a FACT that modern cats properly designed and sailed are at least as seaworthy as Wharrams, better sailing boats and it can be argued safer. Not because they are less likely to break capsize or whatever, but because a well rested dry comfortable crew are less likely to fail and an exhausted sunburned shivering waterlogged crew might.

    Flexible hull connections add nothing but a slack floppy rig. If you take the rig out of the equation (you can on a tri you really can't on a cat unless it's unstayed) the consequences are less severe, but you still have a massive engineering challenge ahead of you. You can't just have your hulls/floats flopping about uncontrolled. In a seaway you'd have a very unpredictable boat lurching about all over the place. So you build 2 structures, like a cars suspension. 1 to flex and another to control.

    We have the technology to easily and cheaply build beams that are very stiff stupidly strong, affordable and fairly easy to make. It's a bit crazy not to unless you have a compelling reason.

    Still if the OP wants to do an experiment that's great. Just don't make the boat unsafe.
     
  5. Ad Hoc
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    Ad Hoc Naval Architect

    Indeed they are!

    That's not correct.
    The strength or how strong, a material is, is only related to its mechanical properties. Such as yeild, shear etc..


    Indeed.
    And you can do with with any material, given enough of it.

    This is why designing with low modulus materials, for example, is driven by the deflection, not the stress.

    As noted previous, it is the structural stiffness, or flexural stiffness, the EI.

    The E, is the property of the material, its strength, or how strong it is.
    The I, is the stiffness, which relates to how much of it and how it is arranged.

    Thus you can't make a structure stronger (unless you change its mechanical properties, like selecting steel rather than aluminium), but you can make it stiffer.
     
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  6. guzzis3
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    guzzis3 Senior Member

    I don't understand what you are trying to say, but this bit is demonstrably wrong:


    What I said is correct. The ability to design structures that don't fail under defined loads is the whole basis of my profession.

    Maybe you can re-word your point ?

    Maybe you are trying to say that for a given size beam changing the material will affect deflection and breaking stress ? If that is the case then who said we are designing a beam of a given width height and thickness ?

    If you go back to the origional question it was for a 40' trimaran. You can work out most of the major loads on that structure. You need to make it generally strong enough to take impact and constant loads from all directions. Now you have parameters for connections that won't break.

    Now we need to consider deflection under that range of loads from all directions and how that will affect sailing performance crew safety, functions on board like sleeping cooking etc.

    Once you have a complete brief of everything the structure needs to achieve you go looking for a solution. This is how all engineering (should be) is done...
     
  7. Ad Hoc
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    Ad Hoc Naval Architect

    It is simply this:

    If this is true, you would, or should know, that a strong material (your words) has nothing to do with stiffness.

    If you take a 50x5mm flat bar, with the 50mm on the vertical, it has a stiffness of 5.208 cm4
    Take the same FB and now ahve the 5mm on the vertical..i.e. it is on the flat, its stiffness is 0.052 cm4.

    Its stiffness - 2nd moment of area - has changed, simply by rotating it through 90 degrees.
    Its strength or how 'strong' it is (your words), has not, as that is related to the material properties, nothing else.

    Merely rotating the FB through 90 degrees does not change its material properties, ergo its strength remains the same.

    So:

    Indeed, but one must either i) change the material (how strong it is)....or.... ii) how stiff it is.
    Or a combo of the two.

    Thus:

    This says,... if it fails, change the material, ....how, well easy from say wood to aluminium, or from aluminium to say steel, i.e. to a stronger material.
    This is related to the material property only.

    You are conflating strength with structural stiffness.
     
    Last edited: Sep 3, 2021
  8. Michael Farmer
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    Michael Farmer Junior Member


    Thanks for the explanation, I see now where you're coming from, it's all about the rigging and maintaining its tension. Wharram cross beams (Tiki 26) are deep"I" section, not a lot of flex in those, but he does use flexible connections to the hulls. Not absolutely sure if the wing sail fitted to that boat uses a stayed mast, but of cours it does have a jib, but to be fair cats have problems in general maintaining tension on the forstay.
     
  9. Michael Farmer
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    Michael Farmer Junior Member

    I would agree with that, especially if you could arrange the mast shrouds to attach to it, this would give a wide base reducing tension in them, obviously, it would increase the load in the beam, alas there is no such thing as a free lunch! I believe this would cancel the objection raised by guzzus3 about flexibility and rigging tension
     
  10. guzzis3
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    guzzis3 Senior Member

    I'm not going to address most of the stuff above anymore. I did my best to be clear but it seems I'm failing.

    Wharram has evolved. He's been dragged kicking and screaming into the 19th century as he watched everyone around him overtake his designs in every respect.

    Look at the "classic" designs. They were full of water traps and if not carefully maintained rotted away. I've seen so many of them beyond economic or sensible salvage. So as timber got dearer and people wanted lower maintenance he begrudgingly offered the glass clad hulls. As this was happening he gradually offered simpler efficient rigs. Then he started designing stiffer beams with fewer rot traps so his boats broke in half less often under the ownership of neglectful owners.

    The reality is most people don't want to be slaves to their boats anymore. People don't want to spend huge hours maintaining and repairing the boat. Here in Australia ply boats are next to worthless for this reason. People who buy cats want them to sail faster than monos because they just don't have time to flop around in a seaway hobbyhorsing horribly and making barely a few knots to windward. There are probably a few people still being seduced by the romance of Wharram's boats. They look wonderful and he is an incredibly good salesman, but his market has always been people with little actual sailing experience who think those boats are quicker easier and cheaper to build than a proper well designed modern catamaran. They just aren't, and while they do sail a properly designed cat of the same waterline length will sail as well or better on every point, be quicker and cheaper to build, and will be more comfortable to cruise both below and on deck.

    Take a look at Mick Waller's boats. They look a bit like wharrams at a glance. His plans are a fraction of the price, his beams are stiff, his rigs sensible, the accommodations vast and while they look like wharrams at a glance I'd guess they would sail rings around a similar size Wharram in any and all conditions. And are similar money to build.
     
  11. Michael Farmer
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    Michael Farmer Junior Member

    Thanks again, I'll take a look at Mick Waller's boats
     
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  12. guzzis3
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    guzzis3 Senior Member

  13. Owly
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    Owly Senior Member

    Unlike Guzzies3, I'm not an automotive engineer.......... but also unlike him, I've worked on vehicles.......mostly pickups and trucks for the last 35 years. Modern automotive engineers are abandoning old proven technology wholesale for inferior technology. Tapered lug nuts for flat faced lugs is a classic disastrous "upgrade" they keep trying and it keeps proving itself inferior. Pickup and truck frames in the past were always high tensile steel channel..... stamped to the desired profile from flat, and everything was hot riveted together. The idea was that the frame itself could twist and flex throughout it's length. Fatigue life is essentially infinite because the deformation is over a great length and well within the elastic limits of the material. It improves the ride because you are not depending entirely on the suspension under an entirely rigid frame structure. Trucks are still made this way. Cars went to unibody many years ago..........and for their light weight it works.... until the attachment points for the suspension fatigue, but that is beyond the service life of most vehicles. Pickups to my horror are now made with rigid welded boxed ladder frame.......... an absolute abortion if you intend to carry any load, but most pickups are just glorified cars these days and never carry any real load........ where I live we actually use them. It is not uncommon to carry 2 round bales on a 3/4 ton pickup using a bale handler that lifts them hydraulically and is mounted to the vehicle, and I've stacked as many as 52 square bales on a pickup, and carried well in excess of a ton of firewood or scrap iron.....but my vehicles are old. Newer ones do not stand up to this kind of treatment on a regular basis.
    The moral of this story is that flex can be a good thing if it is well engineered into a structure. The flex needs to take place through a distance. Ideally it should flex X amount per foot uniformly. That usually means engineering a taper into something... either dimensionally or in the actual load carrying material. If you look at an aircraft wing spar, you will see that it is tapered........ or the spar caps themselves are tapered in thickness.......... as the caps carry the main loading. You may see the wings on the airline "flap" looking at the tips, but that "flap" represents very little flex through the length. Properly designed the flex per foot may be small, but should be uniform over the length of the wing, and of course is cumulative and magnified by distance to result in a rather alarming flap when you look at the wing tip.
    When designing flex into a structure, think it through carefully...... I've re-repaired many structures that someone sought to reinforce, merely moving the failure point to the one or the other end of their reinforcement where there was a hard spot.....usually a weld........ I engineer repairs to they do not fail again....... anywere instead of somewhere else. One of my favorite stories is of a customer who was breaking chains on a machine and wanted me to put bigger chains on it...... I refused and instead suggested that he put a bunch of master links into the chain, spread out, and filed a notch in each master so when it failed it would be a master that failed and be a 5 minute repair...... He instead installed a heavier chain and sprockets..... over my objection. He then twisted the drive shaft off, and I refrained from "I told you so" and kept my mouth shut when he had a heavier drive shaft built, and ultimately destroyed the gearbox. Structural engineering is a part science and part art. Wood is a wonderful material for flexible structures.... depending on the wood..... It really does't fatigue like metals. My aviation background led me to love Douglas Fir and Sitka Spruce, but there are many woods, each with it's own properties.
     
  14. Michael Farmer
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    Michael Farmer Junior Member

    I'm not looking for a boat as such, well certainly, not a 40' Tri the OP is thinking of. My interest in this thread is hull cross beam attachments and the merits of modern (solid) and more traditional approach (flexible)
     

  15. Ad Hoc
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    Ad Hoc Naval Architect

    You should also look at Professional Boatbuilder Magazine, Issue 182, Dec-Jan 2020, they address these issues.

    What you term "traditional approach" is rather a rose tinted glasses view.
    It was more of a case of what materials were at hand and what means of analysis were available... in both cases, going back to those "traditional days".. limited on both counts.

    Im sure if you asked any one of the engineers of those bygone days, if they could use a stronger materials and hence utilise a lighter yet stiffer structure, would they.. and their answer most likley would be... yes.
    But they, like every engineer of their day, they were limited to what was available and how to analysis its response with a higher degree of confidence.
     
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