Displacement and weight
 

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

wrote in message
oups.com...
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?
So, why do people make a distinction 'tween displacement and weight?

This may not be what you are looking for, but......
There are a number of weights and tonnages associated with boats/ships, that
need to be differentiated.
For instance:
Displacement
Deadweight
Lt. Ship
Gross tonnage
Net Tonnage
Etc.
So .... each of these "weights" (and not all are) refer to different,
specific weights/measurements associated with the boat


otn

otnmbrd wrote:
wrote in message
oups.com...
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?
So, why do people make a distinction 'tween displacement and
weight?

This may not be what you are looking for, but......
There are a number of weights and tonnages associated with
boats/ships, that
need to be differentiated.
For instance:
Displacement
Deadweight
Lt. Ship
Gross tonnage
Net Tonnage
Etc.
So .... each of these "weights" (and not all are) refer to different,

specific weights/measurements associated with the boat


otn

Hmmm...."Displaces more water than its weight..." According to
Archimedes Princ. thsi means that there is a net upward force on the
boat instead of a balance. With a net upward force, she would rise out
of the water so I am not sure this is possible. I may be wrong, its
been many years since I thought about this.

wrote:
otnmbrd wrote:

wrote in message
groups.com...

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

weight?

This may not be what you are looking for, but......
There are a number of weights and tonnages associated with

boats/ships, that

need to be differentiated.
For instance:
Displacement
Deadweight
Lt. Ship
Gross tonnage
Net Tonnage
Etc.
So .... each of these "weights" (and not all are) refer to different,


specific weights/measurements associated with the boat


otn


Hmmm...."Displaces more water than its weight..." According to
Archimedes Princ. thsi means that there is a net upward force on the
boat instead of a balance. With a net upward force, she would rise out
of the water so I am not sure this is possible. I may be wrong, its
been many years since I thought about this.


An 8000lb boat will displace exactly 8000lb of water. Basic F=MA, where
A=acceleration due to gravity. The important point is that where the
*volume* of the 8000lb of boat is greater than that of 8000lb of water
(i.e. lower density), the boat floats. Density greater than
water...glug, glug, glug...

Keith Hughes

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


I believe Doug is correct.

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

Courtney Thomas wrote:
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 ? :-)

Sure. A lot of Travel-Lifts have scales built in. Just have one of them
hoist your boat.



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...


That's pretty much it, and of course people raise their waterlines all
the time...


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

Comment ?


Depends on what you mean by "safe." How about positive flotation? Is
"heavy displacement" inclusive of a high ballast/disp ratio?

The main thing that heavy displacement boats offer IMHO is a smoother ride.

A regime that you could persue yourself, with relatively lo-tech &
lo-budget means, is to tape a ruler to your waterline fore and aft.
Bring aboard a known weight, and see how much it takes to immerse the
boat per inch. The next level is to do an incline test and measure how
many foot-pounds of righting moment the boat generates at varying angles
of heel; do this for the boat when stripped light, and when loaded, and
then you'll have some useful information that most sailors take for
granted. It'll also help test how secure your stowage is ;)

FB-
DSK

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 ? :-)

Easiest way I can think of is to ask the crane operator the next time
you get your boat hauled out. Boat cranes often have a load cell so can
tell what the weight of your boat is and avoid overloading their crane.
May not be very accurate though.

Paul

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