5/3/14 Bottom Completed
The most involved part of the project is now complete... a new bottom. I'm very pleased with the results. The new bottom is made of expensive Okoume marine grade (BS1088) plywood. I choose the Okoume because the bottom of this boat is convex shaped with the bow end having some extreme curvature requiring a flexible marine grade plywood. The Okoume is more flexible than the Meranti which I would have preferred as it more rot resistant than Okoume. The Okoume is a far better option than the original fir marine plywood bottom.
The stern to amidships portion of the bottom consists of one layer of ½" Okoume while the bow to amidships portion consists of two layers of ¼" Okoume laminated together with epoxy to allow shaping the bottom to the more extreme curves of the bow. Each side consists of two and one-half sheets in length scarf jointed together. The Okoume is fastened to the stem, keelson, transom, chine and bottom stringers using 3M 5200 marine sealant/adhesive and 1-½" #8 Silicon Bronze wood screws. Screw holes were filled with epoxy thickened with low density filler. The bottom was then sanded, primed with Pettit Tie-Coat, and finished with Pettit Copper Bronze Antifoul bottom paint as when delivered new from the factory. The project involved:
- stripping the entire interior of the boat and flipping it upside down
- careful removal of the existing bottom and splash rails
- repairing the rear most bottom frame
- repairing 3 bottom stringers on the bow end of the starboard side that had been cut off in a previous repair
- repairing a single bottom stinger on the bow end of the port side that had rotted
- making a spiling template of the bow portion of the bottom (this portion of the bottom had to fit exactly in-between the the keelson and chine as the chine is overlapped by the bottom strake in the bow end while it is flush at the stern end allowing me the ease of simply overhanging the bottom and trimming to its final shape)
- scarf jointing together the new plywood bottom pieces and shaping them to fit the bottom of the boat
- laminating the new plywood pieces together ( the bow end of the bottom of the boat consists of two layers of ¼" Okoume marine plywood layer)
- fastening the new bottom to the boat and filling the screw holes (about 1,200 of them)
- replacing the keel
- reinstallation of the splash rails
- installation of hull thru-fittings
- rescribing the waterline
- sanding and applying the bottom coat
With the old bottom off I began installation of the bottom by repairing the bottom stringers in the bow of the boat. It appears that boat had been in a collision of some type far in the past resulting in damage to the starboard side of the bow. That repair effort lead to the all three of the bottom stringers being cut to allow placement of a large plywood butt-block to join the existing bottom with a new piece of plywood cut to fit the damaged area. On the port side, the bottom most stringer had rotted. For the repair I scarf jointed new Philippine mahogany bottom stringers to the good portions of the existing stringers.
I finished the job by using flexible battens to check for fairness of the hull curvature and used a hand plane to shave down the new bottom stringers until I was satisfied with it.
Next was repairing the rear most bottom frames. I found all the frames to be in great shape with the exception of this one. It was rotted on the ends and quite a few of the screws that secured the bottom stringers to the frame had broken-in-two. This indicated to me that this last frame is under a lot of stress which makes sense as the prop is underneath it. I decided that not only would I replace it but also would double it up for added strength.
Next I primed and painted the underside of the frames with bilge paint as it would be difficult to paint those surfaces from the inside after the new bottom was on.
With the bottom stringers and frames all fixed up, next up was actually laying down the new bottom. I began by scarf jointing the first 12 feet (beginning from the bow) of the bottom together. The first 12 feet consist of two layers of ¼" Okoume. I scarfed them using a hand plane and then epoxied them together with a make-shift clamping device using wedges. Unlike typical wood glue, epoxy requires light clamping to hold the shape, so that the epoxy itself does not get completely squeezed out of the joint.
Next was cutting the newly joined pieces to shape. Since the bow end of the bottom of the boat had to fit in-between the keelson and chine, as the lowest plank overlapped the chine ( I did not have the luxury of simply overhanging the bottom over the chine and trimming it to fit after it was secured to the bottom) I resorted to spiling. Spiling is a method commonly used to shape planks on a carvel planked boat. It is far easier to show how to do than explain how to do it so I'll be brief here. The spiling method I choose was:
- using a sheet of cheap ¼" underlayment plywood as a spiling template and clamp it to the frames trying to capture as much of the curvature as I possibly could
- marking the curvature points using a compass
- unclamping the template and placing it on top of the new plywood and transferring the curvature points to the new plywood
- using a batten to connect the curvature points resulting in a line duplicating the shape needed to properly fit the bow
- cutting out the shape
- tweaking it to its final shape using a block plane and a few test fits
Note that I used a track saw for cutting the line adjoining the keelson resulting in a nice clean straight cut.
With completion of shaping the bow end of the bottom pieces, next up is the process of fastening the new bottom to the boat. First 3M 5200 marine sealant/adhesive was applied to the bottom stringers, keelson, chine, and stem. Next the bottom was carefully laid down and clamped in a few strategic places to temporarily hold it in place. Next Silicon bronze screws were driven in. Screw holes were predrilled using Fuller screw bits. As the screw driving process proceeded, I used a clamping device consisting of a 2x4 frame bolted web that went around the perimeter of the hull along with wedges to push the new bottom piece up against the bottom stringers before driving home the screws. I used just enough screws to hold the first layer in place until the second layer is applied, which is securely fastened down with screws spaced every 2" along the stem, chine, keelson, and transom, and every 4" along the bottom stringers.
Next I laminated the second layer by first applying a coat of epoxy to the adjoining surfaces of each layer, then applied another coating of epoxy thickened with silica (the silica acts as both a gap filler and an adhesive) to the piece already attached to the bottom.... sort of like a sandwich I guess you could say. Then I repeated the fastening process for second piece overlaying it over the first.
The stern bottom piece was next. This was far easier to apply than the bow end. I planed a scarf on the one edge to laminate against the new bow end bottom piece and just butted the keelson edge to the keelson and let the transom and chine edges overhang. Notice I temporality inserted a dowel wrapped in wax paper into the limber holes so they would not clog up with adhesive in the process of laying the bottom. Later I trimmed off the the chine and transom edges by rough cutting with a saber saw and then hand planed the edges with a block plane to its final shape. I used my wedge clamping system to epoxy the scarf joint together.
With the bottom now on, I replaced the badly worn keel and then filled all the screw holes with epoxy mixed with a low density filler.
Next the bottom was sanded, smoothing out the scarf joints and screw hole fillings. Then the splash rails were reinstalled. I reused the old splash rails. They were not rotted, although a bit banged up. There were about 80 screws per side holding these to the chine and I thought best to reuse the old chines so I did not have to drill 80 new holes into the each chine. I used silicon bronze screws the next higher gauge (#10's) than the original (#8's) and a ¼" longer that were able to get a secure hold in the original screw holes I used a bedding compound on both the splash rails and the new keel. I found the front ⅓ of the splash rail on both sides of the boat had been replaced some time in the past. This portion did not have a channel plowed out on the underside which produced a gap along the edges that had been filled, so I plowed out channels on both sides using a spokeshave after which I filled with bedding compound. The result was nice, clean, closed edges along the hull.
With the splash rails mounted it was time to reinstall some of the thru-hull fittings. The bronze drain and bronze engine raw water inlet both needed to be installed. It pained me to to drill these big holes right through the new hull I just installed. I carefully measured for their locations and drilled using a good sharp forstner bit. I coated the insides of these fittings with plenty of bedding compound to avoid leaks and fastened them in place and cleaned up the excess bedding compound.
Next up was scribing the waterline. Two thirds of the waterline was marked on the bottom plank, so the process only involved extending that scribed line from the splash rail to the stem over the bow end of the new bottom. I also scribed the waterline across the bottom of the new transom. The process was quite easy using a laser level. I simply aimed the laser level over the existing scribed waterline and leveled the boat using the adjustable boat jacks it was resting on. With that done I simply masked off the laser line with painters tape and then scribed it with a sharp chisel. I also scribed a second line so the boot stripe was completely outlined (top and bottom). Using the laser level gave me two perfect planes but the resulting outline showed a various in thickness of the boot stripe. I found that thickness of the boot stripe varied based on the steepness of the angle of the hull. I wondered if I should make it a constant 2" wide stripe along the entire length of the waterline. Looking for reassurance that I was marking this boot stripe correctly I called my boat school instructor. He affirmed that it was indeed the correct process and that the boot stripe would look better from a distance than it would with a consistently wide stripe.
The last step was painting the bottom coat. In prepping for the bottom coat, I primed the rudder, prop shaft support, engine raw water intake, and drain with Pettit Metal Primer 6455. The rest of the hull above the water was masked off. I applied a coat of Pettit Tie Coat over the entire bottom and primed metal parts.
The owner choose a cooper bronze antifoul bottom coat as that was the factory original finish. I applied three coats of Pettit 1933 Copper Bronze Antifoul paint. You need to mix this copper bronze paint with an electric drill and paint mixer as it contains a lot of stuff (toxins). I intended to put two coats on as recommended by the manufacturer, however I did three because I was unhappy with how the bottom looked after the two coats were applied. The problem was with how I applied it. Even though the manufacturer says it can be applied by brush, it doesn't look very good. I used a roll and tip technique that was taught to me at boat school that I've had A+ results with for top-sides paint. The result with this paint was lots of deep brush marks and streaky black marks due to all the toxins mixed in with this paint. I did some research online and found that there is a consensus that this stuff ought to be rolled on with a thin foam roller for best appearance, or better yet sprayed on. For the third coat I rolled it on with a foam roller, and it looked much better.