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Posted by RonW on August 22, 19103 at 17:09:29:
In Reply to: Re: Butt or Scarf? posted by Kirk out East on August 21, 19103 at 19:46:52:
How about a 3rd. method.I am building a 17 1/2ft. drifter, stretched 16 footer. And like you I do not like the looks of the butt joint, particularly on a stress skin boat like the drifter where they can be seen, although I believe they are probably the strongest and easiest.I also agree on the bottom they impeede water flow and this is a area where dirt will collect and suck moisture from the air and can lead to rot. When plywood manufactures used to make scarf joints, they had machinery and jigs to make accurate and fast joints.So in thinking about this and researching it, I am going to make a double step butt joint on 3/8 plywood using a power planer.Total of 6 1/2 inch lap.Set the plywood on saw horses with 2x4s under for support,and put a screw in each edge to hold firmly, then screw a 2x4 down through ply and into bottom 2x4s to hold everything tight.Use a power planer against the 2x4 as a guide to remove 2/3rds of the ply, then reset 2x4 as guide back the distnce of your planer (3 1/4 inch.)and remove 1/3 of ply, do same to other end of ply and you have a double step joint where the 2 plys overlap each other by 6 1/2 inches.They should fit together perfectly and if not you can trim either edge or even center cut with metal straight edge and razor knife.On 3/8th ply this will give you a 17 1/3 to 1 scarf, much better then a 8 or 10 to one.Easy and accurate and I think you should use the scarf in the high teens.I am using glen-l epoxy grip to glue panels together, and am using 3/4 inch copper tacks to lightly tack joints, with 2x10 under joint for support and tack right through ply into 2x10, let set a day, then slightly set copper tacks, turn over and clench over ends, same as clench built boats.Leave copper tacks and just cut through them,when cutting ply, even a jigsaw would have no trouble in cutting through.This should easily bend as one piece of ply with no hard spots.This will give you a 6 1/2 inch lap to put screws and boat nails into to attach to chine and shear, lot of bearing area.In looking at power planers, all of them have the motor housing sticking out pass the planer, and most have housings that are rounded or at a angle, I did find a delta that the housing is square with the planer and would slide down the 2x4 as a guide accurately.It is $150.My research in this also lead me to find out that some of the plywood kit boats are now using a series of step joints instead of the angled scarf. So this is what I am going to do in about a week.
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