What Is Strip Planking?
(c) 2003 Stephen Redmond. All rights reserved
An Elver receiving the first few strips. Strip planked boats require minimal framing.
I was recently asked about strip planking by a builder who was unfamiliar with the method and wondered if any books covered it. Since most books on boatbuilding only briefly mention the method, some explanation here may be helpful.
What is it?
Strip planking is a method of wooden boatbuilding which uses planks that are more or less square in cross section. Typically strip planked boats will use strips that measure anywhere from 3/4" to 2" in thickness. The strips are laid onto the framing or mould setup and fastened in place. They may also be glued, though there are alternate opinions about whether gluing is absolutely necessary.
Strips are generally fastened by nailing vertically into previously installed strips. Fastenings are also usually driven horizontally into framing. Generally the horizontal fastening is by means of screws, while the vertical fastening requires nails. Horizontal fastenings are not required for every strip. Typically horizontal fastening might be specified by a designer for every 3 or 4 strips.
The vertical fastening method generally utilizes thin gauge nails -- galvanized finishing nails are often used. The nail length is determined by the strip size. Usually nails are about two and a quarter times as long as a strip is deep. This allows the nail to be driven across three strips, with countersinking. If 3/4" square strips are used, the nails would typically be about 1-3/4" long.
Nails are staggered in 3 strip courses so that they won't hit eachother. As an example, the first strip may go on with nails driven every 18" starting at the bow at 18", 36", 54", etc. When the next strip is fastened, nails will be driven at 12", 30", 48", etc. The third strip would then be nailed at 6", 24", 42", etc.
The result of staggering nails in this fashion, and the fact that each nail penetrates three strips is that each plank is effectively fastened on 6" centers to the two planks above it and the two planks below it.
This process of inter-nailing greatly reinforces the cross-grain strength and stiffness of the timber, and strip planked boats can often get by with a signifigant reduction in framing because of this fastening method..
Strip planking is most appropriate for moderate size boats in the size range from 20 to 35 feet in length, though there are examples which have been built both larger and smaller. Generally it is difficult to nail planking any thinner than 3/4" without frequently breaking through the strip sides. If a nail wanders it should be pulled rather than ground off, since a protruding nail offers a starting point for corrosion.
Planking thicker than 2" requires very long nails, and may create problems from shrinkage and swelling. Nevertheless strip planking of a sort has been carried on with very large scantling timber -- on the Chesapeake Bay. Large Chesapeake log canoes and bugeyes have traditionally been drifted together from huge log-sized timbers and then carved to shape. You could call this giant strip planking.
I once took the lines off of a 34' log canoe in Mathews, Virginia. It had extensive checking and rot damage. These boats didn't rely on caulking for tight seams. Massive cracks between planks, and checks within planks can open if they are allowed to dry out. They are tricky to repair because of the through drifts and relative lack of framing. A broken log would require that a chunk of the hull would have to be removed and the drifts cut and new piece graved in, fastening into a backing piece.
At the small end of the traditional strip planking scale are the Lake Champlain pike skiffs. I once owned a couple of these 14 foot boats. Pike skiffs were traditional market fishing double-ended boats used for trolling walleyed pike in Vermont. They used a single oar, and had an long plank keel to keep the boats directionally stable while handline trolling. Both hulls had deteriorated before I acquired them and it was possible to see the internal strip construction through large sections of dry rot in the planking. They were built of individually tapered strips -- wider in the center than at the ends. The planking was as thin as 1/2". Box nails were used in the construction. It required a skilled and patient builder to construct a one of these boats.
Strip Planking Advantages
One advantage of strip construction is that round hull shapes are relatively easy to accomodate. As with all methods of construction gentle shapes are easier to plank than sections with hard curvature. But strip planking can accomodate a variety of hull shapes. The planks do not have to be "backed" -- planed concave on the inside to fit the framing-- though the upper edges do require beveling or shaping
If the wood species is properly selected for dimensional stability it is also possible to produce a boat that can be trailered and dry-sailed, and has reasonably low maintenance requirements. Commercial builders in the late fifties produced strip planked boats like the Amphibette that were meant to compete as trailer sailers in the then-new fiberglass boat market.
A third advantage of the method is that it does not require the highest grade of lumber. The lumber used for strips should be straight grained and clear, but does not have to come from clear, long, or wide boards. It is possible to cut strips from less than perfect stock and discard only the bad sections. Short lengths can be efficiently utilized. Generally the best means of joining strips on the hull is by means of a short "birdsmouth" joint, about twice as long as the strip thickness.
Reportedly, strip planking was invented over a century ago by some enterprising builders in order to utilize the scrap rippings piled outside of box factories. In an early form of recycling, a free source of planking material was utilized. Likewise today it would be possible to build at least small boats from wood discards. I've driven by many overflowing construction dumpsters here near Boston and been amazed at the amount of wood we routinely throw away. Because strips can be spliced as they are applied to the boat with a simlpe slash joint it allows the efficient use of shorter stock than any other method of wooden boatbuilding.
Strip Planking Disadvantages
Strip planking is a relatively slow process. Planking with conventional wood or plywood is faster, though both will require more framing, which also takes time.
On a carefully built strip boat, where epoxy glue is used between the strips, ripping the planks, cutting the splices, beveling the plank, mixing the glue, spreading, clamping, marking, nailing between strips, and fastening to the hull framework can mean that a only a few strips are applied each day, unless a faster production method is adopted.
A hull may require more than a hundred strips to complete. In order to speed the process, some builders have made special bench mounted glue applicators, pre-moulded the strip edges, and adopted the nail gun for fastening through strips.
Because of the curvature of a hull's cross-section the strips will not meet perfectly. The upper edge of each strip is therefore usually beveled to meet the next. If the strips are to be glued as well as nailed, and the curvature is slight, some builders leave a small gap unbeveled, to be filled with glue. Or, an approximate average bevel can sometimes be pre-milled into the strip stock. Generally the bevels do not have to be precise fits. A reasonable fit can sometimes be achieved after a strip is fastened in place by dressing the upper inside edge with a coarse grit circular sander or grinder. Wood planes are difficult to use on strips unless the planking nails are well countersunk.
An alternate method of shaping the joint between strips involves molding the upper surface of each strip concave, and the lower surface convex. This allows the strips to adjust to eachother as they are applied, and provides a gutter for glue if it is used. Glued seams drip otherwise. For beveled strips, it's an advantage to use a glue that glue sets fairly quickly and is thixotropic.
Molded strips should be approximately 30% wider than normal to allow for the additional material removed in the molding process. That means that molded strip boats use about 15% more material than beveled strip boats.
To Glue or Not to Glue
Strip planking is not suitable for hardwood or dimensionally unstable lumber since the entire side may expand as a panel. Softwood usually expands less than hardwood and can absorb some compression. Gaps between the strips can also take up some of the expansion. The wood requirement will depend to some degree on whether the strips are glued with a hard and incompressible glue like epoxy, or a more flexible compound. In extreme cases, where a hardwood is used in a completely glued structure, swelling has been known to force planking off of the transom. Whether it is best to use an adhesive between planks will depend on the design requirements, wood chosen, and skill involved in beveling.
In general, the strip nails are more than adequate to support the wood, and glue is not a structural necessity. Some protection against water ingress and subsequent corrosion for the strip nails is helpful, however, so a sealing compound of some sort is used. Traditional builders used thickened paint between the strips. Modern synthetic rubber adhesives and caulking compounds may be useful as well.
Another disadvantage in strip planking becomes apparent when it comes time to finish it. Because of glue drips and the possibility of nail penetrations, there is almost no alternative to rough power sanding both the exterior and interior. It's no fun to spend the day in a Tyvek suit with goggles, ear protectors, and a respirator finessing a 7" grinder on a hull. It is therefore very important to do a neat and careful job of planking in the first place. A foam backed sanding wheel and 80 grit paper is the usual weapon of choice. There is one great advantage in all of this finish work, however: if varnished, a well built strip planked boat can look wonderful.
An elver being fitted out. Strip planking lends itself well to a bright finished boat.
Cored Strip Construction
Strip planking can be sheathed in fiber reinforced plastic materials. Where the hulls are sheathed on both sides, the panel may act like a cored sandwich, and show additional stiffness and strength, depending on the sheathing material, resin, and laminate schedule chosen. Opinion is divided on whether this is a good idea since once water or moisture penetrates the core material, core degradation or delamination from swelling or shringage may result.
Once wet, the wood core may never be able to dry out. Wood is porous, degradeable, and dimensionally unstable, and is generally not an ideal core material. The resin and laminate materials used to build wood cored boats have varied widely in strength, flexibility and adhesion, and no generalizations can be made about the suitability or longevity of this method without specific reference to the particular design and the actual materials used. During the seventies some notable larger cruising sailboats, like the Quoddy Pilots and Quoddy Pinkies used this method of construction.
On a smaller scale, so-called "stripper" canoes and lightweight rowing boats have also became popular for backyard boatbuilders. These boats generally use edge-glued cedar strips of about 1/4" by 3/4" with no through strip fastenings. They were covered in fabric and epoxy resin inside and out, and are thus coreed laminates. The early experiments with polyester resin over lacquer-coated cedar, as described in Davi Hazen's Stripper Canoe Guide, did not provide enough adhesion to prevent localized delamination. Epoxy resin adheres better in this application.
Stripper canoes are often finished bright with varnish, though the resins used for laminating would eventually photodegrade. There is no doubt that this method produces a very light and good looking boat, that with suitable storage protection could last reasonably well. Nevertheless, it is not a long lived construction and its use is confined mainly to smaller ultralight dry-stored boats.
Tools of the stripper canoe trade: grinder, sander, respirator, staple gun and staple puller. This hull is ready for sheathing.
Traditional strip planking does not treat the wood as a core. Framing is used to add stiffness and transfer loads. Fiber reinforced sheathing does seem to work out well when applied to the hull underbody or topsides for abrasion resistance. Epoxy resin is generally used here for superior adhesion. The planking can breathe on the inside of the hull, since it is not sheathed.
Strip planking is a useful construction method for backyard builders of medium sized boats where access to high quality boatbuilding lumber is restricted. It is less expensive and generally produces a stiffer, tougher, and longer lasting hull than plywood construction, and has the advantage adaptability to nicely curved sections. It is not a fast method of planking, but is simple in concept, and can offer other reductions in building time by reducing framing labor to the construction of a few bulkheads. It can produce a graceful hull, and offers the possibility of utilizing scrap stock, as it has since it's invention. It is not ideal for every boat, but works best when its advantages are maximized by specific design.
(c) 2003 Stephen Redmond. All Rights Reserved