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Fair winds!

Dave and Anke
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Monday, August 15, 2016

Easy Insurance for Ply Boats: Doubler Plate / Horizontal Butt-Strap

Hole-in-the-Wall... *GULP*!

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!


  1. Even before I launched Mistral, my 36 foot plywood dory, I decided to double the (flat) bottom plank from 3/4" to 1 1/2". So glad I did (shoulda done it before anyways). Weight savings should be left to the topsides and cabin, no harm in adding protection to vulnerable areas, out of sight, underwater. It's a sallow draft boat after all and should be designed to take the punishment that implies.

    1. Hi Doryman,

      I agree whole-heartedly! I used to think it was only the bottom that took the brunt, but sideswiping a rock is no joke. Especially as a heavy boat is settling down or lifting with the tide!

      Dave Z

  2. The variation I was planning on is because the copper comes in 36" widths, I was planning on cutting each sheet in half lengthwise. That give 18" then a 2x4 above that as a rub rail for a total height of 21.5". I am going to have a leftover strip of 3/4" but it's not that much wastage. How did you do your side copper sheets?

    1. Hi Dennis,

      We did as you are doing in LUNA, but were able to find 48" sheets for SLACKTIDE and WAYWARD. We bought from in Seattle, WA.

      You're right, ply wast is a lot easier to bear than copper waste! and its likely you can find a use for those strips, somewhere.

      Your project's looking great, BTW!

      Dave Z

  3. What was the type and thickness of copper sheet you bought? When I go on their website I only find 36x96 in the thicker sheets, or 48x96 in the thin sheets (1/16) I remember you posting that you used thinner copper on the sides

    1. Yes, the copper side plates were 1/16in x 48in x 96in (bottom plates 1/8in x 36 x 96in.

      We figured that the sideswipes would have less force than bottom grinding, which seems true in practice. Plus the ply doublers are 'sacrificial' under the thinner copper.

      In terms of longevity due to water action / chemistry, the thicker the better. LUNAs side plates are still looking good, however, at 19 years. I've heard that other areas go through copper faster, but the first example I saw in the lower 48 was 125 years before replacement! Pretty good pro-ration!

      Dave Z

  4. On Kairos, we put the doubling plates on after the hull was constructed, and we extended them from the bottom of Kairos' (7" w by) 9" bilge-keel like skegs (running along the chines a fair portion of the boat's below-water-line length) up the sides to well above the water line.