Life jacket buoyancy is an important and yet often misunderstood concept when it comes to purchasing your next life jacket.
In this post I’ll go over a little bit about how life jacket buoyancy works and then explain how to tell what buoyancy level you really need.
How Life Jacket Buoyancy Works
Many boaters are easily confused by buoyancy ratings. It’s common to see buoyancy in terms of pounds, and usually something in the range of 15-30lbs of buoyancy is normal for an adult USCG approved vest.
For many, that raises a key question:
If the life jacket is approved for 20 lbs of buoyancy, how can it keep a 200lb adult afloat?
Buoyancy Numbers Reflect Dead Weight
The answer is that the weight represented in the buoyancy is in relation to the dead weight capabilities. That is, if you were to place a 20lb piece of lead on top of the life jacket, a 20lb-approved life jacket would keep that piece of lead afloat, but would sink if you put a 21lb piece on top.
Human weight is not dead weight. Upwards of 70% of the human body is water, which is effectively saying that you lose the majority of your weight when you’re in the water.
Buoyancy Also Relates To Displacement
Additionally, buoyancy is also determined by displacement.
Simply put, an object will float if it displaces more water than its weight. This explains why a cargo ship can stay afloat even though a block of steel will easily sink.
The human body will naturally displace a lot of water, and we can influence this displacement manually, by adjusting our position (you float more easily horizontally than vertically) or by controlling the amount of air in our lungs.
How Much Buoyancy You Need
The result is that in order to stay afloat, a life jacket only needs to account for 5% or so of your body weight, but a little more than that if the conditions are rough.
That’s why for inland waters, Type II and Type III vests are USCG approved with as little as 15lbs of buoyancy. For open water, Type I vests are approved at 22lbs of buoyancy.
For the overwhelming majority of people, these numbers are perfectly adequate and are more than enough to keep you afloat. For clinically obese individuals (over 300lbs), however, it may be best to use a life jacket with a higher buoyancy rating.