Here is an article I found in a book by glen L
I am curious if anyone has tried this out and if so how the ply was held in place until the epoxy could cure without a huge clamping nightmare?
Glen L boats:
Fairing a sheet plywood boat framework so the planking will perfectly contact the
frames is wonderful in theory, but virtually impossible in practice. The frame will either
contact the planking causing a hard spot or the reverse. In many sheet plywood boats,
side battens were eliminated or minimized and this accentuated any out of sync frame.
Fasteners driven into an out of sync frame causes a visible bump or dish that will mirror
through the finished planking. This becomes more visible on dark painted sheet plywood
planked sides. Eliminating fasteners and relieving the frames so they did not contact the
plywood solved the problem. Many die hard builders and older texts still retain the notion
that sheet planking should contact frames. The latest practice allows the sheet plywood to
take a natural bend; it isn't being forced against framework to form an unnatural
compound curvature.
27
Many have questioned why we do not advocate fastening into frame on sheet plywood
boats. The foregoing is our reasoning and is proven in practice. Should the planking be
glued to the frame? Why not? Thickened epoxy adhesive on the frame will fill in the void
and make positive uniform contact as developed by the planking. Plus it's a preservative
for the otherwise exposed frame edge.
I am in the process of putting the sides on the Triad boat I am building. I have attached the port side fore sheet of ply so far. The board was approximately 24" by 96" and is 1/4" hydrotech plywood. I used epoxy thickened with silica and wood flower. I attached the side sheet with screws around the outer edge. I put a liberal amount of thickened epoxy on the front bulkhead gusset but did not use screws there. As far as I can tell there is no hard spot or deformation of the boat side and the epoxy bond along that gusset oozed out enough to indicate a uniform bond. I wasn't aware of any controversy about gussets deforming plywood panels I just didn't see the need for screws there. Call it dumb luck or a lucky guess.
Greetings,
The Glen-L excerpt is talking about skinning a frame-and-stringer assembly. The stringers are bent around the frames, and the plywood hull skins are then bent around the stringers. Typically, the stringers run fore and aft, while the frames run athwartship. Frames are usually spaced a foot or two apart, so a boat the size of a weekender could have eight or ten frames. If the stringers are notched to fit flush into the frames, then you have the option of screwing & gluing the skins to both the horizontal stringers and the vertical frames. What they are trying to say is that the skins should be attached to the stringers only, as the stringers are bending to a natural arc.
For a weekender, the closest thing we have to frames are the outside edges of the two bulkheads, one up near the mast, and the other forming the rear of the cabin. Those short vertical stringers attached to the sides of the bulkheads are what they are talking about. If those short vertical stringers stand proud of the curve of the hull skins, you will get a bump in the hull. If the stringers donât quite make it out far enough, then you get a hollow in the hull when you screw the skins down to the vertical stringers.
So what are we to do?
You have a couple of choices. The first is to install the vertical stringers slightly proud of the top and bottom stringers, and then sand it back to perfectly match the curve of the deck and hull bottom stringers. I did this with a sanding board I manufactured from some straight poplar stock to which I affixed some 36 grit sandpaper using 3M 77 spray adhesive. I helped myself out quite a bit here by carefully cutting the stringer to the exact angle required. I clamped a board vertically between the deck and hull bottom stringers in way of the bulkhead and then used an angle keeper to transfer the angle to the saw to cut the vertical stringer. I then pressed that cut vertical stringer tightly up against the clamped board to position it for gluing and screwing to the bulkhead. After the glue dried, a little sanding action produces a perfect fit, but be careful to not sand flat spots in the deck and hull bottom stringers. (a little masking tape helped here). Also, be sure to hold the sanding board perpendicular to the angle that the plywood will be bending to, which should be very close to perpendicular to the deck (angled back slightly from vertical) But this was a lot of work, and would be a pain for more than two frames.
Your second choice is to add the vertical stringers later ⦠after the hull side skins have been attached to the deck and hull bottom stringers. Again, use the angle keeper to transfer the exact angle to the stringer for a tight fit. In this case, you will be gluing the stringer into the corner formed by the bulkhead and the hull skin simultaneously, where will be virtually impossible to use screws for clamping pressure. So epoxy will be required. Epoxy can also fill any small gaps if you do not get the angles to match exactly.
The third choice ⦠the Glen L choice ⦠is to forget the vertical stringers altogether, and to not let the bulkheads touch the hull skins at all. When Glen L talks about eliminating fasteners, they are talking only about the fasteners that would attach the skins to the vertical frames. You still glue and screw the skins to the horizontal stringers. As noted by naval architects knowledgeable in such matters, the Weekenders are way overbuilt, and the omission of 18 inches of vertical stringer on the front and cabin bulkheads will not make much difference in the stiffness of the hull.
Does any of this really matter for a painted hull Weekender? Probably not. Remember, a lot of the Glen L designs are for varnished mahogany runabouts with tumblehome in the topsides. Any deviation from a perfect arc in wide expanses of high gloss varnished mahogany are brutally obvious. As long as you stay away from the really high gloss marine paints, and or paint the hull in lighter colors, small bumps and hollows in a Weekender hull should go unnoticed.
Cheers,
Tom
Well tie me down and feed me cheap rum! I sure read that one wrong. Just goes to show; you see what you want to see at times. But still the concept of a gusset spoiling the curve of the sides is similar. I avoided that by letting the epoxy with fillers act as a bonding agent and a as a self forming gusset edge. So even though it is not what the article talked about I thought I'd add my 2 cents worth.
I think, what Witt and others have done in their texts about early plywood construction, the transition to modern glued seam and modern glued seam (stitch and glue) plywood building methods is to make best advantage of the materials, by using physical properties potential, weight savings, build ease and other clever engineering techniques.
Planking choices, when predominately plywood, offers a different set of good points and bad, compared to solid lumber planking. Plywood is less strong longitudinal, but stiffer and considerably stronger across grain, so it can participate in "hull shell" as a homogenous load bearer, rather then an assembly of longitudinal stiffeners, fastened to an athwart subassembly of stiffeners, assuming the seams are well attached (taped).
So, in some contexts, advice for or against any particular technique, can be a valid argument, depending on the view point.
Glenn-L's comments about planking on stringers carrying the right loads is right on, in it's design specific application. The same is true of glued construction, where a bulkhead may be the only abutment to the flanks of plywood planking, in a substantially open area (no frames or stringers, just hull shell planking).
There's also a transition zone between the early plywood over frames methods, to the new glued seam builds. Weekender and Vacationer are perfect examples of this type of engineering. They're hardly plywood over frames, nor full up taped seam, maybe even stitch and glue. They fall in the "transition" era where the glues and concepts were equally proven, but the engineer in you, tossed a couple of through bolts in it (the design) anyway, just to be on the safe side.
Relying on glue alone has been well proven, but there was a time when designers have faced a choice, "should I trust these glued joint figures", with many (to this day) opting to use a more traditional approach to the problem (like a big, old, hard steel fastener).
There are many designers popping up that focus on "developed" shapes and modern building methods. Some of the new assembly possibilities, with CNC cut materials, are truly clever engineering and a boon to those looking for a 'guaranteed to fit together' easy, build (add glue where indicated).
These boats (Stevenson) can benefit from a taped (and filleted) seam conversion. This would tend to tighten them up and stiffen the hull considerably. The skills developed during this type of construction, would lend well to modifications being successful and the completed boat in general, being more long lived.
On the other hand, they will not be harmed by using the methods and techniques set forth in the plans either. Many a Weekender has been built mostly to the plans, with numerous success stories, so the design works.
It's perfectly natural to want to "improve" things. Don't sweat the details, duct tape it down and if that doesn't work, use a big old nail or if that doesn't work, use some goo in a tube, if that doesn't work . . . welcome to boat building. Problem solving is reality, trusting epoxy fillets is personal and acceptance is the key.