whiskey plank

“Whiskey Line” is a term used by surveyors.  When a parallel lines (based on compass directions) are drawn on a plot, and are extended far enough (depending on the map projection) they are eventually distorted by their conversion from the three dimensional earth’s sphere to a two dimensional drawing and are no longer parallel.
As an example, pick several places on the equator, go straight north or south, and it is easy to see that the none of the lines are parallel.  Even latitudinal lines, which ARE parallel, are distorted when converted to flat projections. 
At some point, to adjust for the distortion, one of the lines must be to jogged laterally far enough to again allow for them to again appear as parallel lines.  That jog to the side came to be called a whiskey line because it looked like an attempt to correct for a mistake made by someone who was drinking on the job. 
Boundaries were first deliniated by geographical features, rivers, mountains, watersheds, oceans.  But with the development of standardized measuring equipment, more and more boundaries came to be established using the compass. 
Look at maps of counties in the plains states where there are often few geographical boundaries to set apart one county from another.  Though the flat country is condusive to laying out a grid, whiskey lines abound.  Notice that some counties and states are narrower on their north sides than the south.  Whiskey lines can be found even at very small scales, even in residential property lines.

There is a tradition of celebrating the completion of almost all major construction projects, not just boat building.  Perhaps you’ve seen Christmas trees that have been hoisted and secured to the top of buildings where the final floor has been finished, or a bridge where the ironwork is done.  They don’t call it a whiskey tree.  Finishing a building doesn’t require the same degree of contortion of the materials used in it as a boat does.  Buildings are much much more regular. 

In order for the “Whiskey Plank” to fit that final space on the hull, it can’t be square.  It has so many angles to it, if you didn’t know it’s strange shape was carefully crafted to fit that irregular space, you might think it was completed by …a drunken shipwright.

It may be that the origin of the term whiskey plank is not because whiskey is drunk after it is fitted, but that it’s shape is not …sober.