# If falling from height into water, at what height would a buoyancy aid be more likely to cause injury?

There are strict requirements for lifejacket / buoyancy aids in the offshore oil & gas industry, requiring them to be worn when working above the water. Some places have been noted to be using a buoyancy aids rather than an auto-inflatable device. I know that theoretically this would cause them to a more sudden stop as they hit the water, increasing the danger, but I can't find any source of information that gives a clear criteria for what height should be considered dangerous.

I know there's a lot of subjectivity, since a slim person with a buoyancy aid could equate to the same force on impact as a larger person, but if we consider like-sized average adult males, one with a 150N rated buoyancy device, another with 100N, 50N and then nothing. What would the difference be?

The forces the person is subject once in the water are weight $F_g$, buoyancy $E$, and viscous forces $F_v$. Without a flotation device, $E$ and $F_g$ approximately cancel out. The only force left, $F_v$, is again strongly dependent on the person's position (how hydrodynamic, or "streamlined" it is) so we can consider it too to be independent of the presence of a buoyancy device.