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I am most likely going to replace the two bottom planks on my 1958 sea lancer. It looks like the planks on the boat are made of one piece that is as long as the boat. I dont know of any place to get marine plywood that long and assume that I will have to use two or more separate pieces. How have others dealt with this? Would I use butt joints over a rib? Thanks, Jim

Jim,
All planks on these style boats are scarfed together, Planks are ground across at an angle usually about 4" back from edge and mated together. These joints were then glued and screwed to ribs.
The glue used on these scarfs on many boats has started to dissolve, I have had several scarf failures and when those came apart there was no evidence of glue of any kind. And for some unknown reason the factory set them so that they open into the force of the water instead of the other way. I think 50 + years on most of these hulls has outlived the original glue. Everyone should be aware of this and keep a close eye on any scarf joint that starts to show itself under paint or varnish.
Here are a couple of shots showing scarfs, one from a boat I owned that had a major scarf failure and sank- green bottom where you can clearly see factory scarf face that showed no evidence of glue, it opened into the water and broke back to next strongest rib. and the other shot of repairs being made on my current hull with new scafs visible.

You can remove any bad panel until you get to solid wood and create your own scarf there. Hope this helps. there were no butt blocks that I an aware of.
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That is a big help. It makes a lot of sense to use scarf joints. Thanks, Jim