Looking at the latest info on Spelljammer coming out through an interview with Lead Designer Chris Perkins it appears that we will not be getting much, if any ship combat rules at all.
"For ship engagements in this setting, we're basically using the core rules of the game. [...] ship-to-ship engagements is, in Fifth Edition—which is more a theater of the mind — there aren't things like Facing or Turning or anything like that. We're not trying to create a game-within-a-game where suddenly you're no longer role-playing, you're engaged in some tactical, "Where's my ship exactly in space" kind of game.
It's kind of like two groups on two small islands, separated by shooting arrows at one another. A ship becomes a platform on which the encounter takes place, like a dungeon room a room in a castle. You can attack the ship, of course, and the ship can take damage, but for the most part, ship-to-ship engagements really have to do with the crews on board. And, despite the fact that your ship may have a catapult and ballistae and whatnot on board, it's still pretty hard to trump a Fireball spell or a Lightning Bolt spell. The characters have so many other options at their disposal for disposing of shipboard enemies. We stay very much in the theatre of the mind with this product for the sake of keeping encounters fluid and fast."
This puzzles me in two respects - first, what is the existing combat system except 'where is my PC exactly' and second, why on earth am I shelling out $65 a pop for the boxes of ship minis for if they don't do anything?
I think there is a core point I agree with in that people sat to the table to play D&D Spelljammer, not some tactical space combat game, sure. However, I think giving people a frame to understand how to stage awesome ship-on-ship combat, when that can be 3D, when it can be two ships flying upside-down over one-another, when it can include ramming from weird angles, all the wrinkles and weirdness - that would be fun.
Looking at the rules as much as has been deciphered from the images of the Spelljammer DM screen, we could read it as pointing to a 'range bands' approach to distance (ships can appear 250, 500 or 1000 ft away at the start of an encounter) or just thats how far out the foe appears, take it from there.
The stat-blocks as they appear in the Spelljammer Academy adventure on D&DBeyond strongly resemble what we have seen in Ghosts of Saltmarsh - AC, Hit points in the 100's (not hull points in the 10's), a damage threshold, speed in feet, cargo capacity, crew and keel/beam dimensions. Key is the fact that the siege weapons just need crew and can fire every round with them. From testing ship combat rules before, this was the major 'speed bump' in AD&D Spelljammer.
Ship schematics and stat blocks from Spelljammer Academy on D&DBeyond
Do these stats imply a really slow, boring combat? How long would it take these ships to fight one another to destruction?

