Screw and Glue questions

Discussion in 'Wooden Boat Building and Restoration' started by Anatol, Sep 20, 2015.

  1. Anatol
    Joined: Feb 2015
    Posts: 191
    Likes: 0, Points: 16, Legacy Rep: 10
    Location: los angeles

    Anatol Senior Member

    I am about to commence building a custom glass over ply experimental multihull. I'm planning to use solid timber keel and stems (something light and strong - probably spruce but am open to suggestions). Then 'persuade' the ply in both planes/axes.

    I'll screw and glue (1/2") ply sides to 'keel'. I'm thinking of using #6 ss screws (3/4 or 1"), like one every 6".

    Some stitch'n'glue proponents who use screws for glue clamping remove the screws - why?

    I am wondering about SS alloys for the screws. 18-8 is standard. 316 is 'marine', what is 410? In any case, I was planning to encase screws and timber in epoxy.

    Predrilling is a pain, I found some nice self drilling screws :
    /Sheet_metal_screws_Phillips_flat_head_self_drilling_Stainless_steel_410_6.aspx - self drilling.

    I've also found a source for 316 SS ring nails.

    Any comments on any of this?
    thx.
     
  2. gonzo
    Joined: Aug 2002
    Posts: 14,150
    Likes: 540, Points: 123, Legacy Rep: 2031
    Location: Milwaukee, WI

    gonzo Senior Member

    If there is enough water intrusion under the laminate to cause a problem with the stainless screws or nails, crevice corrosion will be the least of your problems. Well painted steel lasts for decades in ship construction, so don't worry too much. 18-8 is 302 or 304 stainless. 316 is more corrosion resistant and harder; particularly the passivated type. 410 is harder and usually used for cutlery.
     
  3. Anatol
    Joined: Feb 2015
    Posts: 191
    Likes: 0, Points: 16, Legacy Rep: 10
    Location: los angeles

    Anatol Senior Member

    Gonzo, thanks.
    so are you saying don't bother with stainless? Regular steel or ceramic coated decking screws would be fine if sealed?
     
  4. gonzo
    Joined: Aug 2002
    Posts: 14,150
    Likes: 540, Points: 123, Legacy Rep: 2031
    Location: Milwaukee, WI

    gonzo Senior Member

    There is some moisture in the wood that will eventually rust regular steel screws. Stainless, even 18-8 will provide adequate corrosion resistance under epoxy/fiberglass. The cost difference is very small.
     
  5. Anatol
    Joined: Feb 2015
    Posts: 191
    Likes: 0, Points: 16, Legacy Rep: 10
    Location: los angeles

    Anatol Senior Member

    thanks! good succinct info!
     
  6. gonzo
    Joined: Aug 2002
    Posts: 14,150
    Likes: 540, Points: 123, Legacy Rep: 2031
    Location: Milwaukee, WI

    gonzo Senior Member

    You're welcome. Post photos
     
  7. PAR
    Joined: Nov 2003
    Posts: 19,133
    Likes: 473, Points: 93, Legacy Rep: 3967
    Location: Eustis, FL

    PAR Yacht Designer/Builder

    Your description of torturing the plywood works, but is quite limited. Your screw sizes and attachment approach suggest no experience with this technique, as even a little torque on a 6" screw from "screaming" 1/2" plywood will easily pull the heads off, break the screw or have it rip right through the plywood. Temporary fasteners, typically over pads will do a much better job.

    Yeah, pilot and clearance holes are a pain, but if you don't, you'll quickly realize why you need to, in the first place.

    S.S. screws bonded (technical term) in timber or plywood will not rust, assuming the bonding remains in tact. Bonding is not simply coating the screw with some goo, driving them home and hoping for the best. You're best advised to download the free user's guides from westsystem.com and the epoxy book from systemthree.com. Read them over good, as they'll offer the basic techniques for taped seam construction and working with goo's.
     
  8. Anatol
    Joined: Feb 2015
    Posts: 191
    Likes: 0, Points: 16, Legacy Rep: 10
    Location: los angeles

    Anatol Senior Member

    Hi Par
    points well taken. You're right, though I've got 40 years of construction diverse materials, precision engineering and boat experience, I have less experience with persuading ply. I avoid the term torture as I do not intend to put extreme stress on the ply - enough to gain the strength of bends in both axes, but not enough to risk the kinds of failures you describe. My instinct to use low size screws with big heads addresses this and hopefully mitigates failure along screw lines.
     
  9. PAR
    Joined: Nov 2003
    Posts: 19,133
    Likes: 473, Points: 93, Legacy Rep: 3967
    Location: Eustis, FL

    PAR Yacht Designer/Builder

    Small diameter screws with button, pan or truss heads, will just force the failure in the shank instead of the head. A #6 is a small screw for 1/2" plywood
     
  10. Anatol
    Joined: Feb 2015
    Posts: 191
    Likes: 0, Points: 16, Legacy Rep: 10
    Location: los angeles

    Anatol Senior Member

    410 stainless is high strength so I'd imagined I could reduce diameter one size and maintain strength.
     
  11. upchurchmr
    Joined: Feb 2011
    Posts: 2,943
    Likes: 112, Points: 63, Legacy Rep: 579
    Location: Ft. Worth, Tx, USA

    upchurchmr Senior Member

    Lots of people have tried to "torture" plywood using various means of force and broken their ply.

    Listen to PAR and get the Gougeon book. They have successfully built fairly large multihulls in this manner, and there is a whole chapter on how to do it.
    No where have I seen a solid wood keel with this method. But I haven't seen as much as PAR has.

    http://www.westsystem.com/ss/assets/HowTo-Publications/GougeonBook 061205.pdf Chapter 25
    FREE after I paid for 2 revisions!
     
  12. Anatol
    Joined: Feb 2015
    Posts: 191
    Likes: 0, Points: 16, Legacy Rep: 10
    Location: los angeles

    Anatol Senior Member

    Thanks! I _am_ listening. And I have the Gougeon book. And most of the others: Bolger, etc.
    "No where have I seen a solid wood keel with this method."
    I know, odd isn't it? Seriously though, I'm not thumbing my nose at tradition or experts, just having my own design thoughts. Its not really a 'keel', its an oblique rabbetted timber running about half the hull length midships along the keel line whose main purpose is to hold the side panels at a set angle (about 120 deg) prior to bending (but will provide soem longitudinal rigidity. I'm going for something between a semicircle and a deep V.
     
  13. upchurchmr
    Joined: Feb 2011
    Posts: 2,943
    Likes: 112, Points: 63, Legacy Rep: 579
    Location: Ft. Worth, Tx, USA

    upchurchmr Senior Member

    Are you going to do a scale model to avoid potentially breaking the plywood?
    I'd like to see the model or any plans you have.

    I guess you could equate the wood keel to a fillet and glass as used on a Tornado.
    What about the bow and stern? What shape?

    1/2 " ply must mean a large boat??
     
  14. Anatol
    Joined: Feb 2015
    Posts: 191
    Likes: 0, Points: 16, Legacy Rep: 10
    Location: los angeles

    Anatol Senior Member

    "Are you going to do a scale model to avoid potentially breaking the plywood?"

    making a 1/4 scale model in coming days/weeks. Will post pics.

    "What about the bow and stern? What shape?
    1/2 " ply must mean a large boat?? "

    full disclosure - 30' proa vaka. So double-ender. The general plan is to join side panels (30'x4' scarfed sheets) along the keel line midships to the 'keel piece' (about 10' long), setting the midships keel angle.
    Then bend sides vertical at gunwales midships.
    Then bend sides almost vertical at both ends and fasten to solid timber prow pieces. Prows/stems will be slightly flared to provide more reserve bouyancy. Then fillet the 'keel line' between keel piece and prows.

    Goal is an easy-build, light and strong fast cruiser with some interior space.
     

  15. PAR
    Joined: Nov 2003
    Posts: 19,133
    Likes: 473, Points: 93, Legacy Rep: 3967
    Location: Eustis, FL

    PAR Yacht Designer/Builder

    I just don't understand why you're trying to reinvent the wheel, particularly with a much heavier than necessary approach. Given what you're building, light and stiff is what you want, so toss the centerline keel and simply tape the centerline seam, back it up with a fillet and maybe an internal batten if necessary. Use wire ties, copper wire or other "ties" if you must, though I don't very often any more, then "unfold" the bottom panels and/or side panels. Prop open and continue the filleting process. the net result is lighter and waterproof.
     
Loading...
Similar Threads
  1. missinginaction
    Replies:
    9
    Views:
    2,094
  2. sdowney717
    Replies:
    14
    Views:
    1,603
  3. sab
    Replies:
    6
    Views:
    1,677
  4. brian_n
    Replies:
    13
    Views:
    1,702
  5. Delta Moon
    Replies:
    0
    Views:
    1,170
  6. martinos
    Replies:
    35
    Views:
    10,199
  7. Michael35
    Replies:
    6
    Views:
    1,868
  8. crosscut
    Replies:
    36
    Views:
    18,414
  9. mikefrome
    Replies:
    2
    Views:
    2,504
  10. Jacques Stander
    Replies:
    4
    Views:
    819
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.