Cutting portlights into topsides

Discussion in 'Multihulls' started by bolero105, Jun 9, 2023.

  1. bolero105
    Joined: Jul 2011
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    Location: Caribbean

    bolero105 New Member

    Hi all,

    I would like to add some additional ventilation to the cat that I live aboard with my family. It's a one- off French design, 22m x 11m but it's a pod cat and quite minimalist. Construction is glass over cedar strip with carbon beams and reinforcements.

    The main sleeping cabins (mirrored port and stbd) have one deck hatch only and I would really like to add opening ports in the hull to increase ventilation. These would be approximately 450x250 and placed centrally between a bulkhead and a ring frame that are approximately 1200mm apart. The bulkhead is the main bulkheads that supports the connection of hull to main beam. See pics attached for proposed site of port lights. My plan is to initially install them inboard and then possibly on the outboard side too at a later stage. The outboard ones will be recessed so the frame is set in from the outer hull skin. Forgive the amateur notation on the attached photos, not to scale!

    My question is, how do I know that I am not compromising the structural integrity of the boat? The port light I have in mind is a quality item from Moonlight with internal and external aluminium frames that are bolted together on either side of the hull skin.

    Would love to get some feedback on this before I cut any big holes in my boat! Thanks!!
     

    Attached Files:

  2. bajansailor
    Joined: Oct 2007
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    Location: Barbados

    bajansailor Marine Surveyor

    Hello Bolero,

    Your proposed Moonlight portlights are pretty big really at 450 mm x 250 mm, and will be in the topsides, directly above your bunks...... Moonlight are good quality, and they should not leak (at least not in the early days), but nobody is perfect, and it is all too easy to set off on a day sail to somewhere and forget to close a portlight...... and you might then end up with a wet bunk.

    If you simply want extra ventilation, then a much smaller portlight will still work very effectively, especially if you put a scoop on it to funnel the air into the cabin. Even something not much bigger than a mobile phone in surface area.
    Alternatively, have you considered having a dorade vent box on deck, above each cabin? Or do you want to keep your deck minimalist, with just the hatches, and not have to worry about tripping over dorade vents?
     
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  3. peterbike
    Joined: Dec 2017
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    Location: melbourne

    peterbike Junior Member

    Nice choice of boat :)
    450 x 250 sounds big, but on this size boat it becomes small.
    If concerned about strength, you could laminate another panel (16/20mm) 1200 x 1200 over to spread the load ?
    Does anyone know if the escape hatches remain watertite after being opened & closed a few/lot of times ?
     
  4. Rumars
    Joined: Mar 2013
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    Rumars Senior Member

    You need to reinforce the cutout with additional material to compensate for the hole.
    The elegant way of doing it is to have a NA (preferably the original designer) calculate the amount needed and how to place it, then have a good yard execute his plan.
    The brutal way is to throw on enough material for the problem to go away for sure. For example, a sapeli plywood doubler, the same thickness as the hull, going from bulkhead to ringframe, reaching up to the deck stringer (the step in the overhead) and the same distance down, glued on with epoxy and fiberglassed over with at least the same amount as the current inner skin. The glass should overlap onto the existing structure 10cm, stepped to create a feathered edge.
     
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  5. bolero105
    Joined: Jul 2011
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    Location: Caribbean

    bolero105 New Member

    Hi BajanSailor,
    Thanks for the reply. They are quite big, but I guess I should have said they're not JUST for ventilation. I think the additional light and 'view' would make what is a fairly pokey cabin seem a lot more welcoming. As for forgetting them open, well, yes, it could happen, though after 25-ish years at sea I consider myself to be a fairly careful sailor, besides, even the water through an open 200x100 portlight would be enough to put a bit of a damper on my day!
    I did consider a dorado but it will be quite obtrusive on deck, and won't do much for light etc. Thanks for your suggestions.
     
  6. bolero105
    Joined: Jul 2011
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    Location: Caribbean

    bolero105 New Member

    Hi Peterbike,
    A 1200x1200 doubler seems pretty big. Is there any calculation used to come up with that size or is it a gut feel thing?
    Thanks.
     
  7. peterbike
    Joined: Dec 2017
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    Location: melbourne

    peterbike Junior Member

    Bolero, no calc's - was just meant as an eg.
    If I was to guesstimate on foam, then I would say 3/400mm above window to existing dbl up
    then 250 for window, then another 3/400mm. Possibly vac. infusion on a bench; cut to size & fit.
    Another idea would be to go 1/2 layers of carbon matt ?
    But both options would be worth running past an composites engineer or a naval archi.
    He may have another idea that kicks my amateur butt. ?
     
  8. redreuben
    Joined: Jan 2009
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    redreuben redreuben

    Gut Feel.
    Cut hole, rebate and radius core enough for 4-5 layers of uni around cut out and cover with one layer of Double Bias (150mm) and peel ply both sides.
    Fair and finish.
    Install windows.
    This advice is worth what you paid for it.
     
  9. redreuben
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    Location: South Lake Western Australia

    redreuben redreuben

    Probably a better method would be apply DB from both sides overlapping in the cut out.
     
  10. cando2
    Joined: Nov 2021
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    Location: washington state, USA

    cando2 Junior Member

    Hi bolero. Have you made those big cutouts yet? If not I have a few different ideas that may cut down on your anxiety.
     
  11. waterbear
    Joined: Mar 2016
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    waterbear Senior Member

    I would email the designer of the vessel if possible. Not knowing any better (and to be clear I do not) I would be concerned about the proximity to the high stress beam attachments.
     
  12. redreuben
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    redreuben redreuben

    So another riddle ?
     
  13. gonzo
    Joined: Aug 2002
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    gonzo Senior Member

    A traditional wind scoop will let more air through the existing deck hatch. Cutting large holes on the hull is not a good idea without either the designer approving it, or having an engineering assessment.
     
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  14. fallguy
    Joined: Dec 2016
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    fallguy Senior Member

    I have small vents in my hulls, two per side. They are not recessed, but also do not extend past the rubrail.

    These are Vetus, smaller size, but have screens and nice closure. Just be careful to not overcut! You can decore and reinforce if needed.

    Vetus Porthole Type PM111, Category A1 | MAURIPRO Sailing https://www.mauriprosailing.com/us/product/VTSPM111.html

    As other suggest, the forward escape hatch can be wind scooped and a larger entry can help; although my entries are not screened. The escape and ports are...

    IMG_0765.jpeg
     
    Last edited: Jul 20, 2023
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