That sounds easier than it is when you consider the many variables that need to be juggled during concept design phase (e.g. ambient conditions you expect the ship to operate in, exhaust restgriction, operating speed/power profile, etc.), but it's doable to a fair degree of accuracy.

There are a few primary reasons we did not proceed with air lubrication:

1.) Cost of compressed air. To generate the bubbles you need compressed air. Compressed air can actually be quite expensive to produce, so a fair amount of your fuel savings from reducing friction resistance is consumed by the need to generate the air. That being said, there is generally a decent amount of waste heat on ships, and that thermal energy (or potentially harvested energy from the evironment) could be capstured and used to reduce the cost of compressed air (but that's more equipment to buy & maintain).

2.) Hull shape constraints are very stringent because you need to be able to shape the hull go ensure the bubbles remain adhered to the skin of the vessel over the entire vessel length in order to gain the maximum benefit. Generally that means you want a flatter bottom. However, hull shape is very critical for the final powering resistance, so it's quite possible that if you design your hull for better bubble adhession, you prevented yourself from just using a more hydrodynamic hull shape that would perform just as well as well as the air lubricated hull in terms of total hydrodynamic resistance (friction & form drag).

3.) Real world conditions. You also need to consider that the ocean is not perfectly still, so the pitching/rolling of the vessel due to wind/waves/currents may further hinder bubble adhesion (to what extent is unclear to me).

4.) Concerns about inducing propeller cavitation if bubbles cannot be steered away from the prop in-flow. Cavitation can rapidly deteriorate expensive propellers, so the hull shape should steer bubbles away from the prop. Cavitation is a real, but it was not clear to me how much of a concern these air lube bubbles are; cavitation is really a function of propeller blade shape, size and RPM.

All of the above hull shaping and special curvature can be very costly in production.

I'm not saying air lubrication shouldn't be considered, it should, but a careful CBA must be completed to truly ensure you'll see savings in terms of ship lifecycle cost. Due to hull shape constraints, it's probably better suited to new ship designs vice retrofit.