Thoughts on Lapstrake Planking

Man stands in a red powerboat on the water with pine trees and a rocky shore in the background.

The first boat to come out of our shop in New Brunswick, Canada, was a carvel-planked steam launch completed in 1976. We went on to build numerous boats, many of which were lapstrake-planked. As I think back to those early years, I recall that the books and articles I depended on to increase my knowledge of lapstrake planking left many questions unanswered, even though in some ways that type of planking is more forgiving than carvel.

What follows are some answers that time and experience have cleared up for me, and I hope they will help take some of the mystery out of lapstrake planking for others. I will focus here on planking in which only one edge of each plank is beveled, which is by far the most common method and the easiest to understand.

The drawings all show a hull built upside down, which is a common method, but the references can be confusing: just as starboard is always starboard and port is always port, “up” is always toward the sheer and “down” is always toward the keel, regardless of the boat’s orientation on the building jig.

Analyzing Plank Layout

I find that full-sized drawings are key to my understanding of a new design and a big help in building subsequent boats with similar details. Working from a lofting, it’s helpful to study the intended plank layout all the way from the keel to the sheer, as described in the main article. Here, the drawings show a cross-section amidships at the turn of the bilge, where the hull curvature is the greatest, used in working out the planned plank dimensions.

Note that in drawing No. 1 the comparatively narrow planks are all solidly in touch with the frame where the fastening will be driven through the center of the plank laps.

By contrast, drawing No. 2 shows that the curvature of the frame is too great for the relatively wide planks shown—the frame itself, as shown at point X, holds the lap away from a solid landing, creating the gap shown at point F. You must use a narrower plank in this area. (If the interference is only slight, a shaving taken off the frame may solve the problem.)

Determining the Number of Planks

If you have plans that specify plank shapes, you should stick to the widths and thicknesses given by the designer. Often, however, such decisions are left to the builder. For those plans, and for developing my own designs, I’ve used full-sized drawings to help work out how many planks to use.

Most round-bottomed boats up to about 15′ long have 9 to 11 planks per side. The fewer the planks, the wider they need to be.

Planks that are too wide can create problems at the turn of the bilge, as described in the sidebar at the top of this page, and stock of adequate width may be difficult to find. But it is also possible to use too many planks, which increases the cost of wood and fastenings and, if you’re charging for your labor, increases the cost of the time required for planking. Using too many planks can make their dimensions too narrow at the ends, especially at the stem.

Try to keep the greatest width, including the lap, between 31⁄2″ and 5″ amidships, regardless of the size of the boat. With experience, you may wish to vary the widths of the planks as measured at the boat’s greatest girth. It is common to use wide planks in the bottom and topsides, where the cross-sectional curve is less, and narrow ones at the turn of the bilge.

One simple method for determining the number of planks for round-bottomed boats is to lay them out to be all the same width amidships except for the garboard, which should be wider.

To determine the number of planks and where their upper edges land on the molds or frames, divide the amidship station from the body plan into nine equal divisions from the keel rabbet to the sheerline. Add 1″ to the resulting number to determine the width of the garboard at this station. This extra width allows the top edge of this plank to reach farther up the stem, making it easier to twist into place, reducing the sharpness of its tip at the stem, and reducing the curve in the planks to follow.

Then measure again from the top of the garboard to the sheer and divide by eight to get the amidships width of the remaining planks. Add to this the width of the plank overlap, which should be twice the thickness of the planking, so that you would have a 1″-wide lap for 1⁄2″ planking.

The resulting total width should fall in the 31⁄2″ to 5″ range mentioned above. Also measure from the garboard’s top edge to the sheer at the stem and the transom, and divide by eight to make sure the planks are wide enough to accommodate the 1″-wide rabbeted “gains” in the ends. (Keep reading for more about gains.)

In carvel planking, it is customary to make the sheerstrake wider than the planks below it so that it will not look too narrow when its upper edge is covered by a guardrail or rubrail. This is not necessary with lap-strake planking because the sheer plank, having no overlapping plank above it, already looks wider than the lower planks until the rubrail is added.

Having determined plank widths by this method, check their layout in the critical area of the turn of the bilge by making a full-sized drawing, as shown in Analyzing Plank Layout above. If the drawing for a nine-plank hull shows that any plank lap will be held away from the frame where the fastening will be driven, repeat this process for 10 planks per side. Boats vary in shape; for example, a full-sized drawing for a boat 10′ or less in length with an easy turn of the bilge may show that eight planks per side will work.

It is always a good idea to tack full-length battens along at least every other seam to be sure that your plank lines will be appealing to the eye.

Lapstrake plank thickness chart.

Plank Thickness

How thick should the planking be? A rough guide is shown in the table above, but once again go with the designer’s dimensions if given. If you do not have these, it is always safe to find the average that successful designs have used for hulls similar to the one that you are building.

The thickness of planking changes the geometry of the plank landings on the frames; given two planks of the same width, the thicker one allows planking to tighter curves without posing the problem shown in drawing No. 2 in the Analyzing Plank Layout section. To illustrate this, redraw drawing No. 2, which shows 3⁄8″-thick planking, to use 1⁄2″ planks instead.

The gap between the plank and frame at point F will disappear because the increased thickness changes the overlap geometry by making the angle of the plank less obtuse. However, one of the principal advantages of lapstrake planking is light weight, so it would be better to add a plank rather than increase thickness.

Spiling

Spiling for lapstrake planks (see Apprentice’s Workbench, WoodenBoat No. 176) is similar to spiling carvel planks. For both styles of planking, an essential rule is that at every point along a plank’s length, the spiling batten must lie along the same plane as the plank it is measuring. In carvel planking, the planks lie wholly against the frames or molds, as should the spiling batten. In lapstrake planking, however, only one edge of the plank touches the frames; the other edge is lifted to lap the previous plank.

If the spiling batten is not placed at this same angle, the shape of the plank created will not match its mating plank. Such a plank will probably require edge-set, which means forcing it into place by bending it edgewise, a frustrating and difficult thing to accomplish with the relatively thin and wide planks usual in lapstrake work. Drawing No. 3 shows a spiling batten properly positioned for carvel planking, which won’t work for lapstrake.

Illustrations explaining the placement of a lapstrake spiling batten typically show one edge of the batten landing on the bevel of the previous plank between its edge and its lap line while the other edge is shaped so as not to cover up the marks indicating the width of the plank being spiled, as shown in drawing No 4.

This makes for fussy and time-consuming work in shaping the spiling batten. Doing so also makes it unlikely that the batten can be used again without further shaping. It doesn’t matter that the plank-width marks are covered by the spiling batten, because those can be measured directly after the spiled inboard edge is drawn on the planking stock. What does matter is that the spiling batten lies in the correct plane.

Rendering of scrap wood held against a spiling plank.

Temporary wedges tacked to each mold or to every other steamed frame will hold the spiling batten at the correct angle, allowing the shape and placement of the batten to be far less critical. With this technique, one straight spiling batten 3″ or 4″ wide and one curved batten will usually suffice for spiling the whole boat.

A wedge’s shape can be determined quickly, using a piece of scrap wood, as shown in drawing No. 5; the shaped wedge is nailed in place with a single tack or brad, as shown in drawing No. 6. These wedges can often be reused on subsequent planks.

Rendering of a wedge holding a spiling batten at the correct plane on a plank.

Fastenings

Use your full-sized drawing of planking at the turn of the bilge (as in drawing No. 1) to determine the length and type of fastening you will use to secure the plank laps to the frames.

In V-bottomed, sawn-frame construction, there will be plenty of wood in the frame to use wood screws for this purpose. However, the steam-bent frames for small craft are often 1⁄2″ or less in thickness, and these are best riveted. Screws would not have enough holding power with the few threads that engage the frame.

With frames thicker than 1⁄2″ thick, rivets still work well, although carefully sized wood screws are acceptable. As usual, make a drawing first. When using screws, the best holding power with light frames will be achieved by allowing the screw’s point to just poke through the inside of the frame, where it can be filed flush with the surface. Bungs are seldom used over fastenings in lap-strake planking, which is usually much too thin to permit the counterbores needed to give a bung holding power.

Plank-lap fastenings between the frames can be either rivets or copper clench nails. For the lightest planks (5⁄16″ or less), clenched copper tacks can work. Except for a very light boat, however, rivets are a better choice because the rove’s surface area spreads the load of the fastening and is less likely to crush into the softwood planking, which would allow the lap to open over time.

Rivets consist of a nail and a rove, or burr. The heads of some copper nails sold as rivets are too small to hold securely. Heads should be as large as those on a common nail of the same gauge. Copper nails used in slate roof installations work, since these slating nails have heads of the same size as the rove. I use No. 10 slating nails, driving them flush with a bell-faced hammer. After setting and peening the rove, I give the heads a wash of hot water by paintbrush, to swell out any dimples. This swelling pulls the rivet heads just enough below the plank’s outer surface that they can be covered with a wash of epoxy thickened with a fairing filler.

Bucking Irons

The weight held against the head of a rivet is usually called a bucking, or backing, iron. A heavy hammer head may be used, but because the rivet’s head will likely be drawn slightly below the surface while peening the rivet’s point over the rove, a better choice may be a 1 1⁄2″ round bar with one end turned down to 3⁄8″ on a lathe.

This shape will also accommodate countersunk rivet heads. Riveting is best done with two people, but if you must work alone, try a bucking iron design shown to me by boatbuilder Bill Foster of Hobart, Tasmania. This iron (the one in the photo above made from a discarded propeller shaft) combines the lathe-shaped end with a curved thigh brace.

Beveling the Laps

The tool we use for beveling plank laps is a plane fitted with a guide bar, as shown in “A Magnetic Lap-Planing Guide.”

If a builder has enough experience, plank laps can be beveled completely by eye. Years ago, I watched this done in the W. Lawrence Allen dory shop in Lunenburg, Nova Scotia, where a lap bevel was completed in about one minute using an electric hand plane. This was possible only because the person holding the plane had done the same thing on the same model of boat countless times before. Even then, it was probably not perfect, but it was good enough.

It is not difficult to get the bevel correct at each frame or mold. With those areas shaped correctly, it’s a matter of noting the change of the bevel from one to the next and rolling your plane the slight amount needed to follow the change. It may help to mark each correct spot with two or three dark parallel pencil lines. These help your eye see the change in bevel as you work.

Rendering of a fit stick used to guide a bevel during planing.

A full-length batten tacked to the marks indicating the position of the next plank’s edge will take the guesswork out of beveling between frames or molds, especially if they are widely spaced. Using the shop-made guide bar shown in “A Magnetic Lap-Planing Guide” makes this much simpler. Without that device, an alternative is to use a fit stick, as shown in drawing No. 7, to test the bevel along its whole length as it is being planed.

The batten must lie flat or the bevel indicated by the fit stick will be wrong, and it is sometimes difficult to twist a batten into place to lie squarely on the molds, especially at the forefoot. We have always found that the increased accuracy of using a batten justified the small amount of time needed to tack it on. Using the guide rod described in the sidebar made the process even simpler and more sure.

Gains

Gains at each end of a plank allow the overlap to transition from a beveled fit to a rabbeted one at the stem or transom, allowing the plank surfaces to be flush with each other at their ends. Gains should be at least 12″ long on planking 3⁄8″ or less in thickness and 18″ or more on heavier planking. It is a mistake to make gains too short; a short gain requires a more abrupt dive and twist to the plank end than the relatively stiff wood can follow. This can lead to a poor fit in the stem rabbet or transom and a fuller shape than the designer intended.

Gains can be shaped in several ways, but the following one is the most common. After the lower outside edge of the plank just fastened to the boat has been beveled for its full length (including the area of the gain), use two dry-wall screws to fasten a fence above the lap line at the gain to guide the rabbet plane or chisel. A piece of softwood 1⁄4″ × 1″ by at least as long as the gain will do. Start by holding a rabbet plane or chisel on the plank’s bevel and create a sloping surface with a slight roll that ends parallel with the surface of the stem.

Rendering of an installed wooden boat plank.

The lower inside gain on a new plank must be shaped before the plank is installed, as described below, but the top outer gains are best done on the hull. On a boat with an inner stem and a stem cap, or false stem, as shown in drawing No. 8, the top-edge gain can be finished all the way to the end with a rabbet plane; the planks’ end-grain will be covered later, when the stem cap is installed. A plank fitting into a rabbeted stem, however, must have its gains at least partly finished with a chisel.

Some instructions for shaping gains advise cutting half the wood away in each of the mating planks. In light planking that means there will be very little wood thickness on the upper plank to support a screw—for example, on 3⁄8″-thick planking such a gain would be only 3⁄16″ thick.

It is better to cut the gain on the boat so that only about one-third of the lower plank’s thickness remains in the gain. On the next mating plank, only one-third of its thickness needs to be removed to get a proper fit, leaving two-thirds of the thickness to support the screw.

Rendering of a transom on a wooden boat.

At the stern, flats can be planed in the transom to make firm landings for the planks. Cut the gains here so that they roll from the angle of the bevel to a surface parallel to the next transom flat, as shown in drawing No. 9.

The gains for the mating planks must be cut on the bench. The amount removed will match the amount left on the plank already on the boat (it may be very little at the curve of the transom). This matching gain will have no roll in its shaping as the roll will be created as this mating plank is twisted into place. Gains are more difficult to fit than the rest of the plank’s lap, so it might be a good place to use a bit of adhesive sealant or bedding compound before final-fastening the plank.

Rendering of plank laps on a wooden boat.

Sealing the Laps

The single-beveled lap is the same for glued-lap plywood construction as it is for natural wood. Glued-lap, as its name implies, will have an adhesive between the planks. With solid wood, I strongly advise using no adhesive or caulk of any kind in the laps during planking. Using material that is sticky, and perhaps toxic, will take much of the joy out of one of boatbuilding’s most satisfying tasks. If the material works as advertised, it will make any future repair far more difficult, and the ability to be repaired, after all, is one of the greatest attributes of traditional boatbuilding.

After the planking is completed and the hull is sanded and ready for paint, we seal the laps of our boats with a modification of a method introduced to us by boatbuilder Walter Simmons of Ducktrap, Maine, who wrote about it in his book Lapstrake Boatbuilding. We draw the tip of a medium-sized screwdriver down the length of the lap, being careful to keep the tool parallel to the outer surface of the lapping plank. Try not to cut into either plank, but to compress open a groove in the seam about 1⁄8″ deep. This groove is then filled with an adhesive caulk, as shown in drawing No. 10. We use either 3M 5200 or Sikaflex 291.

With the tip of the caulking cartridge cut for a fine bead, we use just enough caulk to fill the groove. Running a gloved finger along the seam ensures penetration and gives a slight radius to the surface of the caulk. This creates a sort of O-ring to seal the seam. This seal can easily be cut through with a knife if any future repair work is needed. We have sealed the laps of boats from 6′ to 26′ long with this method and find that even after 25 years, planks so joined are unlikely to leak.

Contributing editor Harry Bryan lives and works off the grid in Letete, New Brunswick. For more information, contact Bryan Boatbuilding, 329 Mascarene Rd., Letete, NB, E5C 2P6, Canada; 506–755–2486.

Techniques to demystify the details