RE: [PSUBS-MAILIST] Bouancy (please read)

["Alec Smyth" <asmyth@hsomail.com>]

Katie,

I love this problem! But I disagree with Jonathan, who said:

"salt water should cause anything to be more
bouyant, but it is also more dense which means
the boat would have to plow through more matter
than fresh water."

In fact, it is true that salt water is denser, but according to Archimedes'
principle, the object will displace a lesser volume of it -- it does not
plow through more matter in salt water. On the contrary, while the displaced
mass is the same, the displaced volume of salt water is less, so the object
might move FASTER in salt water due to less hidrodynamic drag.

If the object is moved by wind, whether by design (e.g., sailboat) or not
(e.g., oil tanker cruising empty), you have to consider its DIRECTION OF
TRAVEL relative to the wind. A sailboat for example can move with the wind,
or against it. If it floats higher in salt water, you can argue that it will
catch more wind and move faster... but only if the direction of movement is
WITH the wind. If your sailboat is moving upwind, then floating higher will
only slow it down.

If the object moves solely as a consequence of water currents, there isn't
any difference between sweet and salt water.

Finally, I suggest you tell your instructor that the aswer depends on the
WATER TEMPERATURE. You see, sweet water freezes slightly warmer than salt
water. So if the temperature is just right, your salt-water boat will sail
fine while the sweet-water equivalent will go nowhere at all.

:)


Alec Smyth



-----Original Message-----
From: owner-personal_submersibles@psubs.org
[mailto:owner-personal_submersibles@psubs.org]On Behalf Of Jonathan
Wallace
Sent: Monday, February 28, 2000 1:04 PM
To: personal_submersibles@psubs.org
Subject: Re: [PSUBS-MAILIST] Bouancy (please read)


Well, salt water should cause anything to be more
bouyant, but it is also more dense which means
the boat would have to plow through more matter
than fresh water.  I'd guess that there may be
a theoretical advantage to speed in salt water
since drag would be less (less boat in the water),
but I'd also guess the difference is imperceptable
and possible immeasurable.

ok all you armchair scientists, have at it.

Jon