Composite Forestay Tang

Discussion in 'Fiberglass and Composite Boat Building' started by robwilk37, Sep 10, 2014.

  1. robwilk37
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    robwilk37 Senior Member

    I didn't like the clunky sprit through bolted to the deck, so decided to embed a receiver underneath. 1/4' wall biax and epoxy tube heavily glassed at the bow and at a 1" bulkhead. a longitudinal BH will support the load aft from there. what im thinking is the forestay tang as shown in the pics. the material is G10, would be epoxied together, uni-glassed around the receiver tube and over the gussets. could also overlay (longitudinally) uni carbon.

    the tang is 7/8" thick and 5"x12". the forward mount for up to a 10" furling drum and still clear the solent stay mount ( which would be removable for a hanked-on blade). the boat will also have a removable cutter stay. and a head stay out on the sprit for a furling geneker.

    I like the idea of no more through bolts. what I still don't have my head around is the strength of these materials. the epoxy is good for 30kpsi in sheer and tensile. ive got close to 28sqin of contact surface. the G10 is good for 25kpsi minimum in tension and there is 1sqin at the narrowest point adjacent to the pins - what I am assuming are the weakest points?

    once this area is covered up and glassed in with the rest of the deck my access will be limited so I mocked this up and invite your critical opinions. too much? not enough? of course of I do go ahead with it ill pretty it up a bunch.

    the boat is a 40' 9ton cruiser...

    as always, TIA
     

    Attached Files:

  2. gonzo
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    gonzo Senior Member

    I think that some unidirectional fibers wrapping the parts would be good insurance. For a small piece you can just pull fibers of a piece of fabric.
     
  3. Petros
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    Petros Senior Member

    very nice concept. if it was my boat I would have those gusset blocks run all the way out to the edges of the square tube so it is tangent with the corner radius. This way when you wrap it the fibers are in direct tension with out an prying or peeling loads. it will also increase the bond area available.

    It is not just a matter of strength vs. surface area, but sharp corners, geometry, eccentric loads, etc. can cause stress multipliers that can first weaken a bond, and it will eventually fail in progression from the stress concentrations. use large radius at all transitions, and it should hold together well.
     
  4. Charlyipad
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    Charlyipad Senior Member

    Hi Rob. My understanding is that it is important to alternate the uni with biax or other cross directional material to help keep the uni fibers in column.
     
  5. robwilk37
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    robwilk37 Senior Member

    thanks for the replies all...

    this was just a quick mock-up. the gussets will eventually be cut with a generous radius and be full length and yes, right out to the edge of the tube. ill probably take an inch or so off the overall height to shorten down the lever arm as much as possible, just tall enough for the toggle pins to clear.

    im assuming most of the load is in tension with some lateral sheer. any idea what that ratio might be? might give some insight as to the ratio of biax to uni.

    on another related subject...
    im considering the same general assembly for the bobstay tang. the bob is only countering the geneker, loss of the sprit wont bring down the rig. but id want to attach the bobstay toggle with the pin of a bow/D shackle so the rode could be brought down and aft (off the sprit) while anchored. ive got some more 1" thick G10 stock, would you trust it?
     
  6. groper
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    groper Senior Member

    I can't see what holds the sprit down? It won't matter how well you fix the g10 to the square tube, if the square tube lifts...

    The method often used here in oz, is to wrap the uni around an upside down U shaped material, such as a peice of g10 like you've done, and with a stainless Busch to accept the rigging screw. If lashing it down to a larger post like you have, then the material is shaped so it's a smooth taper all the way to the top, around the stainless tube, and down the other side.

    The potential problem I can see, is that your trying to do it at 90degrees to the way we normally do it here. This means the fibers running over the top and lashing it down to the sprit, are completely cut through and broken as soon as you drill the hole for the rigging screw. Therefore, you have no purchase, other than the strength of the g10 block, to stop the rigging screw pulling out the top of the block.

    If it were me, I would run a stainless tube, sized to accept the rigging screw inside it as a Busch, sitting on top of your g10 block over its full length.Then lash it to the sprit using lots of UD fiber totalling approx 10000gsm, or 10mm thickness. Start with 600gsm Dbias, and again half way through, then finish with 600gsm Dbias. Once it's cured, you have a stainless tube buried under 10mm of UD fiber running transverse to its length. When your ready to rig it, drop an angle grinder down through the glass until the depth cuts through the stainless tube and just deep enough so that the toggle will sit in place, 2 thin cuts for each toggle. Pass a long rigging screw through the tube, and through the toggles. Care should also be taken to align the tube so it's at right angles to your forestay angle. This way the fibre are t their strongest in pure tension.

    This method is commonly used for new boats being built here for both shroud chainplates and forestay chainplates. It has proven to be a very strong and reliable method.

    [​IMG]
     
  7. robwilk37
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    robwilk37 Senior Member

    the square tube is glassed into the hull at the bow and strongly gusseted/glassed at the bulkhead. in addition, the tube/tang assembly will be glassed into the deck when the area is finished off with MDF-ply.

    thanks for the detail of your chainplate lay up. this is roughly how I plan to fab my main and lower chains for coligos deadeyes. I guess with the forestay my question is can I trust the mechanical strength of the G10 to bare the majority of the load without the addition of a lot of uni? using the uni more for attaching the tang to the tube. the G10 in this assembly is not core but structural. the stuff is amazingly hard and strong, i was conservative in the above numbers. tensile is around 40kpsi and flexural around 60kpsi. at what point can the stuff stand alone given the loads likely to be seen on a 40' cruiser? if the point of likely failure is the pin breaking out of the tang itself, then i can overlay carbon uni lengthwise to contain those loads, no?
     
  8. groper
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    groper Senior Member

    Well if the g10 has the same strength as aluminium or greater, then it should be OK to use.

    I'm also worried about the height you have between holes and top of the block. As a general engineering rule pertaining to shear pull out loads like this, you should have at least 2x the hole diameter between the top, and also edge of block, and the hole.

    How does the mechanical properties of this stuff compare to aluminium?
     
  9. Stumble
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    Stumble Senior Member

    The issue I see is the lack of a mechanical tie between the tang and the rest of the boat. I wouldn't trust just the epoxy joing to hold it down, and I don't think wrapping the assembly with uni given the geometric arrangement really looks that good. As the fibers come over the top of the square tube all the bends required to wrap it provide a lot of working room.

    I am not sure what I would recommend however to fix this.
     
  10. robwilk37
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    robwilk37 Senior Member

    G10 vs 6061 look comparable, but I cant say I know enough to interpret the numbers. of course the big difference is the mode of failure, as Al will yield for days and give a lot of indications of rough treatment, while the composites tend to just go all at once. im fine oversizing these parts for piece of mind though.

    as to the mechanical connection, if I do end up with this system, the gussets will be cut full length and radiused for a smooth transition from the tang to the tube. but this goes to part of my original question... ive got almost 60sqin of bonding area in tension, another 48sqin in sheer and my epoxy is good for approx. 30kpsi. at some point cant you just trust the goo? and don't forget the whole thing gets covered with MDF ply and glassed into the rest of the deck, so more surface to spread loads.
     
  11. groper
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    groper Senior Member

    You have to be careful about the numbers for your epoxy... usually those number are for lap shear stress, and your not loading it in shear when you glue it on top, its at right angles to this condition. I've never seen such an important connection made which relies on goo factor alone. There is also dynamic and fatigue considerations aswell. Considering you could loose the rig over this, I wouldn't trust it.
     
  12. Ad Hoc
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    Ad Hoc Naval Architect

    Never assume. Calculate it!

    Also looks too high (hole locations above the deck), so out of plane bending may also be an issue.
     
  13. philSweet
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    philSweet Senior Member

    Terrible design. Don't do it. The stay needs to be anchored to the boat, not the sprit holder. You want to be able to tear that sprit out of the boat without worrying about the forestay. Lets say something big and solid drives into the end of your sprit. The sprit buckles. Does that design handle the buckling force? What will that tang look like after 100' of 3/8 anchor chain has rattled around that joint under load? You need damage tolerance. A long strap that runs down the bow with lots of fasteners that fail like a big zipper. Strap holds the front of a really stout plate about 10" long on deck that has the tangs welded to it. A material that can't be damaged by anchor chain.

    Boats that have sprit receivers or telescoping sprits usually have them offset so the forestay support can be bulletproof and sprit failure won't affect the integrity of the forestay. I wouldn't use a wood sprit in a glass tube. Wood is really tough stuff. In an impact scenario, it will outperform its static rating moreso than the composite. Use a composite sprit in a composite tube so that you can be sure what will break first.
     
  14. dinoa
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    dinoa Senior Member

    As groper mentioned the gusset bottoms would be loaded in tension. Assume roughly only 100psi for epoxy in tension in comparison to roughly about 1000psi for secondary bond shear strength. Redesign for bonded joints to be loaded in shear or wrap with unidirectional cloth as suggested.

    Dino
     

  15. SukiSolo
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    SukiSolo Senior Member

    'Never assume', very wise words. Often structures which appear to be in tension all the time are in compression for some of the time. Whilst in this case it may be simply a case of the rig relaxing, it might well have a high snatch load whilst beating in a big sea, or similar. Also the real world scenario of say another boats boom giving a side swipe on the stay accidentally.

    On something like this might even be worth calculating and then testing with a sufficiently powerful winch to actually check it, to break point or at least yield. Then any calcs are verified or not and any correction factor can be applied.

    Please note another thread with composite (carbon shroud) attachments which had stainless tube inserts. As mentioned before, most holes are 3 X diam from the edge of sheet materials to ensure integrity of load against sheer.
     
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