Quick and dirty Spelljammer for D&D Next

Spelljammer is one of the more interesting and original setting and rules expansions for Advanced Dungeons & Dragons, but it hasn't been officially supported for over a decade, since the 2nd edition of the game was replaced with the 3rd. Often summed up as D&D in space, Spelljammer gives a means and possible motives for a standard dungeon-crawlin' adventuring party to fly a ship (like, literally, a boat, though often an oddly shaped one) out into space by magical means to have nautical-themed space adventures.

That crash-summary really doesn't do it justice, as this relatively simple mechanical change has a huge effect on the scope and feel of the game, especially with a GM who knows how to use it. It opened up doors for whole new races and gave great new depth to some older ones. And since its core is just a relatively simple rules mod, it should be easy enough to adapt it for a different set of rules. This has already been done (unofficially) for D&D 3rd Ed., and I ran a game a couple years ago that spent a couple sessions toying with that. Or more specifically, this:

The Dragonfly-class spelljammer my group hired from some Githyanki pirates in Sigil to rescue a party member who'd swapped bodies with a time traveller from the Illithid homeworld in the distant past. I thought we'd better start simple.

D&D Next is the public playtest of what is due to become 5th edition, and my group have been taking part in it. I like it so far, it's simple and streamlined and doesn't wank over combat so much like 4th ed. did. But I'd like to see how well it works beyond the standard Tolkienesque medieval setting the developers have (quite naturally) focused on. So I thought I'd try mashing it up with old Spelljammer for a couple of sessions.

A couple rules changes are necessary, and this is my summary of those. I'm only covering the bare essentials of conversion for now, so that it's playable but I don't waste days or weeks of effort making a perfect fit for rules that aren't set in stone yet anyway. I'll post an actual play report once we've had a chance to try it out, and if anyone else tries this, I'd be interested in your feedback too. I'm working with the 13 August 2012 D&D Next test pack.

Casters & Spelljammer Rating
How fast a ship goes, its ship's rating (SR) is derived from the magical ability of the magic caster sitting in its special spelljammer helm. For the D&D Next rules, assume this works the same way it did in the Spelljammer rules of old, with the following modifications:

1. Add up all the caster levels a character has, both arcane and divine, to calculate SR; ignore any levels in non-caster classes.
2. Using a spelljamming helm counts as using all non-cantrip/orison spell slots for the day until you've had a long  (8 hour) rest. Cantrips and orisons, the sub-level 1 Minor Spells, being both weak and unlimited, are still available to the caster. (I think this should keep things balanced, placing a significant cost on space travel, while allowing mages to retain some usefulness in off-ship encounters without having to schedule a long nap every time they stop.)
3. Characters without any caster levels but with a magicky Specialisation (Acolyte, Magic-User, Necromancer) gain an SR of 1 that cannot be increased by any means, while operating within a crystal sphere. They are unable to spelljam in the Phlogiston at all. Characters with one or more caster levels gain no SR benefit for having these specialisations.

Ship Vital Stats
Some of the core ship stats can go unchanged, others need rejiggling, as follows:

Built By / Used Primarily By: Stays the same, though obviously a lot of races aren't available for Next yet.

Tonnage: Stays the same.

Hull Points: Stays the same, I think. Some seem a little low for such large vessels, so this will be something to test more thoroughly. (EDIT: There are a few options for calculating to-human-scale HP here listed under 'Ships in Wildspace', but I feel those are overly complicated for a quick conversion like mine, so I'm going to take a wild swing and offer the following crude conversion formula: New HP = Tonnage x Hardness - see 'Saves As', below. This should keep things roughly in line with the revised heavy weapon damages I suggest below, although it might make the duplicated use of Hardness redundant, especially for metal ships. See Edit to 'Saves As'.)

Crew: Stays the same.

Manuevrability Class: Stays the same. I also think it might be appropriate to give ships with an MC 2 or more grades better than an opposing ship Advantage to at least some combat rolls, but I'm not sure what. I'll have to play with this a bit more and see what goes in testing.

Landing - Ground/Water: Stays the same.

Armour Rating: 20 - old AR = new AR.

Saves As:  No equivalent of item saves in Next, and since the 2nd ed. rule this is based on is totally defunct now (and was a lame rule to begin with), I propose borrowing the Hardness rule from 3rd ed. (see PHB table 9-9: Substance Hardness and Hit Points, and look for the best match to your ship's Item Save material type). For those few who missed 3rd ed., Hardness is basically how much damage is ignored per hit by iterms made from different substances. (EDIT: In this case, only use it for saving against non-combat damage, since Hardness is also already built into Hull Points.)

Power Type: Stays the same.

Ship's Rating: Stays the same, plus new caster rules above.

Standard Armament: Stays the same, I guess, except there are no siege weapon rules in Next yet. GMs should improvise. I haven't seen any official siege weapon stats in 3rd or 4th eds, so my best guess is to borrow damage rolls from FantasyCraft's table 4.24: Siege Weapons. This should be at the right scale for individual character damage, but then the standard Hull Points definitely seem too low, even with Hardness added in. (EDIT: Alternatively, there's this table of weapons, specifically for Spelljammer.)

Cargo: Stays the same.

Keel, Beam: Stays the same.

Crew Rating: A slight simplification, there are now only three ratings, Green, Average and Trained, ditching Crack for now. Green crews give a Disadvantage to the ship's Initiative (and any other ship-handling checks the GM thinks appropriate), Average crews have no effect, and Trained crews give Advantage to Initiative (and whatever else the GM likes).

EDIT: You can read about our initial attempt at playtesting these rules here.