Wednesday, September 1, 2010

Preparing The Strakes

After obtaining the strake shape, it is important to scribe the hood (forward) ends of the strakes to mimic the curve present in the stem rabbet. Ideally, the planks should converge neatly with the stem, without any gaps or differences in level. Although this is a simple concept, the builder must be careful to attain an accurate profile on the strake, as any discrepancies between the two components will become obvious later on. To shape these ends, we used the circular saw to make initial cuts and finalized the shape with a block plane, much like the process used on the strake edges. As is often the case, blade sharpness is essential here. Although a keen plane blade will make short work of the task, this step can be very frustrating if one is forced to contend with a dull blade. Because our strakes are mahogany, it was necessary to redress the blade several times during the process of forming all ten strakes. When this is complete, each plank must have “gains” cut into the ends. Usually about 14 inches in length and identical in width to the lap, these long, tapering rabbets effectively remove 50% of the board’s thickness at the stem and stern.

When two gains mate at a lap, the result is a smooth transition from lapstrake to carvel-type planking at both ends of the hull. This allows the builder to form a watertight seal at the transom without going to the effort of coping the stern to accept the laps. Traditionally, this task would be completed with a rabbet plane. However, in the interest of efficiency and accuracy, we opted to design a gain jig for the router. Although it was a highly effective device, we found that it was still necessary to use the hand plane to fine-tune the gains, as, in reality, they must be cut on a slight curve, something that the jig could not provide. For this step, test fitting of the gains on the craft is essential to forming tight joints between the strakes. However, if the task is completed with care to detail, the effect formed by the laps swooping upwards as they taper away at each end of the craft is striking.
Simple inclined plane rabbet jig
Jig in action with router

Once satisfied with all aspects of the first two planks, we finally had the opportunity to begin attaching the strakes to the frame. At this stage, we were eager to begin the true “construction” of the craft, but still had some misgivings about the process. However, although this seemed to be a rather daunting task, it was, in reality, surprisingly simple. At both stem and stern, we secured the planks with 1¼” 316 stainless steel screws, which are purported to have superb corrosion resistance. The same screws were used to secure the garboard to the oak chine. It is truly at this stage that all of the effort exerted during the spiling becomes worthwhile. Because we were able to determine the proper strake shape earlier, we found that each plank readily conformed to the contours of the hull. If a builder neglects the task of spiling, however, he will find that a great amount of force is needed to coax the boards into their proper locations. Stern view of rabbets

Although, by this point in time, we were eager to begin placing the strakes on the strongback, we had to address one final concern—how to seal our craft. In an ideal world, all joints would be perfectly seated against each other, eliminating the need for any sort of bedding compound. However, unless a builder is incredibly skilled and highly accurate, it is inevitable that a boat will have some voids in the joints. This issue can be addressed through two primary means. In traditional lapstrake construction, joints are caulked with textile fibres, such as jute, hemp, or cotton. This system acted upon a simple principle: all of these fibre types swell dramatically when in contact with water. In this way, a craft will “self-seal” due to the expansion of the caulking in its joints. Some builders also believe that this method helps to create pressure within the laps, which subsequently adds some rigidity to the hull. However, there is one major drawback to this approach: the boat will not seal immediately. As a general rule, the craft must be immersed in water for several hours before it will become watertight. In the context of our Willy Winship, a dry-storage daysailer, this means that we would be forced to pre-wet our boat before each sailing session in order to ensure a dry hull. Not only would this be inconvienient, but the retention of water in the caulk during storage would promote the growth of mold and fungi. Fortunately for us, the advent of 21st century technology brought myriad new marine sealants to the marketplace. Builders can now choose from a wide range of polyurethanes, polysulfides, and other petrochemical products that can be handled much like ordinary silicon caulk (note: never mention silicon caulk in the context of a boat, as most builders are vehemently opposed to this material. It is worthwhile to invest the extra $ in a marine sealant, if only to avoid the disapproving looks from other builders). Although some of these products, like 3m 5200, act as both an adhesive and a sealant, the general consensus among the masses is that bedding compounds should have very low adhesive properties and high elasticities in order to allow the wood to move on its own accord. After processing the overwhelming number of opinions that surround this topic (a task unto itself), we finally concluded that we would use Sikaflex 291 on all wood-to-wood joints around the craft. Being an all-purpose, low-strength marine sealant, Sikaflex has the durability and watertight characteristics of a synthetic caulk and the ability to allow wood to undergo seasonal changes unimpeded.

The only drawback to this compound is that it may draw disapproving glances from the devout traditionalists. Fortunately, seeing as the Sika is concealed by the joints, the boat, when finished, will be nearly indistinguishable from a cottoned craft—except, of course, for the fact that Willy, unlike the ships of old, will be reliably watertight.

Joe Lap


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