Its written that already the roman empire used oil to smooth down waves.
The first who did scientific experiments about this effect was Benjamin Franklin,
with the use of only one sponge full of olive oil he managed to smoothen a pond.
So since ancient time, oil was used to smoothen the sea during storms. For example with
packs of linen cloth and ropes soaked with oil (first olive oil, in later times also mineral oils)
that were hanging in the water, or the oil was directly dumped into the water around the ship.
Until the 1950s, every german ship had to take oil with it for the purpose of calming down waves
in emergency. Since the 1970 german naval law forbids to bring out oil because of its toxity.
1. Yes i would believe every smaller ship or life boat had such "wave quelling oil" with them.
2. Depends on the size of the ship.
3. According to a scientific experiment of the University Hamburg done in 1970, a thin oil carpet can decrease the
wave height for about 10%.
4. The oil does not disappear, it does not sink to the ground or mix with the water, so a
ship surrounded by an oil carpet will move in it several hours before new oil needs to be added
(again depending on ship size, ship speed, weather conditions, heavy or calm sea)
5. Forbidden in most western countries,but still in use on asian and african ships, especially small fishing ships.
The trick is also used by clam divers. You may know it from the swimming pool, if there are many waves you
cant see the ground of the pool at all, but when the water is calm, you can see every single tile of the pool ground.
So clam divers use to have a sponge soaked with oil with them, to release a small oil carpet around them, calm
the sea and be able to see the ground and the clams.