Ballasting

There are two major factors that need to be considered for a well-ballasted glider.

The first is its density. Gliders have a limited displacement available for getting up and down the water column (typically around 1L). So the first decision is where in that one liter you want to place the neutral position of your glider. The lazy answer would be in the middle. But the compromise here would be between how much thrust you will have for flying through the water and how much buoyancy extra buoyancy you want to have for making sure glider will get back to the surface. Depending on the location, glider might need to pass through a sharp thermocline or when you expect to have a distinct layer of fresh water at surface, for instance close to a river discharge. So to be on the safe side, you always want to have a little more on the buoyancy side. But sometimes you need as much thrust as you can get to fight through a strong current for instance. So these are decisions that need to be made while performing a mission planning.

To add weight, we typically use lead bars and to add buoyancy, we usually use syntactic foam.

After ballasting a glider for buoyancy, now it’s time to make sure center of gravity and center of buoyancy are in positions where allow the glider to have an efficient and stable flight through the water. This is done by moving the weights and foams up and down the body. The aim here to achieve a neutral pitch position where there is enough room for glider to pitch up and down. Also you want to have most of the weight in the bottom and the buoyancy on top to have a more stable flight.

Slocums are ballasted for surface water at your target area while Seagliders are ballasted for the most dense water at the area.

Slocums can generally be ballasted in a shallow pool.

While Seagliders usually need a deeper pool, since you need to check their surface position for which they are are almost completely vertical.

My favorite way of doing this is to get in the water with glider in a deep enough area with easy access.