Setting: In my world I've introduced some science-defying material. I use it to grant the ability of flight to airships that are less Hindenburg-y and more ship-y.
They are not literally ships that fly, but in order to comply with the aesthetics that I have in mind and still stay afloat, real lifting gases just don't cut it.
These Airships, most of the time, consist of two or more parallelly arranged lifting bodies with most of the mass of the ship hung between them, the center of mass being below the lifting bodies. This arrangement is intended to give them a sort of stability in the air similarly to a catamaran.
There are different classes of ships that refer to different sizes, armaments and tasks of the ship, the largest of these reaching widths of up to 160m and lengths of up to 400m. Aspect ratios vary from smaller ships @7:2 to larger ships @5:3.
Armaments differ between classes, but are similar to WW1/WW2 ship armaments. Similar goes for the ship classes and engagement styles.
E.g. cruisers/battleships will fire heavy shells over vast distances (mostly for land bombardment), while smaller ships feature lighter weaponry to engage in ship-to-ship fighting.
Additionally most ships, down to corvette-sized ships carry a compliment of fighter/bomber biplanes (smaller ships may carry one or two fighter-craft, while larger ships may carry multiple squadrons of fighter/bomber-craft).
Technology: The world is set technologically somewhere between the industrial revolution and the invention of internal-combustion-engines. There's been heavy development and optimizations regarding steam-driven-engines as there is, at least in parts of the world, little to no oil available (also because I like steamengines).
Electricity is something fairly new and so far doesn't go further than being used for creating light aboard airships and being used for telegraphing (little to no electric infrastructure).
While oil is found and burned in other parts of the world, in this part of it the most commonly used fuels burned for heat are coal and peat.
Additionally natural-gas, coal-water-slurry and fishoil are most commonly burned in lanterns to provide light.
Situation: I got the associated lifecycle of the mentioned phlebotinum to a point where I am quite happy with it regarding the occurrence of the element and its behavior(s).
But: The lifting power dictates the element to display either of the following two properties:
A) The element displays antigrav properties, thus allowing it to deflect/repulse/dilute gravitational forces acting upon it or a sufficiently concentrated mass of it.
B) The element displays negative mass, thus gravity does not apply a pull on it but rather a push forcing it away continuously
Both of these explanations are, to my understanding, far beyond anything explainable/discovered in our current understanding of physics. They are themselves not the topic of this question but mainly a fact regarding the environment.
Making use of the gas to lift objects off the ground, applying the laws of buoyancy, I would aim to provide sufficient negative mass to reduce the average weight per m3 of the object-to-be-lifted to the average weight of air at the altitude I'm aiming to lift the object to.
Again regarding airships I want them to be able to lose/gain 'weight' while afloat without these 'weight-changes' having huge influence on their altitude/flying-capabilities (think e.g. of a fleet of aircraft launching from a carrier / returning to it; or a complement of several hundred soldiers and landing boats leaving the baseship / returning to it).
Question: What means are there to regulate lift (overall weight) without having to rely excessively on external means?
I would like to focus on means that are within the technology constraints (they must not necessarily match the time-constraint; e.g. electromagnets can likely be a thing, same goes for electric engines - alas they make little sense when we need to drive steamengines/-turbines in order to provide the electricity anyways).
I would further like to focus on ways that do allow a ship to float at least for a week or two before needing refueling (that means a ship need to be able to carry enough fuel for floating a week or two).