When going to your lumberyard you can find several different plywood grades. It is not always obvious what the different designations stand for but I will try giving you an overview of some of the most normal grades plywood comes in.
Grade N veneers
Grade N veneer shall be smoothly cut 100% heartwood or 100% sapwood, free from knots, knotholes, pitch pockets, open splits, other open defects, and stain. The veneer shall consist of not more than two pieces in 1220 mm (48-inch) widths and not more three pieces in wider panels, and shall be well matched for color and grain.
Grade A veneers
Grade A veneer shall be firm; smoothly cut; and free of knots, pitch pockets, open splits, and other open defects and well jointed when of more than one piece.
Grade B veneers
Grade B veneer shall be solid and free from open defects and broken grain, except repaired smaller splits, openings, knots and cracks or checks. Slightly rough grain shall be permitted. Minor sanding and patching defects shall not exceed 5% of panel area.
Grade C veneers
Sanding defects shall not impair the strength or serviceability of the panel.
Grade C Plugged
Knotholes, worm and borer holes, and other open defects; sound and tight knots; split; broken grains; pitch pockets; plugs; patches and shims shall be permitted. Where grades having C Plugged face veneer are fully sanded, sanding defects shall be the same as admitted under B grade.
Grade D veneers
Any number of plugs, patches, shims, worm or borer holes, sanding defects, and other characteristics shall be permitted, provided they do not seriously impair the strength or serviceability of the panel.
The above list is not a fully description of all the grades available but I am pretty sure it will cover most of the plywood you’ll find at your lumberyard.
Oh, and one last thing. Remember always to buy plywood that are glued with water and boiling proof glue. That is normally stated with a WBP mark.
Happy boat building,
Morten Olesen, Naval Architect