Embarrassing, but this is one of the places we were lucky and not good, so we thought we would post about it in case it helped anyone in their designs.
The initial designs for what evolved into the Blacktip drone had a dry internal cavity. Since a dry chamber requires that the entire chamber be pressure rated, having a large common chamber significantly complicated the design. In general the smaller the pressure chamber the easier they are to build and work with. As a result, as the design evolved it switched to using a flooded internal chamber which in turn contains smaller pressure rated chambers.
Removing a flooded hull from the water requires lifting both the weight of the drone and the weight of the water it is flooded with. Since a design goal is that the drone be able to be loaded in and out of the water by hand, without a crane, a drain point was added.
By adding a lockable bung to the drain, as see in the figure above, the hull is able to be placed in the water dry, and remain that way. This enables the drone to be either run as a surface craft, or just put in the water in a buoyancy positive configuration while it is being worked on prior to mission. The bung can be seen in cross section below. A half twist to unlock it and the hull will flood.
It is embarrassing, but the drain and bung were among the last features to be added, and there we just got lucky in adding a locking bung. Both features tuned out to be lifesavers so we thought calling it out in a post was warranted.
Earlier designs actually experimented with attachable handles, and mount points, for lifting the drone out of the water. However in the end they were cut in favor of a more hydrodynamic hull. What we had not considered was just how hard it was to manipulate a 100+ pound drone (one filled with water) – that was also wet and smooth. The drain made it possible for two people to slowly lift the hull up out of the water, and the bug lets you reach under and lock off the drain from becoming an inlet. This makes actually getting on and off the water without a crane not just easier, but possible at all.