Trimaran crossbeam calculations

Discussion in 'Multihulls' started by langdon2, Jun 5, 2011.

  1. daiquiri
    Joined: May 2004
    Posts: 5,372
    Likes: 239, Points: 73, Legacy Rep: 3380
    Location: Italy (Garda Lake) and Croatia (Istria)

    daiquiri Engineering and Design

    Good thread, I've read a lots of good info and ideas. Thanks guys.
     
  2. Ad Hoc
    Joined: Oct 2008
    Posts: 6,282
    Likes: 452, Points: 83, Legacy Rep: 2488
    Location: Japan

    Ad Hoc Naval Architect

    It depends what the objective is. You mentioned initially to remove the oxide layer, but it forms rapidly so no much point trying.

    If, as Gary already noted, its purpose is to "roughen" up the surface for better adhesion, sounds fine.
     
  3. cavalier mk2
    Joined: Mar 2010
    Posts: 2,106
    Likes: 47, Points: 48, Legacy Rep: 214
    Location: Pacific NW North America

    cavalier mk2 Senior Member

    Thank you all, very instructive and useful knowledge. I think cruising trimarans of course are great cruisers but then not many are being designed currently. The load carrying of skinny hulls is pretty bad, to get the same lbs per inch immersion of my Nicol or a Cross you have to go from 37 feet to 45-50 feet loa using a Newick design as a example. Of course with our immersed amas (at rest) we are in the category Chuck Kanter calls 3 hull catamarans. We've often cruised with 4 and daysailed with 12, everyone had plenty of room and fun. In defense of the Searunner 31 A-frames the bolt holes at the usual 1/32+" oversize had not elongated at all. The corrosion was in the interior backing plates and some on the bolts, If I ever build another main hull I'll reuse the A-arms.
     
  4. terhohalme
    Joined: Jun 2003
    Posts: 512
    Likes: 38, Points: 28, Legacy Rep: 506
    Location: Kotka, Finland

    terhohalme BEng Boat Technology

    Ad Hoc,
    How the oxide layer can form, when there is no air between wet epoxy and alu surface?
     
  5. catsketcher
    Joined: Mar 2006
    Posts: 1,233
    Likes: 107, Points: 63, Legacy Rep: 790
    Location: Australia

    catsketcher Senior Member

    You gotta sand it wet

    Gday Ad Hoc

    The technique in the Gougeon book is to put some clear epoxy on the alloy part and then with your gloves still on get out some sandpaper and sand the aluminium through the wet epoxy. It goes a bit grey even when using yellow paper so I assume we are sanding the alloy oxide layer.

    I do this on my boats with any passivating metal that gets buried. I know that they need an oxide layer but my stanchion bases and other stainless hardware also get the treatment. I reckon that if they are going to be in an area that will be low in oxygen it is better to remove any air from the surface. My chemistry tells me that to have an electrochemical cell we need a reducing agent so I want to remove any possibility of any ion migration to the surface.

    Those fittings I have removed have come up nice and blemish free underneath.

    I am working on the Bucc beams - the under strap seems awfully strong

    cheers

    Phil
     
  6. Ad Hoc
    Joined: Oct 2008
    Posts: 6,282
    Likes: 452, Points: 83, Legacy Rep: 2488
    Location: Japan

    Ad Hoc Naval Architect

    That’s a good point.

    But I assume the bonding is not performed in a vacuum, therefore, there shall always be oxygen present (see more reasons why below). Also, not sure why would you wish to remove the oxide layer for bonding (which is next to impossible), when to bond 2 surfaces the purpose is to present a surface that is prepared ideally for bonding, by whichever method is more efficient. If the oxide layer is partially removed in the process, that is a by product rather than the raison d’être.

    I’m not familiar with the Gougen book.

    However, when looking at to bond two surfaces together, there are 2 ways, Mechanical and Specific adhesion. We can’t rule out mechanical totally, this is where the adhesive penetrates the pores of the adherend. Since the Glass you’re bonding to may not be perfectly “sealed”, and thus may behave like wood or paper being bonding, ie “mechanical”.

    So if we assume it is Specific adhesion, the biggest influence is the wetting; but there are others too. If you apply the epoxy with sand, as noted above, unless you are doing this in a vacuum there shall always be 3 phases present when any liquid is placed onto a surface. 1) Solid, 2) Liquid and 3) Gas (air or vapour from the liquid). These 3 phases are in equilibrium. Therefore for wetting to occur the surface energy of the adherend must be greater than the surface energy of the adhesive.

    That is why it is difficult to bond surfaces like Teflon and silicon, because their surface energy is low. However, excessive roughness can affect the wetting because air becomes trapped and the adhesive cannot ‘flow’ to the base of the porosities. So as your “mixing” “grinding” your mixture, the solid phase is fine, but the other 2 phases present still contain air/oxygen. There is also oxygen in the epoxy too.

    Ideally you need to degrease and pretreat by etching. Unless you’ve done a lot of testing and worked with the supplier, to see what level of abrasion is required, you could be either as noted above, creating unwanted pores or just wasting time as the effect is very minor. Instinctively abrasion is the key, but this is not true for all adhesive and surfaces.

    Several years ago I designed a structure that was all bonded aluminium. Here we just degreased and applied a weak sulphuric acid etch.

    You should also realise that since there shall always be sea water present in the air, via spray, and humidity etc, the joints shall slowly degrade. How much depends upon several factors, namely the quality of the joint to begin with. But the biggest influences are the working environmental (at sea) and the level of stress/fatigue the joint is exposed to, from the sea loads. But the presence of water is the main cause and you cannot escape this :eek:
     
  7. daiquiri
    Joined: May 2004
    Posts: 5,372
    Likes: 239, Points: 73, Legacy Rep: 3380
    Location: Italy (Garda Lake) and Croatia (Istria)

    daiquiri Engineering and Design

    This is interesting, I'd be very interested to know more about that. Why did you choose that joining method? What glue was used and how did it perform mechanically over the years of service in the marine environment?
     
  8. Ad Hoc
    Joined: Oct 2008
    Posts: 6,282
    Likes: 452, Points: 83, Legacy Rep: 2488
    Location: Japan

    Ad Hoc Naval Architect

    Hi D

    I wanted to push the boundaries of what was and wasn't acceptable, with both Class and knowledge of different methods. For the design in question, it was relatively "safe" in the sense that it was not primary structure. It was a major fairing/bulwark on the superstructure/deckhouse. The gaol was for a nice smooth looking side and front, but light weight.

    So, i designed this to be bonded using 3M tape (it was a few years ago - 1997; the options available today are far greater). I did a lot of testing to ensure the best procedure to obtain max strength and we had Class witness and prove the method. Performed mechanical testing to ensure i got good strengths. This was for future use too.

    Went together like a dream. The whole structure was as smooth as a politician on election day, a real mirror type finish. We were very pleased with the results.

    Unfortunately, when the client came to inspect the boat prior to sea trails, he decided he didn't want the fairings, felt they obscured the view from the wheelhouse.... :(

    So my experiment and wanting to push the boundaries came to an abrupt ending.

    Although we did use that info learnt to bond together a deckhouse for a patrol boat with Perflex (I think it was) a year later. She is exposed to extreme heat (60c plus in summer) and humidity and has worked very well so far, for nearly 15 years. All we did was add one or two pop rivets at the ends to prevent peal.
     
  9. langdon2
    Joined: Nov 2009
    Posts: 24
    Likes: 0, Points: 1, Legacy Rep: 10
    Location: UK

    langdon2 Junior Member

    Yes, I have Harris's book and his 40' tri is an informative example to work from. If I read your suggestion right, I think I'm working along similar lines. In my case the tube enters a long clamp arrangement (fabricated from aluminium channel and plate without welding ) at the wing edge of the main hull ( no bolts through the tube) leading to one or at most two bolts through the tube close to it's inner end, almost on the centre line of the main hull. I guess the answer to the fatique problem in the long haul is to just replace the tubes, since my design intention is for them to be relatively easy to remove anyway. I'm trying to choose my compromises rather than have them creep up on me.
     
  10. langdon2
    Joined: Nov 2009
    Posts: 24
    Likes: 0, Points: 1, Legacy Rep: 10
    Location: UK

    langdon2 Junior Member

    Yeah, I agree, however, here's the but and as ever it's set by personal circumstances - just as every boat user has his her own individual set of possible choices from which to select their preferred mix. I have space enough to build the separate hulls at home, dead lucky there, the last boat I worked on, a re-build, the project started off 250 miles away. Not doing that again. I have to put up a temporary structure to cover, but that's an acceptable problem. I am 6 miles from a launch site, so main hull max width is 11' / 3.35 m, it'll go on a low loader okay.

    Yes, I could take it to a yard and in fairly short time glass/carbon in the tubes and proceed to final prep, rig and launch. Then there's the second 'but' - the boat will have to come ashore for storage for fairly lengthy periods and in order to keep the costs to a sane level I want to be able to dis-assemble and use as little yard space as possible. Hence my choice of ally tubes and bolts. It seems the simplest way of doing what I want to do. The built in, non removeable parts of the tube mount system, though, I will be bonding and glassing in and I will also be taking steps to make damn sure no bolts undo themselves.
     
  11. Gary Baigent
    Joined: Jul 2005
    Posts: 2,966
    Likes: 105, Points: 63, Legacy Rep: 509
    Location: auckland nz

    Gary Baigent Senior Member

    Understand your problems, Langdon - another alternative is to lay up glass sleeves (first wrapping plastic or heavy wax around the beam/sleeve area since you don't want adhesion there) and if you have oval alloy beams plus end glass caps, which can be attached/detached from beam ends, you can slide the hull(s) apart - again far superior to any clamping arrangement. This is the setup I had on a 32 foot catamaran (admittedly a lightweight boat) and it worked perfectly. But you have to be able to slide said components apart without twisting or binding - which can take time and you may/will need rails or rollers for such a large hull as yours for movement accuracy. Just another suggestion. Cheers.
     
  12. cavalier mk2
    Joined: Mar 2010
    Posts: 2,106
    Likes: 47, Points: 48, Legacy Rep: 214
    Location: Pacific NW North America

    cavalier mk2 Senior Member

    If you don't compromise you can't be compromised by compromises....;)
     
  13. langdon2
    Joined: Nov 2009
    Posts: 24
    Likes: 0, Points: 1, Legacy Rep: 10
    Location: UK

    langdon2 Junior Member

    another alternative is to lay up glass sleeves

    Gary, I want you to know I am sitting here giving myself a good kicking.

    Why - kick - didn't you - kick - think - kick - of that ? (kick)

    Excellent solution, thank you very much. Simple, neat, more effective. Should weigh less, too. Great.

    And Cavalier ? Errm..yeah, can't think of a smart reply to that one :confused:
     
  14. rxcomposite
    Joined: Jan 2005
    Posts: 2,037
    Likes: 199, Points: 63, Legacy Rep: 1110
    Location: Philippines

    rxcomposite Senior Member

    This thread progressed much faster than I expected. I took a look at global loading rules for crossbeam in Lloyd's but got stuck in the middle.

    The calculations are deeply nested. It starts first with vertical acceleration and ends up with load combination analysis with a) head sea b) beam sea c) quartering wave.

    I got stuck in Section 4.2 to 4.3 where there is a criteria for "partially submerged hull" and "fully submerged hull". What does this mean? Does it mean one hull is fully submerged and the other one is not (as in beam seas) or is it when the hulls gets submerged due to vertical acceleration?
     

  15. Ad Hoc
    Joined: Oct 2008
    Posts: 6,282
    Likes: 452, Points: 83, Legacy Rep: 2488
    Location: Japan

    Ad Hoc Naval Architect

    It is the difference between a catamaran, which has some of the hull below the surface and some above, for buoyancy, and that of a SWATH, which has all of its buoyancy below the surface.
     
Loading...
Forum posts represent the experience, opinion, and view of individual users. Boat Design Net does not necessarily endorse nor share the view of each individual post.
When making potentially dangerous or financial decisions, always employ and consult appropriate professionals. Your circumstances or experience may be different.