Coosa Board Question

I am laminating some stringer cores from 3/4" Coosa Bluewater 26. All along I have been planning to orientate the faces of the laminations in the vertical direction, similar to as if you took two 2x10 floor joists and nailed them side by side to create a 4x10. However, as the BW26 has fabric embeded in the faces (0,90), I got to thinking that perhaps it would be better to orientate the faces horizontally, similar to a glu-lam beam. Ultimately it probably doesn't make a lot of difference as the stringer is designed with a non structural core, but what the heck, I might as well put it in the strongest way. For the sake of this discussion, let's assume the stringer section is 6" in the vertical and 4-1/2" in the horizontal direction.

Sea Jay

 

Assuming you're correct, in that the stringer core isn't structural and serves as a form for the laminate, you could use mashed potatoes within the stringer, if you can get your laminate to cure in the proper shapes.

Adding a stiff core material could cause issues, such as point loading, within a stringer as you've described.

It's an awfully big bit of assuming, on the qualities of the stringer core. Is there a reason you've come to this conclusion? I ask, because most manufactures use a core as a structural element in the laminate. Even a light weight foam is structural, so long as it remains well bonded to the laminate. It serves in compression loads as the laminate attempts to distort under stress. Without the well bonded core, the laminate shell usually collapses, commonly quite catastrophically.

 

Just off hand, Coosa is designed to have the structural stiffness of plywood. When I am working with Coosa, I imagine how plywood would work in the same application. Bluewater-26 is very stiff. I can't see how it would make much difference in either orientation. I have been using Coosa in both orientations for stringers and braces; as you say the glass on the surface will provide a lot of the strenghth as well

 

Par,

I am building to Gerr's scantlings and I probably overstated the "non-structural" element a bit. Gerr does say that his stringers could be hollow but calls for foam formers. The Coosa will be used as the foam where the stringers support the engine.

Bentwood,

You have hit at the heart of the question. I also believe that it really doesn’t make much of a difference but I am curious about this, and the answer will likely be applicable to other Coosa applications. With ply, the strong direction is when the face plies are orientated (in the 8ft direction) with the loads. The 8ft. vs 4ft direction doesn’t apply to Coosa as the face reinforcing is 0,90. So following your line of reasoning of using Coosa like ply, would you laminate strips of ply on edge or lay them flat? My thought is that I would lay them flat so as to have a face ply at the top and bottom of the stack with face grain running parallel to the loads. (For the sake of this discussion it is probably better to ignore the internal plies as unlike plywood, the internal structure of Coosa isotropic.)

 

Chris Craft use to make cores of cardboard with good result. If water got in and rotted the cardboard a paste evantually would ooze out

 

I believe you, my old chrysler 20' center console has decks that were cored with a cardboard honeycomb.

Unfortunately some of it wasn't wetted out with resin and it collapsed, I slapped resin on some of the dry sections and it set up like a rock,
I replaced it all with seacast because I had some left over.

I thought I was imagining things when I first saw the cardboard.:!:

 

Stringers behave like beams. Imagine an I-beam. The flanges of the beam take up tensile and compressive forces in the horizontal plane. The web of the I-beam keeps the flanges a constant distance apart and take up shear forces that occur between the flanges.

In the case of stringers, the reinforcement on the webs take up shear forces and should be oriented in the vertical plane, but oriented at +/-45 rather than 0/90. Orienting the glass faces of the Coosa in the horizontal plane won't help too much for strength since most of the stress is taken up by the reinforcements furthest away from the middle of the stringer (i.e. top and bottom).

I wouldn't rely on Coosa for structural reinforcement because the fiberglass does not get properly wet out with laminating resin and doesn't develop the typical properties of fiberglass laminates.

 

Hjamokey

Thanks for the input. For the sake of this discussion, let’s understand that the stringer laminations will be constructed to a specification that will handle all loads independent of the Coosa core, with the exception of the compressive forces exerted at the engine bearers. I am not relying on the Coosa for structural reinforcement. However since it is going to be in the stringers anyway, I’m trying to understand how to gain the most strength from its orientation. I’ve come to the conclusion that the answer is dependent upon on what sort of strength you are considering.

If we were looking for compressive and tensile strength along the long axis of the stringer, the horizontal orientation would provide some amount of extra strength as the face lamination of the outermost Coosa plies would have 50% of their fiber orientated in fore /aft direction. ¾” of an inch below that would be another face lamination with similar orientation. Of course it would see less of a load as would subsequent faces as you move towards the middle of the beam. With this orientation, the fabric in the Coosa provides virtually no shear strength.

On the other hand, if we orientated the Coosa plies in the vertical direction, we would have very little fabric acting at the top and bottom of the core to assist with tension and compression forces but we would have some at the center (top/bottom) of the core to resist shear.

Although Gerr doesn’t go into the details, he makes the statement that stringer tabbing should be orientated 0/90, not 45/45 as you suggest, so that “50 percent of the fibers (are) along the working axis”. After thinking about this a bit, I suspect that it is because the lamination is designed for stiffness, and in doing so, shear has been amply accounted for.

Thinking this through has been a good exercise for me. Thanks.