John Shafer recently wrote to ask, “Maybe you can point me in the right direction. I will be building a Paul Gartside–designed 22′ centerboard yawl. I’m trying to use wood only from my farm in southwestern Ohio. The two choices for the hollow mast are sycamore or yellow poplar. Both are similar to Douglas-fir in density, but I’m wondering whether the cross-grain of the sycamore adds anything to its strength that is not shown in the elasticity and rupture numbers. Any idea if the wood will be sufficient, or will some embedded carbon-fiber rods be necessary?”
I checked Paul Gartside’s website but could not find a 22′ yawl. Matt Murphy contacted Paul for information, and, it turns out, the boat design was fresh off Paul’s desk and just recently published in Water Craft (www.watercraft-magazine.com). The boat was specifically designed for John Shafer as design No. 298.
The plans show the boat is gaff-rigged with a stayed mast. Historically, conifers such as spruces, pines, or Douglas-fir have been predominant choices for spars and masts (see my columns in WB Nos. 118, 120, 263, and 286). The greater relative stiffness of conifer softwoods compared to deciduous hardwoods has often been attributed to differences in lignin chemistry. Conifers contain guaiacyl lignin while hardwoods have both guaiacyl and syringyl lignin. Since lignin provides stiffness to wood, this is not an unwarranted assumption.
However, this difference fails to account for the fact that lignin is a plastic substance that can become more plastic in wet wood—as is the case in living trees. Moreover, we know that green wood that is heated further plasticizes lignin. Yet conifers still are more recalcitrant and continue to resist bending—generally failing instead. We now know that the spiral arrangement of microfibrils of cellulose in conifer cells (tracheids) significantly contributes to this greater stiffness. 1
The accompanying table compares Douglas-fir with yellow poplar, sycamore, and sassafras, another wood common in southwestern Ohio. I also included a conifer that grows in that area—shortleaf pine. Each of these woods has moderate but differing wood densities as represented by their specific gravity values. Examination of the table’s modulus of elasticity (MOE, a measure of stiffness) column reveals that Douglas-fir and shortleaf pine have the highest values, as expected. The hardwood sycamore, with a specific gravity (SG) value comparable to Douglas-fir, has a considerably lower MOE. Sassafras has an even lower value, but it is a less-dense wood, with SG of 0.46. The surprising hardwood is yellow poplar. It is the least dense of the woods in the table, yet it has an MOE that exceeds the values for sycamore and sassafras and approaches the density-to-stiffness ratios of the two softwoods. Checking for a conifer with specific gravity that matched yellow poplar, I found jack pine with SG of 0.043. It has an MOE value of 9,300 MPa, slightly less than that of yellow poplar.