Simple (but complicated) Foam/Glass Panel Question

Discussion in 'Boat Design' started by CatBuilder, Nov 25, 2011.

  1. CatBuilder

    CatBuilder Previous Member

    My plans call for 1.5" (381mm) foam and two skins of 34oz (1150g) glass on either side of that foam for the bridgedeck.

    The foam is 6lb (100kg).


    I have enough extra 7lb (112kg) foam from my rudders and dagger boards left to do the whole bridgedeck. Over ordered and it was on sale.

    The 7lb (112kg) foam is 1.18" (300mm) thick. The specs call for 1.5" or 381mm foam.

    What would you do? Would you order more foam and hope to use all this foam up inside on tables and whatever? Or would you go ahead with this foam even though it's slightly thinner than spec?

    The designer said it was ok to use it, but it "may" feel springy under foot. He also said that glassing any furniture to the bridgedeck would stiffen it up. However, I don't plan on having a lot of furniture that would cover the whole span. There are 4 bulkheads (2 on each extreme end) of the bridgedeck that would hold those parts up. Also one in the middle-aft. The rest is open space.

    What foam thickness have you built your bridgedecks with?

    My open span area is 16' (4.8m) x 10' (3m) or so...

    Any input?
  2. hoytedow
    Joined: Sep 2009
    Posts: 5,813
    Likes: 376, Points: 93, Legacy Rep: 2489
    Location: North of Cuba

    hoytedow Carbon Based Life Form

    Since you are glassing it anyway, can't you add some battens radiating out from the center underneath to reduce springiness on the floor?

    Attached Files:

  3. waikikin
    Joined: Jan 2006
    Posts: 2,390
    Likes: 153, Points: 73, Legacy Rep: 871
    Location: Australia

    waikikin Senior Member

    Some external longtitudinal stringers/girders with a uni cap etc could be added(with consultation from designer), some Crowther cats had this but could be omitted with recommended(by Lock) increase in core thickness, the increase in core thickness was a heap less work & fuss though & preferred option.
  4. CatBuilder

    CatBuilder Previous Member

    I could, but how do you mean?

    Are these battens in the foam? Outside the glass?
  5. CatBuilder

    CatBuilder Previous Member

    Any idea what most people use for a bridgedeck core thickness?
  6. Bruce Woods
    Joined: Sep 2007
    Posts: 134
    Likes: 12, Points: 18, Legacy Rep: 90
    Location: perth

    Bruce Woods Senior Member

    By deduction it appears as though schionning uses 25 mm with 600 biax each side. See......


    Note schionnings fore and aft underwing stringers.

    I thought you were building for charter. Don't you need to meet survey construction/design standards. IE Lloyds or whatever. Hence construction mods may require
    survey approval, so you may find it cheaper to stick to the plans.
  7. PAR
    Joined: Nov 2003
    Posts: 19,133
    Likes: 489, Points: 93, Legacy Rep: 3967
    Location: Eustis, FL

    PAR Yacht Designer/Builder

    You can add to the laminate thickness to compensate for some of the core thickness lose. You also could shave some of the thinner foam to make up the difference (sandwich). Core thickness will make the biggest difference in stiffness of the deck. The 22% difference is thickness is substantial enough I'd want more core, particularly on a bridge deck.
  8. waikikin
    Joined: Jan 2006
    Posts: 2,390
    Likes: 153, Points: 73, Legacy Rep: 871
    Location: Australia

    waikikin Senior Member

    Yeah, they went on the outside- in the "tunnel". but were lots more trouble to make & extra material so thicker core was the go.
  9. waikikin
    Joined: Jan 2006
    Posts: 2,390
    Likes: 153, Points: 73, Legacy Rep: 871
    Location: Australia

    waikikin Senior Member

    40mm foam is common on boats I've been involved with(for u/wing & deck house tops), but it's very vessel specific to the span etc etc as some boats have blisters for bunks & steps at the blisters(sometimes at 45), champher panels, longti center girders down the tunnel, all these break up the panel size & the smaller the panels the less deflection per laminate, fiber orientation is also a factor in regard to panel aspect.
  10. CatBuilder

    CatBuilder Previous Member

    Thanks for the input, guys. It really helped. I'll have to go thicker. Maybe I can find some thin core to add to the sheets I already have.
  11. waikikin
    Joined: Jan 2006
    Posts: 2,390
    Likes: 153, Points: 73, Legacy Rep: 871
    Location: Australia

    waikikin Senior Member

    I've wondered myself about that on Catbuilders construction, but I think the USA might have a relaxed or soft survey for 6 or less passengers, dunno for sure though, but I know on this side of the world there's a few hoops to jump, my premises are inspected & approved as suitable for commercial vessel construction in composites, not hard to comply but they have a checklist for lighting, ventilation, construction, material storage, lamination log etc.
  12. CatBuilder

    CatBuilder Previous Member

    There is no vessel inspection at the charter level for 6 or less pax in the USA. The inspection comes in the form of a complete vessel survey at the time I purchase insurance.

    However, my designer is the leading catamaran day charter (20 pax, day trip) fully inspected vessel designer in the States. That number of passengers requires a COI (certificate of inspection). That is his niche. He spends most of his time on those, so he is not in the habit if allowing builders to do stupid things that would violate the ability of a vessel to get that inspection.

    The thinner bridgedeck could have been safely made less stiff, but it would feel wrong underfoot to have flex.
  13. rxcomposite
    Joined: Jan 2005
    Posts: 2,423
    Likes: 403, Points: 83, Legacy Rep: 1110
    Location: Philippines

    rxcomposite Senior Member

    You could try adding a continous shear tie in the short side of the supported panel. Usually the panel is longer on one side, adding a shear tie in the short side will increase the stiffness of the panel as it will behave by acting like it is a smaller panel, hence stiffer.

    I have posted that shear tie in the early days of the build but if you have lost it, I will look for it and post it again.
  14. CatBuilder

    CatBuilder Previous Member

    Thanks, Rxcomposite. I will probably just go with the thicker core since I am trying to save time here. The build is taking quite a bit longer than anticipated due to the method of building I have chosen.

    There is more fairing to be done that anticipated before even glassing. Fairing the foam before the glass has been rather time consuming.

    So, I will just buy the extra thickness and add it to the panels I already have (assuming a reasonable cost) and make the bridgedeck nice and thick.

    This is going to be one rugged boat. I've upped the hull and deck from 3/4" foam to 1" foam already, now I will be upping the bridgedeck to whatever the thinest Core Cell I can get is, added to the 1.18" Core Cell I already have. Probably will end up adding 1/2", which is going to put me at 1.68" total thickness, or slightly thicker than the designer calls for. More stiffness.

  15. SamSam
    Joined: Feb 2005
    Posts: 3,900
    Likes: 198, Points: 63, Legacy Rep: 971
    Location: Coastal Georgia

    SamSam Senior Member

    Isn't something like that exponential? Like 22% less thickness means 44% less stiffness or something?

    Adding on a thinner layer of foam to bring up the thickness would bring on shear issues between the two thicknesses of foam, I would think.

    Mainly though, this from Bruce Woods caught my eye...
    I was going to build a charter boat at one time and Coast Guard requirements for certification required periodic inspections at different stages of construction. They also wanted to see the plans beforehand. I don't recall you ever mentioning the CG in any of your threads, but if they're left out of the loop, they get all mortified and it would get sticky getting certs.

    That was 20 years ago and apparently didn't apply to 6 pack boats. But things may have changed. Possibly also the 3 mile offshore and protected waters had a bearing on it all, in that more than 3 miles or in unprotected waters, even 6 pack boats had to have certified plans and certified construction.
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.