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pipeline 06-27-2013 06:48 PM

Does a 4" or 5" inspection port weaken the crossbeam of a trimaran...
I need to drill an inspection port on the underside of one of the trimaran crossbeams, and common sense tells me that it is better to put no holes in those wings, but if I must, better on the top or bottom, rather than the sides re strength of the total crossbeam. Sort of like turning a 2x6 on its side when framing a house, as that is where the strength of the beam is. Wouldn't the same thing be true of the wings?

Also, it would seem that I should use metal rather than plastic ports, as when the inspection port is screwed down, it offers some "metal" strength to the wing being all-chromed bronze - it actually becomes part of the beam, rather than white plastic, while lighter, offers little support?

The naval architect, who is a smart, common sense guy, and I like what he designs, says the inspection ports should be on the side, not the top, and plastic is better than bronze, not taking small additional weight into consideration. Rather than bother him with an explanation, maybe some folks here could tell me where I am wrong in my thinking. Thx in advance, folks.

Richard Woods 06-27-2013 07:31 PM

I think you need to give more details/photos of the boat and exactly where you need to cut the port.

I assume you do mean wingdeck and not crossbeam. But I'm not sure what is vertical in the deck

In general terms you should not cut holes in crossbeams. The most load will be near the gunwales. If you do need to cut a hole in a beam you should indeed do it on the vertical surfaces, not the horizontal.

If you have cut into a structural member then its unlikely that a metal port will "repair" the damage

Richard Woods of Woods Designs

Petros 06-27-2013 08:17 PM

if indeed you are referring to the beam, the top surface will be in primary compression, and hole there will greatly weaken it and could result in buckling it. the under surface would be in primary tension, and a hole there would be less harmful but the beam is still weakened. A hole in the side or vertical face only has a small amount of shear stress and is the best place to put an inspection port. Use a soft plastic plug, a hard "structural" plug will only create a stress risers that would hasten a failure. a metal plug behaves differently that a composite beam and at worst will put compression into the cut end grain possibly causing splits and buckling damage to the fibers.

Your NA has it exactly correct, follow his instructions.

pipeline 06-27-2013 08:22 PM

Thx for the reply, Richard. I changed my wording on my post to be more clear, BTW.

We are speaking about a crossbeam you might find on a Multi-50, say, racing trimaran. The beams are carbon prepreg over balsa.

I understand the there will be a greater load at the gunwales, yes. It seems to me that the vertical surface of the crossbeams provides more stiffness and support from main hull to the ama or float, than does the horizontal, in my 2' x 6" floor joist example above. I am wrong. I just don't understand why I'm wrong (Happens a lot!). Thx in advance.

(Just read Petros' answer above. Thx, guys.)

Petros 06-27-2013 10:41 PM

there is a reason they make I-beams. the vertical element in an I-beam is very thin and light compared to the flanges on the top and bottom. when a horizontal beam is in pure bending, than the bottom and top of the beam take the primary tension and compression loads respectively (top in compression, bottom in tension). the thin vertical web takes only the shear loads.

Consider it this way, if you take a stack of 2x4s laid flat, and you support the ends and put a load in the center of the span, the layers will slide over each other. but if glued together the strength becomes much much higher. that sliding actions is the shear that the vertical web carries, and with them all glued up, the top and bottom take most of the bending stress. when you go into a big box store study the large glu-lam beams, the top and bottom ones will be clear lumber with little to no knots and deffects, but the layers in the middle of the stack up will be full of knots and sap pockets. the middle layers just take shear loads, the top and bottom take most of the bending laods.

In your example, if you take say three 2x6 on edge, it will hold a certain amount of load. But if you took two of them and put one on top, and one on the bottom (forming an I-beam), glue and nailing them together, you will have a beam that is about a hundred times stronger than three 2x6 on edge.

It is a matter of understanding the mechanics of how beams carry loads internally. Again, this is why they make I-beams, and trusses too for that matter; the top and bottom surface or elements of the beam or truss takes the majority of the loads.

If you want to know more there are websites that have more detailed explanations and diagrams. Search for "mechanics of materials", that is what all the engineering schools call it.

pipeline 06-27-2013 10:47 PM

Thanks, again, Petros and Richard. I owe you dinner somewhere...

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