Multihull Structure Thoughts

Discussion in 'Multihulls' started by oldmulti, May 27, 2019.

  1. catsketcher
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    catsketcher Senior Member

    Thanks for the info on the Race boats. Structural issues are always interesting to me.
    I did a couple of articles on Nigel Irens designed tris built by Boatspeed in Gosford. Irens was very interested in using different cores after the 2002 Rhoute Du Rhum where Fujicolor disintegrated. Only 3 of the 18 multis finished with many massive structural failures.
    One of the issues with Fujicolor was that she used carbon skins over carbon nomex. This was very stiff but there was no crack stopping ability with the stiff laminate. So Irens designed Ellen Macarthur's tri - B and Q - with a patchwork of cores - nomex and even balsa in high impact areas. Each area separated with areas where the outer skin was tied to the inner skin to ensure cracks would be stopped.
    I was at a talk give by one of the builders of PlayStation. She was strange in that she used aluminium nomex in between the carbon skins. They had to be careful to put a light layer of glass on to ensure the carbon did not form an electrical cell with the core.
     

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  2. Russell Brown
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    Russell Brown Senior Member

    I think a lot of race boats are built with aluminum honeycomb core. Seems very temporary to me because pre-preg is so porous and trying to seal the skins so that no water ever gets to the honeycomb is difficult. Saw an AC boat getting surgery and there was a lot of white powder in there. It was less than a decade old.
     
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  3. oldmulti
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    oldmulti Senior Member

    Trixia is a John Westall design (designer of the 30 foot Ocean Bird swing wing tri). The tri is 28 x 23.25 foot displacing 4000 lbs and carrying a 39 foot mast with a 415 square foot genoa in a headsail only rig. There are 3 headsails and a storm jib to reduce sail area. The headsails can be sheeted from 8 degrees to 23 degrees on the rear cross beam by sliding track blocks. The main hull length to beam ratio is 15:1 with semi circular underwater hull shape. But the real interest in this design is the articulated floats. The floats pivot on the main cross beam and have a slide arrangement on the aft crossbeam to allow the float to partially pitch independently of the main hull. The same concept was used by the Gougeons later F 40 trimaran Adrenaline.

    In the 1970 Crystal Trophy Trixia finished fourth and in the 1970 Round Britain Race the boat was fast but suffer structural problems and a backstay failure on the second leg. John found a single headsail rig was fast as long as you could use wide sheeting angles for reaching etc. The rig requires rod rigging and a stiff mast to minimise sag in the forestay. The articulated floats worked well but needed an interesting wrinkle. The aft end of the float required soft “springing” (control) in light weather but hard “springing” in heavy weather so the floats could respond to waves as required. If the float has rapid response to small waves in light airs it created more drag than if the float moved over the seas at the same rate as the waves. Adrenaline also had a “wooden” variable spring to control float movement.

    Trixia is built from the sheer down is 6 mm ply and double diagonal ply for the lower hull of 6 mm with a heavy glass cloth covering. The deck is thicker ply with a lighter cloth skin. The cross beams are multiple layers of timber with water stays to add strength.

    The final jpeg is of Trixia after it had been modified to a more conventional float and rig configuration.
     

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  4. redreuben
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    redreuben redreuben

    Russell, when I crewed on offshore boats at Freo the (plate)aluminium boats were called “aspirins” I couldn’t contemplate an aluminium honeycomb myself, perhaps it’s a US thing ? Foam, balsa or cedar in these parts, and for a while now some Paulownia. (Kiri)
     
  5. patzefran
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    patzefran patzefran

    I have owned a Ollier F40, bulkheads where Aluminium honeycomb glass epoxy sandwich, but hulls used foam instead of aluminium. Plane builders use plenty of aluminium honeycomb glass sandwich.
     
  6. catsketcher
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    catsketcher Senior Member

    One thing I can never get over is how someone who can draw a boat as lovely as the 505 could also draw such ugly trimarans. Westell's tris are pretty awful although Triaxia seems the best of them all. It reminds me of a scene in the movie "High Fidelity" where John Cusack asks if Stevie Wonder is still a legend because he wrote Superstition but also inexplicably wrote the dirge that is "I just called to say I love you".
     
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  7. oldmulti
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    oldmulti Senior Member

    A short one. The following PDF is an owners manual for Nacra's 20 foot foiling catamaran setup, maintenance and sailing. This is the "foiling" PDF which is in addition to an owners manual. This is a serious foiler and the manual gives many safety warnings about your health if you don't sail the boat correctly. Page 22 has warnings about how to care for carbon fibre foils which include lines like do not allow them to heat up to much as above 65 degrees C the foils can potentially deform or have CF degradation etc. Informative.
     

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  8. oldmulti
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    oldmulti Senior Member

    High Modulus Fabrics wrote a piece back in 1998 that is very relevant today. I am going to summarise the article for brevity. To meet ABS guide for building and classing of Offshore yachts you need to meet 3 basic categories. Bending strength. Stiffness. Shear strength. Other designer defined criteria include dent resistance, crushing strength, fatigue and impact resistance. Designers need to design structures that meet these requirements at a price that can be afforded by the owner/builder. The basic issue in the document was there is no point spending heavily on wonder materials if the design will be cost constrained elsewhere. Translation don’t build a carbon shell if you cannot afford a good rig and sails to drive the boat.

    So, first principle, what is the purpose of the boat? Is it a bay racer with a bunk or is it an ocean cruiser for 4 crew? If it’s a bay racer do you design for a long life where the boat will operate at 30% load for most of its life and can handle the 50 knot winds once a year or do you design for a max of 25 knot winds with a shell at 90% loading at that stage. Once you have decided on the loads and pressures the boat will handle then you can start to select the materials that will handle the job. You can build 2 boats that will handle the exact same loads but have entirely different materials, weight and costs.

    First you decide on EG the hull shell laminate, does it have sufficient bending strength, stiffness, sufficient pressure and puncture resistance. This is a case of skin and core thickness and type. It is little value in a 30,000 lbs 40 foot cruising cat to build a thin skin carbon fibre nomex sandwich hull shell when the boat will have 3000 lbs of water, 1000 lbs of fuel and 300 foot of anchor chain with a few 50 lb anchors on board. Maybe an e glass skin on a WRC will achieve the design criteria at a 1000 lbs weight penalty which will hardly be noticed on a 30,000 lbs cat.

    Second decision to be made is the longevity of the structure. There is no point designing the perfect structure for 25 knot winds if it will only last for 10 races because of wave impact damage or rigging shock loads because you sail in a gusty area. Here is where you need to understand the shear strength characteristics of your materials and their ability to handle impact damage. Also, some materials are “strong enough” to do the job but are for many unnerving to step on. Decks are a classic. If the deck feels flexible people worry. The requirement is the initial boat design criteria and the ability of the materials to handle the loads to do the job.

    But the final criteria is the ability to build the designed structure. To build a light weight structure properly you need at a minimum vacuum bagging and if possible, resin infusion. Really high performance structures need autoclaves. May I suggest (getting into his flame proof suit) if your going to hand lay up carbon fibre it may not be using the best material for the job. Nomex is not a nice material to build with and takes skill to do well etc.

    Old multi from here. If you have a good design done by an experienced person and you build it well in materials like e glass, WRC, PVC foams and unidirectional glass done with vacuum bagging, the structural weight of a finished shell will be within 20% of a hand laid carbon fibre shell of the same surface area of a boat designed for the same function. The jpegs below show a mono designed and built in NZ that applies a “sensible” structure throughout using materials as required.

    The Elliot 52 foot mono Primo has bulkheads from resin impregnated paper honeycomb cores skinned with e glass. The forward bottom had a balsa core for slamming resistance. The remainder of the hull shell was PVC cores. The decks were a thin layer of plywood over a low density foam core to avoid denting of the foam material. All skins were e glass. An interesting and cost effective solution that provided a very fast boat over its 60,000 miles of racing and 25 years of life.
     

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    Last edited: Feb 13, 2020
  9. peterAustralia
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    peterAustralia Senior Member

    Some thoughts on crossbeam strengths, trimarans, catamarans etc. First up, many will say this is obvious, everyone has known that for years. My response, yes that's true, but some of us dont know a lot, so this is for those. What follow is my opinion, could all be bollocks, but will risk it..

    I will start of think of a crossbeam as a beam.

    Say you have a beam, with one end fixed to a wall and it protrudes out. The further you go out, the more turning moment thus more bending moment, thus more bending moment closer to the wall. Thus everything else being equal you need to make the beam thicker near the wall and have it thinner as you get close to the end, ending up with a tapered beam, a bit like half an arch bridge. So are multihull crossbeams this shape? no. Why? because the analogy is wrong. Firstly crossbeams are relatively light, and the weight at the end is very heavy. Imagine a trimaran, the center hull may weigh 2 tonnes, but the crossbeam might weigh 50kg. Thus there is a strong bending moment on each and every point along the beam, knowing this, makes sense to make your crossbeams the same strength along their length, in other words, same depth, same width etc. Generally this is what most seem to do.

    But say you put the mast in the centre of the crossbeam, like a smaller Wharram cat. The downforce from the mast is forcing the beam to break in two. In this case you need the centre of the beam to be stronger, and it can taper away towards the ends. The extra strength in the centre being for the mast downforce, but there still has to be sufficient strength at the ends for normal catamaran stresses. Thus the Wharram cat should have crossbeams that have a mast on them to be thicker at the centre, but the same catamaran might have other crossbeams that have no mast on them, and these should be of straight section.

    Next loads.

    May sound obvious but if the outrigger is small, it can only push up its volume, otherwise it sinks. Say you have a trimaran and it has small floats, of 200 litres volume each. Since 1 litre of water weighs 1kg (yes it does, known as the metric system), the maximum force exerted by an outrigger is its volume, in this case 200kg. Now say you have limited resources but you want to build a large trimaran. You cant afford huge strong crossbeams because your on a budget. Solution is to use low volume outriggers, examples of this are the Jukung of Indonesia and the Banca of the Phillipines, both of which use bamboo as outriggers. Low volume outriggers, less performance, but lower loads, thinner, lighter and cheaper crossbeams

    But say you want to go fast, and you want large volume outriggers that do not submerge. Here the load is based on the weight. If your outrigger has a volume of 3000 litres, but your multihull weighs only 2 tonnes fully laden, then your maximum bending load is based on the 2 tonne figure. The bending load is the 2 tonnes times the distance from the outrigger to the center of gravity. Note that the sailor may choose to load up your boat with heaps of stuff and go over 2 tonnes, and still push the boat very hard, so perhaps when doing the calculations is best to take into account overloading.

    Now say you have a 2 tonne cat, and a 2 tonne trimaran. Chances are the trimaran will be wider. Thus the trimaran will have higher bending stresses, so maybe thicker beams? However the trimaran stresses are concentrated only on the section between the outrigger in the water and the main hull, with the other hull is doing nothing. Thus now the catamaran although has lower bending stresses these loads are exerted over a longer beam, thus maybe there is more chance of buckling, so maybe the catamaran beams need to be thicker to deal with buckling?

    Shock-loads. Say your sailing along, going fast in a big sea. Your boat hits a big wave. The inertia of the hull can hit the water and cause high shock loads. I think it is possible that these shock loads might be higher than normal displacement loads. Also say you have a flat bottomed hull, it slams down fast onto a a big steep wave. The floats has a lot of mass as say a catamaran does, the shock loads are going to be high. I would think that a hull with a medium vee or deep vee would have lower shock loads compared too a hull with a flat bottom. So maybe a multihull with flat bottomed outer hulls needs stronger beams to deal with larger shock loads?

    Next rotational interia. Say your 2 tonne trimaran is rolling around. Most of the mass is concentrated in the center, with relatively light amas. Your rotational inertia is likely to be less than say a catamaran with weight in outer hulls. When a catamaran hull slams down hard on a wave it will have a lot of rotational inertia that has to be dissipated by the water as it hits. A 2 tonne cat might have a hull that weighs 900kg and it goes from going down fast, to hitting the water, and then to going up again very quickly. Here large loads are put onto the hulls and also the crossbeams. So maybe catamarans without vee hulls need stronger crossbeams than trimarans to deal with shock loads of slamming hulls caused by going fast in big seas. (is this correct? this is just my educated guess). So maybe because of higher rotational inertia a 2 tonne cat needs bigger crossbeams than a 2 tonne trimaran?

    What about flaring out the top of the hull, and making the crossbeams shorter? This is OK, but it does not permit the crossbeams to be any thinner. The crossbeams still have the same large bending moment from the center of mass to the pivot point, irrespective of any flaring of the hull. But there can be some advantages. Firstly more internal volume, so that is good. Secondly it can reduce the length of unsupported span, thus minimising buckling. Lastly imagine you have a deep narrow hull that gets hit by a surging wall of water hitting side on. This wall of water will hit the side of the hull and cause stresses everywhere, but in particular where the crossbeams attach to the main hull (connectives). The wall of water hits the hull below the crossbeams causing a turning force, the hulls want to bend inward say. Having the top of the hull wider means this turning force is spread out over a larger area, thus the peak forces at the connectives from a surging wall of water travelling parallel to the sea are lessened

    wood. Say your on a budget and you build your crossbeams out of wood. You go all high tech for wood and make box beams. You look up the tables for wood strengths for various species, some rough figures might be a tensile strength of 80MPa with the grain, and a maximum compression strength of 40MPa with the grain (these figures are approx, but reasonable for quite a few species, some species higher, some lower). Now just say your on a budget and build low volume outrigger, allowing for lower loads and thinner crossbeams. You build your outrigger to be a size of 700 litres.

    Next you build your crossbeams 4.5m long and 30cm thick. Your main hull weighs 2 tonnes, but because your outrigger can only extert 700kg upwards (any more and it will sink), you can use 700kg as your load. What load is on the bottom and top of your crossbeams? 700kg x (4.5 / 0.3). Answer 10.5 tonnes, Convert this to Newtons and you get 103,000 Newtons.

    Say you build your box beam with two planks of wood that are 30cm apart, and each plank is 40cm wide by 2.5cm thick. Is this strong enough? Firstly assume that all the loads are taken by the front crossbeam and zero by the aft crossbeam, because you want to be sure. I think this is a reasonable assumption.

    Strength of plank is 40MPa x area (use the lower 40MPa not the higher 80MPa, note MPa means Mega Pascals, or millions of newtons per square meter). Strength = 40,000,000 x 0.4 x 0.025 = 400,000 Newtons. Thus you have a factor of safety of 3.8. The factor of safety has to take into account shock loads, poor quality wood, potential rot, collisions, fatigue stresses etc. Conclusion for this outrigger volume with this length of beam, your crossbeam seems strong enough

    Again all the above is just my thinking, could all be wrong, am no naval architect. There are no real tutorials out there to tell you how to design a multihull crossbeam. It would be nice to know what the shock loads are in high seas. To get that data would mean placing accelerometers onto multihull outriggers and logging the results.

    Lastly I link to a couple of websites that give some tables of timber strengths. The last link is to a large tacking outrigger from Madagascar (Madagascar being the large island on bottom right of Africa - yes I am referring to you Thomas, who needed to consult a map). Note that large main hull and the relatively small outrigger, I use this as an example of using a small volume outrigger that exerts less loads on the crossbeams. The downside is that sail too fast and the outrigger will sink, thus maximum speed is less. Aside this type of vessel though with a different rig is the de-facto everyday boat in Madagascar.

    https://www.fpl.fs.fed.us/documnts/pdf2001/green01d.pdf
    Wood, Panel and Structural Timber Products - Mechanical Properties https://www.engineeringtoolbox.com/timber-mechanical-properties-d_1789.html
    Pirogues - Madagascar discovery https://www.pirogue-madagascar.com/en/pirogues/
     
  10. Ad Hoc
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    Ad Hoc Naval Architect

    There is an article in Proboat Magazine, Issue no 182 on this very subject.

    I applaud your efforts. But...
    when designing with low modulus materials, such as composite and especially wood, stress is not the driver. It is deflection.
    Next time you fly...look at the wing tips...they are bouncing up and down very merrily and happily. But, imaging this same deflection at the ends of your outriggers...not good!
    Yet the stress levels are satisfactory.

    Thus the deflections are the key to beginning to understand the design of cross beams, it is not just the stress.
    You also have to deal with rotational moments both longitudinally and transversely at the same time too, this exacerbates the simple cantilever load case scenario, you note above.
     
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  11. peterAustralia
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    peterAustralia Senior Member

    OK, all good, so what formula do you use then?
     
  12. Ad Hoc
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    Ad Hoc Naval Architect

    By knowing the strain energy in the beam with a given deformation and a summation of the strain energy of all the cross beams, you can establish an angle of twist, per unit length.
    Then knowing the work done by the applied torsion is proportional to the angle of twist.

    It is not a simple one liner calculation.
     
  13. peterAustralia
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    peterAustralia Senior Member

    Many thanks , I dont doubt all that is true. My trouble is I dont know how to do all that calculation as the maths and engineering. Given how complicated it is all getting I can start to see why boats built to professional plans sell for higher, the amateur may/may-not have the maths skills to get it all correct first time. Maybe better to buy professional plans than have your boat break up in two (hello Hot Rod)

    I suspect that a tiny bit of deflection could be useful as the beam has a small amount of give thus spreading shock load force over a longer time period, thus lowering the peak shock load figure, think of a shock absorber. As there are at least two crossbeams in a trimaran (or tacking outrigger), the crossbeams would have upwards deflection, cant see any twist in the beams themselves. There could be twisting forces in respect to the boat as a whole.

    For small outriggers, my first one was 18ft, I just massively overbuilt it and was done with it, was a tacking outrigger so the extra weight of the crossbeams was useful when going on the wrong tack, two lengths of 3 x 1 timbers, 2 inches apart, with a 220L ama, crossbeams were overbuilt and thus too heavy and hard much work to assemble. For larger boats, I guess again you overbuild, and visually compare to what other boats do, traditional boats in Madagascar (I think they are called Lakarnas?), Indonesian boats, Sri Lanka boats (Orowas), traditional Polynesian tacking canoes, modern Fijian tacking outriggers, Freedoms Child outrigger, Wharram cats, Kismet tri's , modern Hawaiian outrigger canoes. Gaia 2 tacking outrigger, etc

    Note that the beam described above, 4.5m long, two lengths of timber, 40cm wide, each plank 25mm deep has a volume of 90 litres (use 10cm as 1 unit, one cubic unit = 1 litre, note two planks, thus volume in litres= 2 x 45 x 4 x 0.25). Assume a density of wood of 0.8 and your crossbeam weighs 72kg. Need to add some extra weight for the spacer blocks between the two planks, so maybe 80kg. Aft crossbeam maybe can be bit thinner as it is less likely to be subjected to the high upward load that is exerted on the crossbeam as the bow of the ama spears into a wave and pushes up, causing large upward force on the front crossbeam. So maybe 50kg for the aft one, giving total of 130kg, not too bad for 2 or 3 tonne boat. Boat thinking off was something like that large Madagascar gaff rigged tacking outrigger. Now 130kg of wood total for both crossbeams is not super super expensive, more so if using recycled timber

    IMHO it would be prudent to make the 'planks' out of many strips of thinner wood, so any rot is confined to small area. Note that traditional tacking outriggers often have very heavy front crossbeam and thin (often very, very thin) aft crossbeam, probably because the front crossbeam has a lot of outrigger extending in front of it, causing high twisting moments (diagonal tripping forces) and these are forced entirely onto the front crossbeam. The aft crossbeam has little outrigger trailing behind it, thus less twisting moment from a steep following sea lifting up the stern and lifting the trailing aft end of the ama. It is always possible to make a beam a little thicker, say 33cm deep not 30cm. However in the example I used my gut instinct is that the beam is sufficient

    I will see if the pro boat magazine issue you referred to can be back ordered

    Going back to alternatives to maths, looking at other vessels can be sound too. Say a boat has crossed an ocean, then maybe copying that could be an OK start. I looked at the Freedoms Child tacking outrigger, crossbeam is 12ft long, but is only 6 inches deep. Thus 24 times long as deep. Furthermore the ama is 20ft long. Now say this small hull has a volume of 1200L (maybe more), that means 29 tonnes compression in top and 29 tonnes tension in bottom of box beam. or 280,000N. Just assuming same dimensions of the beam, 40cm x 25mm, then the safety factor comes out at 1.4. Now given that the 20ft outrigger probably has a volume of more than 1200L, seems they were cutting it fine. Can only assume they were not crossing oceans, which was the case as it was limited to the carribean. The crossbeams were towards the end of the ama, they probably shared the load making is safer, but maybe was best they never attempted crossing an ocean whilst pushing it very hard

    Gaia II has three beams each about 8 inches deep and CL to CL beam is approx 16ft, thus a length/depth ratio of about 20. Beams were narrow but solid wood, boat weighed 3 tonnes. Have not done calcs on this. The Wharram cat Tama Moama has five solid spars of 140mm diameter, seems on light side to me.

    I know the professionals on here may look down on this discussion and say tick-tick, and say this and that has been overlooked, but we have to start somewhere

    At time of writing a very, very nice 71ft proa is for sale asking 53,000 USD, assume real price a bit lower. Hard to build a 71ft multihull for that kind of money, especially when u look at all those electronics etc. So maybe when it comes to larger boats, all these calcs become redundant as it can be better to just buy
     
  14. oldmulti
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    oldmulti Senior Member

    Peter. Lock Crowther told me about Italian government approaching him for advice on multihull boats as several people were building or buying them and the Italian authorities had no formal measures of build or craft safety. Lock provided excellent documents on how to calculate both the structural qualities and boat safety qualities which the Italians duly formalized in regulations. The amusement was Lock took the more difficult path of calculations from first principles in his documents which virtually gave him a lock on the Italian market for a few years. Other designers eventually worked out the short cuts that gave the same results with a quarter of the work. Translation what is not being said so far there a few simple rules of thumb that will give you an over engineered but realistic result without much work. But if you want a really good result you need to do the real maths with realistic expectations of what the boat is expected to sail through and an understanding of the true materials strength/ deflections etc in real live built condition. Lock Crowther had a university mate who built a Kraken 40 that had load cells over many parts of it to understand the real loads and deflections on the trimaran when sailing in many conditions. EG the exact same build materials on a foam glass shell structure have different strength and deflection characteristics depend on it being hand laid or vacuum bagged. If you change the fabric layup order eg move the uni directions from under a biaxial to outside the biaxial next to the foam using the exact same layup method it will change the strength deflection etc. Designers either by a lot of experience and failure or really good technical skill earn their money and are reluctant to hand over their hard won knowledge to quickly. My suggestion do a lot of reverse engineering of similar existing designs, you will start to find a lot of common patterns. But when you do the reverse engineering real understand the materials. EG a western red cedar cross arm without glass is structurally is different to a douglas fir cross arm of the same dimensions with a layer of glass.
     

  15. Ad Hoc
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    Ad Hoc Naval Architect

    Understood.
    But don't try to dumb it down so you can understand, as the baby is thrown out with the bath water!! There is a reason why people spend many years at uni and study - some things are not a simple one liner. If it were that simple - everyone would be doing it!!

    It should be simple enough or their online digital editions.

    As for the rest...you're deluding yourself that copying other similar deigns will cover it.
    Each vessel is unique to its own requirements, displacement, speed, dimensions etc etc. And more important the EI, the E being the Young's modulus of the material. In composite each is unique! Your layup may well be on paper the same, but the quality control vastly different thus your E will be grossly in error.

    You may get "a feel".... but it is like putting a Ferrari engine into a small 2 door family car and not understanding why the performance isn't the same as the 358!!
     
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