How come there are different rules for the same vehicles?
Take the Sailing Ship, for example. Ultimate Equipment lists it as being 20x75 feet. The Skull and Shackles Player's Guide, on the other hand, lists it as 30x90 feet.
Since hit points are based on the ship's size, the former has 900 hp while the latter has 1620 hp.
Why is there a discrepancy? Which set of stats should we players/GMs be using?
Sailing ships come in a dizzying variety of designs and sizes. Even two "identical" ships can have different characteristics. Those ships listed are probably "typical". If you're buying, shop around. Otherwise, just pick one at random.
The "ship system" is, as the "Kingdom Building" system very abstract.
So a lot of place for homebrew stuff. :)
Not only is it abstract, it is inconsistent, which is what bothers me.
If the sailing ship from Ultimate Equipment has 2 masts and 30 squares of sails, that would mean the Man's Promise (a 3 masted sailing ship) would have 45 squares of sails, right? Then how come the game designers tell you to use the sailing ship stats from the S&S Player's Guide, which has a whopping 90 squares of sails.
What's more, Ultimate Equipment makes it clear that sails have 5 hit points per square, and its sailing ship abides by this, but the Player's Guide sailing ship has 360 hit points for some reason (if it truly has 90 squares of sails then it should have 450 hit points).
What's worse, the Dominator (a four-decked, four masted sailing ship) has 120 squares of sails with only 960 hit points. If it truly has 120 squares of sails then it should have 1200 hit points (double normal due to magical treatments).
Why on earth isn't anything proportional or consistent!?
This ship and this ship are both single mast sailboats. Clearly, that doesn't mean they should have the same amount of sail squares.
"Sailing Ship" is a bit like saying "car".
To follow the car analogy, at the moment you're comparing a convertible and a minivan and complaining that they are both referred to as "cars" when clearly they do not have the same trunk space.
As a very general rule I'd say that a larger ship will field larger masts which would translate to more sail squares. However there can and will be significant variations and exceptions to this rule based on who built the ship, how it's designed, and what its intended purpose is.
Finally, the sailing ship in Ultimate Equipment is described as between 75 and 90 feet long, yet the cost stays the same. There's flexibility written into the description, most likely specifically to give the party and the GM some leeway in how he/they describe the ship in question.
Look no further than the reason of inconsistency between books and APs. You'll find the ACs different by 10, too.