Beam Design.... Again!

Discussion in 'Multihulls' started by cookiesa, Jun 29, 2015.

  1. cookiesa
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    cookiesa Senior Member

    After much searching I have been unable to source alloy for my cross beams locally. I was never really sold on using alloy beams as I am building on ply/epoxy and would like to make the beams myself. (The main beam is ply/epoxy anyway)


    I'm hoping those more knowledgeable might be able to help with design of the beams.

    The plans are for T6 Aluminium fore beam 200mm outside 5mm wall. Aft beam 101.6mm x 6.35mm (both 3.4m long)

    One thought was to make them ply 4mm with a 900gsm uni (running the length of the beam) with 400mm 9mm (bulkheads) in an oval shape, overall 200mm diameter. For the aft beam, same dimensions however with an extra 9mm "flat" on top for mounting the main track (filleted to the beam)

    My main concern is ensuring good bonds internally making the beams? ( do not have the room or inclination to do infusion)

    The other option could be a laminated Oregon (fir) beam, however I think the ply option would be lighter overall?
     
  2. Tom.151
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    Tom.151 Senior Member

    Simple or complex approach

    Hi

    Your beams look relatively light duty -- tell us what the boat dimensions and what the fully loaded weight is expected to be?

    For DIY beams it's either simple design or complex build requirements --

    Simple design to me means to resist all the main loading by using the top and bottom laminates -- typically in glass uni -- and rely on the wood elements only to separate the top and bottom elements. Increasing the height of the beam (the separation of the top from the bottom) is MUCH more efficient than adding gobs of laminate - that's why you see beams much taller at the main hull and less tall at the amas. For me it makes sizing of laminates more clear and predictable - you don't have to worry about the type of wood, it's quality, uniformity, etc. as the uni-glass is carrying all the tension (bottom ) and compression (top), using wood/ply just to be the web between the two.

    For me, trying to engineer the wood structure to carry the main beam loads can let you down (unless you add huge safety factors) because you can't build it as well as you can engineer it -- but for the uni-glass parts you can probably get within 90% of the expected engineered strength.

    I have used the continuous fiberglass roving (as used to feed f/g chopper guns) to reinforce 20' long akas on two of my 30 foot trimarans. since that material comes in a continuous length (on a roll) there is near zero waste. I just put nails in the beam ends (like the teeth of a comb) and stretch the continuous roving off the roll through the nails from one end to the other.

    Since you know the Alu beams specs, you can calculate the max bending strength exactly -- then just duplicate that with an amount of roving and beam height.

    Best of luck -
     
    Last edited: Jun 29, 2015
  3. cookiesa
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    cookiesa Senior Member

    Thanks Tom,

    Using the ply to seperate the glass is certainly my intention. It is more a case of getting the correct spec alloy is not easy here. I'm building the rest so figure, why not! The main beam is already a timber/glass design so happy with that.

    The expected weight is 950kg with a displacement of 1600kg.

    Thanks
     
  4. catsketcher
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    catsketcher Senior Member

    Gday Cookie

    Going composite for the fore and aft beam is a fine idea. You should do some calculations though. You need to work out the second moment of area of the current beam

    Forebeam = 1.45 x 10^7mm^4

    Then you multiply this by the Youngs modulus of the material to get the Section modulus

    Aluminium E = 69GPA

    You can just multiply if you keep on using the same units later

    = 100

    Unlike Tom I think wood is a better material for beams than it is given credit for. It is easier for a designer to know the basics of good wood than it is for widely varying laminating styles.

    If you want to use timber like Hoop pine ply

    Youngs modulus is 12 GPa. So you need to increase the Second moment to get 100 = 8.3 rather than 1.45

    Increase size is one way with increase in thickness too.

    Lets say 250 mm diameter with 12.5mm walls. This gives you a Second moment of 6.6 rather than 8.3. If you go 15mm walls you are almost there at 7.7.

    With some uni glass laid on top and bottom you will be there. How much uni? Do the same thing. Keep the section modulus the same for each part of the beam. I have done this with a few beams in my cats and it works well.

    Be careful of putting lots of uni on top and bottom of a beam without doing some calcs. You need to make sure the shear strength of the ply is not neglected where the caps glue on to it. A box beam is a great idea especially for an aft beam as it is great in torsion like the alloy beams. You can't go too thin on the side walls - about 4-6mm ply would be a quick guess for a box beam with 300 mm depth and about 100mm fore and aft.

    Look at JE Gordon "Structures" and "Stress without tears" by Bernard Rhodes for well worked examples of beams.

    cheers

    Phil
     
  5. cookiesa
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    cookiesa Senior Member

    Thanks for the input, that makes a lot of sense!
     
  6. aussiebushman
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    aussiebushman Innovator

    When I was considering sliding beams for the new cat, I hunted around and finally found that Capral here in Oz have the sizes you might want, though I agree with Phil that timber is probably a better choice and a lot less expensive. The size I checked was 152.4 X 76.2 outside but they have other sizes. One length of code 817251 is $346 plus GST

    Although the wall thickness is only 3.7 mm. this would be more than adequate. They are 6061 grade and are generally in 5.5 metre lengths.

    Call them - they are very helpful

    Alan
     
  7. cookiesa
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    cookiesa Senior Member

    Thanks for the heads up.

    Not much luck with them so far... Not enough demand
     
  8. cookiesa
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    cookiesa Senior Member

    Ok, still not getting far with the T6

    Looking at a ply box beam with the following to replace the 3.4m 200OD (5mm thick) t6061 fore beam.

    450mm centre section (mounting point for forestay, centre 38mm bulkhead, 18mm bulkheads each end, 35mm solid Oregon striker mounted above, 9mm reinforcing pad top and bottom) (giving a 18mm top and bottom for the middle section between these bulkheads)

    Main beam 9mm ply sides, 9mm ply bulkheads. 20x20 Oregon stringers (corner mounted) two layers of 900gsm unpaid lengthwise, 2 extra 900gsm layers around middle 450mm section. half cut pvc pipe resiined to front of beam with 450gsm double bias covering around beam (more for aesthetics, create a D beam)
     
  9. MikeJohns
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    MikeJohns Senior Member

    Phil

    That gives you EI which is the flexural rigidity, not Section Modulus (Z).

    Section Modulus is just I/y where y is the extreme fiber distance from the N.Axis. It's handy as the extreme fiber stress is simply bending moment(M) divided by Z.

    You probably remember that in retrospect........
     
  10. Ad Hoc
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    Ad Hoc Naval Architect

    These two statements are at variance with each other.

    The DESIGN of a structural member is totally different from how you MAKE it.

    Thus what is your beam designed to do...what loads and how is it attached and what life span etc...the answers of these dictate the design. How it is made, or even indeed which materials to use, is the second question...not the first. And can only be decided once you have the former.
     
  11. cookiesa
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    cookiesa Senior Member

    Hence the design would be as per the last post, made via wood, (marine ply and Oregon/Douglas fir), epoxy and uni cloth.

    As the post is in boat design, under multihulls and refers to the front beam of a catamaran one could assume (sometimes dangerous) that it would be for attaching a forestay to and provide additional bracing for the main hull. (Cat is of a bridge deck design, standard sloop configuration)

    Cheers

    Sorry attachment will be wrapped to bulkhead (the design has bulkheads directly under the beams. These will have a large fillet underneath (final size of beam will dictate this) then wrapped from bulkhead, full width up and over then down, each layer staggered 25mm 600gsm double bias, 900gsm uni, 600gsm double bias. This will be over a "sleeve" 500mm wide on the end of each beam 6mm thick 600gsm double bias. (Bulkhead is 220mm wide at the attachment point)
     
  12. Ad Hoc
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    Ad Hoc Naval Architect

    Okay...i'll assume your catamaran is just 1.0m long and weight 10grams and has paper for sails.....would that be correct?

    Or I could assume your catamaran is 100m long, weights 500 tonne and has more sails than i can shake a stick at....would that be correct?

    Ergo...you're missing the whole point of DESIGN.

    If you're seeking help/assistance, please read and reread my first and this post again.
     
  13. cookiesa
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    cookiesa Senior Member

    Signature kinda gives it away to... But hey thanks for the helpful lesson in ensuring I use the correct terminology.

    Very well aware of the difference between design and build. Not missing the point at all. Perhaps I should have been clearer and said how about a design of.... Built using etc, however I think most seem to have taken it the right way and not bothered to get bogged down in the use of a certain terminology in the wrong place.

    Thanks to those who have actually provided useful information! Ad hoc, perhaps you have a positive contribution to make?
     
  14. catsketcher
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    catsketcher Senior Member

    Thanks Mike - I didn't know I had that one wrong.

    Cookie

    I would again start with the design of what the boat has and then go from there.

    Your arrangement sounds well thought out but it could be over or under engineered. The best way is to see how it would compare to the alloy beams.

    Many designers do not follow Ad Hoc's approach as it is very hard to work out what the design loads are on a small cat. So most of the time empirical evidence is used from previous designs and then re-engineered for a new approach. There are many cases in Australia of designers going too far down the engineering approach and having beams fail when new approaches are made.

    We have been through this thread before and if you want you could use the Edmunds method where you design the boat so it can be tied to a seawall and the tide ebbs leaving one hull in the air. The beam shouldn't fail under this load.

    This seems like a good approach but I have asked many designers of great boats what they do and no one has given me a straight answer. It could be that they are hiding proprietry information but it is probably also true that they use their experience tempered with engineering and boatbuilding clout.

    So start with the alloy beams - work out the flexural rigidity. Make sure the wooden beams aren't extreme in shape and get them to be the same or better as the alloy ones. It is fine to want to use 4mm or 6mm ply if you want, 6mm is common in beams on these sized cats - just make sure you don't load the web too much.

    cheers

    Phil
     

  15. Ad Hoc
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    Ad Hoc Naval Architect

    It is actually very easy indeed. Even more so if one follows the ISO or Class rules based upon yachts as a guide. These now give very good approximations of the loads and then one can add a FoS to suit.

    Perhaps because they don't know themselves???!!

    When using low modulus materials like composites, it is not the strength that dictates the design, it is the deflection.

    Hence my comment.
    What is the SOR, or what is the boat DESIGNED to do?...once you know that, the rest is easy. But it appears, as is so often the case, many wish to put the cart before the horse, simply because...well, it is tangible to them, the former is not. Thus start within the comfort zone. The trouble is one is most likely to fall into the trap of:

    Simply because slapping on a few layers of cloth is easier to understand that trying to establish the fundamentals of what the beam is designed to do. Nowt wrong with that, because not everyone can design one and over engineering si then your best course to follow. But do not fool oneself into thinking just because I can make it and make have equivalent EI, is all there is to it. There is a lot lot more to it than that.

    Trying to prevent that classic mistake of hubris, seen so many times on this forum.
     
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