Doug,

Is there any way to determine 'real' displacement as a practical matter
for a typical sailor, other than loading the boat per it's intended use,
and seeing what happens ? :-)

For is it not typically the case that... someone buys a boat, is
uncertain as to what and how much loading will then ensue, but then sets
about their sailing regime...

This might seem to indicate that...the safe thing to do is to buy heavy
displacement.

Comment ?

Thank you,

Courtney




DSK wrote:

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.


Yep.

... This means that my boat that weighs 8000 lbs must displace
8000 lbs of water in order to float.


Yep.

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


Not necessarily.

There is a relationship between weight, displacement, and immersed
volume. The "displacement" in reality is the immersed volume, and the
weight of that water *will* equal the actual weight (or mass would be a
better term) of the boat & all it's contents.

So, if your boat weighs 8000#, and you add another 8000# (doesn't matter
if it's inflooding water, bricks, or feathers) then your boat might sink
*if* it does not have the hull capacity to create that new larger
immersed volume, and thus displace that much more water. Capische?


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


Usually what people mean by "displacement" is shorthand for
'displacement when floating at the boat's designed waterline.' That
could be light, with no people or stores; it could be fully loaded ie
all tanks full and crew and stores, or it could be something like 'half
load displacement' which is defined as 1 average sized person for each
bunk, no stores, and all tankage half full.

Most boat builders quote the lightest figure for displacement and leave
the buyer to guess what the boat's capacity for stores etc etc really is.

Fresh Breezes- Doug King



--
s/v Mutiny
Rhodes Bounty II
lying Oriental, NC
WDB5619