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Lapstrake Construction
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Lapstrake planking refers to the method of attaching planks
to the frames and to themselves. The planks actually "lap" one edge over
another as opposed to butting together at their edges as in standard
batten-seam or straight-carvel construction. This method, used for
hundreds of years and sometimes referred to as "clinker-built," adds
strength to the hull in that where the lap occurs the fastened doubling
of plank thickness essentially forms a "stringer" at each plank edge.
These inherent longitudinal stiffeners allow the boat to be designed
with lighter plank stock of half the normal molded thickness
(inboard-outboard measurement) and framing that is usually steam bent
rather than sawn-sistered and thus about one third the size and weight,
molded and sided (fore-and-aft dimension). Many of these boats use a
combination of steam-bent and sawn frames, especially forward at the bow
and aft at the transom. The bottomsides and topsides are usually one
continuous curved shape with round bilges instead of a hard chine at the
turn of the bilge. The inherent lower cost of materials and labor
produced a boat the average man could afford.
Those who are familiar with driving lapstrake boats know that they have
more stability and sea kindness than flat-bottomed, hard-chined power
boats. Each one of those protruding laps at the bow acts as a sort of
shock absorber, turning the force of breaking water to the side and,
following dozens of stabilizing fins. There must be something to this
because brand-new fiberglass speed boats also feature these protruding
laps.
The discussion here applies to Chris-Craft Sea Skiffs, Lymans, Centurys,
Grady-Whites, Thompsons, and dozens of other marques. Lyman began their
lapstrake inboard line in 1930, though they had been using clinker
construction since the 1800s. They continued their plywood lapstrakes
until the early 1970s. Chris-Craft began producing their plywood
lapstrake Sea-Skiff line in 1953. Starting at 18 feet, these boats
eventually were built to 43 feet. Century introduced their Viking in
1954 and later produced the Raven, which was built of solid mahogany
plank, as opposed to plywood.
Lapstrake planks are usually screw-fastened to the frames where the
lapped planks intersect the curved outboard surface of the bent frame.
This means that the planks are only fastened to the frames at the top
edge and bottom edge of each plank, unlike carvel construction where the
full width of the plank is tight against the frames with three to four
screws to hold the plank from cupping. Plywood, because of its multitude
of grain directions, actually makes a much better plank stock than solid
wood for lapstrake boats. It is less liable to bow or crack
longitudinally where lapstrake construction leaves unsupported.
The real downside to this type of construction is the gap that is left
between the frame and center of the inboard surface of the plank. This
leaves hundreds of handy catchalls to collect dirt and debris, which can
hold moisture above 30 percent and allow rot spore to flourish.
As far as I've seen, in these mass-produced plywood lapstrake, the oak
frames were steam bent into the hull and fastened in place, with the
outboard surfaces of the frames never getting any sealer or paint. In
most cases, removal of planks shows that the boat also did not get bilge
paint until after assembly. This means that the very spots that are
prone to trap moisture and foment rot are not sealed against this.
Remember: The factory simply didn't expect these boats to last as long
as they have.
It is especially important to keep this type of bilge clean and dry. The
greatest problems occur at the turn of the bilge just beneath the
floorboards. Dirt, debris, and the biggest offender, leaves, get past
the edge of the floorboards and accumulate in these plank-to-frame gaps
just out of sight, and never get washed into the center of the bilge
where you can see them and clean them out. Unfortunately, this area at
the turn of the bilge is where the unsealed oak frames were forced to
take the greatest stress in the bending of the tightest curves. Here,
the outboard faces of these frames probably exhibited surface checking
from day one. Could there be a more perfect for rot spore to get deeply
into the frames?
 One fellow brought his 25-foot Sea Skiff to my shop and asked that I
replace four planks that his surveyor indicated needed replacement
before insurance could be written. The survey indicated two identical
rotted plank sections just below the turn of the bilge on either side of
the boat. I removed each plank section and sure enough, there sat a
little pile of leaves on each side of the boat, returning to nature and
taking the plank with them. Directly above each little offending pile
was a deck vent with its screen missing. If your lapstrake did not come
from the factory with a piece of brass or copper screen beneath the deck
vents, you should add them. Come to think of it, all wooden boats should
probably have these.
Cleaning your entire bilge annually (not just the center under the
hatches) is probably the best thing you can do for any type of wooden
boat, lapstrake or not. If you have a lapstrake wooden boat and you want
to keep it in sound shape, I highly recommend removing all floor boards
at the end of each season and thoroughly hosing out and vacuuming dry
the bilge. Leave the floorboards out until spring outfitting. You may
have to cut your linoleum or rubber flooring to fit these and use
oval-head screws set flush, but your hull will last longer. If you are
storing the boat outdoors, cover it with a well vented tarp and make
sure it does not collect leaves.
Between frames, the planks, where they lap over each other, are
typically fastened together with two or more fasteners. These are
usually not screws, though, because the plank edges (at least one of
them and most often the upper outboard edge), is beveled so that the
next unbeveled plank (most often the lower inboard edge) can make a
tight wood-to-wood joint. The inner beveled edge, therefore, has been
reduced too much in thickness to hold a woodscrew. Because the builders
did not want this shingled effect to be present at the stem and transom,
the planking bevel would be increased in degree and applied to both
plank surfaces so that the planks would lie nearly flat against one
another at the stem and stern. This increases in bevel is referred to as
"gains."
The standard method of fastening these planks between frames is with
 clinch nails, where a nail is driven through and "clinched" or bent
over. A common earlier method was with rivet and rove. With this method,
a nail, usually copper, was driven through and a "rove" or "burr," a
tight-fitting washer also made of copper, was set tightly and the nail
was snipped short and its end "peened" over to form a head against the
rove. A third method used by some manufacturers is a machine screw with
washer and nut on the inside. Some used a "PM" nut, a combination of
washer and nut stamped from sheet metal with protruding teeth to keep
them from spinning when set.
The bulk of the lapstrake boats that were produced after World War 11
were planked with fir marine plywood, and their lapped seams were coated
or "paid" with a flexible adhesive sealant to maintain a year-round
seal. Chris-Craft and Lyman used an early version of 3M 5200, a two-part
polysulfide called Thiokol. So much of this fir marine plywood was made
just for Chris-Craft that it was stamped and marketed as "Chris
Craft Grade" marine plywood. I purchase some of this once from an
ex-Chris-Craft employee for repairs to a 1955 Sea Skiff. (How he got
that into his lunch box, I'll never know) |
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