Is weight and Displacement the same?
Robert Taylor
Hello I'm trying to do some calculations on my Meadow Bird 16' and
there is no displacement given in the plans, only that the weight is 650
pounds. I assume this does not include the weight of the optional
ballast of 300 pounds. Does the displacement of a boat normally include
the ballast? Is there a difference between the weight of the boat and
the boats displacement?
Thanks Robert
theparlins
Yes there is a big difference! The weight of your boat is what it weighs if
you put it on a scale, and the displacement is the weight of the water that
the vessel displaces when floating. Your weight better be less than your
displacement or you will be sitting on the bottom.
You usualy don't hear about the displacement weight of a vessel until it is
a large commercial vessel, such as a 25 ton Tug or a 100 ton passenger
vessel. I operate a Fire Boat which is "10 Tons Gross" and I believe that is
the maximum amount it can displace safely.
Correct me if I'm wrong???
Be safe.
Robert Taylor wrote in message <358818CC...@spiritone.com>...
Anthony Gondola
Whoa!!!
Weight and displacement is certainly the same. A boat that weighs 5000 lbs.
will displace exactly 5000 lbs. of water. The confusion comes in when people
start quoting unloaded, dry vessel weights verses fully loaded design
displacement.
A C Gondola, NA
Northwest Marine Design
Wayne Redditt
Robert Taylor <gree...@spiritone.com> wrote in article
<358818CC...@spiritone.com>...
Is there a difference between the weight of the boat and
> the boats displacement?
No. Archimedes principle states that a floating object will displace its
weight in fluid in which it floats. In other words, the weight of the boat
must equal its displacement. The only argument you will receive on this is
from the nuts that debate Thames tonnage or some other weird cargo capacity
calculation from the industrial revolution. It is not scientifically valid
though. Archimedes most certainly is proven.
Steven H. Jackson
The displacement is the weight of the water that the boat displaces when it
is floating. The heavier the boat
the greater the displacement. I think of the displacement as being variable
because depending on how the boat is loaded it will displace varying amounts
of water because it will sink lower. Figure the displacement for a given
waterline by calculating the volume of the hull below that waterline. If the
boat and all its gear weigh
the same as the water, it will float to that waterline.Calculating the
volume of the hull is easiest to do by measuring off the body plan drawing
and figuring the area of each station shown. Then average all the areas
by adding them together and dividing by the number of sections. Then
multiply by the length of the waterline. This will come close to the actual
volume of the hull for a specific waterline. Multiply by (approx.) 64lbs/
cubic ft of water and this will be the displacement. If you know the weight
of the boat and the volume of the hull you can calculate whether it will
float to a given waterline.
So the weight of the boat and its displacement is not the same thing
Robert Taylor wrote:
> Hello I'm trying to do some calculations on my Meadow Bird 16' and
> there is no displacement given in the plans, only that the weight is 650
> pounds. I assume this does not include the weight of the optional
> ballast of 300 pounds. Does the displacement of a boat normally include
> the ballast? Is there a difference between the weight of the boat and
> the boats displacement?
>
> Thanks Robert
RHayes7782
I say the first answer wins. Same, Same
Bob
Al Gunther
In article <35888...@news1.annapolis.net>, "theparlins"
<thepa...@annapolis.net> wrote:
> Yes there is a big difference! The weight of your boat is what it weighs if
> you put it on a scale, and the displacement is the weight of the water that
> the vessel displaces when floating. Your weight better be less than your
> displacement or you will be sitting on the bottom.
The design displacement of a recreational vessel always includes the
ballast, normally includes the weight of crew and what ever the designer
included for ground tackle, fuel, water and provisions. Actual
displacement is literly what the boat weighs if you were to take it out of
the water. If you weigh a bare boat, without all the stuff normally on the
boat when it's loaded with crew and ready to shove off, it will weigh much
less, but if placed in water it will still displace exactly the same
weight of water, so the actual displacement will be less. Some designer
figure the fuel tanks half full for design displacement so in that case
your actual displacement might be more if your tanks are full and you are
provisioned for a two week cruise.
Most "blue water" cruisers that I've seen are over their design
displacement most or all of the time. I had a friend who loaded up for a
trip to San Diego, and he had to divert his sink drain from the main head
into the bilge, because it fell below the waterline of the boat and would
not drain even when level.
> You usualy don't hear about the displacement weight of a vessel until it is
> a large commercial vessel, such as a 25 ton Tug or a 100 ton passenger
> vessel. I operate a Fire Boat which is "10 Tons Gross" and I believe that is
> the maximum amount it can displace safely.
> Correct me if I'm wrong???
I believe you are correct.
> Robert Taylor wrote in message <358818CC...@spiritone.com>...
> > Hello I'm trying to do some calculations on my Meadow Bird 16' and
> >there is no displacement given in the plans, only that the weight is 650
> >pounds. I assume this does not include the weight of the optional
> >ballast of 300 pounds. Does the displacement of a boat normally include
> >the ballast? Is there a difference between the weight of the boat and
> >the boats displacement?
> >
> >Thanks Robert
> >
--
Al Gunther, Kingston, WA <---- 47° 52.7'N, 122° 30.9'W
Please remove the "z" in my return address to reply.
Gary Zwissler
Not quite right. Net tonnage is vessel weight. Gross tonnage is not the
same as net tonnage. Gross tonnage is a measure of enclosed area
available for the carrying of passengers or cargo. One gross ton =
100 cubic feet. Actually the regulations also state that the area
available for passengers or cargo is that area not dedicated to
proplusion systems, tankage, or mechanical equipment. Gross tonnage is
not an indicator of overall size. A 65 foot charter fishing boat may be
200 gross tons (and require the operator to have a USCG 200 Ton Masters
license), while an 80 foot tug may only be 100 gross tons since it has
significantly less interior space for passengers or cargo (all its
interior space is mostly engine and machinery).
Gary Z.
theparlins wrote:
>
> Yes there is a big difference! The weight of your boat is what it weighs if
> you put it on a scale, and the displacement is the weight of the water that
> the vessel displaces when floating. Your weight better be less than your
> displacement or you will be sitting on the bottom.
> You usualy don't hear about the displacement weight of a vessel until it is
> a large commercial vessel, such as a 25 ton Tug or a 100 ton passenger
> vessel. I operate a Fire Boat which is "10 Tons Gross" and I believe that is
> the maximum amount it can displace safely.
> Correct me if I'm wrong???
> Be safe.
Peter Doherty
"Steven H. Jackson" <jax...@together.net> wrote:
>The displacement is the weight of the water that the boat displaces when it
>is floating.
Which is, of course, the same as the weight of the boat.
> The heavier the boat the greater the displacement.
Yup.
> I think of the displacement as being variable because depending on how the
> boat is loaded it will displace varying amounts
>of water because it will sink lower.
Well, you made the 'boat' heavier by loading it. Of course it will
'sink lower' (displace more water).
>Figure the displacement for a given waterline by calculating the volume of
>the hull below that waterline. If the boat and all its gear weigh the same
>as the water, it will float to that waterline.Calculating the
>volume of the hull is easiest to do by measuring off the body plan drawing
>and figuring the area of each station shown. Then average all the areas
>by adding them together and dividing by the number of sections. Then
>multiply by the length of the waterline. This will come close to the actual
>volume of the hull for a specific waterline. Multiply by (approx.) 64lbs/
>cubic ft of water and this will be the displacement. If you know the weight
>of the boat and the volume of the hull you can calculate whether it will
>float to a given waterline.
What you've done there is determined the weight at which a given hull
shape will sink to a given waterline. You figured out the volume of
water below the waterline and computed the weight of that water. This
gives you the weight (both above and below the surface, of course)
that will cause that much water to be displaced since any object will
always displace a volume of water the weight of which is equal to the
weight of the object.
>So the weight of the boat and its displacement is not the same thing
You didn't show that. You didn't even really address that question. In
fact, you used the fact that they are to show that you can figure out
how far the boat will sink into the water. One can use the method you
describe to determine the waterline that a boat with a given hull
shape will sink under a given load.
For the guy that wrote that they aren't because if they were you would
sink...the trick is that the density of the boat is less than that of
the water, which means that after the boat has displaced a volume of
water equal to it's own weight some of the boat is still sticking up
above the surface (that's good!).
For the formally minded, the OED says:
displacement (dIs'pleIsm&schwa.nt). The act of displacing or fact
of being displaced.
(other definitions cut here)
b Hydrostatics. The displacing of a liquid by a body immersed in
or floating on it; the amount or weight of fluid so displaced
by a floating body, e.g. a ship.
A discussion relating to stuff that floats (bouyancy) includes:
Archimedes' Principle
Archimedes' Principle is an application of Newton's Third Law of
Motion. Gravity is the force of attraction towards the Earth. When
an object is placed in a fluid, e.g., water or air, there is a
force exerted on that object which pushes it up. This force is
called buoyancy. Buoyant force is the force which causes objects
to float. Buoyant force is caused by the displaced fluid pushing
back against the object. Strictly speaking, buoyant force is the
mass of the displaced fluid multiplied by gravity.
When an object floats below the surface of the fluid, e.g., an
object floats in water below the surface, the weight of the object
is equal to the buoyant force. When an object completely sinks,
the weight is greater than the buoyant force, even though there is
still a buoyant force on the object. When an object is floating on
the surface of a fluid, e.g., ice floating on water, the weight of
the object (mass times gravity) is equal to the amount of fluid
pushed out of the way times gravity.
macnaughton.com
The weight of the boat in pounds or tons will always be exactly the same as
the actual displacement of the boat in pounds or tons. Those who cite
Archimedes understand this. Old Archie and all us naval architects since his
time understand it too.
Tom MacNaughton
http://www.macnaughtongroup.com
Steven H. Jackson wrote in message <35889412...@together.net>...
>The displacement is the weight of the water that the boat displaces when it
>is floating. The heavier the boat
>the greater the displacement. I think of the displacement as being variable
>because depending on how the boat is loaded it will displace varying
amounts
>of water because it will sink lower. Figure the displacement for a given
>waterline by calculating the volume of the hull below that waterline. If
the
>boat and all its gear weigh
>the same as the water, it will float to that waterline.Calculating the
>volume of the hull is easiest to do by measuring off the body plan drawing
>and figuring the area of each station shown. Then average all the areas
>by adding them together and dividing by the number of sections. Then
>multiply by the length of the waterline. This will come close to the actual
>volume of the hull for a specific waterline. Multiply by (approx.) 64lbs/
>cubic ft of water and this will be the displacement. If you know the weight
>of the boat and the volume of the hull you can calculate whether it will
>float to a given waterline.
>So the weight of the boat and its displacement is not the same thing
Bateau2
In article <358818CC...@spiritone.com>, Robert Taylor
<gree...@spiritone.com> writes:
> Does the displacement of a boat normally include
>the ballast? Is there a difference between the weight of the boat and
>the boats displacement?
A boat displaces its weight, there is no way around that but I like to specify
a weight and a displacement. The weight is the complete boats but without crew,
gear, provisions, fuel etc. while the displacement is the weight of the boat
ready to go with everything and everybody onboard. The difference is the
capacity, a very important point for cruising. Always ask about these two
figures before buying or building a boat.
Jacques Mertens - Boat Plans OnLine
http://bateau.com
Ron Dwelle
Robert Taylor wrote:
Is there a difference between the weight of the boat and
> the boats displacement?
No. They are identical. (we're talking actual displacement, here, not
some weird tax or legal or tonnage "displacement.")
Steven H. Jackson
Robert Taylor wrote:
> Hello I'm trying to do some calculations on my Meadow Bird 16' and
> there is no displacement given in the plans, only that the weight is 650
> pounds. I assume this does not include the weight of the optional
> ballast of 300 pounds. Does the displacement of a boat normally include
> the ballast? Is there a difference between the weight of the boat and
> the boats displacement?
>
> Thanks Robert
Isn't this MEADOWBIRD 16 really a Sam Rable TITMOUSE ? I wonder if
BOATBUILDING IN
YOUR OWN BACKYARD has the displacement figures for this design.
Bruce Gennette
On Wed, 17 Jun 1998 22:23:50 -0700, "Anthony Gondola" <ac...@aa.net>
wrote:
>Whoa!!!
>
>Weight and displacement is certainly the same. A boat that weighs 5000 lbs.
>will displace exactly 5000 lbs. of water. The confusion comes in when people
>start quoting unloaded, dry vessel weights verses fully loaded design
>displacement.
>
Can I try to simplify this a little -
A boat is a hull, equipment and a whole lot of AIR contained within
the hull. The weight of the hull, equipment and the air inside the
hull shape has to be less than the weight of an equivalent volume of
water for the boat to float. If the boat and its equipment is heavier
than an equivalent volume of water the boat will sink.
Put another way, the average density of the boat has to be less than
the density of an equivalent volume of water for the boat to float.
The boat pushes aside (displaces) its own weight of water, while
taking up less volume than the (on average) denser water.
If you can separate the concepts of mass, volume and density in your
mind you can see the whole thing clearly. [d=m/v or v=m/d or m=d*v]
Anyway displacement (of water) = the total mass of the boat.
Bye.
Dan Bollinger
Robert;
For marine engineering purposes, they are slightly different concepts, no
need to bother with the finer points here. For your purposes they are the
same.
If your boat weighs 650 and displaces 650 and part of the boat is still
above water, great! You have a boat. If your boat weighs 650 and only
displaces 450, oops, you paid too much for that anchor!
The ballast is always a part of displacement. If it is removeable water,
like in my former Sea Pearl 21, then the mfgrs. have probably not added it
to their 650 number. Add the weight of the ballast if you use it.
Dan the Man
Robert Taylor <gree...@spiritone.com> wrote in article
<358818CC...@spiritone.com>...
Brian Prescott
Hi!
You're right, of course. I suspect what the original questioner is wondering is
whether the displacement listed in plans and specs for boats is the weight of
the boat as built, or the weight including the useful load. In my experience it
may be either, and there seems no easy way to tell which.
Brian Prescott
macnaughton.com wrote:
macnaughton.com
Mr. Gennette,
You do understand that the displacement of the boat and its weight are
identical don't you? This was the original question.
Tom MacNaughton
Naval Architect
http://www.macnaughtongroup.com
Bruce Gennette wrote in message <35899d89...@news.tafensw.edu.au>...
Al Gunther
In article <358a5...@newsfeed1.cybertours.com>, "macnaughton.com"
<mcn...@nemaine.com> wrote:
> Mr. Gennette,
>
> You do understand that the displacement of the boat and its weight are
> identical don't you? This was the original question.
>
> Tom MacNaughton
> Naval Architect
> http://www.macnaughtongroup.com
If I understand what he is saying, it's that *if* the boat's total weight
exceeds it's maximum displacement, it will sink. Visualize filling it up
with lead shot.
Of course if the boat had enough foam in it, you wouldn't have enough room
for enough shot to sink it. ;-)
Jonathan Klopman
"element". Water density determines the "effective" weight of the
vessel. A hull will actually displace less in salt that fresh. The term
displacement, therefore is the most accurate way of desribing a vessel's
mass in terms of performance. Not that this would transform a Tahiti
Ketch into a speed demon if you plunked it in the Great Salt Lake.
JKK
John McCoy
"macnaughton.com" <mcn...@nemaine.com> wrote:
>The weight of the boat in pounds or tons will always be exactly the same as
>the actual displacement of the boat in pounds or tons.
Two minor quibbles here - first off you have to make sure the specified
tonnage is in fact a weight, not a volume (i.e. register tons); and in addition
this does not apply to a planing hull at speed.
Granted, neither of these notes applies to the original question :-)
John
Steve Cork
OK, time for my $0.02 worth.
The displacement of a boat is the same no matter what medium it is
floating in because the displacement is measured in units of mass.
If the density of the medium varies then the volume of that medium
displaced will vary with inverse proportionality but the mass of the
medium displaced will not change.
The weight, or mass, of the vessel and it's contents, effective or
otherwise does not change either.
Steve
Jonathan Klopman wrote:
> not to be smarmy, but I understand that displacement depends on the
> "element". Water density determines the "effective" weight of the
> vessel. A hull will actually displace less in salt that fresh. The term
> displacement, therefore is the most accurate way of desribing a vessel's
> mass in terms of performance. Not that this would transform a Tahiti
> Ketch into a speed demon if you plunked it in the Great Salt Lake.
>
> JKK
--
Steve Cork | Ph: 61-8-9237-3041 | "All I ask is a tall ship
AlphaWest | http://www.alphawest.com.au | and a star to steer her by"
Consultant | Fax: 61-8-9382-1688 | (John Masefield)
Bateau2
In article <6mn37e$dck$1...@nnrp1.dejanews.com>, lawso...@my-dejanews.com
writes:
>Actually, displacement does depend on medium. Salt water weights approx
>64lbs/cu. ft, fresh water is 62 lbs/cu. ft. a boat weighs a specific amount
>(mass times gravity) at sea level on earth weight(mass times constant
>gravity) will not vary, but it is the MASS that is fixed.
Therefore, it always displaces the same weight floating in fresh, saltwater or
beer. When expressed as a weight, it is always the same except that some are
now going to say that weight being mass*g and g changes slightly gna gna gna
and we can go on forever.
Sander Wissing
OK, so how is this for splitting hairs? Some navigable (fresh water)
systems are higher than sea level, thus moving the boat further from
the earth's centre. So now the boat will actually weigh less than at
sea level. Now your boat can have different displacements in salt and
fresh water. Of course, it will still be the same as its weight,
though ...
Cheers
Sander
per.c...@gentofte.mail.telia.com
Just a Question ;
I you place a one ton boat on the MOON, (they say that there are water there)
--- What would the displacement then be, if weight and displacement are the
same ?
Guess that it don't matter, that there are no air.
Best regards
Per Corell
Cyber-Boat
http://w1.1396.telia.com/~u139600113/INDEX.HTM
Dan Bollinger wrote in message <01bd9b00$e7a816e0$11d9f8cc@oemcomputer>...
>Robert;
>
>For marine engineering purposes, they are slightly different concepts, no
>need to bother with the finer points here. For your purposes they are the
>same.
>
>If your boat weighs 650 and displaces 650 and part of the boat is still
>above water, great! You have a boat. If your boat weighs 650 and only
>displaces 450, oops, you paid too much for that anchor!
>
>The ballast is always a part of displacement. If it is removeable water,
>like in my former Sea Pearl 21, then the mfgrs. have probably not added it
>to their 650 number. Add the weight of the ballast if you use it.
>
>Dan the Man
>
>
>Robert Taylor <gree...@spiritone.com> wrote in article
><358818CC...@spiritone.com>...
>> Hello I'm trying to do some calculations on my Meadow Bird 16' and
>> there is no displacement given in the plans, only that the weight is 650
>> pounds. I assume this does not include the weight of the optional
>> ballast of 300 pounds. Does the displacement of a boat normally include
>> the ballast? Is there a difference between the weight of the boat and
>> the boats displacement?
>>
>> Thanks Robert
>>
>>
-----== Posted via Deja News, The Leader in Internet Discussion ==-----
http://www.dejanews.com/ Now offering spam-free web-based newsreading
Daryl1953M
>OK, so how is this for splitting hairs?
I don't know if it can be split . . . . but I'm pretty sure it can be shoveled.
Best wishes,
Daryl
Bateau2
In article <35930de6.97814299@firewall>, san...@icon.co.za (Sander Wissing)
writes:
>OK, so how is this for splitting hairs? Some navigable (fresh water)
>systems are higher than sea level, thus moving the boat further from
>the earth's centre. So now the boat will actually weigh less than at
>sea level. Now your boat can have different displacements in salt and
>fresh water. Of course, it will still be the same as its weight,
>though ...
I admit that it sounds right at first and I was extremely worried: my designs
may not float right on lake Titicaca.
I was ready to add a note on our dinghy plans like those cake recipes for high
altitudes: for sailing above 4.000 meter, please add 10 kg of ballast . . . but
thinking of it, is that water also not subject to a lesser g? And will
everything not stay the same?
The waterline will be the same but we should have asked Tristan Jones . . .
Bateau2
In article <6mr8v4$lp6$1...@nnrp1.dejanews.com>,
per.c...@gentofte.mail.telia.com writes:
> you place a one ton boat on the MOON, (they say that there are water there)
A cruise on the Sea of Desolation? Is that what they call astronomic
navigation?
Russ Turpin
-*-------
While it is true that the same mass weighs less the further
removed it is from the earth's center, the difference in this
between sea level and the world's highest lake is negligible.
Russell
--
This new ship here is fitted according to the reported increase of knowledge
among mankind. Namely, she is cumbered end to end, with bells and trumpets
and clock and wires, ... she can call voices out of the air of the waters to
con the ship while her crew sleep. But sleep Thou lightly. It has not yet
been told to me that the Sea has ceased to be the Sea. -- Rudyard Kipling
Tan PS
The weight of the boat will be 1/5 of earth, that will be 1/5 ton. And
earthling Pascal's law will still hold, the boat will displace its weight of
water, ie, 1/5 ton of water....unless there's some Moon person's law.
This does not mean that it is 1/5 the amount of water. The volume of water
will be same as on earth since the same amount of water will also weigh
lesser at 1/5 ton.
Now all we need to do is find the water and we sgould eb able to start a
space colony and make atmosphere with it.
Regards.
per.c...@gentofte.mail.telia.com wrote in message
<6mr8v4$lp6$1...@nnrp1.dejanews.com>...
>Just a Question ;
>I you place a one ton boat on the MOON, (they say that there are water
there)
Bateau2
In article <6mtlsm$q4o$1...@thetimes.pixel.kodak.com>,
tur...@temporarily.unavailable (Russ Turpin) writes:
>While it is true that the same mass weighs less the further
>removed it is from the earth's center, the difference in this
>between sea level and the world's highest lake is negligible.
Please don't spoil our pleasure with logic and common sense!
Sander Wissing
Yes well, the boat will weigh less, as will the water so it will float
to the waterline. But the displacement on paper will be less. As
someone said, maybe not a hell of a lot less, but what the hell.
Cheers
Sander
j...@dynamicpictures.com
In article <3599869c.22611473@firewall>,
To be fully pedantic, displacement is measured in mass units, which are not
relative to your local gravity field (mass times gravity gives force, which is
what makes you tired at the end of the day). Force varies with gravity. Mass
stays the same. If your boat displaces X Kilos of water at sea level in
Chesapeak Bay, your boat will displace X kilos of water in one of those big
underground sailing lakes we need to build on the Moon.
More cheers.
-john
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Steve Cork
I think you're wrong, John. A boat's displacement is a function of the
gravitic attraction between it and whatever it's floating on. The boat
will 'weigh' less but at the same time the water's ability to support
a body will be proportionally less as well. The volume (and hence mass)
of water displaced will remain constant but the 'weight' of that water
in the local gravity field will be less than on earth. The water will be
supporting less 'weight' and hence the displacement will be less.
IMHO :-)
Steve
j...@dynamicpictures.com wrote:
>
> To be fully pedantic, displacement is measured in mass units, which are not
> relative to your local gravity field (mass times gravity gives force, which is
> what makes you tired at the end of the day). Force varies with gravity. Mass
> stays the same. If your boat displaces X Kilos of water at sea level in
> Chesapeak Bay, your boat will displace X kilos of water in one of those big
> underground sailing lakes we need to build on the Moon.
>
> More cheers.
> -john
--