Fill kerfs in foam?

Discussion in 'Fiberglass and Composite Boat Building' started by massandspace, Oct 21, 2022.

  1. massandspace
    Joined: Sep 2017
    Posts: 53
    Likes: 0, Points: 6
    Location: Salt Chuck

    massandspace Junior Member

    Hello

    I am building a custom foam/epoxy catamaran. I have secured .75” foam with kerfs and will use it in a strip plank fashion to create the hulls.

    Question: Do I need to fill in all the kerfs with thickened epoxy? Of can I save weight and leave them empty, then skin both sides?

    Thanks,

    David in Seattle
     
  2. Blueknarr
    Joined: Aug 2017
    Posts: 1,383
    Likes: 361, Points: 83
    Location: Colorado

    Blueknarr Senior Member

    FILL
    Or accept the strong possibility of delamination.
     
    fallguy likes this.
  3. massandspace
    Joined: Sep 2017
    Posts: 53
    Likes: 0, Points: 6
    Location: Salt Chuck

    massandspace Junior Member

    Thank you……but….

    Some foams are, I believe, NOT kerfed, and boats built with that type of foam do NOT delaminate, correct?
     
  4. bajansailor
    Joined: Oct 2007
    Posts: 3,097
    Likes: 1,206, Points: 113, Legacy Rep: 37
    Location: Barbados

    bajansailor Marine Surveyor

    Is this what the designer specified?
    Are you the designer?
    How big is this catamaran?

    Many foams are not kerfed.
    Every type of foam sandwich construction CAN delaminate if it is subjected to forces outside of it's safe operating range, or if it is not built properly, or.........
     
    fallguy likes this.
  5. Blueknarr
    Joined: Aug 2017
    Posts: 1,383
    Likes: 361, Points: 83
    Location: Colorado

    Blueknarr Senior Member

    Unfilled kerfs are voids. Many delams start as voids or bubbles and grow.
     
    fallguy likes this.
  6. fallguy
    Joined: Dec 2016
    Posts: 6,784
    Likes: 1,389, Points: 123, Legacy Rep: 10
    Location: usa

    fallguy Senior Member

    The purpose of the kerf is for resin channels for infusion. Usually kerfs cost more.

    Did you buy the foam second hand?

    The outside of the hull 100% needs the kerfs filled, or you will be building the most perfect expected hull failure in history.

    The inside is less certain. If there are kerfs on both sides, things like walking loads also create a risk for delamination.

    If you are strip planking, the orientation and width of the planks could shift the kerfs to glue joints, but then your strips would be only 0.75".

    Filling the kerfs with fumed silica and resin is lighter than resin only for infusion, so your query is somewhat nonsense.

    If you take the wrong shortcuts in foam sandwich construction; you will be forever sorry. I bought my foam for wet bagging and later wished I had kerfs for infusion. Once you commit to the foam purchase; changing methods is not arbitrary.

    Filling the channels with thickened resins sucks a bit because most all of your bonding will become secondary to the core as well.

    In order to offer you more guidance, you'd need to provide more details about the foam and the build method like planking width, etc.
     
    Last edited: Oct 22, 2022
    bajansailor likes this.
  7. fallguy
    Joined: Dec 2016
    Posts: 6,784
    Likes: 1,389, Points: 123, Legacy Rep: 10
    Location: usa

    fallguy Senior Member

    Further, the number one way for foam hulls to delaminate is impact or a void in the built laminate below the waterline. Water pressure then moves the area and rather quickly hydraulics erode the good bonds. It is not really a question.

    However, how you build the strips is...if you cut away one groove for each plank or you can turn them that would be good.

    The Diab GR4 is a 20mm center with 2mmx2mm channels. In terms of density, say an 80 kg density foam, use 160 kg for fumed silica and 1150 kg for resin. Using 2-1 for the mix ratio and the density of the filler is about 500 or a bit less, but less than epoxy only as intended by the product.

    The 20mm centers mean if you rip the stuff down woth a 2mm blade, it would end up at 18mm wide, then the only remaining channels are in the glue joint the short way.

    If you have a different product, let us know what it is...
     
    bajansailor likes this.
  8. massandspace
    Joined: Sep 2017
    Posts: 53
    Likes: 0, Points: 6
    Location: Salt Chuck

    massandspace Junior Member

    Thanks to those who have replied so far..

    So I am understanding that the problem in NOT filling the kerfs is that all those areas are starting points for possible delamination, is that correct assumption?

    If so, then how about this idea: Float a thin layer (1/8” or so) of thickened epoxy over the raw foam. Let it sit a bit. Apply the glass with some unthickened epoxy and squeegee out. Add peel ply and squeegee again. In the end the thickened epoxy should fill the very tops of the kerfs, leaving the bottom 90% empty. (Please see attacked sketch). Does this solve the problem of the voids?

    Again, the idea is to save weight, and in addition will save some material costs.
     

    Attached Files:

    Last edited: Oct 25, 2022
  9. fallguy
    Joined: Dec 2016
    Posts: 6,784
    Likes: 1,389, Points: 123, Legacy Rep: 10
    Location: usa

    fallguy Senior Member

    Noone can help you perfectly if you cannot provide the product datasheets and what and how you are laminating because you are forcing assumptions.

    If you are trying to use scrim; it would seem to me to have some inherent weakness if you do not intend to fill it, but raw epoxy will run out of scrim as well. But like I said, without more detail we are guessing.

    Scrim is horribly expensive and typically, for vertical structures; you'd be using thermoforming.

    I used scrim for roof decks and the way I did it was to fill the edges with thixo, then use an excess of raw epoxy to fill the grooves. My laminate ended up resin rich, there was no way around it unless I had groove filled it all as you suggest, but I also was worried about not getting enough into the grooves and opted for more resin on the roof.
     
  10. bajansailor
    Joined: Oct 2007
    Posts: 3,097
    Likes: 1,206, Points: 113, Legacy Rep: 37
    Location: Barbados

    bajansailor Marine Surveyor

    Just a reminder re my earlier questions.

    "I have secured .75” foam with kerfs and will use it in a strip plank fashion to create the hulls."
    -------------------------------
    Is this what the designer specified?
    Are you the designer?
    How big is this catamaran?

    Also, can you post any sketches or drawings of the design?
     
  11. massandspace
    Joined: Sep 2017
    Posts: 53
    Likes: 0, Points: 6
    Location: Salt Chuck

    massandspace Junior Member

    The size of the boat and such is not relevant to my question…..my question is simply the best, lightest way to securely bond glass to kerfed foam.
     
  12. bajansailor
    Joined: Oct 2007
    Posts: 3,097
    Likes: 1,206, Points: 113, Legacy Rep: 37
    Location: Barbados

    bajansailor Marine Surveyor

    With respect David, my reason for asking was to find out some more information about this cat that you are building, never mind that you think that my questions are not relevant.
    Why be so secretive?
    Is it a model catamaran, or one that is 20 metres long, or....?
    There is a lot of expertise on here, and this expertise might be able to help you in other aspects of your design and build, to possibly avoid you making an expensive mistake (or two).
     
  13. massandspace
    Joined: Sep 2017
    Posts: 53
    Likes: 0, Points: 6
    Location: Salt Chuck

    massandspace Junior Member

    Yesterday I laid up a test piece using the “thin layer of thickened epoxy to just fill in the tops of the kerfs” idea (as in the drawing I included in an earlier post) and it seemed to work very well. So only the very tops of each kerf are filled….glass on top.

    Before I start to mass produce the strips can anyone comment on whether leaving “air pockets” (the lower 90% of each kerf) UNFILLED will, in the long run, cause any issues. It will be a large weigh savings if I can do this….
     
  14. bajansailor
    Joined: Oct 2007
    Posts: 3,097
    Likes: 1,206, Points: 113, Legacy Rep: 37
    Location: Barbados

    bajansailor Marine Surveyor

    Read Fallguy's post #6 above.

    If you are still not convinced, go back to the basics where stress = force / area.
    If you only have a very small filled area at the top of each kerf, then for a given stress, the force acting on it will be much higher than if the kerf was filled (completely).
    And the higher the force, the higher the potential for something to shear (break) or delaminate.
    Ok, this is a very simplistic way of looking at it, but I think it helps to illustrate the difference between filling and not filling.
     
    Blueknarr likes this.

  15. Blueknarr
    Joined: Aug 2017
    Posts: 1,383
    Likes: 361, Points: 83
    Location: Colorado

    Blueknarr Senior Member

    Many high performance craft use extreme measures to increase lightness.

    They sacrifice durability.

    How much potential lifespan are you willing to give up for being a lightweight?

    Or

    How much weight are you willing to have for a longer service life?
     
Loading...
Similar Threads
  1. IdahoBoats
    Replies:
    6
    Views:
    951
  2. Jason Rodgers
    Replies:
    9
    Views:
    1,109
  3. DogCavalry
    Replies:
    26
    Views:
    2,074
  4. Heynow999
    Replies:
    8
    Views:
    1,887
  5. garage monster
    Replies:
    9
    Views:
    2,526
  6. L'eau.Life
    Replies:
    4
    Views:
    2,169
  7. midcap
    Replies:
    10
    Views:
    5,353
  8. reelpleasure
    Replies:
    5
    Views:
    3,467
  9. Ebrahim2020
    Replies:
    10
    Views:
    6,903
  10. magentawave
    Replies:
    21
    Views:
    5,621
Forum posts represent the experience, opinion, and view of individual users. Boat Design Net does not necessarily endorse nor share the view of each individual post.
When making potentially dangerous or financial decisions, always employ and consult appropriate professionals. Your circumstances or experience may be different.