Prevention is better than cure.
- Desiderius Erasmus
Easy Insurance for Ply Boats: Doubler Plate / Horizontal Butt-Strap
In TriloBoat StudyPLANs (for square boats built of plywood), I suggest installing doubler plates.
Judging by questions I field, this isn't a self-explanatory concept, so I elaborate. But first, a yarnlet...
We occasionally get a wild hair to do something really... well... ill-advised.
This time, it was to shoot Hole-in-the-Wall, a narrow break in the levee separating an estuary from a passage between islands, in ZOON, our ex-Bolger LONG MICRO. At 19ft6in x 6ft6in, it was a tight squeeze. We wanted to do it just before max ebb, too, which generated a three foot water drop gushing through the heaped stone wall. This left us plenty of water, both beneath ZOON's flat bottom and to propel us with adrenal force. We thought of it as a training mission.
To our credit, we'd scoped it well, and were counting on a sand flare on the down side of the gap; if we overshot the sharp right turn necessary to clear to deeper water, we'd fetch harmlessly up on sand for a tide cycle. Fail-safer.
As we approached, however, we could see someone in a power skiff, doing something-or-other near that sand-bank. We hovered on standby, but didn't abort (as we should have) to see what he was up to. Once he zoomed off, we decided the increasing water flow was still manageable, and to go for it (without further reconnaisance!).
Well... it was satisfyingly gripping, and all went according to plan. Except. Except, the skiff guy had parked a log on our fail-safe, awaiting the turn of the tide!
We were not able to make the turn, and bumped the log, rather than the bank. But we skid along it, under force of water flow, and shot past into the clear.
Anchored and congratulating ourselves on surviving our foolishness, however, Anke discovered a drippy leak at the edge of our bunk. Turns out we'd encountered a (mercifully) short branch spike just above the waterline, and had punctured our 1/2in (12mm) side.
A bit of scavenging and jury rig later, the hole was patched and we went on our merry way, not as sad as we might've been, but wiser.
Since that time, we've been installing what I call doubler plates - an extra layer of plywood installed along the lower hull, at least doubling hull thickness to well above the waterline.
Their weight is low, contributing to ballast stability, their volume more than floats themselves and being outboard, contribute to form stability. The fact that they overlap the bottom edges means it protects then and its glue join is considerably improved. We view them as more or less sacrificial... while they are constructed as hull proper, being add-ons, any scrapes or dings in them can be easily repaired or filled with little concern for the hull's integrity.
Twenty-four inch (1/2 sheet) and sixteen inch (1/3 sheet) are convenient heights given plywood's 48in sheet width.
Doubler plates of 3/4in or thicker provide good 'bury' for any fasteners used while gluing them in place.
For square boats built from plywood, another feature soon became apparent.
Many hull sides are taller than a single sheet of plywood, and must be extended upward via a run of full or partial width ply. These strakes are often joined with a horizontal buttstrap (narrow strip of plywood straddling the seam where they butt together), but these must be notched into bulkheads requiring careful placement and carpentry, and they interrupt the smooth interior wall... vertical joinery must be further notched around them.
By placing the narrow strake low along the chine and using doubler plates which are taller than the lower sides, a longships rabbet (notch) is formed. The upper strake can be lowered into this notch and glued and fastened in place along it. All this without any notching, and a smooth inner wall results. In my opinion, this feature alone pays for the increased cost/effort of installing doubler plates.
We're building WAYWARD with 5ft sides, calling for 4ft plus 1ft. The 1ft strake is run low, and our 2ft doubler plates form a very deep notch. Given draft designed to range from 12 to 16 inches, the top of the doublers will rise 12 to 8 inches above the waterline. In our case, we've chosen to copper plate the bottom and sides to the top of the doublers, for excellent combined protection.
A further perk: Since the doubled lower hull is quite low, it facilitates the option of completing the lower hull upside-down, flipping it, and building upward from there. Temporary frames may be used, or clever types can divide their bulkheads into upper/lower portions.
This simple upgrade improves puncture resistance, simplifies construction and makes the inverted build option more attractive and manageable.
Not bad for a coupla extra sheets of ply!