Fresh water and sea water need different designs

How a small density difference becomes a trim problem.

Fresh water and sea water need different designs because the two fluids differ in density by only a few percent, and a hull trimmed to sit exactly level and neutral in one of them will sit measurably higher or lower in the other, a gap small enough to seem irrelevant on paper and large enough to be obvious the moment the same vehicle is lowered into both.

Salt, temperature and a two and a half percent gap

Sea water carries dissolved salt, and that mineral load, spread through every litre, makes typical open-ocean water about two and a half percent denser than fresh water. That sounds tiny next to the density gaps engineers usually worry about, the eightfold gap between steel and water or the roughly eight-hundredfold gap between water and air. It matters because a hull's whole purpose is to sit at the exact point where its own weight and the water's upward push are equal, and changing the density of the surrounding water moves that point.

Put numbers on it and the gap stops looking small. A vehicle weighing 100 kilograms and trimmed exactly neutral in a fresh water tank displaces 100 litres. Lowered into the sea, those same 100 litres now push up with the weight of about 102.5 kilograms of water, so the vehicle needs roughly two and a half kilograms of extra ballast to stay neutral, and without it will rise steadily toward the surface. A percent or two of total weight is, as the earlier article on ballast showed, easily enough to see by eye once a hull is sitting in the water.

Temperature nudges the figure in a smaller way. Fresh water is densest at around fridge temperature and grows slightly less dense as it warms, so the identical hull can drift a fraction of its trim across a single season in the same harbour.

Floating on your back in a swimming pool and then in the sea shows the effect directly: the body rides a little higher in salt water, because the denser fluid pushes back harder for the same volume displaced, and a body close to the balance point between floating and sinking is sensitive enough to feel the difference. A hull does the same arithmetic, but it cannot kick a little harder to compensate, because its trim is fixed by its shape and weight.

Test tanks that measure the wrong target

A vehicle intended for both fresh and sea water needs either adjustable ballast sized to bridge the gap, or an accepted compromise trim that sits slightly buoyant in one and slightly heavy in the other. Vehicles tested only in a fresh water tank during development are especially exposed, because every indoor trim check is measuring against the wrong density if the working environment is the sea, and a design that looks perfectly balanced through a whole test campaign can arrive at its first open water trial floating visibly high. The trap is easy to fall into because a tank is cleaner, closer, and available on demand, so the setting that makes iteration fast is the same one that quietly measures the wrong number.

The fix follows from the ballast article's logic: keep some of the corrective weight adjustable, so the same hull can be re-trimmed for whichever water it is about to work in. The adjustment is small, since the gap it corrects is small, but it has to be planned from the first drawing, because retrofitting adjustable ballast into a hull never designed to carry any is far more disruptive.

Estuaries and enclosed seas

The two and a half percent figure holds for ordinary fresh water and open ocean, but brackish estuary water sits somewhere between the two and drifts with rainfall and tide, and the saltiest enclosed seas are dense enough to open a gap close to ten times wider than the open-ocean one. A vehicle designed against a single assumed water density is designed against an approximation, and that matters most in water whose salinity is not known in advance.