80 foot cargo harryproa

Discussion in 'Multihulls' started by lucdekeyser, Nov 15, 2020.

  1. rob denney
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    rob denney Senior Member

    Try this, which is the second image down on the cargo proa web page Cargo Ferry – Harryproa http://harryproa.com/?p=2561.

    We weighed the components a year ago. From the November #2 update 2021:
    "We borrowed a set of accurate crane scales and weighed the bits. Lee hull ends incl rudders (2 x 125) and middle (310), ww hull (350), beams (2 x 82), masts (2 x 122), wings (2 x 10 x 4.5kgs) and rudder blades (2 x 32). The toybox and 8m/28′ tender were not weighed, but bathroom scales say 150 and 250 kgs. Total is 1,880 kgs/4,200 lbs. Strings, blocks, winches, o/board, safety and nav gear to come, but looks like <3,000 kgs/6,600 lbs ready to sail. Materials cost $AUS50,000/$US33,000."

    One of the advantages of an all fibreglass/carbon/epoxy boat is that it is easy to tell how much weight has been added. Conservatively, it is twice the amount of epoxy used. Since it has been here, I have used 35 kgs of epoxy to join the lee hull, add fittings and replace several items when we figured out better ways to do them. Max of 70 kgs added, plus 10 kgs of pultrusions to make the tender davits.

    There are plenty of people keen on publicising the boat, but I don't have the time or inclination to teach and monitor them. There is no publicity required until the boat is launched and proven, and even then, it looks like there will be enough interest to not need to generate additional publicity. I write the updates as a least effort way to keep 200 interested parties up to date. I post them on discussion groups for the same reason I visit and talk to island villagers and other involved parties. I want as much feedback as I can get to improve the boats and save unnecessary work on ideas which are flawed. I look forward to your (and others) contribution. ;-) Cargo-proa-sketch-1536x958.png
     
  2. rwatson
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    rwatson Senior Member

    I remember that page.
    Do you think you will ever try the Truss style crossbeams, or has that been superseded ?
     
  3. rob denney
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    rob denney Senior Member

    Absolutely. It is a brilliant use of low cost composites, once the bugs are out. The main one is that to get sufficient buckling strength, the members need to be thick. The best, but not the only way round this is to use tubes, but there are then issues with attachments. It is a fascinating problem. Unfortunately, we did not figure it out in time so went with the pultruded carbon with fibreglass sides. The production boats will almost certainly have truss beams, with 'glass tops for walking on and ends that are tailored to their use.
     
  4. Barry
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    Barry Senior Member

    I noticed the V cross section on the prototype and the same cross section on the trussed image Post 151. This cross section is not the optimum to bending strength and might be worth addressing it from a design perspective.

    I am wondering why you went with attaching the cross beams to the mast. The mast tube which does not (appear) have a large diameter to resist bending, (depending on wall thickness)
    In this case you have loaded additional forces into the mast, first the sail force/stress loading and then any further forces caused by flexing between the hulls in waves.
     
    Last edited: Dec 14, 2022
  5. rob denney
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    rob denney Senior Member

    The thinking is that the bottom of the beam is in compression, needs to be thick to resist buckling and to give a simple point of contact on the mast. Would not work so well on a cat or tri, but seems logical for a proa.

    The beams attach to the mast to reduce the required hull size and concentrate the loads. There is a lot of reinforcing where the beam is attached to the mast. The mast tube is 400mm/16" dia which is huge for a 4m chord, but looks small on the 24m boat. The beams are tied to the masts, so any flex stress 'should' be dissipated.

    The boat is a prototype with a unique set of operating requirements (easily built and handled, shallow draft, low cost to build and maintain, easily disassembled for carrying up the beach to name the major ones), resulting in some unique solutions (wing sails, tied on rudders, beams attached to the masts, etc). It is meant for testing ideas, which is why the finish is rough. We will see what works and what doesn't and change the latter.
     
  6. Barry
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    Barry Senior Member

    Sitting at rest in the water with a cargo load between the hulls, the bottom of the beam will be in tension. Additional forces on the beam will be vertical shear, shear flow through the cross section and compression in the upper part of the beam.
    Introduced into the mix underway will be any forces that the mast to beam joint will add, forces due to the dynamics of hitting waves, ie pitching of each hull, forces due to the hulls following wave troughs, the dynamic forces due
    to the cargo being accelerated from the boats movement. I am sure I have missed some. (probably several)

    I cannot recall seeing very many beams with this V shape if designed for strength and acting without further support throughout its length.
     
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  7. Ad Hoc
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    Ad Hoc Naval Architect

    Indeed.
    Since the lower 'tube' would have to have the same stiffness and shear area as the upper two 'tubes'.
     
  8. rob denney
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    rob denney Senior Member

    The load is carried in recycled plastic boxes on the long hull between the masts. The only time the beams will see significant reverse bending is if the rig is caught aback and something jams, preventing them weathercocking. We are doing what we can to make this impossible.

    True, because they are no good for beams loaded in both directions, such as cats, for the reasons you state. But for primarily one way loads like proa beams, where twisting is not a big concern (no shrouds to lossen/tighten), they do the job. The primary loads are the tension in the top, compression in the bottom and shear in the sides. These are all accounted for. We will see if the design has missed any of the secondary stresses you mention and add laminate.

    We built some triangular truss beam samples and tested them (http://harryproa.com/?p=3788&page=6) bottom of the page. We also tested the full size beams by attaching a line to one masthead and a tree and lifting the other hull. No problem. If they don't work on the boat, we will try something else. As it gets nearer to sailing, we are getting much more interest from among others, engineers, NA's and Uni engineering Departments, so should have some interesting discussions before the production boat design is finalised.
     
  9. rob denney
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    rob denney Senior Member

    Some pics of the test section attached. The failure mode was the lower rod (not tube) failing in buckling. A conventional square truss of the same weight (and 1/3rd more labour) would have the single lower rod replaced with two significantly smaller diam rods and another set of diagonals. I did not do the numbers, but logic suggests it would have buckled at a lower load than the triangle.

    This reasoning is based on the second test we did. Turned it upside down and the original top with 2 rods the same weight as the single bottom failed at about half the load, again in buckling.
     

    Attached Files:

  10. Ad Hoc
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    Ad Hoc Naval Architect

    Without knowing the load and stiffness/scantling details, hard to say..but it could be, looking at the images, a compression failure that leads to instability (buckling).
    You can mitigate the buckling several ways:

    Having 1 member joining the nodes, like so:
    upload_2022-12-15_13-25-30.png

    That helps to transfer the load, but may still be insufficient to prevent compression failure/buckling.

    Thus, this would be more advisable:
    upload_2022-12-15_13-27-18.png

    2 cross members, reducing the long.t tube's span in half, and then joining them, for additional load path.

    More work...but...
     
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  11. rob denney
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    rob denney Senior Member

    There are several options, but it was designed to be the other side up and that buckled right where it should have, and only a few bricks weight below the designed load which did not include safety(fudge) factors or allowances for material or manufacturing flaws.

    Truss design is a lot of fun, trading off nos of diagonals and verticals with size of horizontals. Made much more interesting if all the elements can be varied in diameter along their length. Add in a few non cantilever loads (the toybox and the tender which hang off the beams) and it gets even more interesting.
     
  12. rob denney
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    rob denney Senior Member

  13. rwatson
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    rwatson Senior Member

  14. rob denney
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    rob denney Senior Member

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  15. cando2
    Joined: Nov 2021
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    Location: washington state, USA

    cando2 Junior Member

    Thanks for the update, Rob. It looks like you have found a good match, using your abilities to help others. It really is a good feeling giving to others and having them appreciate your efforts, gives meaning and spice to one's life. Please post more often; I enjoy seeing the evolution of your boats to more appropriately meet the needs there.
     
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