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J-4 or J-5 adventure vessels?

Two hundred tonne hull.

It might get a little tight.
It does. Build at 199Td and it's single-handed, plus a gunner if desired. RAW you get two staterooms and one ton of cargo, instead of four SR and three tons of cargo. The fuel shortfall trick allows it to have the same SRs, cargo, and air/raft of a Type S so it can be plugged into scenarios calling for a Type S but needing J4 for whatever reason.
You seem to be running into 2 problems simultaneously.
  1. The ship isn't football/eye shaped. It's circular saucer shaped. The square cargo deck and hexagonal engineering/crew decks fit into that circular saucer shape so it can have a convex ventral and dorsal curvature to it.
  2. The crew deck sits on top of the cargo deck while the engineering deck is "half and half" of those two decks
    Deck 1 = Crew Deck
    Deck 1.5 = Engineering Deck
    Deck 2 = Cargo Deck

So in this ship, the cargo space is 60+5 tons (the 5 tons is a set aside for the mail vault).
60 tons = 120 deck squares = 10 x 12

The collapsible fuel tank is 0.6 tons when it is not filled with fuel. This amounts to 1.2 deck squares worth of area (basically a 20% slice of one side of 6 deck squares in a row in a straight line) along one edge of the cargo bay. You've got like 8x2 squares (plus a little extra) devoted to the collapsible fuel tank, which would be like 8 tons and capable of expanding to hold 800 tons of fuel (clearly way too much).

The crew deck is composed of:
  • Bridge: 20 tons (4 tons actual bridge, 4 tons avionics, 2 tons ship's locker/airlocks, 10 tons of access corridors)
  • Computer: 3 tons
  • Staterooms: 8 tons
  • Fire Control: 1 ton
  • = 32 tons
This amounts to:
Bridge: 8 squares deck space
Avionics: 8 squares deck space
Ship's locker: 2 squares deck space
Dorsal Iris Airlock: 1 square deck space
Ventral Hatch Airlock: 1 square deck space
Corridor Access Spaces: 20 squares deck space (distributed as needed between all decks)
Computer: 6 squares deck space
Staterooms: 8 squares deck space (some of which is corridors)
Fire Control: 2 squares of deck space (most of which is in the turret above Deck 1)

The engineering deck basically amounts to 29.7 tons of drives or 60 deck squares of space.
Divided up into 6 "repeating module" sections of 60º each, you wind up with 6 discrete "rooms" of 5x2 on the engineering deck.

One of the problems you're running into is the idea that you have to have a ring of access corridor space between all the different engineering spaces. You don't. All you really need is a "half deck ramp" (or steps) down from the crew deck to the engineering spaces. Set things up such that you only need 1 half deck access every 2 vertex of the hexagon instead, saving a tremendous amount of wasted space (so only 3 spokes instead of 6 plus ring corridor).


Up on the crew deck, you move the Bridge area from being along a side of the hexagon into an open corner vertex between the half-deck ramp/stairs down to the engineering spaces. So you wind up with something more akin to this instead (obviously the scale is a mess here, but I'm just doodling on your schematics):


Something like that would probably wind up working out better on the deck plans needing to fit into a 3D space.
Might need to flip the cargo bay to put the mail vault on the right of the image, but other than that ... :unsure:

Oh and access corridors ought to be a single deck square wide, not 2 squares wide.

Hmmm ... the cargo hold is 65 tons and the crew+engineering spaces are 61.7 tons, with the remainder taken up by the fuel purification plant (another 5 tons) and fuel. So it's kind of like 1/3 cargo, 1/3 crew access spaces, 1/3 fuel systems.
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It's coming in more like 300 tons than 200 tons, back to the drawing board possibly.
My is coming in at 300 tons also - (for Cepheus Engine)

TL 11 Explorer Modified
4,200 m^3
44m long, 11m diameter
14.7 decks: 1 (0.7) Electronics, 2 Bridge, 3 Ready Room, 4 Quarters, 5 Vault, 6-12 Fuel (7 Decks), 13-15 Engineering

300 ton (6/6 Hull/Structure) 52 Weeks Build Time
Streamlined (Fuel Scoops included) 13.2 MCr
Armor TL 10, 15t, AV 4, 2.4 MCr
Self-Sealing 3 MCr
Stealth -4 DM, 30 MCr
Total 15t, 48.6 MCr

Engineering Section
Jump Drive F, 35t, 60 MCr: J4
Maneuver C, 5t, 12 MCr: M2
Power Plant F, 19t, 48 MCr
Fuel 144
Total 207t, 128 MCr

Passenger Compartment
Bridge 20 tons 1.5 MCr
Computer Model 3/bis-fib, TL 11, Rating 20, 4 MCr
Evade: -2 DM, TL 11, Rating 15, 2 MCr
Fire Control: TL 10, Rating 10, 2 Weapons or 2 +1/1 +2, 4 MCr
Jump Control/4: Rating 20, 0.4 MCr
Advanced Electronics TL 11, +1 DM, 3t, 2 MCr
Total 23t, 13.9 MCr

Crew 7: Pilot, Nav/Comm, Engineer, Engineer/Gunner, 2 Gunners, Medic
Staterooms 7, 28 tons, 3.5 MCr
Emergency Low Berth 1 ton 0.1 MCr
Total 29t, 3.6 MCr

Fuel Processors 7t, 0.35 MCr
Ship's Locker
Vault 12t, 6 MCr
Total 19t, 6.35 MCr

3 Turrets, 3 tons 1.5 MCr
A Particle Beam 1 ton, 4 MCr
Double Laser 1 ton, 2 MCr
Double Missile 1 ton, 2 tons magazine 12 missiles, 20 sandcaster barrels 0.03 + 0.01 MCr for 12 smart missiles, and 20 barrels
Total 8t, 7.54 MCr

Cargo 3 tons (2 folding electric motorcycles 2t, 0.01 MCr)
Total 3t, 0.01 MCr

Total tonnage 300 tons, 208/187.2 MCr Life Support 0.14 MCr per month, Yearly Maintenance 0.208 MCr

Note, lands on tail. Modified Explorer Scouts are used by the Scout Corps for Group B missions, thus their getting the nickname of "Wild Bees".

Explorer Decks 1d and pic.jpg

Inspiration is Elson's "Pebble in the Sky"
You seem to be running into 2 problems simultaneously.
Thank you very much, I think where I went off the track was here...
"The layout of the engineering "mid-deck" outboard of the cargo bay all the way around would be done in an eqilateral hexagonal arrangement with M-Drive plates and HEPlaR elements ringing the perimeter of the circular hull."

I think I read this as mid-deck being hexagonal on the perimeter. Will take another look!
I wonder if there are ship designs out there for vessels between 200-400 tons, either lab-ships, merchants or scout ships, ships that characters could acquire on mustering out. Ships with better than J2

ITTR that in CT: AM6 Solomani some characters could have a j3 courier as mustering out benefit...

As per MgT (1E), see my own IISS Intelligence Courier. It may serve you if your group is small enough...
It's coming in more like 300 tons than 200 tons, back to the drawing board possibly.
Think I've found a 3D mistake I made with my description.
Hmmm ... the cargo hold is 65 tons and the crew+engineering spaces are 61.7 tons, with the remainder taken up by the fuel purification plant (another 5 tons) and fuel. So it's kind of like 1/3 cargo, 1/3 crew access spaces, 1/3 fuel systems.
Upon trying to "square the circle" (while fitting a hexagon in too!) in a 3D space, I realized that a single deck square cargo bay precludes the option of a hexagon deck stacked on top and have lateral "half deck" space around that hexagon and still deconflict withe the cargo bay below.

The cargo bay is just to large and flat to allow an "in between deck" outboard of the cargo bay, so the 1/1.5/2 deck scheme doesn't work because of the shape (and size) of the cargo bay as a big flat square.

Solution? A 2 deck cargo bay.

So instead of a 10x12 square single 3m deck height ... instead you would be working with a 10x6 square double 6m deck height for the cargo bay. Do that and you have a 1/2/3 deck deck arrangement, with the crew deck on top (1), the engineering deck in the middle (2) surrounding the cargo bay in the center underneath (2+3). At that point, it then also makes some sense to abandon the hexagon arrangement for the crew deck and engineering deck and simply move to a more "conventional" orthogonal square deck arrangement with lift shafts between decks 1 and 2 extending all the way down to deck 3 for cargo bay access.