Wooden Boat Construction

Black Locust for Boatbuilding – a Native Treasure


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The molds were up, the ribbands in place, it was time to put in the frames, and I was a little worried. I was building a melonseed—a great little traditional sail boat with shallow draft and a hard turn of the bilge in the aft sections. I was framing with steam-bent Black Locust, as I do with all my boats, and I just wasn’t sure if the 3/4” x 5/8” frames would be able to take that much bend. I knew from experience that good Locust bends beautifully, and I had good stock—nice, even grain with little or no run-out—but this was going to be a severe test. I was thinking about all this as I loaded frame after frame into the steam box, one every five minutes. The first frame came out of the box after twenty-five minutes. Hot and flexible, and nearly glowing green with that strange color fresh-cut Locust has, it took up its midship position without too much of a fight. A good start but the turn would get harder as I moved aft. There was no more time left to worry about it though, the box was full of frames and the next was due out.

Two days later and forty frames in the boat, I just couldn’t believe it…only two failures in the whole batch! I don’t think many other woods could do it, and fewer still that are acceptably strong and rot resistant for boat use. Even good white oak might have given me trouble, and the locust is a better wood.

Despite the fact that it grows in nearly every part of the country, few people have any experience with black locust (Robinia pseudoacacia L). Some farmers know that it makes good fence posts (make sure you turn the cuttings upside-down or they will start growing again!), and many people recognize the tree itself. However, the lumber is almost never sold commercially and has become somewhat forgotten.

Boatbuilders are among the only people I know who think of black locust as a useful wood, and most of them only think in terms of trunnels, cleats, or other small items. These things make good use of the wood, but this tree is a native treasure with far more potential. Locust is an excellent structural lumber for the deadwood and framing of boats up to 35’ (the specie’s relatively small size limits larger applications), and it is a readily renewable source of timber. Locust grows very rapidly on what are often considered waste soils, and because of its almost non-existent sapwood, even juvenile specimens are of use. Furthermore, being widespread, the tree is commonly available. Anyone with a little motivation can go out and get it. Yet locust is largely ignored and builders instead pay for costly white oak or expensive exotics of dubious moral character. To me this just doesn’t make sense; I truly believe black locust to be a superior alternative. It really is ideal timber for small boats. Nearly every quality it possesses is of advantage to the builder.

To begin with, it is highly rot-resistant. Rot-proof would almost be more accurate. I have cut into logs that have been lying on wet ground for years and years, long enough to become nearly buried, and inside they were fine and sound. There is an old New England saying that locust will last seven years longer than stone. It isn’t all that much of an exaggeration. This is a highly desirable quality in a boat wood. Decay may well be the number one drawback of wooden boats; this remarkable species nearly eliminates that concern where it is used. Team it up with cedar planking and stainless, bronze, or copper fastenings, and you have a vessel that will really last.

Black Locust is also extremely strong. In many respects it may be the strongest native lumber per dimensional unit in the United States. Without getting too technical, suffice it to say that in most types of strength testing locust tests 30-50% higher than oak.[1] That’s pretty impressive considering how strong white oak is. Like many high strength woods, Locust is also very hard, making it resistant to abrasion and crushing damage from blows.

The wood is hard due to its density, and it is correspondingly heavy. The additional weight can be a drawback in some applications where low weight is a priority. You can, however, gain back some advantage by reducing scantlings, given locust’s superior strength. The up-side to density is that properly pre-drilled fasteners have outstanding holding power. But please be careful, as undersized pilot holes can lead to splits. The dense fibers of this wood simply will not crush and deform to make room for the fastener.

Fasteners have great holding power in locust, but take note that glues do not. Like white oak and other dense woods, black locust glues poorly. The hard, smooth surface does not provide much for glues to grab hold of. This seems to be especially true with epoxies, with which I believe it may be chemically reactive, and I have had experience with the combination giving incomplete cures. Polyurethanes seem to get the best grip on it, but I don’t like to use locust in applications where I depend on a glue bond for long term integrity to a piece. This makes locust a poor choice for laminations.

Fortunately the tree’s natural growth form provides a wealth of grown shapes, eliminating the need to build such shapes up from laminations. It is a common sight in the groves near my home to see me wandering about with an armful of patterns for stems, knees, floors and various other curves. I go from fallen tree to fallen tree looking for a specimen with curves that exactly match the shape I want. No need for compromises here, where the choices are plentiful. All trees provide this to a certain extent but not like Locust does. A single tree can provide enough crooks for all the odd shaped pieces in a boat. The crooked growth form is so pronounced that it is one of the characteristics I use to identify the trees from a distance or in winter. An added bonus is that locust tends to develop flaming wavy grain and fiddleback patterns in its crooks that really spruce up a small open boat.

In part, convoluted growth is responsible for the little attention this species receives and limits its usefulness in larger boats. It is rare to get long clear straight pieces. If I can get a ten foot plus clear run I count myself lucky and set it aside for keels and rails. Or, if the piece is good enough, I mark it for planking. Last year I built a 16 foot Boston Whitehall using some exceptional pieces of locust for the keel and garboards. It was heavy, but stiffened the boat remarkably, and gave it a bullet proof bottom that will be appreciated along the client’s rock shoreline. The garboards had come from the same tree as the framing stock for the melonseed, and bent in just as beautifully—and there is a LOT of twist in that plank.

More common to the species are stretches of 3-6 feet of good wood between defects. This is more than adequate length for most framing applications. If you are willing to use a band saw to cut your framing stock instead of a table saw, you can cut with the curve of the grain to utilize pieces that are not “dead straight”.

I like to cut my framing wood just before I am ready to frame so that it’s still nice and green (wet), which helps it bend better. Other pieces I cut as I need, although I do keep some random leftover stock for the occasional job requiring dry lumber.

For the most part dry wood is not necessary. Locust is quite stable as hardwoods go, showing 7.2% tangential and 4.6% radial shrinkage from saturation to oven dry (compared to 10.5 and 5.6 for white oak or 5.1 and 3.7 for mahogany)[2]. This translates into little enough shrinkage to 12% “air dry” that my stem rabbits don’t deform noticeably, especially when the piece has been sawn radially. I have been using green locust for several years now, and none of the boats have given me trouble. But you have to use it judiciously, for relatively “narrow” pieces. Planking or wide expanses such as transoms should use seasoned lumber.

Rot-resistant, strong, plentiful, and stable. It almost seems too good to be true. Although I have been singing the praises of this wood, there are some precautions to bear in mind. Rot resistance comes at a price. Locust is toxic. It will not poison you, but breathing the sawdust can make you feel pretty sick or give you a nasty headache. This is fairly easy to overcome by wearing adequate dust protection when machining or sanding…which you really should be doing anyway. Sawdust of any sort is not particularly good for the lungs.

There is also the fact that the tree grows under a fair bit of tension. It is a good idea to cut pieces roughly to size first and let them “rest” a day, then lay out final shapes and jointed surfaces. This releases the tension and minimizes any shape-changing.

The heart of the matter of course is the wood itself. The small band of sapwood is usually nearly invisible, comprising only a year or two of growth. When it is visible, it is creamy white (and lacking in the durable qualities of the heart wood) The heart wood is a bright yellow green with a distinct astringent odor. The color when first cut is striking, and many people find it unattractive. Fortunately, the wood loses the green cast as it oxidizes to a rich honey brown, which is quite handsome. The color change is in large part a photoreactive process, and even a day in strong sun will have a noticeable effect. When I use the wood for brightwork (which I like to do in place of teak or mahogany) I give the finished pieces a quick “suntan” before I put on the first coats of varnish. Once coated, the UV inhibitors in the varnish slow the reaction.

Locust frequently has an interwoven grain. This, combined with the wood’s hardness…well, locust can be difficult to work with edge tools. It’s not too bad with green wood, but seasoned timber requires a little more attention. If you keep your blades sharp and expect to touch them up frequently, it shouldn’t be too much of a problem. And it is a good idea to keep a scraper handy for any particularly difficult areas.

Unfortunately, as stated earlier, Locust is generally not commercially available. Which means if you have become interested in using it, you will have to cut it yourself or have a local sawyer cut it for you. Unless it is a really big project, cutting it yourself makes the most sense. I can generally get all the hard wood I need for a small boat from one tree, with some to spare. You will probably have to at least find the log yourself, as a logger won’t usually fill such a small order. Which is for the best, because then you can find a tree with just the right sweeps and bends for the particular boat being built.

I primarily use a chainsaw mill (see WB153) for harvesting dead and down trees, but the locust logs I use are often small enough that I just cut them up on my 14” bandsaw. If you don’t have the equipment to cut it yourself, or just don’t want to, look into hiring someone with a portable bandsaw mill to come out and cut the log(s) you found. But unless they come recommended, I would make it a point to be there checking the planks as they are cut to make sure they are even, straight and without cup or twist. I’m sorry to say some of the guys who offer this service may not be as familiar with their equipment as they should be.

Grown shapes can usually be ground out of small branches (6-10” in diameter) using a 6” joiner and 12” portable planer. It’s primitive and a little slow, but working in from side to side lets me decide exactly where in the branch to take the piece from. This higher degree of control allows one to avoid defects as they are revealed. On the smaller branches, defects are of course more common, and it is a good idea to get a few extra crooks. For bigger sweeps on larger boats I shift to the chainmill or a large bandsaw.

Finding Black Locust trees is easy, they are quite distinctive once you recognize them. Its leaves are small, oval, and dark blue-green. They are compound leaves, meaning that each leaf is comprised of many pairs of the oval leaflets. It has deeply furrowed bark, which is forking and somewhat corky. The bark will commonly host a yellow lichen on the north (shady) side. In spring, the tree is covered in showy white flowers; and like all members of the legume family to which it belongs, these ripen into pods of small beans. Locust can grow from 40-60 feet high and reach diameters up to 2.5 feet, but trees with 10-16” diameters are more common. The tree spreads prolifically by both seed and root suckering, so they are commonly found in pure groves or localized patches. If I see a grove or even a few trees I will drive around the area on the lookout for more. I especially look around river banks, old farms, and railroad right-of-ways. The tree is opportunistic and likes to sprout up on disturbed soils and sunny areas. Locust is relatively short lived, and there are usually a number of downed or standing dead specimens in any grove. Because the wood is so rot resistant these trees are mostly still sound and should be the first choice for harvesting. Take a cut into a “rotting” log and you are likely to find sound wood 1/4” in. But locust grows so fast that there is little guilt in cutting a living tree.

It is rare to find a species of tree so well suited to a specific purpose as Locust is to boatbuilding. The precautions listed earlier in this article are a small price to pay for such a gift. Given the environmental concerns we face today, I can’t help but be grateful that it comes in a package that is local and renewable. I feel we should be embracing this native treasure, not ignoring it. It would be great to see the species actively cultivated and available at the commercial level. I can even envision it edging into the market for pressure treated lumber used for outdoor construction. But until that happens I admit locust takes a little more work to get hold of. I believe in building boats that will last a lifetime and then some. That starts by using the best materials. Sometimes it’s not easy to get the best, but it’s always worth it.

[1] Understanding Wood, Bruce Hoadly. The Taunton Press, 1980 p.111

[2]Understanding Wood p.74

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