or Giving Your Boat Fresh Underwear
by Sherwood Heggen
Most of the antique and classic inboard wooden boat bottoms, with the exception of Century boats, have double-planked bottoms. That is, the bottom consists of two layers of planking. The inner planking is made up of thin, narrow planks of mahogany set edge to edge at about a forty-five degree angle to the keel. The outer planking runs longitudinally. Typically, the inner planking of an original bottomed double-planked boat will be buckled and rotted, especially in the lowest point of the bilge.
Now, if you are a purist, the inner planking would be replaced with the same style and kind of planking as what the factory provided, but then you would be asking for the same inherent problems the boat had when it came from the factory.
Modern times has come up with a better idea. This highly regarded method is that of nailing down Okume plywood in a bed of 3M 5200 on the chines, keel, and transom base and then screwing down new outer planking over that.
What are the advantages of that? Your hull will be watertight and it will be stronger. It is not difficult to do, but there are details to the process that should be understood to get the best results. Lets run through the process.
It should be understood that a restored bottom includes new frames, lower transom bow, keel, and chines where necessary. When is it necessary? Whenever there is oil soaked wood, a hint of dry rot, or damage, it is necessary to replace the subject part(s). You should expect the restored bottom to last for decades. It would be horrible to have to tear all this apart because you saved yourself the work of not replacing a couple of marginal frames that you thought would last.
Okume plywood is a high quality plywood that is ideal for replacing the original mahogany inner planking. The original inner planking on Chris Craft and others was three-sixteenths of an inch thick and the Okume 6mm plywood is a bit thicker than that. It is said that 4mm plywood is sufficient on boats under 20 feet and 6mm is better for boats over 20 feet. Take your choice.
When you are sure that the bottom framework is solid, it is time to start fitting the plywood. But first, mark the center of the frame locations on the chines and keel with a felt tip marker. This will allow you to draw frame location lines on the inner planking after it is fastened down. When you start driving screws, you will know exactly where the frames are located. Now, starting at the aft end, lay a sheet of plywood with an edge against the keel rabbet and the aft end even with the end of the lower transom bow. Clamp down the forward end of the sheet to a frame to draw it into the concave shape of the bottom frame if it is so shaped. Then, with a marking pen, mark the outer edge of the chine on the underside of the plywood. Also, reach inside and mark the forward edge of the most forward frame under the plywood and the rearward most edge at the transom. Now flip the plywood over and adjust the marks so the panel will fit the confines of the rabbet on the keel and chine after it is cut to size.
Do so by moving the chine line in by the dimension of the outer edge of the chine rabbet to the outer edge of the chine. Then, move the line drawn against the frame back about half an inch so the forward end of the cut panel lies on the center of the bottom frame. Cut out the piece of inner planking plywood and do final trimming with a block plane for a loose fit. Follow the same to make matching panel for the other side. Temporarily screw these pieces in place with a few one inch long sheet rock screws. Now move on to fitting the next piece forward. There will be a point where the chine starts to curve up about half way forward of the transom. This is the point where the front edge of this piece angles forward at about forty-five degrees to the keel. Every piece in front of this must be done in strips of three to four inches in width to accommodate the double curvature of the forward portion of the bottom. If you are doing a Century or other boat that does not have a concave or convex bottom frames, then it is not necessary to fit the remaining forward section in diagonal pieces. Large panels can be fit, as they were fit aft, since there is only one curve to follow. Back to the concave bottom framed hull, cut and fit these two pieces and temporarily screw them into place as you did the aft pieces. Now comes the harder part of this task, cutting and fitting the diagonal strip planking. Each strip must be individually fit edge to edge and fit well in the keel and chine rabbets. Before you start cutting and fitting these strips, prepare the intermediate frames that will be screwed between the main and auxiliary frames. These are made with flat sawn white oak. They are typically nine-sixteenths by seven-eighths inch if for a Chris Craft. Because the hull is more concave in the forward area, it will be necessary to steam bend these few forward intermediate frames. Set up a bending jig with the curvature of the second frame back from the stem. This will be nearly correct for all the frames creating a bit of over bend, which is OK. Cut the stock for steaming longer than necessary and trim them to length as you fit them in place. Steam the frames and let them cool and dry for a day. Now, run some Okume plywood through the table saw to make three inch wide strips. Cut off a length from a strip to approximately fit the length of the first diagonal strip. Lay the strip edge against the previously fit sheet edge. Mark a line to indicate the angle to the keel and then cut and fit to the keel rabbet. It might take some trimming with a block plane to get the right angle. Once it fits, place the keel end in the keel rabbet, push the strip against the concave bottom frame. Then, mark and cut the chine end. You need to do this because the overall length is longer when the strip is pushed against the frame. Cut the strip and then hold it in place against the edge of the previously fit sheet.
Do the edges fit tight? If not, take your block plane and trim the edge of the strip to fit reasonably well. Continue this process with additional strips until the forward space on both sides is filled. During this process, cut and fit and temporarily install the intermediate frames in their correct positions with one inch sheetrock screws. Pre-drill a hole for the screw to prevent splitting the intermediate frame. These frames need to be included in the fitting process to create correct shape for the forward diagonal planking.
When you have satisfactorily fit all diagonal planking strips, remove them from the frames, setting them aside in order. Now, apply a couple of coats of Smiths penetrating epoxy to both sides and all edges of the plywood inner planking. Let it dry for a day. When it is dry, apply a bead of 3M 5200 on the outer edge of the frames and spread it to the full width of the frame with a putty knife. Notice: This is where you will learn how messy 3M 5200 is. Keep at hand a generous supply of paper towels and naphtha for cleaning purposes. If you dont immediately clean your self and/or tools of any unwanted 3M 5200, it will soon be everywhere. You will carry and spread it to every place you touch for the next 12 hours. It will find its way to places you never realized you touched. You can wear latex gloves, but the mess will still find its way to your clothes, tools, car seats, and pets, so clean off any mess immediately. Enough warning, if you heed it.
Where were we. Oh, yes. With the 3M 5200 spread on the frame outer edges, lay the plywood panels and strips carefully in place one a time. With a straight batten long enough to span from keel to chine, draw a line to indicate the frame location using the marks on the keel and chine you made earlier. Now, using these frame location lines, nail the plywood in place with one and one quarter inch copper ring shank nails. Use enough nails to draw the plywood snug to the frames from chine to keel. When you have fastened the last piece in place, you are finished and the work should appear something like the picture below.
hat is one method of doing the inner planking job. If you wish you can install the intermediate frames at this time. Apply 3M 5200 to the mating surface of the frame and screw it in place with one inch sheet rock screws. As you screw the outer planking in place into the intermediate frames, these sheet rock screws will be removed. Some restorers screw the intermediate frames to the plywood before screwing it to the main frames. Take your pick.
The job isnt all that difficult, although somewhat messy at times and you can feel good that you are providing a bottom for your boat that will last for many years to come. Now, the bottom is ready for the outer planking. Stay tuned. Next time we will take on the challenge of fitting new bottom planks that will make you proud.
If you have read this far, you are likely one of those boat lovers who would rather restore than destroy an old wood boat. My hat is off to you. Spread the word of restoration and protect the unrestored boats. If you have a question about restoring your project boat, give me a call at 715-294-2415 or e-mail me at Heggensj@Centurytel.net. I look forward to hearing from you.