David Tyler wrote: Very interesting, and it's good to hear it like it is, from a man who knows from first hand experience. So I went to www.triloboats.com to learn more:
What was all that about plate copper sheathing?
Plate copper provides great mechanical protection for grounding, and non-toxic anti-fouling.
Note: I've read that elemental copper is non-toxic (repelling growth via electrical field, which goes with nobility). So called 'copper' bottom paints use Cu2O (cuprous oxide) which is toxic. It is red, in this form, and does form along the waterline under patina... I try not to scrub hard enough to expose it (which suits my lazy nature).
One more factoid: I've read that CuNi (copper nickle alloy) is preferred for marine use as having better mechanical properties, less prone to electrolytic degredation, and stronger anti-fouling. Internet searches pop up hulls built of the stuff. More expensive, but something to consider.
It is initially expensive (at this writing, it's running about $5.25/lb), but pays for itself quickly in haul-out avoidance (haul-out, yard fees, paint, brushes and such, treats = $$). It dissipates a lightning strike when bonded to your protection system. 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's heavy, so you'll make up some of the cost on ballast savings. Nail, screw or glue it on (be sure to test your glue on copper, first) and cover the chines with bronze angle.
Could you expand on the "nail, screw or glue it on", please? I would guess at silicon bronze gripfast nails, but what kind of glue, or at least, sealant to keep the water out of the nail holes? Do you use hard, or half-hard copper sheet?
Hi David,
Thanks for the reminder that I could, once upon a time, get to the point! 8)
Copper plate has been half-hard (rolled).
It ranged in cost from $2/lb (LUNA 1997) to $5.25/lb (SLACKTIDE 2008 and present project). Cheaper than good cuts of meat, in our area, and about equivalent to a gallon of diesel. The spread between commodity and manufactured prices has ten-folded since '97.
We've been happiest with #6 x 1+in silicon bronze wood-screws, countersunk flush into the plate. Longer wouldn't hurt, but I doubt there's advantage as they seem to work mostly in sheer. We've used no pilot holes into the wood, but start with an awl punch. Tar (asphalt based, wet surface roof patch) and Irish Felt are favorite underlayments.
Unfortunately, Irish Felt is getting rare and very expensive to obtain. We're trying a rubberized asphalt sheet adhesive roofing underlayment (GRACE brand... used in marine for gaskets in deadwood, and the like), this time. Haven't launched, yet, so no feedback. Some adhesion difficulties keep me from recommending this, yet (may work out, but not as positive as hoped, especially in cool weather application).
Chances are, tar and about any solvent resistant fabric (preferably synthetic?) would do to replace Irish Felt. Tar alone has been used by others with apparent success. Some paint before tar, others tar the bare wood. I've heard that tar paper turns to mush, eventually. Most of the articles I've found the subject were from WoodenBoat Magazine.
My second choice, as of now, is a thick 'gasket' of poly-urethane construction adhesive applied to the wood (can be relatively cheap in tubs, as it doesn't have to be highly elastomeric).
Seawater intruding behind the plate itself seems to present no problems whatsoever.
The theory of these underlayments is to seal the fasteners and prevent water intrusion into the wood. My guess is that water slowly does intrude, with the methods we've tried, but aside from extra weight, doesn't seem to be a problem. One reason to go lighter Cu is in anticipation of acquired 'water-ballast' from bottom saturation. Could be only settle over the years for the usual reasons of middle-age-spread, and that water intrusion is minimal.
We had a glue fiasco (quoted here from correspondence with a fellow 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 ionic interaction with copper 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.
One advantage of screws (our present choice) is that plates are relatively easy to remove for inspection or maintenance.
Bronze ring-shank nails are traditional with copper foil, and they work well. Problems, to my mind, is that they're harder to sink flush (overcoming their ease of driving). If not sunk, they're like a field of small barnacles, and their heads could be prone to grinding off when grounding. Still, so far so good on SLACKTIDE's fix at six years despite rough living (we countersunk the nailheads, slightly, with a flat-ground bit).
*****
We first started thinking along these lines when a 125 yr old schooner hauled out next to our project (1st boat). It had been coppered with 1/16in nailed over Irish Felt (no extra tar), and was just now starting to 'perish' (holes showing). Never recaulked in all that time, and the owners were apprehensive about what they might find.
As it was 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.
Now, so far as I know, we're the only ones who've tried it with either plate or foil on plywood - would love to hear of other experiences!
*****
If you're interested, for another project I worked out a scheme for plating a 5 panel (flat bottom) hull, with the grounding plate protected by 90deg bronze angle. Come to think of it, I'll do a post on copper plating, soon, at triloboats.blogspot.com.
Hope this helps!
Dave Z