Edge rebate for glass tape joint

Discussion in 'Materials' started by Stefano Dilena, Aug 19, 2021.

  1. Stefano Dilena
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    Stefano Dilena Junior Member

    I believe you are rigth.. I am studying like crazy on the topic and the amount of information is overwhelming.
    Some answers are not easily found unless stupid questions are asked.. That is part of the learning process
     
  2. Rumars
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    Rumars Senior Member

    The Duflex panels are made to be laminated over. The initial Z joint is to create full size panels (boat length) from the shipping friendly sized ones, like you would scarf plywood. Then you laminate over the entire boat to achieve final skin thickness. The Z joint is appropriate for what it's supposed to do. If a boat is engineered to use only the factory supplied fiberglass skin, then the joint is good enough as supplied.
     
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  3. rxcomposite
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    rxcomposite Senior Member

    The one illustrated by Tansl is the best method applicable on foam cores as it is being used by Class Society. However the rules states that if the joint falls under a major substructure such as frames or bulkheads. If it does not fall under major substructure, and high strength cores (balsa and plywood is a high strength core) is used, it is scarfed with an angle greater than 45 degree and glued, before lamination. The glue to be used must be equal or greater than the shear strength of the core.

    If it is a prelaminated board, the above rule is to be followed. The joint on the broken face laminate must be compensated. The rule is 50 mm on each side of break, plus a light extra layer because the rule followed is that joints must have 1.5X of the original skin thickness whenever there is a break. Knuckles cannot be avoided. If you want it nearly flush, glue the board and grind the original face laminate to a 6:1 taper minimum and strip laminate.

    They are good shop practices.
     
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  4. fallguy
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    fallguy Senior Member

    I am a bit confused. Does the class society not offer a solution where the core is made thinner and then the glass simply thicker there @seams not in same plane or chines?

    In my build, we used a 12mm core and our tape or seam reliefs were 2:5" wide on each panel and 3mm deep or a bit less. Ideally, the reliefs would have been tapers, but this I botched and instead put a radius on the planer knives for a smooth glass transistion. This allowed for a 5" tape margin and two four inch tapes were staggered. Probably more fitting would have been a 4" and a 3" tape and the angle.

    The relief was thus tapered at 5:1 or perhaps a bit more. But all done to raw core, prior to any glasswork.

    All that is happening is the core is made thinner at the seam and more glass is used. Perhaps this is why tansl got confused?

    It is rather conventionally understood a butt join of epoxy in foam core is always stronger than the core. So, important to mention I believe the rule cited is hd core?

    Does the class society dictate core thickness?

    I feel like we are talking about the moon and you the sun, no? Sorry. Truly trying to understand.
     
  5. fallguy
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    fallguy Senior Member

    ps...I am better now..it was a hard week
     
  6. rxcomposite
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    rxcomposite Senior Member

    Attached is the LR rule that Tansl cited. Note that it said if the Shear is in "excess of the core material". Core shear is greatest at the base of the stiffener so it must be sized correctly. Some calculate core shear at midspan where the elements go against each other. That is horizontal shear. Under the base of the stiffener, it is vertical shear which is 1/2 of the load divided by the area. This is the point where in a uniformly loaded beam, ends fully fixed, is like a punch and die method. More like shear by cutting the edge with scissor. Most designer check the vertical shear strength of the core to be used. Balsa has very good vertical strength.

    Note that in 1.13.13 it says where can also be fitted. Class rule are always conservative. They add a bit more than what the calcs show.
     

    Attached Files:

  7. fallguy
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    fallguy Senior Member

    Do you see anywhere on this picture where this is used? We are referring to the tape relief in the hullside at the gunwhale. That tape relief is what the OP asked about. We did not see a relief in the picture you cite.

    There is a chine halfway down where a tape relief was also used, but I did not use the 45s because there is already angles present and making angles here would offer little.

    Just trying to clarify if we are talking about the same thing..or if reliefs are not allowed.

    Any load test of this joint would end up destroying the hullside and not the join.

    795360BC-9B79-48BD-807F-C0DFAD662E79.jpeg
     
  8. fallguy
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    fallguy Senior Member

    Tansl addressed me privately and I agree my language to suggest my bonds were not absolutely stronger than the hullsides was a bit heady.

    But our offline dialogue brought me to an interesting way to help understand the issue.

    Let's propose a hullside of 2 meters and a 10 meter hull. If we add a tape seam to the top and bottom edges without relief, let's say the tape seam is 2mm. This is 2•10•0.002 or 0.04 cubic meters of fairing fillers required. Some fuzzy math, about 10.6 gallons of filler. Roughly 10 pounds per gallon with epoxy and cabosil is around 100 pounds of fairing filler each side of the hull.

    There must be something happening in translation here because I don't believe LR requires you to use 200 pounds of fairing compound to hide these tapes.
     
  9. fallguy
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    fallguy Senior Member

    My math on the compound weight is off..let's say 3 gallons epoxy and 7 gallons cabosil..more like 30 pounds of fillers per side, and a lot of time...
     
  10. rxcomposite
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    rxcomposite Senior Member

    2 mm thick is about 1800 gr/m2 of fabric. What are you using? Boat cloth? The key is to use thin plies, especially in the outermost layer.

    LR will not tell you how to make your boat beautiful, just structurally sound. Technique depends on the boatbuilder.

    For example, while in the aerospace industry, we have to finish the aircraft to the best/practical smooth finish. Though the layers are only 0.6 mm thick, we have to fair it as there are unavoidable overlaps. We were using using microbaloons laden primer before painting. Later, we switch to very fine prepreg veil cloth with microbaloons. Gave us a very fine finish.

    Boats, no problem, weight is not that critical. Outside layers are usually resin rich layer of CSM and veil cloth for waterproofing and to hide the fiber weave underneath. It is a sacrificial layer. There is a lot of leeway there to fair it off. Plus, microbaloon putty works.
     
    Last edited: Aug 22, 2021
  11. fallguy
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    fallguy Senior Member

    I am referring to two tapes of 600/225 biax tape with mat to bond the hullsides to top and bottom. A 2mm estimate for two tapes each top and bottom joins is conservative for discussion. If you make no relief, you are forced to fair that entire amount.

    Respectfully, I still am not sure we are communicating well.

    We are talking about modifying the core dimension prior to any laminating to allow for the tapes.
     
  12. rxcomposite
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    rxcomposite Senior Member

    Yes, it is a necessary evil. You have to plan ahead. And why 2 layers of 850 gr/m2 at the same time? what happened to overlap? 850 gr/m2 is about 1 mm thick. That thick, you are going to have problem fairing it.

    You say gunwhale? Is that deck to hull connection? There are many ways to approach it. Or to hide the seam. Download Marine Composites by Eric Greene and associates. There are illustrations on how to do it.
     
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  13. fallguy
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    fallguy Senior Member

    I followed specifications for the hull for the tapes. The tapes must exceed the hull laminate as well. Hull is 900g... They probably could have been made with less tapes for sure, but this is all digression from allowing for [a] tape. They are also staggered and not stacked.

    It it a hull to deck connection; sort of, but not a walkable section here and not even accessible from below; so probably not able to follow/meet LR, but also we are digressing.

    All I have been attempting to do is explain to the OP methods of doing it, so the Green document might be helpful.
     
  14. rxcomposite
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    rxcomposite Senior Member

    I guess we are not on the same page then. The OP posted a connection of foam cored laminate and Tansl responded on the proper method to join cored laminate based on Class rule. I responded on the same topic.

    If it is a deck to hull joint or a top part to hull mating, the rule does not apply as it is a very low side pressure. It is an impact pressure analysis and this type of joint is usually protected /hidden by rubber or wood bumpers.

    Marine Composites has two pages of illustrations on this type of joints. There is and always a last layer that will be proud. It has also several illustrations on tapered laminate repair. Theirs is a 12:1 taper.

    I have the hard copy. 2 actually. The newest digital version might no longer include this. Let me know so I can scan it.
     
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  15. fallguy
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    fallguy Senior Member

    The OP did not realize his image was wrong...it took me a while to understand there was a communication problem..

    I hold you guys in high regard, I am just a builder..
     
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