|Copper Bright on Launch Day|
(high blocked for trailer)
Copper Plating a Plywood Hull: What I Know and Don't
Near the beginning of our journey, a then 125 year old schooner hauled out near us.
It had been coppered with 1/16in strips, nailed over Irish Felt (no extra tar), and was just now starting to 'perish' (worn to holes). Never recaulked in all that time, and the owners were apprehensive about what they might find.
As the copper and felt were pulled away, a like new, DRY bottom was revealed!
Local caulkers (pronounced 'corkers' out here) pulled some cotton (and oakum?) and said it didn't need reworking! No one could tell if it had been initially oiled (no paint), so they went for it this time round (don't remember the oil), new felt and copper, and away they sailed.
Leaving us VERY impressed greenhorns in their wake.
Inspired, and guided by clues (mostly) from WoodenBoat Magazine articles, we're now copper plating our third hull. At this writing, LUNA is seventeen years old, and SLACKTIDE six -both rode hard for much of their lives... they and their copper in good shape.
Now, so far as I know, we're the only ones who've tried coppering with either plate or foil on plywood - would love to hear of other experiences!
Here's what we've learned, induced, guessed and gone for... everything I know (or don't) about it all.
- Copper provides wood borer protection, non-toxic anti-fouling, and vastly reduces the work involved.
- Plate copper provides great mechanical protection when grounding (by choice or not).
- It provides ballast, as low as possible in the hull, proper.
- It's high on the galvanic scale, so you need fear no "hot" neighbor. If your motor lifts out, you are mono-galvanic (copper and bronze) below the waterline, so no zincs.
- It dissipates a lightning strike when bonded to your protection system.
Copper is initially expensive.
We tell ourselves that it's cheaper by weight than good cuts of meat, a gallon of diesel, a Big Mac. Most times it is. But it still staggers us when we go to pay for it.
- The initial outlay is offset by savings on ballast, and costs of alternatives
- It pays for itself quickly; in haul-out avoidance (haul-out, yard fees, paint, brushes and such, treats = $$).
- It pays for itself slowly; longevity makes for a long, profitable payout.
- It's ability to protect the hull constitutes fantastic, on-board insurance.
- It's a commodity metal, with market value. Unlike most construction materials, it can be cashed in for a good chunk of your initial outlay. If prices rise, you may actually profit. It's incognito and off the radar of (most all?) would-be thieves.
- It adds value at resale, not only for its virtues... but with a certain cachet.
Construction (Roofing) Copper - Any copper with a minimum copper content of 99.5% to be used in building construction. Copper has been valued for centuries as a roofing material, for its aesthetically pleasing green patina that forms over time. It is also very economical, as many copper roofs have lasted up to 100 years and it is 100% recyclable. The roofing copper specification, ASTM B370 allows a slightly broader chemistry than does ASTM B152. Copper sheet 10 oz through 48 oz in our inventory is referred to as roofing copper. -- Alaskan Copper and Brass Company (Seattle)
Copper sheathing's primary purpose is to counter wood-borers, followed closely by anti-fouling.
Elemental copper (I've read) is non-toxic, repelling growth with its intrinsic electrical field. Cu2O (cuprous oxide), the active ingredient in 'copper' bottom paints, is toxic and reddish in color. We do develop Cu2O along the waterline, under greenish patina, but light scrubbing (my favorite) won't expose it.
Borers are kept at bay by the presence of copper. It's inboard face is as potent as outboard, if not more so.
Anti-fouling is less than most paints, but keeps growth down to a light slime, during the year. We scrub the slime away two or three times a year, at about 15 minutes per side. Or, we hang out in fresh water for a few days to kill it, and it sloughs off when we get under way.
We experience zero growth under the chines (at least on our square boat hulls). It's possible that the combination of copper stress and low light exceed a threshold of viability. This saves immense effort in scrubbing the awkward under-hull, and limits area to mere hull sides!
Copper is high on the galvanic series. The higher on the series, the less a metal has to worry about electrolytic degradation. Galvanicly similar metals, such as silicon bronze and some alloys of stainless steel react negligibly with copper, so are used as fasteners.
Like any material, copper erodes over time through mechanical or galvanic processes. Longevity is hard to predict. I've heard predictions that seem remarkably short, but have witnessed lifespans ranging from impressive to phenomenal (as in that schooner's case).
Longevity will be proportional to initial thickness. After that, such factors as time spent at rest vs underway, oxidation, groundings, mechanical scrubbing, growth quantity and type, electrical environment and... juju? I've heard a lot of theories! 8) I'm sure there's some expert opinion out there.
Copper is milled in differing tempers. We've used a temper referred to as 'half-hard' as a balance between the need to bend and work it against it's mechanical durability. I haven't been able to confirm that this is a well-advised choice, in part because our application is virtually unheard-of.
I've read that CuNi (copper-nickle alloy) is preferred for marine use as having better mechanical properties, is even less prone to electrolytic degradation, and stronger anti-fouling. Internet searches pop up hulls built of the stuff. More expensive, but something to consider.
Copper-Nickel alloys are recognized primarily for their ability to withstand seawater corrosion, erosion and biofouling and therefore are widely used in marine and offshore industries.
C70600 - (90-10) Copper-Nickel is widely used in marine applications where resistance to both corrosion and biofouling is important. -- ibid
Foil and Plate Sheathing
Hulls are sheathed with thick-ish foil or thin-ish plate.
Very Curvy Dogs tend to foil or very thin plate - generally in diagonal strips (possibly spiled like planks), fastened along their overlapped, longitudinal edges - which can follow their voluptuous forms. Strips are laid from aft forward for streamlined laps, and may change pattern at various points for ease or appearance.
Plywood and plate are both sheet materials, so both assume what is called a 'developed' shape (don't ask). This means that the plate can be much thicker and wider -up to uncut full-size - and still conform to the hull. Beyond a certain point, laps may be forsaken for butts (edges pressed tight, each against the next).
Triloboats - being based on sheet dimensions - plate up with the same ease they plank up.
Foil, and perhaps thin plate is usually fastened with bronze ring-shank (annular ring) nails.
Plate can be nailed or screwed. Either can have their heads countersunk to flush with the plate surface, which smooths water flow, and protects the heads from grinding when taking the ground.
We prefer wood screws for ease of repair/replacement.
Transverse edges are fastened every six inches, and about every foot mid-sheet. Along the chines, we use heavier fasteners passing through bronze angle (see below) and plate, fastening and linking them.
My thinking is that there is relatively little stress in tension (force pulling a fastener directly out of the wood). It would be greatest on the bottom while afloat... weight alone is roughly sheet weight divided by number of fasteners. Some suction forces are likely present, but have never seemed to amount to anything significant.
Our present (most sparse) pattern averages less than three pounds per mid-plate screw in tension.
Thus, we tend toward shorter fasteners, than not, with a preference for about 3/4in (18mm) minimum. We have used much shorter ones on the sides, into 1/2in (12mm) ply. We avoid poking through.
While I've not seen evidence of it, I suspect that (bottom) fasteners could undergo more stress in sheer. Therefore, we tend toward thicker fasteners than not. Shouldered wood screws make the most of the fastener where it passes through copper.
What to put between copper and wood?
The purpose of underlayment is to... um. Lotta factoids floating around out there.
As copper protects the wood from borers, and seawater is itself preservative, I'm not totally convinced that any underlayment is necessary. However, given the dry hull of that ol' coppered beauty, it's a property to shoot for. The underlayment, must, in this case, seal the fasteners, and isolate wood from seawater.
A survey of articles from WoodenBoatMagazine (as I recall... it's been a while), some paint or oil the wood. Some don't. Red lead and creosote (both currently illegal, most places) were 'common' (meaning at least two cases reporting). It's remarkable that, despite a wide range of approaches, all report success. Pretty forgiving.
Some tar the wood (with asphalt based roofing tars, mostly), and maybe more.
Irish Felt (tar impregnated wool felt) is often used, with or without extra tar. This is my personal first choice, but has become rare and is not always in economic reach. My guess is that any solvent resistant fabric and tar would do reasonably well.
We've used a thick layer of PolyUrethane adhesive on the ply to form a gasket. PU is elastomeric, which appeals to me, waterproof, and cheap in bulk tubs (sometimes sold as sub-floor adhesive). This is currently my second choice.
Presently, we're trying GRACE brand adhesive roofing underlayment - a rubberized asphalt product. It's used around here for bedding hardware or deadwood and the like. Unfortunately, we're having (minor) adhesion issues. I'm hoping warmer weather will improve its grip. Stay tuned for results.
I'm unsure whether our waterproofing efforts have been successful. Every boat has slowly settled. Whether that's 'middle aged spread' to which live-aboards are prone, or the slow absorption of 'water ballast' is unclear. Never had to rip into one of our bottoms, so it remains a mystery.
My guess is that we are absorbing water, which does not appear to be a problem. One approach might be to design for a saturated bottom, anticipating the weight as water-ballast.
|Copper/Angle Schemes for Square and Deadrise Ply Hulls|
Underlayments not shown
Where plates come together along chines, their butt is exposed to trouble. They can be covered with protective angle.
Right angle chines (square boats forever!) can use heavy, 90deg bronze angle (L-section lengths of bronze metal). This is great for withstanding heavy grounding stresses. For example, sweeping across the bottom while settling or lifting, one can encounter a salient rock with a lot of force. Nice to know that heavy metal is on task!
In the above diagram, a scheme for right-angle protection of the edges of a ply grounding plate is shown. The lower reaches of deadrise are still vulnerable, but not so much as they might have been without the salient plate and angle.
Other angles may have to make do with a strip of copper, creased along the mid-line, to span the gap.
Either may be darted to help conform to a curved chine. A good machine shop can roll angle to spec (well done, it's a beautiful thang!).
They may be bedded in tar or PU to fill any voids on the concave side.
We like heavy screws to fasten angle, alternating side and bottom, about every 6 to 9in (150 to 225mm).
Stress and Scantlings
Bottom stresses will vary with Pounds per Square Inch involved (or their equivalent).
A bigger boat will generate higher PSIs, all things being equal.
A rockered bottom will generate higher point loading, when grounded, along its transverse line (band, really) of contact with the ground. The more the rocker, the narrower the band and the higher the PSIs.
A deadflat, tends to spread the load across its whole area. The bigger the deadflat, the lower the PSIs.
Transitional bottom (up-curving toward the ends or transverse deadrise) are exposed to rocks toward their lower areas. Increasing angle, however, will decrease PSIs.
It's when we settle down on a tall standing, pointy rock that PSIs skyrocket. Especially if we're whumping up and down on one in the surf. Our best laid plans often expose our bottom to such high stress point-loading. Scantlings had best cover this eventuality.
Given that (with one truly exceptional situation) our plate has shrugged it off, I suspect that the following is more than adequate:
LUNA (5 ton, high rocker bottom at 1/4in (6mm) amidships and 1/8in (3mm) toward the ends)... the 1/4 now seems excessive, though doesn't hurt more than wallet and waterline.
SLACKTIDE (4 ton, deadflat bottom at 3/32in (about 4.5mm))I've come to prefer 1/8in (3mm) for these sized cruisers in general, but economics have led us to round down.
So far, even the thin end of the range has stood up well to hard groundings. It seems that, if it's solidly backed by ply, it's pretty durn tough. If the ply crushes (can happen), the copper will deform, but has never yet holed. In fact, there are very few scratches, even after years.
We've used 1/16in (1.5mm) plate for our (vertical) sides, with full satisfaction.
Screws we (now) use are #6 x 1in (25mm) for sides and bottom, and #14 x 2in (50mm) for angle.
LUNA and SLACKTIDE's copper has held up over years and through hard use.
We 'take the ground' often in an area short on sand. Even the best beaches usually have some of the planet protruding. We sail year round, in a place where weather can get uppity. We like to run the fringes where the charts are, charitably speaking, approximate.
Copper plate has our back... er... bottom!
PS. We've also used so-called copperpoxies - both DIY and commercial - in which powdered, elemental copper is mixed with epoxy and applied thickly to the hull. Once cured, the surface is intermittently sanded to expose fresh copper particles.
Our experience has been positive, but not in the same ballpark as sheet copper.
It is fairly expensive, and nasty as any epoxy. Sanding is unpleasant and difficult with plastic residue, especially after the first time. It's anti-fouling properties are far less than foil or plate - or even standard anti-fouling - and require considerably more elbow grease. It's longevity and mechanical protection are much less than copper.
Where copper sheathing has eliminated under-the-hull growth, copperpoxies do not.
Still, it's initially cheaper than copper, lighter, consistent with hull encapsulation, and outlasts most anti-fouling paints. Once cured (and between sandings), it's non-toxic. It can stand extended haul-out, unlike some paints.
We'd still consider it for dinghies and camper cruisers.
PSS. On SLACKTIDE, we tried gluing the plates on with PU, but it was a fiasco (story quoted here from correspondence with a friend considering epoxy):
RE epoxied copper - I'd be apprehensive about expansion differentials stressing the bond. Not so much while immersed, but once dried out, especially in hot weather. But then, I've grown paranoid.
I don't know if you heard about our fiasco gluing copper w/ polyurethane (on theory that elastomeric bond would be preferable)? Did great, both on tests and while dry. But once launched, virtually every plate failed. Lots of oxide intrusion patterns from the edges on those that were still partially adhered.
Almost lost one, but mechanical fasteners along the chines saved the day. Ended up nailing (bleah!) over the PU 'gasket' (original glue, which remained firmly adhered to the ply).
My hypothesis is that minute intrusion at the edges oxidized the copper, which voided the bond and advanced the leading edge for another round. All in all, it went quickly. Possibly some chemical interaction among copper, salt and PU ingredients?
We did not, however, acid wash our plate. It came protected with a 'mill oil' film, and was 'new penny' bright, We washed with acetone before gluing (best guess at the time, no precedents I could find). We'd certainly acid wash if trying again. While dry adhesion was excellent, it's possible an oxide film allowed the initial edge-failures.