Bulkheads and stringers stiffen the hull and carry the loads from the rig, the keel and the engine. They're fastened to the hull by tabbing, fibreglass laid over the joint. If the tabbing lets go, the hull loses the stiffening it was designed with and starts to flex.
Why it fails
Dave Gerr, writing for Good Old Boat, explains that tabbing makes a secondary bond, which is never as strong as a primary bond. Doing it well needs the surface ground, vacuumed and cleaned, and every sharp inside corner rounded with a fillet. Skip that and the bond can be pulled off by hand; his photos show a bulkhead panel that did just that.
Once a bond starts to crack, flexing grows the crack. Gerr's line on engine beds: sooner or later, cracks occur, and as the flexing continues you have a true structural failure.
How a surveyor checks it
Follow every bulkhead and stringer you can reach where it meets the hull. Look for tabbing that's lifted, cracked or separated, and tap it: a hollow sound along a bond means it's let go. Pry gently at lifted edges.
Then look for movement. Doors that bind, cracked joinery, and a chainplate bulkhead that shifts when the shroud is loaded all point to it. On a sea trial, listen for creaks. Check stringer cores for water where you can.
What decides the grade
What's loose and what it carries. A structural bulkhead or stringer that's separated or moving is an A. Lifted tabbing over part of its length, or a wet stringer core, is a B. A small lifted edge on a sound bond is a C.