Bag
Bag turns up in 2 fields: boilers, maritime. Here is what it means to the people in each one.
Bag in boilers
A local bulge in a boiler tube, furnace or shell plate where overheated metal has been pushed out by pressure.
When scale, sludge or oil stops water from cooling a patch of metal, the steel overheats, loses strength and bulges toward the fire side under boiler pressure. Inspectors measure the size and depth of a bag and decide whether it can be heated and pushed back, patched or must be cut out. A bag is a warning that water treatment has failed.
"Inspector found a bag on the furnace about the size of a dinner plate, right over the burner, the boiler's down till it's repaired."
Bag in maritime
To drift or drop away from the intended course due to wind or currents.
A vessel experiences this when leeway pushes it off its steered heading. It happens most often when sailing close-hauled or when a strong crosscurrent pushes against the hull. Navigators must calculate this lateral drift and steer slightly upwind or upcurrent to compensate and maintain the true track.
"We're starting to bag to leeward, so bring her head up a few degrees."