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  1. #1
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    Two layers of 3/4 Coosa vs a single layer of 1.5" Coosa

    Okay so I FINALLY found a distributor of Coosa board, SORT OF in my area.

    They sell 4x8 sheets, 1.5" thick for $500, or 3/4 thick for $260.

    So my question is.... if I go with 1.5" thick, it's less work, as I need to cut fewer things. The stringer would likely have a butt joint, and then glassed over it. The transom would be one giant piece.

    If I go with 3/4 thick, I could double layer it on the transom getting to 1.5" thick, and on the stringer, I could potentially stagger the butt joints then laminate two 3/4 stinrgers together to make one single stringer. That said I have read most of the strength in a stringer comes from the glass, so maybe I am just adding weight/time/complexity for little benefit.

    I'm leaning towards the 1.5" thick sheet.

    Thoughts?

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    1.5” is a heck of a thick stringer. I’m using 1/2” for example. If you buy the 3/4 it’s easy to laminate the layers and you could do a layer or two of 1708 in between to increase strength. My two cents
    Hydrostream dreamin

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  4. #3
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    Yeah, 1.5" is like inboard stringer thickness. Not really necessary for and outboard boat. I would spend the extra $20 and have more versatility with 2 sheets of 3/4" ...even though you will have to laminate it for the transom.

    My $0.02

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    Couple more bits of info:

    The factory stringer on my SST was a 2x8, 12' long or so. That's it the only stringer and wood in the hull, other than the transom, so yeah, it was beefy. I was thinking I'd just use the same size as the factory did.

    The other driver is that, again, the factory transom was 1.5" thick. If I dropped down to a 3/4 thickness, I'd still need two sheets to build up the transom to 1.5", but yeah, I could theoretically run a thinner stringer... it's just sort of tall to do a 1x8; seems like the proportions wouldn't be as strong.

    Thoughts?

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    Quote Originally Posted by ADDvanced View Post
    Couple more bits of info:

    The factory stringer on my SST was a 2x8, 12' long or so. That's it the only stringer and wood in the hull, other than the transom, so yeah, it was beefy. I was thinking I'd just use the same size as the factory did.

    The other driver is that, again, the factory transom was 1.5" thick. If I dropped down to a 3/4 thickness, I'd still need two sheets to build up the transom to 1.5", but yeah, I could theoretically run a thinner stringer... it's just sort of tall to do a 1x8; seems like the proportions wouldn't be as strong.

    Thoughts?
    Yes, you are correct. I would not build a 3/4x8 stringer...
    Construction, of all types, relies on materials available. 2x lumber is (or has historically been) easily accessible everywhere in the U.S. for cheap. That is why it was good for holding up the glass for making stringers. Boat builders can get it, even if its oversized, and glass it in. Without knowing the guy's thought process when he designed the hull I cannot tell you if a 8" stringer is really necessary for strength. I would guess its not, but if you need to flush out with adjacent surfaces you have no choice.

    I hope that makes sense. If you do end up doing anything taller than like a 6" stringer then yes, I would go 1.5" not 3/4"

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  8. #6
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    Buy 1 sheet of each that way you’ll have some for your next project. Price is not coming down, I guarantee you that. Plus I like spending other people’s money.

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  10. #7
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    lmao. yeah buy it, says the guy not paying for it

    Yeah, if you look at the cross section the height is required because of the tunnels/floor:



    So... I'm leaning towards 1.5". Just seems like way less work, and I would think 1.5" would be plenty strong compared to a double layered 3/4" transom. Work smarter not harder.

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    Yeah based on your section you could do perpendicular joists to support the floor and then knees to tie the transom in to the tunnels and the floor. If I was completely re-doing the boat thats what I would do to make it lighter. But, thats more work.

    But as mrichartz said...they used the 2x8 originally cause it was cheap and fit, not necessarily cause it was required.

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  13. #9
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    I would use the 2 layers of 3/4". Much stronger with glass laminated in between and more versatile when running the longer stringer. You could stagger length for strength. Use 2 layers of mat in between Coosa layers as well as a layer of 1708 then encapsulate the Coosa complete in glass utilizing layers of 1708 or equivalent. Also when bedding Coosa in as transom to glass utilize multiple layers of mat probably 3-4.5oz total. As for bedding stringers and fillet utilize cabosil, milled fibers and 1/4" strand glass fill then 1708 over Coosa, joint and into hull.

    Also could I ask what localish distributor had Coosa in stock. Thanks

    Good luck

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  15. #10
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    Quote Originally Posted by 25two.stroke View Post
    Yeah based on your section you could do perpendicular joists to support the floor and then knees to tie the transom in to the tunnels and the floor. If I was completely re-doing the boat thats what I would do to make it lighter. But, thats more work.
    I'm interested in what you're describing; I'm planning on doing knees anyway. Are you suggesting that if I do knees the main stringer isn't even necessary?

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    I mean...to say for sure I would have to see exactly how that boat is framed, but yeah there are plenty of boats with no stringers. Let me see if I can sketch something.

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  18. #12
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    Now I am no professional boat builder, but I have built a few boats and restored others. So by no means is this right, it is just how I would do it if it was my boat. If you get a sheet of 1.5" then the ribs I label as ~1x4 I think could be 1.5"x3"

    Sorry for the poor ISO of your transom... that tunnel design is one complicated thing to sketch. I hope you get the point.

    Click image for larger version. 

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    Attached Thumbnails Attached Thumbnails SRichard Av21061010290.jpg  

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  20. #13
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    You can see what I'm working with in this video:



    Doesn't get much more simple than this, but I'm interested in saving even more weight.

    I'm ready to order the coosa today, so yeah, gotta make the decision soon.

    Right now I'm still leaning towards one giant 1.5" 4x8 panel... that will give me enough to do the transom (roughly 6x2) leaving me a 2'x'8 section big enough to make one 12' long stringer that tapers from 6-7" tall down to nothing, and use whatever is left over from the transom cut (2x2) to make some knees... though ... now that I'm writing that out, in order to do the knees I think I need a bit more. :\

    If I get two 3/4 sheets, I can layer the transom up, which should be insanely strong, so maybe knees would just be adding weight at that point?

    I supposed two 3/4 sheets.. if I sketch it out...

    the transom would take almost 1 entire sheet, (2x6)x2, leaving TWO 4'x2' sections for knee/joist building.... and then I'd have the other sheet to make the stringer, but doubled up. I think that would leave a scrap of 3x4'

    Okay, two sheets it is. I like the idea of having more left over for knee/joints/stuff out of it.

    I'm still going to do the floor out of wood, purely because I know it holds fasteners really well, and I'm not worried about it rotting since this won't be left outside.

  21. #14
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    Quote Originally Posted by mrichartz View Post
    You don't mean opposite the way the stringer is do you? You'd be puting pressure points in the wrong places..
    And depending how think the bottom is will end up with cracks or worse..
    Oh ok just saw it..only thing is if he doesn't make the boat rock solid any twist will start making cracks.
    Not for 25two...Think of grabbing the boat by the transom and pulling it to lift the bow.
    And think of boat hitting a wave wake etc on one side and it twisting even just a bit.. pressure points.
    Yes, you are right about that flexing, which is why its critical to understand the design of the whole boat before you change how its reinforced. They often have a strong direction and soft directions..or as you said, the whole thing is rigid. I was assuming it is a rigid build cause it looks super heavy. You really would need to understand how the bow works and the direction of forces there...that will tell you how they intended the twisting and angled loads to be distributed.

    A way to eliminate those punching points at the end of each rib would be laying them on top of the tunnels, but your floor would rise by the height of the rib.

    If you think the bottom is relying on that stringer for support and does not have a wood core/is thick enough to support itself, then I have no idea what to say. That would be one sad "race boat."
    Given that they typically did not epoxy the plywood floor down to the stringer (I am not sure if when you demo'd it you found the floor attached to that stringer or not) that piece is not really capable of providing strength to the the bottom of the hull. It resists downward forces from the floor, but does little in upward from the bottom. It would simply push up the ply and belly the floor. Now it would help distribute the load across the length of the boat, but that can be easily accomplished with those additional glass corners and maybe a set where the floor meets the inside of the tunnel.
    Last edited by 25two.stroke; 06-10-2021 at 12:43 PM.

  22. #15
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    Quote Originally Posted by 25two.stroke View Post
    Now I am no professional boat builder, but I have built a few boats and restored others. So by no means is this right, it is just how I would do it if it was my boat. If you get a sheet of 1.5" then the ribs I label as ~1x4 I think could be 1.5"x3"

    Sorry for the poor ISO of your transom... that tunnel design is one complicated thing to sketch. I hope you get the point.

    Click image for larger version. 

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    WOW. Thank you so much. And yeah, impressed in the isometric view man. Very helpful and beyond what I was expecting.

    So, I'm still planning on doing the floors out of wood. I'm unable to do a through bolt for the seats, due to the tunnels, so I like using a lot of large aggressive thread pitch but short screws.

    I like the idea of not running a stringer, my only concern be the keel of the boat hitting waves and flexing a bit. I'll think about it.

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