11/16/13 Transom Mounted
By far the most challenging, fun, and stressful part of this project has been the replacement of the transom. That was completed this week and I'm pleased with the end result. Remaining work is plugging the screw holes with mahogany bungs and staining/varnishing to match the rails. Cutting out a major portion of somebody else's pride and joy was worrisome indeed... I desperately did not want to make any mistakes.
The original transom was made of ½" marine plywood with a mahogany veneer (I think it was Honduras Mahogany) framed with upper and lower white oak transom bows and left and right with Philippine Mahogany cheeks.
The process was involved as the transom is curved and had to fit perfectly into place. The amount of curvature was too great to bend modern marine plywood, as they're made of additional thinner ply's than were done back in the 60's making today's marine plywood's stiffer (and better). So laminating two sheets of ¼" marine plywood together was in order. My first choice for material was marine grade ribbon cut Sapelle Mahogany plywood... it is a gorgeous wood grain pattern. However I found it in short supply locally and to ship the material from the closest supplier was cost prohibitive. I settled on Keruing Mahogany marine grade plywood that I found available just west of Minneapolis at Midwest Boat Appeal in St Bonifacius, MN.
I used a vacuum press technique for the lamination process. It involved making a mold to match the curvature of the original transom and sealing it with epoxy to prevent air leakage during the vacuum process. For the lamination adhesive I used Entropy Resin Super Sap Epoxy as I have no allergic reactions to it as I do with the benchmark West Systems Epoxy.
With the mold completed it came time to laminate a ½" thick plywood transom out of two ¼" sheets of Keruing Mahogany marine grade plywood panels that I rough cut to size. Epoxy was applied between the two sheets of plywood which were covered in release fabric and breather fabric. A vacuum manifold was placed over it and connected to a vacuum pump. All that was sealed with mastic tape and vacuum film and then a vacuum was created using the vacuum pump creating the required pressure for the lamination to bond properly. Also, the upper and lower transom bows were steam bent to shape.
After the lamination process was complete, the transom bows (upper and lower) and the transom cheeks (left and right) were screwed in place to the laminated plywood effectively framing it. The bows were made from White Oak, and the cheeks were made from Vertical Grain Fir. Once framed it was cut to its final shape, then the edges were carefully beveled to meet the angles of the adjourning lapstrake planks, bottom and deck. A template was drawn from the existing transom using clear velum and transferred to the new transom for an accurate pattern.
Then the moment of truth came... cutting out the old transom, notching the lower transom bow of the new transom for the chine log, keelson and stringer pockets, and finally mounting the new transom.
Next up... replacing the bottom.