Escape hatch reinforcement

Discussion in 'Multihulls' started by simon, Nov 6, 2009.

  1. simon
    Joined: May 2002
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    simon Senior Member

    I have posted this question in the Fiberglass and Composite Boat Building without success.
    My question is:
    How are composite hulls locally reinforced to take the escape hatches on the inside of catamaran hulls. These are areas that will take a lot of pounding in waves and the hatches are critical for the watertightness as they are close to the waterline.
    My catamaran has developed delamination because the cutout was not reinforced, which I guess lead to movement in the hull. How should I reinforce the escape hatch area?
    Also I wonder what type of hatch to use. I am not really happy using a deck hatch.

    Thanks
     
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  2. Manie B
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    Manie B Senior Member

  3. catsketcher
    Joined: Mar 2006
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    catsketcher Senior Member

    Not an engineer but here are my ideas

    Gday

    If you have a few cracks then that is good. It is the boats way of telling you that the material is under too much load for it too take. You can't take away any load probably so you have to add some material. Stress that leads to movement in the hull has caused the cracks - it is not the other way around. Now the movement is exacerbated as the structure has to deform more to take the same load.

    I would firstly check that the inner and outer skins are bonded. If the structure shows signs of too much load per unit area (stress) then you can add some more stuff in there. I am guessing the hatch is square and the cutouts are about 50mm radius. I would increase the laminate around the hole.

    Taking another guess (no photo) I guess you would be reluctant to put the boat on the hard and start grinding away on the paint outside and increase the laminate inside and out. It is easier to do this inside but you still have to tie the layers together. I have successfully removed the foam core in a cutout to a depth of 30mm (use a screwdriver or similar) and then laid down a bed of glue mix epoxy just to smooth the rough bottom of the trough. Whilst this is malleable I have then laid in a rope of unidirectional carbon around the insides of the cutout where the foam used to be. Uni glass would be fine. Make sure the uni is well adhered to BOTH skins as you wind it around and also make sure that it overlaps the start and finish of the tape.

    This can be a messy job but keep poking and use bits of foam to keep the top edges wedged in. I would also urge you to increase the radius of the corners by burying the tape in deeper cutouts where necessary. Corners increase stress in cutouts.

    Use filler to cover up your uni job and then increase the laminate on the inside of the hull with about twice the laminate staggered from the cutout about 100mm. (bottom layer 200mm from cutout top layer 100mm from cutout) This last bit is a total guess - but would make me feel better.

    Just increasing the specs on the inside laminate will lead to assymetric loading but by tying the two together you will do a better job.

    I hope this helps

    Phil
     
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  4. sabahcat
    Joined: Dec 2008
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    sabahcat Senior Member

    All good advice, but what I have seen on other boats which may be even easier is to use hairy bog.

    Get unidirectional glass and cut it all up into 1 inch lengths seperating the fibers, mixing this in with some highdensity filler and then jam that in.

    It is still all tied together and incredible strong.
     
  5. catsketcher
    Joined: Mar 2006
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    catsketcher Senior Member

    Sounds a lot easier. My advice comes from me making holes in a tris crossbeams. I made cutouts and then vacuum bagged the unis into the holes. Your method is a good compromise between strength and ease of building.

    cheers

    Phil
     
  6. simon
    Joined: May 2002
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    simon Senior Member

    The sketch should explain a bit the current situation and on the right I have put a possible solution. Currently there is only a reinforced laminate on the inside, but just a single layer on the outside. I was thinking about routing a groove into the cedar and leaving a bit standing to facilitate putting in the uniglass rope. Then the edge would be rounded and the inner corner filled, so that the outer laminate is joined with the inner laminate.

    Thanks for the suggestions.

    Simon
     

    Attached Files:

  7. catsketcher
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    catsketcher Senior Member

    Looks good

    Thats looks good Simon. The use of the rope and the joining of the skins will certainly help in the delam situation. I am surprised that the core is cedar - I was thinking that cedar would be able to handle the shear fine and that foam would only do this. I have learnt some more again.

    The only thing I would like to do is to spread the load some more. Why not overlap the tape that joins the two skins onto the inside skin some more to help avoid a stress concentration - you have to feed the loads in and out in a broad way. If the loads hit the very stiff rope then you may get a stress concentration that you are trying to avoid. Less glass in the rope and a bit around the cutout would assist with load sharing in the broad laminate.

    cheers

    Phil
     
  8. simon
    Joined: May 2002
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    simon Senior Member

    Thanks for the support,

    catsketcher: what is the procedure to prepare the uniglass rope and how do you glue it into the groove?

    Simon
     
  9. Luckless
    Joined: Mar 2009
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    Luckless Senior Member

    Are the bolts held in place by only a few layers of fiber glass? Is there no added structural collar around the hatch to help insure even stress load spread over a wide area?
     
  10. catsketcher
    Joined: Mar 2006
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    catsketcher Senior Member

    Luckless and I are thinking similar thoughts and would like to see a bit of reinforcing around the inside of the cutout. I would also like to see the uni rope inside the laminate.

    As to how to make uni rope - it is really easy. If you have some biaxial glass then just pull off the strands running across the roll to get a whole heap of 1.27 m long strands. Then put them down on a clean bench and work them through with epoxy. When wet out grab one end and then use your other hand to pull them down into a rope. Do this a couple of times to get a nice smooth rope. If you don't have biax then buy some uni tow. It is really cheap and do the same as above.

    I like to put the rope onto a bed of epoxy glue to ensure there are no voids. Push it well beyond the surrounding laminate so you can fill over it and make it look nice.

    cheers

    Phil
     
  11. simon
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    simon Senior Member

    how would you reinforce the inside with uni rope? or would you put a doubler on the inside? I have very little space around the inside, otherwise I would have to cut the interior lining out.

    Simon
     
  12. catsketcher
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    catsketcher Senior Member

    Uni rope goes in between the laminates. It replaces the core. The reason you need the rope is to move the stress around the cut. It is best to tie the laminates together to do this. As for doubling - do what you can. I would use 600gm double bias as it is good to use and tough.

    If you can't double and you want to bolt into something then do the oversize thing. Do this anyway. Do this after you have finished the uni rope and glassing doublers as much as you can inside.

    Usually you remove the core and insert a high density filler like ply or epoxy glue mix. For your job I would be careful to remove the core below any bolt hole and insert epoxy glue here. Then you can either drill a small hole and tap a thread into the glue or drill a slightly larger hole and insert a epoxy covered bolt into the hardened glue.

    In the end you should have the bolts in the epoxy glass/glue matirx around the cutout. This should spread bolt and cutout load.

    cheers

    Phil
     
  13. simon
    Joined: May 2002
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    simon Senior Member

    Today we routed on the outside a groove, about 7mm outside the cutout edge, all around the cutout. The groove goes down to the inner laminate, so that it did remove all the wooden core.
    The groove is 12mm x 12mm. Then we filled it with uniglass. The uniglass was divided into three bunches of strands to make the filling of the groove easier and that way we could make sure that the void would be filled.
    We ended up with a nice blue transparent ring, slightly standing proud. The next step will be sanding the ring flush with the strip planking and then applying the 2 layers of biaxial glass to the outside and bringing it in onto the inner laminate. The uni-ring is attached to the outer biaxial and the inner laminate.
    I did forget to take the camera, but hope to take some pictures the next time.

    Cheers

    Simon
     
  14. Alan M.
    Joined: Jan 2006
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    Alan M. Senior Member

    BSI Moonlight hatches make some specifically for multihull escape hatch use. They have thicker lenses for their size, and are marked as suitable for area 2b

    http://bsi-dk.dk/site.asp?sideid=201

    Other brands probably have a similar product.
     

  15. simon
    Joined: May 2002
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    simon Senior Member

    Here some photos of the repair, half way through

    cheers

    Simon
     

    Attached Files:

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