Results 1 to 17 of 17

Thread: What's the procedure to attach a plywood bottom to frames?

  1. #1
    Join Date
    May 2021
    Location
    Hawaii
    Posts
    49

    Default What's the procedure to attach a plywood bottom to frames?

    I'm planning to build a 12 feet long, 4 feet wide flat bottom Jon boat using 3/8" marine plywood for the bottom. In thinking of the build process I hit a snag.

    The plan is to glue and screw the bottom to four 3/4" thick frames that make up the 2 bench seats (2 frames per bench seat). But now in thinking of the actual build procedure in step-by-step detail, I'm not sure how to insure that the screws through the bottom will be centered in the frames.

    When attaching the plywood sides to the frames, it will be easy to butt a piece of 3/4" wood to a side and scribe the outline on the inside of the side. Then drill holes in the side centered between the outlines. Since the bottom is not installed yet, it should be easy to position the frames between the outlines and then drive screws through the sides into the frames.

    But with the boat upside down to install the bottom, what's the way to insure that screws through the bottom will be centered in the 3/4" frames?

  2. #2
    Join Date
    Jan 2000
    Location
    Cushing, Maine
    Posts
    4,113

    Default Re: What's the procedure to attach a plywood bottom to frames?

    Scribe your frames on the bottom right side up. Then drill pilot holes for the screws. Set up a jig to hold the frames in position then turn it over. Smear on the glue, put on the bottom and put in the screws.
    Ben Fuller
    Ran Tan, Liten Kuhling, Tipsy, Tippy, Josef W., Merry Mouth, Imp, Macavity, Look Far, Flash and a quiver of other 'yaks.
    "Bound fast is boatless man."

  3. #3
    Join Date
    Mar 2017
    Location
    Loon Lake, Washington
    Posts
    206

    Default Re: What's the procedure to attach a plywood bottom to frames?

    I've done this three times on three separate builds now, and I don't recall any difficulty hitting the frames -

    Here I'm butt-strapping the bottom of the "200 dollar" version of the Bolger Featherwind in place. You can see the next frame to the rear.
    I bet I simply made a pencil mark on the side of the boat denoting the center of the frame, and put a straight edge across the bottom lining up with the marks on the sides and drew a pencil line to indicate the center of the frame.

    SAM_6408S.jpg

  4. #4
    Join Date
    May 2021
    Location
    Hawaii
    Posts
    49

    Default Re: What's the procedure to attach a plywood bottom to frames?

    Quote Originally Posted by Ben Fuller View Post
    Scribe your frames on the bottom right side up. Then drill pilot holes for the screws. Set up a jig to hold the frames in position then turn it over. Smear on the glue, put on the bottom and put in the screws.
    Could you describe the jig to hold the frames in position? I can't envision what that means.

  5. #5
    Join Date
    May 2021
    Location
    Hawaii
    Posts
    49

    Default Re: What's the procedure to attach a plywood bottom to frames?

    Quote Originally Posted by Etdbob View Post
    I've done this three times on three separate builds now, and I don't recall any difficulty hitting the frames -

    Here I'm butt-strapping the bottom of the "200 dollar" version of the Bolger Featherwind in place. You can see the next frame to the rear.
    I bet I simply made a pencil mark on the side of the boat denoting the center of the frame, and put a straight edge across the bottom lining up with the marks on the sides and drew a pencil line to indicate the center of the frame.
    Putting marks on the sides denoting the center of the frames sounds like a workable solution. Thanks.

  6. #6
    Join Date
    Jul 2007
    Location
    Wongawallan Oz
    Posts
    16,377

    Default Re: What's the procedure to attach a plywood bottom to frames?

    Do you really need to screw them down as well? Epoxy and fillets will hold the ply to the frames - if it’s a flat bottom it’s not like you're having to hold a curve in place and if you have a slight curve weight will do the job.
    Larks

    “It’s impossible”, said pride.
    “It’s risky”, said experience.
    “It’s pointless”, said reason.
    “Give it a try”, whispered the heart.

    LPBC Beneficiary

    "Keep away from people who try to belittle your ambitions. Small people always do that, but the really great make you feel that you, too, can become great!"

  7. #7
    Join Date
    May 2021
    Location
    Hawaii
    Posts
    49

    Default Re: What's the procedure to attach a plywood bottom to frames?

    Quote Originally Posted by Larks View Post
    Do you really need to screw them down as well? Epoxy and fillets will hold the ply to the frames - if it’s a flat bottom it’s not like you're having to hold a curve in place and if you have a slight curve weight will do the job.
    The bottom is completely flat in the aft two thirds until it starts curving upward to the bow about the forward one third. All of the seat frames are within the flat area. I would like to glue and screw the bottom to the frames without having to use epoxy fillets and fiberglass tape to secure the bottom to the frames.

    I want to start down the center line and screw into the 4 frames. Then work outward to have the bottom ply lie completely flat and in full contact with the frames without any convex shape that could occur by screwing the outer edges only. The screws will act as clamps holding the bottom firmly to each frame.

    Thanks.
    Last edited by DayTripper; 05-13-2021 at 06:02 PM.

  8. #8
    Join Date
    Jul 2007
    Location
    Wongawallan Oz
    Posts
    16,377

    Default Re: What's the procedure to attach a plywood bottom to frames?

    I tend to think gluing with epoxy and filleting the corners of the frames with the hull right side up would be less work than it would be screwing the ply down with the hull turned over and then bunging and/or filling over the screw heads (and holes where you miss the frame?) to seal them.

    You don’t need fibreglass tape for the fillets and clamps should hold the curve coming up to the bow.....or as you say just use the screws for the curve at the bow. The screws are really just there to hold the ply in place while the glue goes off anyway, at 12’ this boat won’t be supporting any massive loads or pressures.

    Just an option to consider that may simplify the task for you - ultimately it’s whatever works best for you.
    Larks

    “It’s impossible”, said pride.
    “It’s risky”, said experience.
    “It’s pointless”, said reason.
    “Give it a try”, whispered the heart.

    LPBC Beneficiary

    "Keep away from people who try to belittle your ambitions. Small people always do that, but the really great make you feel that you, too, can become great!"

  9. #9
    Join Date
    Nov 2011
    Location
    Salt Spring Island, BC
    Posts
    7,986

    Default Re: What's the procedure to attach a plywood bottom to frames?

    Hold the bottom in place with weights. Crawl under and scribe both sides of each frame. Remove the bottom and drill the holes between the lines. Dry fit it, including the screws. Remove it and apply thickened epoxy to the frames and the ply and try to put it back in place without smearing the glue all over the place. This will be a good time to have a helper for a few minutes. Go back under and make the squeeze out into mini fillets with a popsicle stick. Clean up any excess before it hardens.

    If you anticipate ever having to replace the bottom use pan head screws and once the glue is set up remove them and pound snugly fitting square dowel into the glue filled holes. Fill the holes with a WEST syringe.

  10. #10
    Join Date
    May 2021
    Location
    Hawaii
    Posts
    49

    Default Re: What's the procedure to attach a plywood bottom to frames?

    Quote Originally Posted by Larks View Post
    I tend to think gluing with epoxy and filleting the corners of the frames with the hull right side up would be less work than it would be screwing the ply down with the hull turned over and then bunging and/or filling over the screw heads (and holes where you miss the frame?) to seal them.

    You don’t need fibreglass tape for the fillets and clamps should hold the curve coming up to the bow.....or as you say just use the screws for the curve at the bow. The screws are really just there to hold the ply in place while the glue goes off anyway, at 12’ this boat won’t be supporting any massive loads or pressures.

    Just an option to consider that may simplify the task for you - ultimately it’s whatever works best for you.
    Thanks for sharing an option. It's good to have different suggestions to choose from.

  11. #11
    Join Date
    May 2021
    Location
    Hawaii
    Posts
    49

    Default Re: What's the procedure to attach a plywood bottom to frames?

    Quote Originally Posted by Gib Etheridge View Post
    Hold the bottom in place with weights. Crawl under and scribe both sides of each frame. Remove the bottom and drill the holes between the lines. Dry fit it, including the screws. Remove it and apply thickened epoxy to the frames and the ply and try to put it back in place without smearing the glue all over the place. This will be a good time to have a helper for a few minutes. Go back under and make the squeeze out into mini fillets with a popsicle stick. Clean up any excess before it hardens.
    I planned to set the boat on the floor of our carport to make it as simple as possible without having the boat elevated above the floor. I might reconsider my building plan with further input from others.

    If you anticipate ever having to replace the bottom use pan head screws and once the glue is set up remove them and pound snugly fitting square dowel into the glue filled holes. Fill the holes with a WEST syringe.
    This will be a one time build and if the bottom needs to be replaced, I'll just discard the boat.

    Thanks for the ideas.

  12. #12
    Join Date
    Jul 2007
    Location
    Wongawallan Oz
    Posts
    16,377

    Default Re: What's the procedure to attach a plywood bottom to frames?

    Quote Originally Posted by DayTripper View Post
    Thanks for sharing an option. It's good to have different suggestions to choose from.
    Just another thought - if you can glue the frames down right side up as I described you will be able to achieve a much neater finish inside with clear access to everything - ie without having to crawl under it getting glue in your face while you clean up after gluing and screwing down an upturned hull
    Larks

    “It’s impossible”, said pride.
    “It’s risky”, said experience.
    “It’s pointless”, said reason.
    “Give it a try”, whispered the heart.

    LPBC Beneficiary

    "Keep away from people who try to belittle your ambitions. Small people always do that, but the really great make you feel that you, too, can become great!"

  13. #13
    Join Date
    Jan 2011
    Location
    Norwalk CT
    Posts
    1,794

    Default Re: What's the procedure to attach a plywood bottom to frames?

    If you are attaching the sides first just mark the frames on the sides. Strike a line across the bottom for the center of each frame.
    Feels lie you're overthinking it.

  14. #14
    Join Date
    Jan 2000
    Location
    Cushing, Maine
    Posts
    4,113

    Default Re: What's the procedure to attach a plywood bottom to frames?

    Quote Originally Posted by DayTripper View Post
    Could you describe the jig to hold the frames in position? I can't envision what that means.
    I was assuming that your frames are not attached to the sides, in which case you need to make something that is what people sometimes call an eggcrate, longituidinals with slots. If your frames and sides are fastened together in the traditonal bottom last skiff build, then you don't need to do it. Noted recommendations in this thread for marking the frames on the sides, laying on the bottom, springing it to shape with weights / props drawing parallel lines across then drilling your pilot holes, which will work, To get the screws to pull in hard, the hole in the plywood works best if it is little larger than the threads so all the pulling is done on the frames.
    Ben Fuller
    Ran Tan, Liten Kuhling, Tipsy, Tippy, Josef W., Merry Mouth, Imp, Macavity, Look Far, Flash and a quiver of other 'yaks.
    "Bound fast is boatless man."

  15. #15
    Join Date
    Mar 2017
    Location
    Loon Lake, Washington
    Posts
    206

    Default Re: What's the procedure to attach a plywood bottom to frames?

    If you are attaching the sides first just mark the frames on the sides. Strike a line across the bottom for the center of each frame.
    Feels lie you're overthinking it.
    Boy, you got that right!

    I said just mark the sides and snap a line in the second reply above.
    Building a boat like this is as easy as it gets -

    Butt the plywood together and cut out the sides -

    SAM_6354S.jpg

    Make the frames and transom. For a john boat, you'll have a bow and stern transom.

    SAM_6378.jpg

    Attach the frames to one side (you should have carefully marked where they go when you laid out the sides) and bend on the other side.
    I use drywall screws with small plywood "pads" to temporarily put things together. When I'm happy with the way it's going together I slather with glue and nail! Sometimes I use brass or stainless screws, but nailing is fastest.

    Suddenly, it looks like a boat!

    SAM_6390S.jpg

    Dry fit the chine longs and wales to see if they will go on without breaking. If they are good to go, slather with glue and nail 'em on.
    This is when it's nice to have lots and lots of clamps.
    Flip, and fit the bottom as shown above in my first post.

    I do recommend you fiberglass the bottom. Plywood loves fiberglass! I use ordinary Bondo polyester resin. I'd also recommend putting a stiffener on the bottom -

    On this boat it's a 1x4 glued and screwed to the bottom. Those odd shiny things are lead weights ( I gather scrap lead and cast them in an old muffing tin, and use 'em for bullet casting).

    On this boat I fiberglassed each side of the bottom up to the center line reinforcement or "skeg" or "keelson" or whatever you want to call it.
    I did not fiberglass the 1x4 itself. Being solid wood it doesn't need it and it would get ground off anyway because I'm always grounding and dragging this boat.

    sailing 021.jpg

    The bigger boat shown above got three runners on the bottom to stiffen and protect it.
    I fiberglassed the bottom, then glued and screwed the runners on.

    I didn't sand the fibergalss before I painted it, and on the very first trip the paint came off the bottom!

    That's how I learned about the wax in polyester resin. It must be sanded before painting.

    SAM_6464S.jpg

    I recommend ordinary exterior latex house paint for the boat., I use Titebond III glue myself, works great, it's less expensive and much easier and safer to use than epoxy, and it perfectly fine for a boat like this.

  16. #16
    Join Date
    Mar 2017
    Location
    Loon Lake, Washington
    Posts
    206

    Default Re: What's the procedure to attach a plywood bottom to frames?

    The big boat above was put together with two pounds of galvanized roofing nails. Talk about cheap!
    There is no way to set roofing nails below the wood and putty them. Instead I simply covered each run of nails with 3" wide fiberglass tape and poly resin. If you look closely at this photo you can see the lines of nails under the fiberglass and varnish (my wife told me to varnish that boat!).

    You can also see a line of nails on the outside of the boat going into a frame in the photo above showing the paint coming off the bottom...

    They would be less visible under paint, and who cares if you can see where the nails are?

    Besides, covering the outside of the butt joints on the sides with a 3" wide strip of fiberglass tape is a good idea, another point in favor of doing a little fiberglassing.

    SAM_6501S.jpg

    Day Tripper,

    Is this a Jim Michalak design you are building?

    Plans for his john boat is here - https://duckworks.com/jonsboat-plans-instant-download/

    He also has plans for a 20 footer and a 12 foot version, among a very great many other boats.

    If you can, let us know how your build is going, I know we'd all love to see photos of it!

    Good luck building and have fun on the water!
    Last edited by Etdbob; 05-14-2021 at 03:43 PM.

  17. #17
    Join Date
    May 2021
    Location
    Hawaii
    Posts
    49

    Default Re: What's the procedure to attach a plywood bottom to frames?

    Quote Originally Posted by willin woodworks View Post
    If you are attaching the sides first just mark the frames on the sides. Strike a line across the bottom for the center of each frame.
    Feels lie you're overthinking it.
    No, not overthinking it. Because until Etdbob suggested marking the sides, I had not thought of that idea.
    As I replied to him, "Putting marks on the sides denoting the center of the frames sounds like a workable solution".
    If I had not started this thread, I would not have received his suggestion.

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •