Here's a neat trick .
It was taught to me by my landlady, ( she does stained glass as a hobby ). Take a piece of black artists chalk and shave it over a basin of Warm water big enough to put your ship in . Now measure the drawings of the ship from deck to boot stripe top.
Now mark the ship at the same spot with a " Sharpie" fine point , once aft , once forward , both sides! . Now set the ship in this water and the black chalk will stick to the hull showing you where to paint your line .
Why the warm Water ? well , she says the warm water seems to soften the charcoal or chalk or something , making it stickier and easier to do this way .She uses it to dip large sheets of glass so both sides have a cut line !
Hey , guess what ? It has worked flawlessly for five ships now , for both the bottom then the top , of the boot stripe or just plain waterline . Kudos to Crossover ideas ! Aha , By the master of Tanks built to float and carry things , Why , of course , it's the - - - - Tanker - Builder