: Hi Paul, I wonder if that 1.3% difference can be extrapolated to large
: vessels...even the largest only draw 50 ft. or so, so the possible
: difference due to water density is only 8". Doesn't seem to make a
: difference to ships or kayaks, as long as you are not in the dead sea!
: Thanks for the explanation, and next time someone says it is easier to
: swim in salt vs. fresh water, then I'll have to think about it some more.

: John B.

If you get that 8 inch answer by figuring 1.3% of 50 feet then you may be using the numbers incorrectly. the displacement of the boat would need to increase or decrease by this amount. Since the hulls are rarely shaped in straight lines, then the increase in draft, or the amount the boat would sink to displace the additional amount of fresh water, could be a little, or a lot.

With some of these supertankers, a 1 percent change in the cargo weight is measured in dozens or hundreds of tons.

In regards to swimming, though, the difference becomes a bit more interesting. Disregard for the moment the amount of bouyancy that changes as the lungs are alternately filled or emptied of air. We evolved from sea creatures, and our bodies are mostly water, with a specific gravity close to that of salt water. With some people, their bodies are not able to float in fresh water, they must move constantly (swim) to keep their mouths or noses above the waves. For them the slight additional bouyancy of the salt water allows them to float. In these cases it is simply that the specific gravity of their bodies falls between the specific gravity of salt water and that of fresh water. Since muscle tissue is denser than fat, you might look at it this way: If they were more muscular they would not float in salt water, either. Or, if they had more body fat they could float easier.

The 1.3 percent difference in bouyancy does not mean they will sink 1.3% deeper, it means they will sink completely, period.

PGJ