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Swimming in JP-8
Name: Adam
Status: Other
Grade: Other
Location: PA
Country: United States
Date: April 2006
Question:
I am a fuels troop is USAF. There is a rumor that
I am trying to squash, that it would be impossible to swim in jet
fuel (JP-8) because it is less dense than water and your body is
comprised partly of water. I have countered that your density is
not what makes you swim, it is the upward thrust generated by the
motions you make. The density of JP-8 jet fuel is 6.7 lb/gallon @
60 degrees F, and I have found the density of water is around
8.35 lb/gallon at the same temperature. I have contended that
this difference is not enough to render the thrust generated by a
swimmer ineffective.
Replies:
I do not believe myself fully qualified to support or debunk you
position, but I would like to add a couple of details and ask a
question or two.
How viscous is JP-8? If it is very thick, swimming would be
relativly easy, if it is very thin, well, it will be much harder to
effectivly apply force to the fluid.
Being less dense than water would definately (by itself, ignoring
viscosity) make swimming more difficult, though probrably not
impossible. An interesting paradox arises though as anyone who is a
good enough swimmer to stay afloat in jet fuel probrably is not the
kind of person to risk their health swimming in jet fuel.
Ryan Belscamper
The principle is this a body is buoyed up by a force equal to the
volume of water (or JP-8) displaced by the body. So shape plays a
big factor. Obviously, ships are more dense than water but still
float. Icebergs float, but only about 10 to 12 percent of their
mass is above water. Remember the ill fated ship Titanic.
Submarines and other submersibles float or sink depending upon how
much air or water are in their ballast tanks. I do not know the
density of JP-8 fuel but it is probably about 7.5 to 8.0 lb/gallon,
so whether a person could displace enough JP-8 to remain afloat
probably depends upon his/her body volume and how they are
oriented. It is also the case that by "dog paddling" so as to give
the body an upward thrust that a heavier body will float on a lower
density fluid, but there is a limit to how much upward thrust one
can generate. Remember, of course, that helicopters "float" in the
atmosphere by thrusting massive amounts of air downwards.
Vince Calder
Dear Adam,
I am sorry to be so late, but I think I can add some insight to the
archived answers to this interesting question.
Since most people have roughly neutral buoyancy (I can float, but
can sink by exhaling deeply), their average density is about that of
water. A 200 lb person would thus displace about 24 gallons of
water (water has a density of 8.35 lb/gal). This person would then
displace 24 gal of jet fuel which would generate a buoyant force of
about 160 lb (assuming his volume is unchanged).
This means that he would have to exert an upward force of 40 lb with
his arms and legs to keep from sinking, I should think most people
would find this VERY difficult and would soon sink.
Incidentally, a ship must have an average density less than that of
water; otherwise it would sink.
Best, Dick Plano, Professor of Physics emeritus, Rutgers University
Dear Adam,
I believe you would find it very difficult, if not impossible, to swim in
jet fuel for more than a very short time, even ignoring problems
your body may have coping with the fumes and itching. You are
correct, of course, that you can use arm movements to try support
yourself in jet fuel, but I think you would find that the 40 lb of
vertical thrust you need would be difficult to maintain.
I get the 40 lb by first assuming the human body has the same average
density as fresh water (62.5 lb/ft^3 = 8.35 lb/gal). A 200 lb man
would thus weigh as much as 24.0 gallons of water and so would just
float when immersed in water. However, 24 gallons of JP-8 weighs
only about 160 lb (24 gal times 6.7 lb/gal), so this 200 lb man
immersed in JP-8 would feel a buoyant force of only 160 lb, and
needs an additional 40 bl of vertical force to keep from sinking.
Best, Dick Plano, Professor of Physics emeritus, Rutgers University
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