Hatch Deflection

Discussion in 'Boat Design' started by pyoung, Aug 19, 2020.

  1. pyoung
    Joined: Aug 2020
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    pyoung Junior Member

    First post so I am just going for it.

    When working towards a lam schedule for a hatch lid, is there a particular deflection range that is considered comfortable underfoot without feeling "soft"?

    Right now I have a 57" x 12" span. This will be an all composite material construction. Based on the lam schedule I have I am at approx 0.092" deflection assuming a 16psi point load.

    I searched around the forums and never really found what I was looking for.
     
  2. bajansailor
    Joined: Oct 2007
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    bajansailor Marine Surveyor

    Welcome to the Forum PY.

    0.092" does not sound like a lot, but it works out to 2.3 mm - which I think would be noticeable underfoot..
    I am surprised that you have this much deflection, when the lid is only 12" wide..
    What is your laminate schedule? Is it cored?
     
  3. JSL
    Joined: Nov 2012
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    JSL Senior Member

    The hatch should feel like the deck. If it feels 'spongy' (deflects) under foot you can expect some accidents or perhaps a failure.
     
  4. fallguy
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    fallguy Senior Member

    How exactly is the hatch going to seal without some movement? Usually surgical tubing or some seal will have up to 1/8" of movement. Although a compression latch should help shrink that to under 1/16".

    And 16 psi for a man with a say 11" by 3" shoe is 33 sqin or 500 pounds+.

    I have a 12mm M80 core deck with 22 oz triax each side. Max span is 18". It doesn't really move at all. All my in floor hatches are made with 3/4" aquaplas and getting glassed with 2x 12 oz biax. The hatch spans are a bit larger, well, on is 18x24. Anyhow, I am using high density core for all hatch lids to reduce movement on hatches because they WILL move on their seals a bit.
     
  5. pyoung
    Joined: Aug 2020
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    pyoung Junior Member

    I agree is doesn't sound like much but its also not far off from 1/8" which is no good.

    A little bit more info. This lid will be an RTM part. It was recommended to use a 3mm Rovicore in the laminate. Beyond the Rovicore it is cored with a PVC material. .

    LAM Schdl:
    Gel - 18-22mills
    Rovicre 450 (3mm thickness)
    E-LTM 1808
    Gurit PVC Core 1"
    E-LTM 1808
    Rocivore 450
    9oz 2x2 3k Carbon Twill (strictly aesthetics)
    Gel - 18-22mills

    BINGO!! Yes 16psi was unnecessarily high. Even going with a 330lb point load over 33sq in (which is still a guy standing on one foot) I am now at 0.01" deflection. This lets me move to a 3/4" core and I am seeing 0.05" deflection. I had to get my cross section thickens down for a compression latch to work.

    Thank you guys for your help.
    I will try and post more often and help if/when I can on others issues. I will also try and get a pic up of this boat once I figure out how to link in a photo. This is all for a 47'cc w/ quint 450's
     
  6. fallguy
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    fallguy Senior Member

    The span distances are important.

    Anything over about 18" and you ought to consider a higher density core than the pvc.
     
  7. pyoung
    Joined: Aug 2020
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    pyoung Junior Member

    Right now the Gurit 80 (5lb/ft^3) PVC is on the line of a shear failure. My main purpose was to dial in a thickness so I can finish the gutter detail work and the original LAM thickness was a no go. I can bump up to the 130 (8lb/ft^3) and overcome the shear failure by a good margin.
     
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  8. bajansailor
    Joined: Oct 2007
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    bajansailor Marine Surveyor

    Please do post some photos PY - that is a seriously impressive (and large) centre console vessel at 47' - and with 1,800 hp on tap, she should be pretty fast.

    To attach a photo, click on the 'upload a file' button in the bottom right corner of the screen, and then select a photo (eg from your 'Documents' on your computer).
     
  9. pyoung
    Joined: Aug 2020
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    pyoung Junior Member

    Screen Grab for now. First hull is out of the mold. Infused carbon, lots of uni fabric in strategic locations. I will try and get to the other shop this week and get some real pictures. Console mold is done, hardtop mold is in tooling. Working on fishing up deck details to get that into tooling. Lids, tubs and console liner still left to dial in.

    We thought this boat was going to be a quad engine set up, but first owner wanted quints once Cigarette came out with the Tirranna running 6 engines. So we have 8 Merc 450Rs sitting on the shop floor that were supposed to be for the first two boats. If we can get the tooling buttoned up the actual build should move along quickly. I will try and keep it updated.
     

    Attached Files:

  10. fallguy
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    fallguy Senior Member

    I would like to see your gutter plan/drawing/section. I made some gutters from the same 20mm material as the covers. I am a bit worried my gutters are small so might double or 4x drain them.
     
  11. fallguy
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    fallguy Senior Member

    No seal in the picture. I used the same laminate schedule decks n core, so they shoukd be close at final with a seal. But like I said; think they are a bit undersized.

    I expect you are going solid glass. But I spent less time making these than making a mould.

    7C9A30E5-1C50-4CC0-86FB-8976FC917511.jpeg 7079E40A-D8AF-467A-8BEC-746571810544.jpeg
     
  12. pyoung
    Joined: Aug 2020
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    pyoung Junior Member

    Yep I got you. Let me finish up my drawings and I will get you a screen grab this afternoon.
     
  13. SamSam
    Joined: Feb 2005
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    SamSam Senior Member

    It might depend on it's surroundings. It's subjective, but if a deck in general feels solid but the hatch has a noticeable deflection, that might qualify as soft. Anything different than the surrounding area might qualify as soft. They might still be serviceable, but feel soft.

    As far as forces - Subjects jumped from a box 0.3 metres high to land on a force plate. Results showed that there were no significant differences (p>0.05) across gender, activity levels, and type of sport played. Across all subjects, the mean peak vertical GRF was 4.5 bodyweights (SD: 1.7).
    Normative data of vertical ground reaction forces during landing from a jump - ScienceDirect https://www.sciencedirect.com/science/article/abs/pii/S144024409980187X

    I take that to mean if a person jumped from apprx. 1 foot onto the hatch, the momentary pressure would be 4.5 times the body weight. If a boat crested a wave and dropped out from beneath you and then bottomed out and came up to meet you coming down, there would be a large jump in momentary pressure.

    I would have no idea how to numerically calculate acceptable flexation of a hatch, but it does seem if you have 1/10" movement 6" from the edge of a hatch, that might be a bit much. I would wonder what would happen to the laminate and the hardware/fastenings (seals, hinges, clasps and fasteners) with repeated flexing.

    Here is how forces are distributed from the foot to the ground/hatch...it doesn't look like you can take a simple size like above, 11" x 3" , and spread the weight evenly over that to get an accurate psi.
    [​IMG]
     
    Last edited: Aug 20, 2020
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  14. fallguy
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    fallguy Senior Member

    I like the post, but the scale next to the foot is confusing me or I am not sure how to interpret. A shoe is not a 58 pound sqin load.

    If you used say the front of a foot at 4 square inches; you'd be closer to the original 16psi number using the 400kpa, but when you juxtapose that across the 33 square inches in a standing load estimate; you get over 500 pounds per foot or a 1000+ pound person, or a safety factor of ?4-5.

    pretty sure my hull isn't rated at 58 psi

    All I am saying is the data seems a bit much for designing. But I am not designer.
     

  15. SamSam
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    SamSam Senior Member

    I'm not a designer either but the info seemed like it might be useful.

    The foot image came from Wikipedia
    Pedobarography - Wikipedia https://en.wikipedia.org/wiki/Pedobarography

    From what I can tell, that scale goes from 0 psi to 58 psi.

    Ground pressure - Wikipedia https://en.wikipedia.org/wiki/Ground_pressure

    Whoa, that's almost 1,900 psi. for the spike heels. 60 - 80 kPa equals 8.7 psi to 11.6 psi, 4.5 times that (jumping from 1 foot high) is 34.8 psi to 52.2 psi

    So a hatch that deflects .092 " at 16 psi would deflect .3 " at 52.2 psi. Maybe, there must be other factors a designer/engineer might know.
     
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