POSTMORTUM OF A NEWPORTER APRON: A GREAT DAY WORKING ON A NEWPORTER

[bob mitchell]

     I have previously used the word KEELSON for the 6 layers of 3/4" plywood that runs the length of the boat.  Keelson will now be replaced with the word APRON, in an attempt to establish a common vocabulary consistant with what the original builders used at the Dorchester Shipyard.

      And now, without further adoodoo, a cross section of a Newporter 40 APRON in the area of the main mast step.

The razor blades define the actual width of the Douglas Fir deadwood below the Apron. There is 3/4" vertical plywood sheathing on each side the deadwood right below the razors themselves. THE GOOD NEWS!!!!!!!!!! THE DOUGLAS FIR DEADWOOD, FROM WHICH I INTEND TO START REBUILDING, APPEARS SOLID AS A ROCK - MADE MY PRY BAR RING UPON IMPACT!!!!

           But the obvious question is why is the douglas fir deadwood solid, while the bottom three layers

of plywood in the apron are shot. One theory might be that plywood and its glue are much more vulnerable to electrolytic currents between the bronze keel bolts and the galvanized floor bolts. It is a mistake to put those two metals in such close proximity, especially under the conditions found in the bilge. What I found in most cases was that the galvanized floor bolts had not deteriated that much, but that the wood around them was mush. The same was true for the plywood around the bronze keel bolts. If you really want to know what is going on you can read Degradation of Wood by Products of Metal Corrosion, U.S.D.A. Forest Service Research Paper FPL 229.  Here are two photos, the first of a galvanized floor bolt through frame 12, still with 90% of it's diameter good steel, and the second of what I call an electrolytic tunnel around a bronze keel bolt.

Notice in the first photo of this post that the douglas fir deadwood around the bronze keel bolt is in great shape (as compared to the plywood in the photo above. I can postulate two ideas: 1) plywood is more vulnerable to electrolytic currents, or 2) there was a minimum of electrolytic current because the galvanized bolts were not in the douglas fir deadwood. (there were occasional 3/8 inch galvanized spikes going through the apron and into the deadwood, but the douglas fir around them was fine. My guess is that the plywood allows a lot less resistence to the movement of charge, and that the movement of charge (electrolytic current) breaks down the glue.

     But that still doesn't explain why the bottom three layers are so deteriated. My theory on this is based on the ratio of end grain to board width. Consider the top three layers in the first photo - 12" wide, and only 1 1/2" of end grain. Look at the bottom three layers. Calculate that they go from 5 inches width to 12 inches in 2 1/4 vertical inches (3 x 3/4). So each one has a lot of end grain exposed along the bevel, but not much board in between.

     Another factor might be that those three deteriated layers were at the bilge water/air interface of continual drying and wetting, but I question that because the vertical plywood on the sides of the deadwood also appears gone. And finally, the areas of wood surrounding the bolts, both bronze and galvanized, would have been subjected to initial crushing, as well as compression set later. I think that idea probably has a lot to do with the wood surrounding the bolts.

        Regardless of my speculations, the plywood is shot, and the douglas fir is in great shape. I am just not sure that plywood was the best choice for the apron. I am seriously considering laminating white oak as an alternative.

         So here is the final shot of the days work (4 hours). I moved at a detectives slow comfortable pace.  Finding the douglas fir in great shape is a wonderful thing. I might even sleep good tonight. By the way, the first two keel bolt nuts unscrewed easily AFTER heating them with a propane torch and then cooling them with WD40 while tapping with a hammer - don't screw up the threads.