Beginner question here: So my partner and I are trying to design a buoyancy engine where a motor operates a ball screw that draws and pushes syringes in order to draw and push water in the syringes(water from the pool the engine is submerged in). Would this be classified as a buoyancy engine? The reason I'm doubtful is because the official definition of a buoyancy engine includes an "external bladder"(something like a balloon) which our design does not have.

If not a buoyancy engine, would this be classified as a hydraulic system? Thanks!

  • 1
    $\begingroup$ what difference does it make if it can be called a buoyancy engine? $\endgroup$
    – jsotola
    Mar 31, 2022 at 0:10
  • $\begingroup$ If you refer to something, please cite it. In this case, please provide a link to your "official definition", including who the official is who's making the definition. $\endgroup$
    – TimWescott
    Apr 1, 2022 at 20:58

2 Answers 2


I found a paper describing a buoyancy engine that specifically notes an external reservoir. I did not find an "official definition," however. In the past, I have attended an autonomous watercraft competition, the name of which escapes me. One of the exhibitors in the display tent promoted water glider kits for students. The description you've provided regarding the syringes is nearly identical to the mechanism used in the water glider kit.

Pulling in water from outside into the "engine compartment" results in an increase in weight as well as an imbalance based on the placement of the water portion of the syringe. The resultant sinking nose down will cause the glider to move forward (via wings) until the desired depth is reached.

Pushing the water out increases buoyancy, again at the nose, creating a climb and forward "flight" in the water.

Lacking an indication of "official definition," I would suggest that what you describe and what I've seen qualify for a buoyancy mechanism, if not a buoyancy engine. Propulsion via buoyancy seems to be the correct assignment of the terminology.

I'm not quite sure why the external reservoir is part of the system. External to what? Does it float on the surface or is it secured to the bottom of the ocean?


I would use the example of a porpoise. It has its own internal air reservoire and by controling its inflation (among other things) it changes its buoyancy's CM.

Then using its fins it takes advantage of tipping up and down caused by that change for propulsion.

If you syringes do the same thing, loading your float in a way to make it undulate and you take advantage of this for propulsion, then you can call it a buoyancy machine.


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