Another complication is that almost all steel ship models show the ship in a condition minutes before the dry dock is flooded. Then marine growth, dockside rubbing, friction while underway and general scumminess take over. If you are ever around a ship or boat newly made dry; they stink. Not an attractive thing to model.
The Cap'n will correct me if I'm wrong, but during the War period a boot stripe, or boot topping, was confined to Navy ships. The vast majority of auxiliaries, those that sailed under the Maritime Commission and were not "USS", didn't have them.
I'll opine that 10 markings at 6" each equals 60 inches, or five feet. That's a pretty common dimension.
One small thing, I've never heard any suggestion that on Navy ships in WW2 they were any color other than black.
I've always assumed that the bigger the ship, the taller the stripe, although not in a linear fashion. Some smaller ships, as the Cap'n notes like a T2 tanker, grossly change their displacement when laden or unladen. I've seen photos of T2's, which had the engines way astern, sticking the bow completely out of the water when empty.
Other ships like aircraft carriers probably don't change much.
Another thing. Be aware that a boot stripe does not have a constant width. In other words, in almost all cases an continuous width of tape all the way around as a mask will produce poor results.
Rather, while sitting on an even keel a boot topping will encompass the vertical dimension between the least and highest displacement, plus some. Pretty much the only way to get this right is to scribe a pair of lines representing those extremes (plus) on the hull while it is rigidly mounted on (or under) an even keel.
As for what color red to paint the bottom. I don't know the answer, no doubt others do. I find the smaller the model, the more true red I go. But that's just me. Tamiya Hull Red is brown, Tamiya Dull Red is redder.
Rustoleum primer is brown however quality issues with the paint itself have lately made me avoid it.
Ace makes a knock off version, is grossly red but I think looks swell for the red primer that used to be ubiquitous on merchant vessel decks. Here's a model with both.
