On 15 Feb 2005 14:34:42 -0800, wrote:

Brian Whatcott wrote:
After numerous ship losses, Plimsoll legislated a hull mark
in Parliament, the "Plimsoll Line" which accounts for salt and fresh,
warm n cool water, beyond which a commercial vessel must not be
loaded. Unaccounted ship losses were much reduced thereafter.

The difference between salt and freshwater displacement and waterlines
makes sense to me, since the salt increases the density of the water,
right? But why is there a difference between summer and winter?
Thanks, Brent


Warm water is less dense than cold water. Water expands more with
temperature than the metals, so a ship sits lower in hot water.
Moreover, water's rate of expansion increases as the temperature
rises. Another factor: water like oil, gets considerably less
viscous when warm, so a hull might be expected to be livelier, and
maybe the swell higher....

Brian Whatcott Altus OK

Brian
Are you sure about this. It would seem to me that the cold water
would be less dense. It expands as it freezes.
JR

Brian Whatcott wrote:
On 15 Feb 2005 14:34:42 -0800, wrote:


Brian Whatcott wrote:

After numerous ship losses, Plimsoll legislated a hull mark
in Parliament, the "Plimsoll Line" which accounts for salt and fresh,
warm n cool water, beyond which a commercial vessel must not be
loaded. Unaccounted ship losses were much reduced thereafter.

The difference between salt and freshwater displacement and waterlines
makes sense to me, since the salt increases the density of the water,
right? But why is there a difference between summer and winter?
Thanks, Brent



Warm water is less dense than cold water. Water expands more with
temperature than the metals, so a ship sits lower in hot water.
Moreover, water's rate of expansion increases as the temperature
rises. Another factor: water like oil, gets considerably less
viscous when warm, so a hull might be expected to be livelier, and
maybe the swell higher....

Brian Whatcott Altus OK

On Wed, 16 Feb 2005 01:40:22 GMT, Brian Whatcott
wrote:

Warm water is less dense than cold water. Water expands more with
temperature than the metals, so a ship sits lower in hot water.
Moreover, water's rate of expansion increases as the temperature
rises. Another factor: water like oil, gets considerably less
viscous when warm, so a hull might be expected to be livelier, and
maybe the swell higher....

So the worst case scenario for a loaded ship would be a laden tanker
enduring a Red Sea or a Persian Gulf cyclone? I vaguely remember that
those areas are the hottest oceanic bodies on Earth.

By contrast, on a calm day in zero C. Antarctic water (ice-free,
however), the same laden ship would ride high(er) and dry.

Interesting!

R.

In article .com,
wrote:

Maybe I've killed too many brain cells over the years or just forgotten
some basic stuff but my understanding of Archimedes Principle is that
the buoyant force on an object is equal to the weight of the fluid it
displaces. This means that my boat that weighs 8000 lbs must displace
8000 lbs of water in order to float.

This is true and is a basic principle of floating objects.

If she takes on 8000 lbs of
water, she sinks cuz the buoyant force doesnt balance the weight.
Right?

I suspect that's false. Your boat may or may not have that much reserve
buoyancy. Our 3.5 ton boat is rated to carry at least 5 tons. Would take
somewhat more to sink her in calm water. Hull (without keel) is a bit
more than a foot submerged. We have 3'+ more freeboard and the hull gets
FAT up there.

So, why do people make a distinction 'tween displacement and weight?


Sounds better, and is more technically correct.

Reminds me of a hairy old story: New engineer is told to find out how
much more of something is on a barge. After hours of laborous
measurements and calculations, the foreman comes by to see what the
matter is and points out the numbers marked on the hull -- showing
displacement.

--
Jere Lull
Xan-a-Deux ('73 Tanzer 28 #4 out of Tolchester, MD)
Xan's Pages: http://members.dca.net/jerelull/X-Main.html
Our BVI FAQs (290+ pics) http://homepage.mac.com/jerelull/BVI/