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

Question

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?

Answer 1

It shouldn't change the impact much.

The breaking force due to surface tension is the most damaging, and we can probably suppose this force would be the same in all three cases (that wouldn't be true if the flotation device were large, but that force is anyway highly variable, depending on the position with which the person hits the water surface).

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.

Thus the buoyancy rate of each device (be it 50N, 100N, or 150N) is the precisely extra force the person will be subjected to, once submerged. These forces don't seem large and are fully present only after the initial impact with the water surface, so they shouldn't offset the advantages of wearing them.