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| 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.
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| 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.
Appreciate the detail in your articles - a person would have to be a martian if they can't figure out how to build a barge after following this series of articles- Thanks!
ReplyDeleteHi Linn,
ReplyDeleteThanks for your kind words!
In this blog, I'm afraid it's both dry and a little staccato at the same time. So many details all run together! I'm glad you're finding it useful.
Nanu, nanu!
Dave Z
[Posting on behalf of JOHN]:
ReplyDeleteHello Dave,
Another type of fastener that some boatbuilders find useful are composite (plastic) nails and staples, such as those made by RaptorNails.com. They may not be quite as strong as steel or bronze nails, but being plastic they don't rust, and they won't harm your edge tools if they are hit with them. If they break through the wood they can easily be cut off or sanded down. Two disadvantages are they are more costly than some types of nails, and apparently they require a pneumatic nailer to drive.
John
Hi John,
DeleteThanks for the tip! Looks like composite fasteners have come a long way since last I looked at them (unreinforced plastic, in those days).
Reading some threads, I gather than they have some issues... not deal busters, but considerations:
They appear to be brittle in sheer (not a problem if solely used for clamping pressure).
More seriously, users complained about tendencies to follow grain lines, diverting the nail. I could find nothing about driving through ply thicker than about 1/4in (I'm sure it's out there, just didn't find it in the short look-see)... that diversion problem might be worse in ply. Several stated that they don't draw the pieces being joined tight, as with regular nails.
Clamping and predrilling solved these issues, but both slow the process down, and take the fun out of a nail-gun.
They sure make an interesting alternative to look into, especially where galvanic considerations are in play. Bronze and Monel aren't hard to undercut, price-wise. I'd consider renting a nailer and buying a couple pounds/coils to do some testing before diving in with them, whole hog.
One solution to the expensive tool problem is to sell at project's end. Usually, one can recoup a goodly chunk of the intial outlay. We often sell at full list price, merely eating the S&H costs. It's often possible to pre-sell the whole power set to one of the looky-loos who want to gear for the project you've inspired in them!
Dave Z
BTW:
ReplyDeleteI just came across this BRILLIANT method for the modern trunnel (tree nail)! I'm generalizing the text, below... otherwise in the original words.
A fellow used square douglas fir pegs. Square pegs because they are easy and cheap to make. The way it works is make a 1/4" x 1/4" peg. Drill a regular round 1/4" hole. [Adjust sizes according to use.] Taper the peg (use a pencil sharpener) and drive the square peg in the round hole with a dead blow hammer. Add some gorilla glue (which expands) in the hole. Make the pegs from scrap so small dollars.
To this, I might add that tight grain limbs from close to the trunk make fasteners especially strong in sheer and may, in some cases, be worth the extra shaping.
These square pegs would be especially useful in larger, longer fastener sizes, especially where 'exotic' metals or plastics are required.
Another idea for the toolbox!
Dave Z
What the old boat builders do here in NZ,to make trunnels - especially driving hard wood into hard wood, is to make them square, as you suggest, and then to drive them through an appropriately-sized pipe or tube to get them roughly round. That way you need less force to drive them into the holes in the boat.
DeleteLove the details,thanks for sharing. Do you seal the bottom before adding the water shield and copper plate? Do you use primer / paint, epoxy or another coating?
ReplyDeleteLD
Hi LD,
DeletePreviously, we've used a thick layer of polyurethane under the copper, and nothing else. There's a chance we should have stuck with that.
Now, we're trying to decide whether to prime, first, to seal or go with bare wood. Both options are allowed by the directions.
Our test patch allowed water underneath, via the wood fiber (we allowed this, on purpose). This showed us that, once adhered, it still stuck to wet wood, albeit without much grip. We'll try another patch later, with primer (we don't have the type we'd use on hand). It did NOT adhere strongly to TiteBond III, though possibly adequately.
The ultimate goal is to exclude water, which the PU did not. It's not necessary, but dry wood is stronger and a lot more buoyant. The watershield adheres very strongly to itself... if it fully gaskets the nails, there's a fair chance we can keep water from intruding.
We'll know in a few years!
Dave Z
[Posted on behalf of JOHN:]
ReplyDeleteHi Dave,
You responded to LD, in part:
"Our test patch allowed water underneath, via the wood fiber (we allowed this, on purpose). This showed us that, once adhered, it still stuck to wet wood, albeit without much grip. We'll try another patch later, with primer (we don't have the type we'd use on hand). It did NOT adhere strongly to TiteBond III, though possibly adequately."
In an earlier blog (Water-Based WaterCraft: Options in Boatbuilding ) you commented on your experience with various types of adhesives, and said you planned to use TiteBond III as a sealer as well as a glue. In a follow-up comment I suggested that polyvinyl acetate glues (such as TiteBond III), after they have hardened, can’t easily be glued onto again. Your response to LD (above) corroborates my understanding, as you noted that you weren’t seeing good adhesion between the copper and TB-III-coated plywood. So I just wanted to reiterate my concern in your using TB-III as a sealant, if you subsequently want glue (or maybe even paint) to adhere well to that sealed surface.
I've heard of folks using the construction adhesive PL Premium (has to be Premium, not any of the other PL construction adhesives -- now sold by Loctite, I think) as a sealant for the nylon skins of skin-on-frame kayaks. I've used it myself as a underbody sealant on my car to try and hold back the formation of rust. It sticks tenaciously, and I believe it can be re-applied to itself. Even if you were interested, I suspect this suggestion is probably too late for you :(
John
Hi John,
DeleteOne minor correction in your comment... we weren't trying to adhere copper, but rather Grace Water and Ice Shield (self-adhesive). The copper will be screwed on over and through that layer.
You're likely right about getting stuff to adhere to TBIII. The Grace did okay... it took some effort to peel it off, but it didn't hurt the Grace to do so. We prefer to see it all tear to pieces before the glue fails! 8)
We're only likely to be using the TBIII as a sealer where we leave it bright. We've been looking for a cheap, easy alternative to varnish, mainly. We're planning on using it on our decks, though, and will paint over that.
We tested TBIII with latex primer. Couldn't tell the diff between one side we painted 'green', and the other when fully dry. Did some sand/no sand samples, too, and no diff. All cases adhesion was excellent. Unfortunately, we didn't have oil primer (what we've ordered for the boat) on hand... I hoping it will work as well.
We use quite a bit of PL Premium (lood for polyurethane... they have a few other options, too). Only quibbles are that it's not as elastic as 5200 or Sikaflex, and tends to get bubbly if there's any moisture in the mix. Cheap, though, and seems tough enough.
We even glued on half tubing as a dory chine guard (no other fasteners)... worked pretty well, though one side eventually stripped off. Hard use, though.
And your right about 'too late'... our die is cast. Crossin' our fingers!
Dave Z