: If I decide to use non-marine plywood it is 8 feet long. Marine ply
: is 8' 2 1/2" long, correct? This means I should add 2 1/2
: inches to the bow end side panels, correct? Thanks in advance.

It is not a matter of marine ply vs. non-marine. It is related to metric vs imperial/ english/ inch measurements. 3mm and 4mm plywood is obviously specified in metric measure. Those panels are likely 2500mm x 1250 mm -- again, metric sizes-- which are roughly 98.4" x 49.2". That is slightly larger than the 1/8" plywood sold in 4' x8' sheets.

But nothing is consistent in the world of lumber where 'nominal' sizes rule. Measure the stuff yourself.

To answer your question: If you are joining two panels for those side panels then each one will need to be stretched out by 2.5 inches, or you can join the two 8' pieces and add a 5" piece.

You have some other options, too. It may be possible to redo the layout of the cutting plan so you can get the extra wood you'll need from a 'scrap' area on another panel. To get the length you need out of 8' sheets you are going to need to piece together 3 pieces--but one can be 8' and the others can be any combination that adds up to the final size. So, if you are using a butt joint with two 98 .5" parts you'll end up with a part 197" long. If you are trying to create that part from 96" plywood you use one 96" piece and add two more to make up the remaining 101". That might be a 48" piece and a 53" piece. Or it might mean using some other combination of lengths.

With butt joints you probably want to avoid splicing a 5" piece to the end of the panel. If ther is a block behind it to reinforce the joint, that block might get in the way of your stem connections. However, if you add your 5" piece to the center of the boat , and reinforce that itha an additional piece of plywood glued to the inside of the boat, then you can significantly strengthen
The cockpit area-- which can be a good thing.

If you are using scarf joints then you'll need to refigure the size of your pieces to allow for the extra material lost to the 2 joints.

Hope this helps.

PGJ