Buccaneer 24 Trimaran

Discussion in 'Multihulls' started by Samnz, Dec 1, 2008.

  1. ThomD
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    ThomD Senior Member

    Speaking of wing spars, I have always been struck by the one the Gougeons built for a Searunner 25 that used just paper core between really thin aircraft ply all bagged into a teardrop shape in a single step. It lasted great, and makes use neither of lots of stringers and bulkheads, or really expensive lam schedules in carbon. More broadly it means there are any other number of cored masts that could equally be constructed using whatever core options people prefer. Hardly a surpise when one considers the number of different options that exist for beams. I know my own head was locked in a box by the fact that the Gougeon stresskin/carbon plans were out there and seemed to imply simpler options wouldn't work. I'm sure many others have made such spars in cedar etc... are there good threads on these kinds of options, and scantlings?
     
  2. Samnz
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    Samnz Senior Member

    In retrospect they are very light, we were going to make them just one layer of 4mm ply all around with a layer of 10oz boat cloth but (happily) decided to beef them up a bit. Any chance you can post some scantlings you mention?

    They are very stiff but im not sure how close to breaking they are... I have thought about foils or making the flots higher volume but dont think this is safe without reinforcing the beams. Question I have is where/how would they break so where would the reinforcing be needed....
     
  3. ThomD
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    ThomD Senior Member

    I can dig up some scantlings, after I dig out some plans. My 24 footer is so fine in the main hull that I was told it wouldn't support the weight of unstayed beams. The stayed beams it has are very similar to the ones on the Buc, with slightly less diameter, and slightly more wall thickness over 18.5' beam. I will see what I have, all by very major designers, but I may not mention names since I am not sure how I feel about publishing scantlings for plans that are all still being sold as far as I know. I think I have about 5 unstayed beams worth.

    Your beams look good and yet are really large in section. Since strength goes up to the square, and stiffness to the cube, you may be doing better than my guess. The windage doesn't seem to be killing your boat's performance... :) Conventionally you could have used some ribbing in the vertical segments that would have created a truss effect with the nailing flanges. That would have given effectiveness near that of having the flanges be solid. So if you have 3/4" flanges and add appropriate truss segments to hold them apart, you get an effect like nearly solid 1" beams, for and aft for the sake of a few more feet of stringer. At least that is what I understand the engineers to be saying. Further, if the tops of the beam rather than being plywood with the veneer run-out problems therein, and the 30-40 percent transverse fiber, were say 8mm strip with the glass you speced, that might also be a step forward.

    One other detail I wonder about is what would happen if one epoxy bedded cable in a groove in the nailing flange. There are trusses built that way as elaborated by the Gougeons in an epoxyworks article, and as they use in their strongbacks. For relatively low additional weight one could be loading up some cable. Not sure that would be all that useful because it might not get loaded before the surface had ruptured. Always wanted to try it though.

    I will dig around tonight.
     
  4. ThomD
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    ThomD Senior Member

    So here are the first few scantlings from a very famous designer. The boat is about 24'x17'. The beams are either 6.5" aluminum with 3/16" wall, 6061-T6. or they are 5.5 square in quality timber, with top caps of 27 mm, sidewalls of 18 mm.

    The third beams come from Funky Tri, as built by Jan Gougeon and JR Watson in 1973. The beam dimensions are not given but they look to be 6" x 3", and the walls are described as 3/4". The article I have says these beams are unsupported by wires or struts, though there appear to be wires in the accompanying photos. Funky Tri was 25' x 14'. I made a serious effort to visit her, but the owner was always aboard her on trips to the Florida Keys, and didn't seem interested in making contact. Possibly the simplest trailerable tri ever built.

    The fourth example really doesn't count because it had wingdecks right out to the floats. The separation was 11 inches at the main hull to about 8 inches at the floats, with 3/8" plywood in deck surfaces and bulkheads and with nailers of 3/4" stock.

    OK, I have two other examples, but they may take a little longer to find. More to come.
     
  5. Samnz
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    Samnz Senior Member

    Uh... so its only 17' wide and has 27mm caps, compaired to my 8mm and 18mm walls compaired to my 4mm....

    That scares me a little bit :eek:

    do you mean 5.5" by 5.5" or?
     
  6. ThomD
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    ThomD Senior Member

    Right, 5.5" x5.5" So on the scale thing your greater dimension would be more than 2x stronger, and more than 3 times stiffer. But your walls are a lot thinner which would scale that back down to something weaker overall. So it possibly comes down to a mater of how heavily used the boat it. Even mooring can have a wearing effect.

    By caps I mean the top and bottom of the beam are solid D-fir of 27mm, and the sidewalls are solid 18mm. Plus glass, paint etc... It is a solid box beam without stringer etc... A fat wooden tube. It also tapers in the normal way. Each beam weighs about 30 kg at the lightest.
     
  7. bruceb
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    bruceb Senior Member

    buc 24 beams

    Sam and Thomd, I have been following this tread with great interest- sooner or later, I want to replace my beams too. I think Capricorn's beams scare me some, but by my very crude calculations, they are a little stronger than Crowther's original design. The 1/2" bolts in shear on the water stays are the limiting factor on the Buc 24s design- and the 2.5" (5/16x 8") cross section of the top and bottom plates of Sam's beams seem to be about 20% stronger (front beam) . I think if they failed, it would be from the sides distorting first?? That does not take in to account dynamic loads, but I think Capricorn has been "tested" hard and fast:). I am really interested in any other scantings that anybody can produce, the only boats I have near me are production Corsairs and they are very heavy solid glass and use water braces. Bruce
     
  8. ThomD
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    ThomD Senior Member

    Ok, so these are the last scantlings for 24' tri beams.

    I heard from a guy who sailed one of the earliest versions of this tri down to Baja that the front beams were made out of a 4x4. In the old days the haircut on a 4x4 was more like 0-1/8" than the current half inch, and back in the 50s or 60s, on the west coast, one could find wood that today might not be out of place on a guitar. So keep that in mind as to quality and size.

    A famous designer's kid, told me his 24 foot cruising tri, had a pair of 2x6s up front, and a single 2x6 in the back. This would be not that long ago when a 2x6 was 1.5x5.5.

    Both of the above boats probably had classical beam of 66% at best.

    My last contribution is for the beams on a rugged cruising tri with fixed, separate, fore and aft beams. These were triangular "boxes" with the the truss being 12" high at the wings, and 6" inches at the amas. I think the web was 3/8" ply. There were three stringers stacked at the top and the bottom of the truss, each 3/4" square, six stringers total. This truss would hold the major loads, and was further structured as part of a triangular plywood tube. the opposite side was 12" at the root, as mentioned, the hypotenuse was 19", and the adjacent side was whatever it would be. The leading edge where the hyp. and the adj. met had two stacked stringers of 1/2" x 2". Because of the acute angle a fair amount of that material got planed out. The rear beam was similar but 9" high at the root, and 5.5" high at the ama. I don't recall, but I think the non truss sides of the triangle were 1/4" ply, I have video somewhere. The key thing here is that the major loads are carried by the stringers/longerons (the distribution of these could be better. As a result the ply on all three side is transverse grain orientation. The box is stiffer if the fiber takes the shortest distance. It's like a footbridge, the decking is hung from side to side, there wouldn't be much point in the rest of the bridge if the boards you walked on each hopped the span from one side to the other. Or so the story of these beams goes.

    Now if you want some insight into how beams get designed, consider that the 4x4 beams, and the hollow wood beams in the earlier post, both have the same cross sectional area to them. By one calc (depends what 4x4 means and how weight was moved around longi wise) they both come out at 16 square inches. Not saying, but that kind of coincidence comes up all the time when one review design work. (The 2x6 beams are 16.5 inches). Designer B comes along, looks at the existing field, and simply reconfigures the original weight budget to do a better job. Cuts down on all the structural analysis back chat. I call that managing the structural problem. No numbers needed. Structure A worked and Structure B is a better distribution of material, but harder to build, no problem. Designer A and Designer B are probably the two biggest names in american tris, yet somehow they landed on the same square. Presumably extensive strutural analysis in a lab somewhere. Capricorn"s beams have approx 7.4 square inches in them at max section. So that is kinda in the great leap forward catagory of scantling.
     
  9. ThomD
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    ThomD Senior Member

    Bruce B, I'm not capable of doing the calcs, but I'm a little surprised at what they tell you about a wire and tube beam. The big problem with wire and tube beams is that they are pretty directional, and we also all know rigging fails catastrophically more often than, say, hulls. So there are problems with that kind of structure, but I thought the strength of the wire/shackle was pretty much the least of our worries. so far I haven't seen a version of this with spectra. That would be interesting, since it could be home maintained more easily than rolled fittings.
     
  10. bruceb
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    bruceb Senior Member

    wire and tubes

    Thanks ThomD. Crowther's design uses a stainless strap for the waterstay instead of wire, but the thought is the same. The attaching bolts or pins are subject to failure and the inner pipe joint gets very loose after a while. They are also heavy, one of my 19'x 4.5" aluminum cross beams, waterstays and hardware weighs about 90 lbs and is not very good at locating the float. I am sure 90 lbs of wood and glass would make a very strong beam and not need the waterstays. I imagine that there are some designers watching this thread and our seat of the pants attempts at beam design:confused:, one side guessing that we will turn our old plywood box boats into toothpicks, and the others thinking that they can save some weight in their next design:) Sam's Capricorn coming off a wave at 20+kts certainly is doing some real world testing. Thanks for the scanting info, that gives me somewhere to start. Bruce
     
  11. ThomD
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    ThomD Senior Member

    The system on my KHSD is lighter. It uses 4.5" tubes with 1/8" walls. There are chainplates through the boat that are pretty stout. The load goes to a shackle, that is rolled to a 3/16" wire that is rolled to a J hook that can be engaged with the tube through a flush to the outside fitting. My boat has curved 1/4" main hull, and 3/16" floats. This system is probably the lightest out there and would adapt to the Buc. It allows the tubes to be simply inserted through the boat, though I think a half tube method as on the Buc has a lot to recomend it. I thought the Buc was similar, I guess I superimposed my system onto the plans. Kurt has this system on boats up to 30 feet in beam, and possibly larger.

    It's unlikely capricorn gets anything like the cycles that would indicate it is reliable for the kinds of redundancies one wants in an average design. I find it an exciting scantling because it has worked so far, and there are lots of boats on moorings being raced weekly that can use this kind of thing. A lot of our summer weather up here has very light winds. I would just make these beams for a Buc right away if I had one. It would be very cool on a boat like Toy. I think I would make the caps out of DF linear, and the walls out of 1/4".

    "I imagine that there are some designers watching this thread and our seat of the pants attempts at beam design,"

    As I mentioned above, when one starts to study the masterpieces of the past, they often look one part Hereshoff, 1 part marine curves, and one part Piver scantlings.
     
  12. bruceb
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    bruceb Senior Member

    Wire stays

    Thanks Thom, I had wondered what size wire your boat used in the waterstays. ( You have a 23?) I think PBO or other synthetic would be even better, but I have a roll swedge machine and lots of wire:) if I go that way. I think you are right on the beams for my 24, and I think I need a set soon. I am still trying to make my boat fold, probably not a complete system like the corsairs, but some arms/stays that would speed the process up and allow one person to do it and still not add much weight. My boat takes two people about 2-2 1/2hrs to set up or take down, and it is a pretty busy 2 hrs. I have been making models all winter, but I am not satisfied yet:mad: Bruce
     
  13. ThomD
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    ThomD Senior Member

    "Thanks Thom, I had wondered what size wire your boat used in the waterstays. ( You have a 23?)"

    Right, it is sort a 24 because it is made out of 24' panels minus a few inches at the edges where the vac pressure isn't perfect. But then more material is added back in the bow and stern.

    The rule of thumb seems to be the stays are one size larger than the rigging wire (yeah, I know, eyes are rolling out there), but in fact my wires are 1/8", the stays are 3/16". 3/16" wires, and 1/4" stays. That tended to be what I saw in surveys. And obviously they are related. If one put enough rig on my boat to demand 3/16" wires, it would at least be wise to design the waterstays with it in mind that the loading on the beams could also go up.


    " I think PBO or other synthetic would be even better, but I have a roll swedge machine and lots of wire"

    Pushing green with envy button...
     
  14. Samnz
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    Samnz Senior Member

    Not very stiff however....


    And you think the alloy tube and wire setup is?

    The Hughs 23 over here I race against has the standard alloy beam and tube setup and it has caused him a fair few issues allready...

    In saying that I do like the hughs beam setup because its so easy and cheap to build!
     

  15. Samnz
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    Samnz Senior Member

    The Hughs 23 or 24 or whatever it is, here in Auckland used 5mm dynex when the 5mm ss wires broke in a storm, (while on a mooring) and it was (as expected) hopeless, as the creep is far to much for this application, pbo would be the only way to do it in synthetics and you'd have to replace it every year or two at most so would just be a pain I think...

    I think the Seacart 30 uses 8mm stainless wire or rod, not PBO just to put it into perspective!
     
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