Designing and Constructing Crossbeams

Discussion in 'Multihulls' started by mcm, Jan 19, 2007.

  1. mcm
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    mcm Senior Member

    I want to learn as much as I can about designing and constructing multihulls.

    But I haven't come across any books on how to specifically design multihulls.

    True, much of what I've read in all the general yacht design books can be applied to multihulls, but I have yet to find any information about how to design and construct crossbeams and their hull connections.

    The books I've seen never discuss the loads and stress-points of crossbeams and their hull connections.

    I want to design and construct a pair of arched box-beam crossbeams for a 40' (12.3m) cruising cat out of a core of cedar strip planks skinned and stiffened with cold-moulded cedar veneers and glass.

    I want to design for a 7 ton displacement on a 24' (7.38m) beam-over-all with a center-pod cockpit stretching between the forward and after crossbeams.

    Does anyone here have any information that can help me, or know where I can find it?
  2. joe_cope
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    joe_cope Junior Member

    I am also interested in this subject. I was wondering if anyone had ever used plywood beams for crossbeams?


    I know that they have a high strength to weight ratio, but I'm not sure about flexability and I worry about water damage.
  3. Raggi_Thor
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    Raggi_Thor Nav.arch/Designer/Builder

  4. mcm
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    mcm Senior Member


    Well, we use plywood I-beams alot in construction. They are used to span long distances between posts where they act as headers for heavy load bearing walls.

    So I'm sure that if you're not using aluminium or pure composite core and skin you would use ply, and probably ply I-beams or D-beams.

    Raggi Thor,

    Thanks for the link. Eric W. Sponberg seems to understand it well, so I'll follow his suggestion and look for Arthur Edmund's "Designing for Power and Sail". I saw it at our library, but with "Power" in the title, I passed it by.

    "X-Beam" thread on the front page of this same forum, Yipster has a great cross section photo of 'Team Philips' carbon fiber D-Beam. I wonder what the core material is on their beam

    Also on that same thread, brian eiland shows a cross section photo of a trimaran's forward and after D-Beams. Those beams look paper-thin, and they look to be glass over foam. If he has that kind of confidence in beams that thin, then crossbeams must be easier to design and construct than I would have supposed.

    Moreover, that trimaran sure is starting to look good! And so does Team Philips look good even with the forward third of their port hull sheared off.

    I like their biplane rig. And thats a multi-million dollar, highly engineered (almost), racing cat. So that says they thought highly of the biplane rig's ability. Hmmm.
  5. yipster
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    yipster designer

  6. mcm
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    mcm Senior Member

    Thanks for the Shuttleworth link. I could spend all day digesting that web-site.

    Shuttleworth stresses fabric direction of unidirectional fiber. When I picture a crossbeam in my mind I see all forces, whether vertical or horizontal, attempting to crack the lengthwise fibers. So, if I'm understanding Shuttleworth correctly, running the unidirectional fabric the length of the crossbeam, at least till it meets the hull, would be the way to go. I suppose that's also the way to orientate the grain if the core of the crossbeam is cedar strip-plank.

    Why did Team Philips break where it did?
    Because they didn't have a compression strut across the bows?
    Because they wanted to be light, and chose to not to carry the additional weight of the crossbeam to hull connecting lay-up very far up the hull?

    If that was the reason, then I wonder what should have been the lay-up, and how far up the hull they should have carried it? And, to feather-out the edge stresses, how long and fine should one taper the edges of a crossbeam to hull connection lay-up?

    Any one have any examples?
  7. fhrussell
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    fhrussell Boatbuilder

    I once spoke with Woody Brown (inventor of the modern multihull) about crossbeam design. He said that his source for practical information in that department came from his knowledge of flying and building gliders. He was a champion glider pilot in the '30's.
    You may want to study some books on homebuilt and experimental aircraft wing designs. Try the EAA (Experimantal Aircraft Assoc.) and any homebuilt aircraft 'how-to' book... also a great resource for light wood/epoxy and composite construction!
  8. OldYachtie
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    OldYachtie Junior Member

    I started the other thread about catamaran crossbeams, and it was I who posted the imformation that Edmund's book contained a formula for this, but I also mentioned that his formula left out how he determined the wall thickness of his beams, mentioning only the outside dimensions of his beams. In his example, he said that he used 1/2" thick fiberglass, but not why, or how he arrived at that figure. Anybody have an answer for this?

    It is pretty basic engineering, I would think, and any civil engineer should be able to answer it. Basically, it is the same engineering as designing a cantilever balcony, or any other cantilever structure.
  9. fburton
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    fburton Junior Member


    There’s a great little program “Beamboy” (freeware)

    Specify the beam length
    specify the support (cantilever) at one end
    with the hull load at the other

    You can then state the beam details; type (tube, solid beam), dimensions
    and the modulus of the material (steel = 210 GPa) (Douglas Fir = 13)

    You then get displays for moments and deflections

    It’s a nice simple program.
  10. Trevlyns
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    Trevlyns Senior Citizen/Member

    MCM – An excellent multihull book is Chris White’s The Cruising Multihull. There are a couple of sections dealing with beams.

    Joe_Cope – a lot of James Wharram designs incorporate ply crossbeams. See
  11. Richard Woods
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    Richard Woods Woods Designs

    I wasn’t able to respond to the earlier discussion on multihull crossbeam design as I was making my fifth Atlantic crossing, this time sailing a 33ft catamaran from the Canaries to Panama.

    However I am now back in my design office and catching up with forum questions.

    There are two stages in creating a successful crossbeam solution for your boat.

    First you must DESIGN the structure, only then can you CALCULATE it. The former is usually the more important and certainly the one most people get wrong.

    There are several factors to consider when designing crossbeams:

    First, you need stiff crossbeams, not just strong ones. Fortunately stiff beams are nearly always over-strong. By stiff I mean one without any obvious deflection. Engineers normally consider that to be 1-2% of length.

    Why a stiff beam? Well imagine crossbeams made out of rubber. They would never break, but would be so flexible you could never keep the two hulls in line and the mast would fall down.

    How can you make a stiff beam? Well, actually it’s not just the beams that you want stiff, rather it’s the boat as a whole. I’ve found that the best way to do this on an open catamaran is to have two crossbeams plus a separate one to take the mast loads.

    The actual positioning of the beams is also very important. Although crossbeam size and placement is often complicated by rig and accommodation considerations, the beams must take priority!

    If they are too near the middle of the boat then the bows can flex up and down and you cannot keep the rig tight. If too close to the ends (especially to the bows) there isn’t enough boat to take the loads and, furthermore the beam cantilever is longer.

    Having the first crossbeam almost at midships was the most spectacular mistake the Team Phillips designer made. Pete Goss and I are members of the same sailing club and other members were upset when I expressed concern about its design. After the breakage they realised what I was on about.

    Once you’ve designed the structure it’s really a trivial problem to calculate the necessary scantlings. If you use a strain energy analysis you’ll find that the loads will dissipate quite quickly into the hull. Indeed it’s extremely rare for beams to break off the hulls. Usually the problem is the beams themselves breaking.

    Fortunately it’s very easy to check the strength of catamaran beams once you’ve built them. You simply jack the boat up with a support under each bow and each stern. Then take one of the chocks away. The boat shouldn’t move appreciably. It looks scary, and is certainly a load that you wouldn’t get at sea. But is very reassuring all the same.

    You can see a photo on my website of exactly this test that I performed a couple of days ago on my own 25ft Merlin catamaran. Go to and then the “Latest News” page.

    The Merlin is similar to my Strider design and uses two aluminum tubes with inertias around 500cm4. I usually use ply and timber beams as they are easy to make and to attach to the boat (and of course to attach boat to them), but they are heavy (approx 1.5 x the weight of aluminum tubes). On a 40ft, 7ton boat, beams that are strong enough may weigh 400kgs. Ones that are too weak will still be heavy - they may weigh 300Kgs. It’s NEVER worth trying to save weight in your beams. Carbon beams may weigh 250Kg at a huge cost. Is it worth spending that extra to save 150Kgs at best? I’d rather spend the extra money on better sails and deck gear.

    In the earlier discussion someone suggested using two 6in OD aluminum tubes with 3mm (1/8in) wall on a 25ft catamaran. That is exactly the size beams I use on my Strider (cruising weight about 1000kgs), none of which have broken during the last 25 years.

    Perhaps that’s because the hull spacing was much less than the writer suggested. A Strider has an overall beam of 4.3m and a CL hull spacing of 3.3m approx. However the cantilever is less than that, as it runs from the CL of the windward hull to the inner gunwale of the lee hull. In the Strider case that’s about 2.8m. The writer was going to use 4.5m for this distance, which relates to a 6m overall beam, which is very wide for a 7.5m catamaran.

    I hope this helps the discussion.

    Best wishes
    Richard Woods of Woods Designs
    1 person likes this.
  12. Trevlyns
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    Trevlyns Senior Citizen/Member

    Richard, how very good to have you back after your reported ordeal!

    And thank you for your valued input.

    Take care!
  13. Raggi_Thor
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    Raggi_Thor Nav.arch/Designer/Builder

    Richard, very informative, thanks :)
  14. OldYachtie
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    OldYachtie Junior Member

    Thanks, fburton, I appreciate the tip! :) Tim Dunn aka OldYachtie


  15. Ahmed Wahab
    Joined: May 2007
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    Ahmed Wahab Junior Member


    Dear All
    I'm new to the forum but reading since longer time.
    I downloaded the beamboyand I have difficulty with the input in the beam property .
    If sombody can give an example for a plywood beam !!!!
    Thanks in advance
    Ahmed Wahab
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