scarf joint question


I want to use cherry for the gunwhales on my next s&g canoe but I can't get it any longer than 10 ft. I plan to make them 3/4 by 1 inch. The scarf joints on the 1/4 in. ply are 2 inches long. Does this mean the joints for 1 inch should be 16 inches long? This doesn't sound right to me. Is there a formula? Thanks for any replies.
It's been shown that a well-glued 8:1 scarf can essentially create "seamless" plywood as far as flex and strength go. For dimensional lumber like gunwales, that's typically about as short as you want to go and often a 10:1 (ten inch scarf for 1" stock) or a 12:1 scarf are better. In some cases (aircraft) these ratios will be specified, to make sure your wing spars don't come apart.

I've owned factory-built canoes which came with gunwale scarfs as short as 3"-4" on 1"-1.25" stock. If they are positioned in areas that don't get very much bend, they can work, but eventually they usually come apart. It's generally well worth the time to make the longer scarfs if you want them to work and hold up well.
Todd makes a good point about length of scarf and placement.

I have an OT Yankee in my shop that had scarfed joints, pic attached, they did it with a jagged edge which most don't do today. The joint is about 1/3 the length of the canoe and you reverse sides. This Yankee was built in 1929.

I think E.M. White was most known for scarf joints on his 18 1/2' and longer canoes.

Good luck,


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