joinining boards learning to make puzzle vs scarf joints

I am making a hybrid wood duck 12, not from a kit. I am planning on cutting out the pieces from marine-grade plywood. Since these pieces are bigger than the boards they will need to be joined together. It seemed like the general recommendation was to use puzzle joints to connect them as this would be a more “foolproof” method. I am a little nervous about this step as I have not done this before and I feel like there are many opportunities to make mistakes when cutting the joints. Most tutorials use precut scarf joints or precut CNC puzzle joints. I had talked to someone who made this boat and I thought that they cut puzzle joints by hand, but I now think I am mistaken, as it seems like most tutorials say you need to use a CNC machine to get the cuts tight enough. I do not have a CNC machine but I can ask around to see if someone has one that I can borrow or rent. However, I am beginning to think learning how to do a scarf joint may be better. Does anyone have any good advice for the most foolproof method for joining the pieces together? I am willing to use scarf or puzzle joints and I know I will need to practice either skill a few times before using it for this project to make sure I can do it correctly. Any suggestions on what technique to use, how to learn it, and how to practice so I can become proficient in this skill would be greatly appreciated. 
Also is it better to join the plywood boards before or after you cut out your pieces? 


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RE: joinining boards learning to make puzzle vs scarf joints

Hi Annym,

If you are not building from a kit, then you should do a scarf joint or a butt joint.  As you point out, a puzzle joint really is a technique where you have a CNC machine.

Further, as you point out, it is best to join your board prior to the your final cut.  The reason is mostly about not ending up with a piece that is too short due to a scarf or butt joint that needs to get reworked a bit.

On the scarf joint,  at 1:50 gives you an overview of a scarf joint.  This video shows how to do a scarf

For this boat, I think a butt joint could also work.  For a butt joint, you can see the following video:    For taping, I use two pieces of 3 inch fibreglass tape overlapped 1 and ½ inch over the joint.  like the video (which uses one strip of glass over the joint)  make sure you set that tape up to be on the inside of the boat.  

I have built kayaks with butt joints before and had no issues.  It is important however, that the joint is eventually glassed on both sides.  So on a kayak, the glassing on the outside occurs when you glass the assembled hull, and usually there is another layer of glass on the inside of the boat as well.

i hope these videos and description helps



RE: joinining boards learning to make puzzle vs scarf joints

Stricyly speaking, you don't need a CNC machine to cut puzzle joints, but if you don't have one you need a lot of skill. It's high precision joinery using thin plywood. While theoretically it could be done using nothing but unpowered hand tools, a router would make it a lot faster. But then you have to make a precision router jig. You'd also have to practice and dial in all the jig, router and clamping settings, In the time to do that you could have cut the scarphs for 5 or 6 boats. So take Howard's advice and go with a different joint. Based on my WD12 build, I'd go for a scarph instead of a butt joint and try to keep any joints towards the middle of the boat. This is because of the extreme twisting at the ends of the boat.

If you really want the scratch build experience but with puzzle joints, CLC sells full plywood sheets with precut puzzle joints. You simply glue them together, then do the layout and cutting as if you had an extra long sheet of plywood. I used them to build an 18-ft schooner and they worked very well. Since I live within easy driving distance of CLC and could jut pick them up at the store, they weren't that much more expensive than just buying the wood. Your situation may vary.



RE: joinining boards learning to make puzzle vs scarf joints

I have made similar puzzle joints by putting one piece over another then cutting "puzzle" pattern with a scroll blade on my saber saw.  It gives a joint with the smoothness of your hand and a gap of the saw kerf. Which can be filled with epoxy thickened with wood flour providing a dark line. A layer of cloth and epoxy on both sides and it was done.    A scarf joint is easier to get a finer line. Puzzle joint has a lot of opportunities for sloppy joints. 

Otherwise you have to cut and sand one side then place that cut piece over the other and scribe the line with a fine knife. Then cut the second, proud, and sand down to the line. Sand both to fit. 

RE: joinining boards learning to make puzzle vs scarf joints

   Thank you all so much for the information! I think I will try using a scarf joint (after practicing a few times). 

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