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From: Tom MacNaughton
Date: 29 Dec 2007
Time: 07:38:31 -0500
Remote Name: 66.252.37.86
Unfortunately here you are forgetting that these vessels are sheathed strip construction. In other words they are sheathed transversely inside and out with glass fiber and epoxy or carbon fiber and epoxy. Thus they have all the longitudinal strength that they need from the strip planking and because of the transverse sheathing they can be considered transversely as similar to a cored hull with an exceptionally high shear strength core. If you were to eliminate the sheathing and add cold molding on the outside you would then have to add frames on the inside and you would have a much heavier and much more labor intensive vessel. This is why more and more wood and epoxy boat builders are turning to sheathed strip. It is the strongest for a given weight or lightest for a given strength of any of the non-exotic materials construction forms. It is also much easier and quicker to repair than any hull can be that has cold molding added. It is difficult to be sure what you mean when you say that cold molding would “minimize the visual effects of lines appearing through the hull due to timber shrinkage”. I suppose you could be referring to shrinking and swelling of boats that were built with strip planking, either glued or not, over frames and with no sheathing such that the seams could shrink and swell and thus crack the paint and show. You could also be referring to the “print through” of the wood grain often seen on wood and epoxy boats that have not been constructed in a manner which minimizes it. Actually both of these conditions can be effectively cured by a better understanding of proper strip planking procedure whether you are talking strip planked fully framed hulls or sheathed strip hulls. Unfortunately many people do not understand proper planking procedure for strip planked boats. Strip planked boats should always be built with square section strips. That is the thickness of a given plank exactly equals its width. If you take a few square section strips and examine them, turning them a quarter turn at a time so you can examine each side in turn you will notice that two of the faces, opposite each other back to back, will normally have long smooth grain running the length of the plank and the other two faces a quarter turn away have “wild” grain. If one ignores grain run and just puts the planks in place every which way, you will get a crazy quilt of wild grain run. Some will appear to run the length of the strips and some will appear to run crosswise to the strips. In a vessel where planks can shrink and swell a boat planked like this will shrink and swell a great deal because planks with wild grain on their inside and outside surfaces will maximize transverse swelling. On the other hand if when you pick up each strip you turn it a quarter turn as necessary to get the nice smooth long grain on the inside and outside surface you will minimize shrinking and swelling in the transverse direction on the hull surface. Hulls built this way will tend to look beautifully fair and are often finished to a much higher gloss than anyone would dare put on most planked hulls. Next let’s consider the effect of the same turning of strips to get the best grain run inside and out when used in a sheathed strip hull. In this case the grain and the seams all run in the same longitudinal direction. Given the way the human eye works and that the planks will change shape the least as everything cures with this type of grain run, you will have automatically minimized “print through”. Obviously the wide flat veneers used in cold molding cannot be turned to get the best grain run. In this case the only way to minimize print through is to use veneers cut out of quarter sawn wood. This can be done but rarely is. Assuming you have minimized print through using the above method in a strip planked hull, will the hull be a perfect mirror finish? No it won’t, not the first year. Even in a wood and epoxy hull it takes time for the planking and sheathing to fully finish curing and taking its final set. Given the sensitivity of the human eye to very slight variations, the few thousands of an inch of slight adjustment everything makes in the months between the finishing of the hull and the launching of the boat can produce a bit of surface irregularity when viewed in low angle light. However this is not a disaster. The last coat or two of paint should be left to just before the boat is launched and when the paint is applied you should use a semi-gloss or even quite flat paint. This will make it much harder to see these few thousandths of an inch irregularities. Then next year, when you can be pretty sure everything has taken its final “set”, you can sand with fairly fine paper removing very little paint and add another coat or two. Sanding will level out virtually all the very fine irregularities and the new paint will level over anything that’s left. You can use a higher gloss paint if you want and will have very nearly a mirror finish. I should mention that for many reasons sheathed strip is taking over all over the world. We’ve sold a very large number of copies of our sheathed strip scantlings rule to designers and builders all over the world and I assure you the results reported to us have been very gratifying. For further reading see our “Sheathed Strip Scantlings Rule” and our article reprint “The Sheathed Strip Alternative”.