Dick-
Have you received any response on this?
Yes - you can assign “Read Only” rights to visitors of a blog - or you can allow for comments. It is set up at time you begin your personal blog.
Just haven’t spent any time out there. Perhaps after the snow flies?
I wonder what is the ideal cross section for cross beams. Most of the crossbeams on models seem to be longer, and therefore stronger, in the horizontal plane. On full size trimarans, the beams seem to have a box section and I suspect that most of the carbon is in the vertical plane. My feeling is that in full size trimarans, the main stresses on the beams are in the vertical plane and the reason why they still look fairly flat (at least towards the float end) is simply that the designers have added a non-structural fairing as streamlining. In models, the ultimate stresses may be different, since we regularly crash our tris into an object (the bank, rather than another boat, one hopes), and at that point the stresses build up explosively in a horizontal plane. I am currently building some 1.8 meter crossbeams using foam and two vertical sections of 270 gm carbon fiber forming a box section. I intend to add some streamlining fairing on the front but keep the aft side vertical and then glass it with light glass. My tri is about 1.8 meters long and will weigh I guess around 7 lb. (It used to be around 1.65 meters and weigh 6.5lb, but I have gradually extended it.) I hope to have it back on the water by next summer. So my question is: does a cross beam have to be stronger in the horizontal plane, or in the vertical one?
Any suggestions?
Colin,
My thought is that the the beam needs to be as strong as possible in the vertical direction to try and resist any twisting of the hulls (ie in your case the carbon needs to be on the top and bottom). The compromise is then making the beams thin, particularly near the floats to minimise water (+air?) resistance. I then figure the beam is stong enough horizontally to cope with most events.
Colin -
I am using a thin ply for a “spine” for the majority of the beams. Basically, when viewed from the front, the ply is cut to that “gull wing” shape. This gived vertical stiffness to the beam. I add foam to front and read of the beam and shape to desired “look” and then add glass. This glass “could” be carbon, but regardless of what you use - there is a vertical and a top and bottom at right angles, so if one were to cut the beam - you would see an almost “I” beam like configuration made from top glass/carbon, vertical ply, and bottom glass/carbon. The foam merely fairs things out to look good, although it does add a bit to the rigidity of the beam.
Dick and Jon, thanks for your replies. It seems as though the I-beam has merits. I was concerned that if there is only one spine, it can twist easily. That is why my beam has two vertical spines. The result unfortunately is that it is too heavy (6 oz). I may try making another that has a single spine and a D shaped cross section. I’ll let you know the results one day. Thanks, Colin
Colin -
here is a cross section idea from a web site that doesn’t open any longer. It is some Spanish and some Portuguese.
Regardless, here is the idea offered. Foam wrapped in glass cloth, with plywood spine and epoxied aluminum tube to slide retaining bolts through the beam.
Detalle Fijacion Brazo = detail fixed arm (Cross beam detail)
Espuma = foam
Agujero = hole
Contrachapado = plywood
Tubo alu = Aluminum tube
Epoxi = Epoxy (Note fillets at top and bottom of aluminum tube)
I had a set of completed, laminated wooden beams ready for the F-48, but after reading posts and giving more consideration, I am going to spend time to do a set of foam beams too. Will then be able to judge weights, stiffness, etc.
Photos show actual wood beam, and then the foam where the shape has been transferred. Foam is 2" thick (about 50.8 mm) and front will be shaped to fit. I plan to cut some thin 3mm (1/8 inch) ply to this basic shape and add foam to front and rear to be shaped before glassing. More photos will be added as I proceed.