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

Wrong!

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

Because your empty boat won't sink until it fills up with more
weight than the hull shape can float.

Your boat may weigh 6000 pounds, but it won't sink until you pile
another 10000 pounds on it. At that point, it's maximum floatation
value, it gets to the point where one ripple on the water, one more
fly landing on the mast head would cause your boat to instantly sink.

Shippers in the past found out that there must be some reasonable
(there's that word, again) value of vessel size and overall weight
including cargo must be considered as some sort of maximum under
some rule somewhere to ensure a decent chance that a vessel, and her
crew, might arrive safely in her next port. Insurance companies got
together to determine how they would deal with unscrupulous
corporations who might easily decide to take on an outrageous amount
of otherwise valueless cargo for the sole purpose of having the
vessel and it's "valuable" cargo sink conveniently at sea for a
guranteed return on insurance, taking witnesses and rotton planks
and all to the bottom, amongst those others who tell no tales.

So, displacement is not defined as the weight of the boat. It is
calculated according to practices and rules acceptable to those who
buy and sell safety. It is measured in barrels, because up until
recently, all cargo was shipped either alive, in cabins or pens, and
in barrels, volume units considered to contain average weight
material, probably rum (which floats, by the way), so that
inspectors could measure a ship's shape and determine what she might
carry safely and for tax purposes, given actuarial studies of what
has been safe up until today.

Some "ships" are rated and documented for insurance purposes, and
some are not, being pleasure craft.

How much a floating boat actually displaces while it is afloat is a
different measure, and it will, naturally, be equal to the weight of
the boat and it's contents. Archimedies figured this out just before
he invented the word to describe the other guys at the public bath
house ("You-reek-a") and the practice of public nudity. S'truth!

Measures and practices continue to evolve, but the customs remain
the same.

Everything about boats is a compromise.

Terry K