plywood bulkheads

Discussion in 'Wooden Boat Building and Restoration' started by need-help, Jan 19, 2011.

  1. need-help
    Joined: Jan 2006
    Posts: 7
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: new jersey

    need-help Junior Member

    Good morning,
    Starting in on a first build – a 20ft catamaran designed for plywood over frame construction.

    After the first complete read of the plans I’m ready to start focusing on the bulkheads. Although the plans are dimensionally detailed – there are areas that could use more specifications for a first time builder.

    In regards to the bulkhead construction - The plans specify 6mm marine ply with perimeter framing of 1” x 2” (on one side) and the bulkheads are to be notched to accept the stringers and gunwale.

    Is 1x2 framing material the same as stringer timber?

    Other than “screws and epoxy” – there are no further specifications for joining the 1x2 framing to the 6mm ply. Will a (non 2part adhesive) (PL or Titebond3) be acceptable in this application for bonding the framing to the bulkheads?

    What would be a recommendation for applicable screw type (profile), and spacing?

    If framing is only required on (1) side of the ply bulkhead – would adding it to the other side provide increased rigidity and larger bearing surface (frame+6mm+frame) for the stringer notches – or would it be needless weight added?

    Thanks in advance.
     
  2. alan white
    Joined: Mar 2007
    Posts: 3,731
    Likes: 122, Points: 0, Legacy Rep: 1404
    Location: maine

    alan white Senior Member

    The plans you are using should specify a lot of the details you are asking for.
    Regarding the 1 x 2 perimeter framing, one side should be enough.
    Screw spacing is less important if epoxy is used to adhere the 1 x 2. However, I can imagine #8 x 1" flat head bronze screws 4" on center would be adequate.
    Stringer timber means wood dimensioned to be used for stringers. If the dimensions are the same and the wood species the same, then the same pieces can be used elsewhere, but the pieces are simply 1 x 2s at that point. The word, "stringer" is only applied if the wood is going to be used for a stringer.
    PU adhesive such as Gorilla glue is probably your best adhesive if you are avoiding the use of epoxy. Titebond 3 is claimed to be waterproof but I have no experience with its use in this application. Others may comment on that.
     
  3. PAR
    Joined: Nov 2003
    Posts: 19,133
    Likes: 488, Points: 93, Legacy Rep: 3967
    Location: Eustis, FL

    PAR Yacht Designer/Builder

    I'll generally agree with Alan, though I'm not a PU adhesive fan, because of cost and the foaming action it makes. TiteBond III is an option, but I'd not trust it in the bilge.

    A 1x2 is a 1x2, though a stringer is also a stringer. The use of the term 1x2 is purely dimensional, the term stringer is fairly specific. If the plans call for a "nailer" of 1x2 around the perimeter of a bulkhead (a common practice) then this is usually just for bonding and fastening surface area. You should have some bulkhead drawings or at least a few images of a bulkhead that show the gunwale and stringer notches. If not you need to visualize the project a bit more, maybe a scale model first.

    Detailing plans for the novice builder is a difficult and tedious thing at best. For example I assume that someone building a boat knows how to size the length of a fastener properly, such as the perimeter framing on your bulkheads. This is a reasonable skill assumption and part of the process. I do spec out "odd" fasteners, like keel bolts, etc. but at some point it has to stop and the builder has to be trusted to select a fastener that will hold the piece on without bursting through the other side, splitting the part or other wise compromising the joined structural elements. The plans can always do better in regard to some novice builders, but then the call outs and scantling specs would become monstrous and difficult to wade through.
     
  4. alan white
    Joined: Mar 2007
    Posts: 3,731
    Likes: 122, Points: 0, Legacy Rep: 1404
    Location: maine

    alan white Senior Member

    I have a question for you, PAR. When a 1 x 2 is specified, I think in terms of one full inch by two full inches. In building construction, it's a 3/4" x 1 1/2". However, with boats, I've always leaned towards the full dimension. Hence, the #8 x 1" screw sizing.
    t
    This may be an unanswerable question, given the fact that it has to be clarified every time you talk to someone new anyway.
    This is like the question, at what size boat does the displacement include the crew?
     
  5. PAR
    Joined: Nov 2003
    Posts: 19,133
    Likes: 488, Points: 93, Legacy Rep: 3967
    Location: Eustis, FL

    PAR Yacht Designer/Builder

    When I spec a 1x2 it's the nominal dimension and I expect the builder to use a .75" x 1.5" piece of the species I've recommended. This would be true of all the common dimensional stock sizes, such as 2x4's, 1x6's, etc.

    On the other hand if I call for a 6/4's x 5" piece, then I expect a 1.5" x 5" piece. Many designers aren't using the 4th's dimensioning standard any more, so I've changed to an actual "call out" for 1.5" x 5", to use the previous example. This eliminates confusion, but to further reinforce the measurement, I include metric with the call outs, so a 1 x 4 would have 19 x 89 mm dimension in parenthesizes under the SAE dimensional call out.

    Is this a standard, oh hell no, but I try and I think others do as well. So things just have to be assumed, such as the screw size I mentioned. A good rule of thumb is to have at least as much thread in the piece you screwing to, as is in the the piece you are holding down. I prefer to have a bit more and have rules of thumb for too much screw as well. These are very basic carpenter and craftsmen practices, that can be derived from entry level books about woodworking, which the possible exception of rough house building carpentry.

    I'm late to the game with metric and still struggle with it, though am getting more comfortable. From my European friends I've discovered that centimeters are generally a conversational usage, but typically not used in call outs. So my call outs are millimeters or meters. About the time I'm ready to die, I'll probable get squared away with metric, which is easier to work with, but still something I just can't "see". It's a bit like the fat, ugly woman making the better wife, but you just can't get comfortable with it, so you choose the skinny, pretty one and spend the rest of your life, hoping the next heart attack will be effective enough!
     
  6. need-help
    Joined: Jan 2006
    Posts: 7
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: new jersey

    need-help Junior Member

    Par and Alan,
    Thank you for the guidance.
    In regards to Par’s last comments on material thickness (nominal dimension and 4th dimensioning) the plans mixed metric thickness for the plywood with the dimensional spec for the timber. While I initially thought the 1x2 for the frame would/could utilize the nominal dimension lumber (3/4”) I’m assuming since the plans had given total linear (meters) of 1x2 which included the stringers and gunwale – that the actual sizing was more inline with a true 25.4mm x 50.8mm. I suppose if a “notch” depth on the bulkhead was dimensioned and not just shown – then that would have eliminated any questions.

    While searching for suppliers of marine ply and lumber – perhaps I overlooked it, but could not find 1x2 size listings. I found larger 1x4 and 1x6 sizes, so am thinking that it may be a standard technique to (rip) larger material down in half or into thirds.
    I’m probably over thinking the basics...lol

    Thanks for the fastener info too.
    Interestingly – I would have selected a pan head or truss head screw. My assumption for head selection would have been to think that a screw with a larger diameter head would have distributed the clamping force along the surface of the plywood better than making a counter sunk hole and having the taper of the flat head held within the plywood.

    Thanks
     
  7. alan white
    Joined: Mar 2007
    Posts: 3,731
    Likes: 122, Points: 0, Legacy Rep: 1404
    Location: maine

    alan white Senior Member

    You often can't get 1 x 2s at the lumber yard---- they can be gotten out of 1 x 4s with a small amount of waste. Generally, suppliers of wood for boats will be selling wider pieces, assuming the builder knows best how to rip what they need, since anyone building a boat is going to own a table saw.
    Species of wood like mahogany and white oak, along with some cedars and certain other woods might be considered boat woods, but a light construction such as a multihull is almost always going to be made up of light woods that can be found at typical lumber yards. In terms of weight, pine, spruce, and fir are among the lightest. For what you mentioned, the perimeter pieces, clear eastern white spruce would be my choice (available in my area).
    Regarding the screw type, if the heads don't interfere with FG taping (which they could easily do) then I personally believe they (truss, pan head) are superior to flat heads in solid wood (not plywood, which will not split).
    If you have to counter bore to sink the head of a pan head screw, you are in trouble, as the c'bore will substantially weaken the wood and the connection between two pieces of wood. In other words, plan on using a lot of flat head screws, especially when attaching the hull panels to the bulkheads and chines/longitudinals.
     
  8. PAR
    Joined: Nov 2003
    Posts: 19,133
    Likes: 488, Points: 93, Legacy Rep: 3967
    Location: Eustis, FL

    PAR Yacht Designer/Builder

    A flat head screw will impart more pull out strength then any other type of screw head, truss and pan heads included.

    1x2's are commonly called furring strips. The obvious way to get these are to purchase 2x stock and rip the edge off at .75" on a table saw. In fact this is the best method to get good wood from cheap stock, as you end up with quarter sawn stock in the 1x2's which is dimensionally more stable then the flat sawn board it was ripped from.

    The wise builder "re-saws" most of the stock they get, to the dimensions they need, rather then buying "milled" stock. If you need 4/4's x 3 then rip the edge off a 4x4 or 4x6 post at 1", you'll have to make a second table saw pass at 3", but hey, welcome to boat building. Buying stock milled to the dimensions you need can cost a fortune. The only time some think this is viable is for stripper builds, where the stock is also cove and beaded. I can see someone purchasing 100's of strips, with the routed edges as an advantage, but 1x2's?
     

  9. need-help
    Joined: Jan 2006
    Posts: 7
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: new jersey

    need-help Junior Member

    Thank you PAR and Alan very much. I appreciate the guidance.
     
Loading...
Similar Threads
  1. AwJees
    Replies:
    1
    Views:
    109
  2. sdowney717
    Replies:
    4
    Views:
    320
  3. Sunburned One
    Replies:
    38
    Views:
    2,133
  4. sdowney717
    Replies:
    30
    Views:
    1,599
  5. Travis Grauel
    Replies:
    26
    Views:
    3,215
  6. Paul D
    Replies:
    13
    Views:
    2,261
  7. Floatything
    Replies:
    0
    Views:
    1,524
  8. juan manuel luna
    Replies:
    21
    Views:
    2,615
  9. Old salty
    Replies:
    4
    Views:
    4,097
  10. Vesimakara
    Replies:
    2
    Views:
    2,220
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.