Count the blocks on your ship, size the balloon, add propellers or thrusters, and see whether it flies, the altitude it settles at and how fast it goes. The 3D model is one plausible ship built from your counts, flying at that altitude, with the clouds at Y 192 going past at its top speed. Weight, lift and thrust come from the mods' own code: ServerBalloon, BlockEntityPropeller and Sable's block mass tables.
Lift vs weight by altitude
Masses come from Sable's block tags: every block counts as 1 unless a tag says otherwise. Items inside chests don't count. Rope and pulley magnets weigh nothing.
A balloon holds at most its enclosed volume in hot air; each burner adds up to 500 at full redstone, each steam vent up to 5,000. Envelope and wool blocks you count in the weight table also add air drag.
Leave cruise altitude blank to use the height the ship settles at. Thrusters use full redstone and a clear 10 blocks behind them.
Sable's gravity is 11 m/s², not 9.8. Every lift and thrust value is already scaled to it, so a balloon's "1.5 per block" means it holds up 1.5 units of block mass at sea level.
weight = Σ block mass × 11 balloon lift = 1.5 × filled volume × 11 × p(y)DimensionPhysics.DEFAULT_GRAVITY · ServerBalloon
Air pressure is 1.0 at sea level (Y 63), about 0.46 at Y 256 and 0 at Y 320. Balloons, propellers, fans and sails all scale with it, so a ship rises until lift × pressure equals its weight.
p(63)=1.00 p(150)=0.71 p(263)=0.45 p(320)=0DimensionPhysics.createDefault
Each levitite block cancels up to 10 units of weight, but the code caps its lift at the ship's own weight, so it can hover a ship but never make it climb. It ignores air pressure.
levitite lift = min(10 × blocks, ship mass) × 11FloatingBlockController.applyLift
A propeller's push falls linearly to zero as the ship's forward speed reaches its airflow speed. That's a hard top speed no drag reduction can beat.
thrust = RPM × (1 − speed / (0.1 × RPM)) × p(y)BlockEntityPropeller.getAirflowScaling
A propeller bearing's thrust grows with sails^1.5, but its airflow only with √sails. Doubling sails gives 2.8× the push, but only 1.4× the top-speed ceiling.
thrust = 0.2 × sails^1.5 × RPM airflow = 0.05 × √sails × RPMBlockEntityPropeller (bearing)
Every envelope or wool block adds 0.33 × pressure of drag per m/s. On a balloon ship that's usually most of the drag, which is why big balloons are slow.
envelope drag = 0.33 × blocks × p(y) × speedfloating_materials · FloatingBlockController.applyFriction