I'm new to the hobby of wooden boats. Over the summer I was able to pick up on older boat that needs alot of work. I got it for a good price. It is going to need a new bottom due to the previous owner allowing rain water get in the bottom and sit and it eventually rotted out. My first question is what is the best way to take the old bottom off and secondly are there any templates on the web so I can cut out and place a new one. The boat I have is a 15' Tatman. Sorry I don't have any pictures of it right now.
No need for a template Wayne. Here's how you do it.
Turn the boat upside down.
Remove the outside chine batten.
Run a snap line across the bottom where each of the frames are. You don't have to be exact. You want to know where you have to be careful. Use a jig saw to cut the bottom out between each of the frames. There should not be any screws in this area because there is nothing to attach to, so just go for it. Leave about 1 inch of bottom from the frames and the inside chine log.
This will allow you to see the chine log and the frames as you work on the next stem. Use a router with a flush trim bit. Set the depth to cut the bottom and the roller bearing to rest on the frame or the chine as you cut. The frame and the chine log will guide the bit to cut the bottom off flush with the frame and chine log.
Next, remove all of the screws that are in the bottom. Then use a chisel to carefully pry or chip away the bottom from the frames and the chine log. The bottom was likely glued down with 3M5200 so be careful that you do not chip away the chine log or frame. I the glue is hanging on then chisel out the bottom leaving the last layer of plywood.
Once that is as clean as possible with only 1/16 or so waste left from the bottom you can belt sand the bottom frames and chine fair the same as you would if you were building a new boat.
Forgot to say why you don't need a template. Once the boat is ready for a new bottom you lay the scarfed plywood on the boat. Use the boat to scribe a line where the bottom meets the chine. Take the plywood off the boat and cut it to shape leaving about a 1/2in (or an inch of you are a chicken like me) of waste outside the edge of the line.
Then screw and glue the bottom to the boat. After the bottom is installed you trim the waste to the chine of the boat. I use a power plane. Some use a flush cut router bit with an oversized bearing. Either way leave a bit proud so you finish with a sander to match the boat sides to the bottom.
1/2 marine grade fir. I suggest you glass the bottom with 20oz triaxail fiberglass cloth. At our shop we also coat the inside boat bottom with Durabak-18. This way the bottom is soaked in epoxy and protected inside and out.
Some on this forum like Plascore. While I am a fan of Plascore for decking and its lightweight qualities, the bottom of the boat is not the place to save weight. Good boat design/performance requires a low center of gravity. The extra weight on the bottom improves performance in the water.
Some will argue that adding ballast to the bottom (insert beer joke here) is easy to do; however I believe that a boat should perform at its best when loaded light or loaded heavy. Many white water day trips in the Northwest are not expeditions and require very little gear.
Randy has made some good answers to this question. I follow up by suggesting that the 1/2" Fir bottom be fiberglassed with 6 oz. S type fiberglass and epoxy on the interior of the bottom prior to installation. If the owner desires they can then varnish over the epoxy/glass bottom when installed. It is very had to see the glass. Or, the epoxy/glass bottom can be coated with graphite prior to bottom installation. Then the boat sides and interior can be oiled. Our feeling is that the bottom must have fiberglass on the interior for the best impact resistance.
Some time ago one of the members of this blog punched a hole in the bottom of his boat on the Rogue River. It was a beautiful Sapele plywood boat. It would have helped if the boat had the type of coating I am suggesting. It is our feeling that Douglas Fir plywood is the strongest plywood for boat bottoms especially when coated in the manner suggested.