 |
| Sizing for the different situations around the boat. We decided 1 3/4in nails over 1 1/2in for better bury. 3in spikes (left) are sized for mechanical assistance to glue. |
Praise the Glue but pass the Fasteners!
- George Beuhler
Nail where you can; screw where you have to; bolt where you must.
- Cap'n Pete Culler
5200
- Name of a DIY boat on the Port Townsend beach
Epoxii vincit omnias (epoxy conquers all).
- A boatbuilder friend's mantra
Glued, Ply/Frame Construction
Glue holds our boats together. Glue. Glue, glue, glue.
I have to keep reminding myself!
If one is to follow GB's advice, fasteners must be sized to function as if without glue. In other words, if the glue fails, the fasteners must take over. Completely. Otherwise we just wast money.
The problem is that both glue and fasteners are spendy items. For many of us, if we invest in one, there's less available for the other.
The holding power of simple, embedded fasteners is technically a function of surface area, but practically, it's linear, and proportional to the mass of the fastener and holding strength of the woods involved. Piddly, that is.
A 'clench', rivet or bolted fastener improves the situation immensely, but only so long as the metal of the shank remains substantially intact. Over time, even noble metals deteriorate, leaving 'icicles' to shoulder the burden. More shank diameter is expensive. Over the years, one of the most expensive refits is to 'refasten' a wooden vessel... something we've seen a lot of in other's boats.
One more consideration is that fasteners, of themselves, don't make a water tight joint. They require some mastic, gasket or caulking, none of which are known to be permanent.
Anke and I put our faith in glue.
The grip of glue on wood is proportional to area, a squared function vastly superior to mere linear ones. Glue's holding power is measured in PSI (Pounds per Square Inch), and modern glues are way the heck up there. With any appreciable area, standard fasteners' holding power is quickly dwarfed (same goes for lashings, BTW, multiplying turns for total tensile strength).
Tape and glue methods are the next logical choice. Unfortunately, resins are generally the only reasonable choice, and they are expensive, messy, and toxic. In addition, fillets and fabric take high volumes of glue, compared to films.
That leads us to quick and dirty, glued ply/frame construction. Frames provide gluing faces and material integrity for joining sheets into girders.
The goal in glued ply/frame design is to maximize the glue surface area, within reason, and complicate it, if possible (I'll get back to this).
The quality of the wood frame (now only supplying gluing surface) can be much lower than structural frames (such as a deck beam, which must support weight). All it has to do is hold together... it needn't be stiff, or hang on to its fasteners. Tight knot grades with high annular ring counts (mostly to resist rot) are fine.
Let's look at the chine logs, for instance - a 2x4, say, framing the hard, inside chine.We observe the following:
- The bottom/side connection is the highest stressed join in a box barge. We want this one to stand up to anything the sea, and some of what the shore, can throw at it.
- The 'weakest' face is the 2x face... the 4x face has nominally twice the holding power (a bit more, in fact). We might consider whether a 2x2 would be just as strong (is more than the weakest link a waste?), with half the material. Or should it be beefed up to a 4x4? [2x2 has worked for us up to 32ft LUNA].
- All joint rigidity is supplied by (ply) sheets joined into girder arrangements. Until this girder fails, the chine log itself is not substantially stressed. Once it's failed, the hull's come apart and the gig is up (we engineer to prevent this!).
- Complicating the joint adds glue surface AND mechanical strength to the joint. For this reason, Anke and I favor 'doubling plates' along the lower hull sides. These complicate the joint and 'double up' side thickness as anti-puncture insurance. Side impact above the bottom would have to 'peel' the doubling plate, as well as sheer the glue before failure.
- Fasteners do no harm and contribute mechanical strength, especially in sheer (across the join like a pair of scissors, aka shears). The copper plate helps distribute stress, as well. While we don't rely on either of these, they add a layer of security.
 |
| Section View of Bottom/Sides. Note outboard doubler plate overlapping bottom... ... nearly doubles glue area and complicates joint from line to L. |
So the role of fasteners - at least in terms of hull construction - is, for us, to provide clamping pressure until the glues set. Clamps are expensive and slow, and often very hard to arrange.
We prefer ringshank nails... they have holding power near that of screws but are much faster to drive. Because of copper plate, we use bronze to the top of the doubling plates, and stainless above. Stainless for relative economy.
(Hot Dip) Galvanized is an economical choice, but is more likely to cause problems down the line. Still, for a 'low road' boat, it's a big contender, and if it came down to a choice, I'd go galvanized and get out there!
The opening illustration shows how we size our fasteners. Everything's going into either 1x or 2x framing. Ply thicknesses vary by use.
Obviously, we don't want the fastener so long as to poke through. Reasonable bury is a good thing, but can be lower if used only for clamping/sheer support. Too much is expensive waste.
In our case, we had a choice between 1 1/2in and 1 3/4in nails for middle cases. Some were only clamping/sheer, but others could conceivably be called upon to resist pulling (acting in tension). Rather than order two varieties, we went with the longer ones.
One thing we try to do is keep the variety low. Each variety represents a chance to run short, while one fastener for several jobs means 'round-up' can be applied to the next job. Even if it costs a bit more per fastener, it tends to save in efficiency.
Glue is tougher. Manufacturers give coverage values (in square units), but whether it's applied a micron thicker or thinner makes a big difference (we're applying by eye, of course). We tend to round up quite a bit. Extra glue, if any, can usually be sold at the end of the project, or kept on for future maintenance.
Remember to count the gluing surface, not what's being joined. For example, hold up three fingers, representing three layers of ply. Count the spaces between them (two) to find the gluing surfaces, and multiply area accordingly. A bottom, say, made up of three layers would take two times the bottom surface area in glue... not one or three.
Anke and I are somewhat outside the pale, when it comes to glues. We use three kinds: 3M5200 for exterior hull joins (all around the permimeters); TiteBond III for faces wider than 2x, (non-structural) filleting, sealing, foam and bonding fabrics; Gorilla Glue for sheet lamination. This combination has worked well for us, and so far (knock on ply) we've never had a join fail.
But I've never run into ANYONE else doing something similar in cruiser-sized boats. Epoxy/polyester folks are happy with their choice, especially the tape 'n' glue crowd.
It's all good.
*****
When using fastetners, here's a cheap trick to multiply their holding power:
Angle every pair of fasteners slightly toward or away from the other.
This means the force required to pull one is opposed by its partner.
Make sure the join is tight before driving, as any gap will be impossible to close. Often, you can walk a quick clamp down the line as you go, one set per pair of nails.
