On Tue, 15 Feb 2005 05:44:51 -0700, Keith Hughes
wrote:

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...

Exactly why it's hard to fly a steam-powered airplane and to sell a
ferro-cement boat. No matter how good they are, something seems to be
defying the laws of nature in a concrete canoe. G

R.

On Tue, 15 Feb 2005 09:07:50 -0500, DSK wrote:

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.

An illustrative example or two.

My boat is a light/medium displacement 33 footer rated for 8,800 lbs.
displacement. I am guessing this is "empty".

On the slings it weighs 10,000 lbs. on the nose with lots of tools,
spares, full gas, no water except beer and pop, and three anchors,
chain and about a thousand feet of line. Add 400 lbs. for crew and she
sits on her lines and sails quite well. Fin keel ballast is about 47%
of "empty" weight.

Buddy of mine in a Wallstrom-designed ketch checked with Wallstrom
himself who said it should displace 24,000 lbs. About 30% of that is
in the cutaway forefoot full keel.

Well, in the slings it weighs 36,000 lbs., which should create a new
category of "liveaboard displacement".

Still sails properly and is only a couple of inches south of its
waterline, however.

R.

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

No real difference, as you suspect. Design weight = design
displacement. Dry weight = dry displacement. On real ships,
of course, attention is paid to salinity/density of water,
which influences draft, buoyancy, etc.

But shipbuilding has been around a long time and I wonder if
a boat's weight was once measured by the actual displacement
of water in dry dock. Did ship builders years ago really
weigh ships? Would it have been worth the trouble? Did the
process of building models, measuring actual displacement of
the model, and scaling turn out to be "better" than scaling
a ship's weight from the model? Probably.

Today, of course, almost any marina can tell you what a lift
gauge reads, FWIW.

I thought I might repost "A drinking man's tunnage" from several years back:

Tonnage has nothing to do with the weight of your boat. It is a measure
of how much wine you can carry.


The word 'tun' was originally a size of a cask used to ship wine from
Spain & Portugal to England. In 1347 a tax of 3 shillings per tun was
imposed and this was called 'tonnage'. A ship's size became known by the
number of casks it could carry, and the word tonnage started being used
to describe a ship's size.


It was found that if you took the length x the breadth x the depth of
the hold under the deck and divided by 100 it was close to the number of
casks. That is where we get the "Measurement ton" of 100 cubic feet per
ton.


There are several kinds of tonnage: The first two are used by the tax
collector. The next two are used by designers. The fifth and sixth are
used by freight salesmen and canal operators and the last one is used by
the USCG for documenting boats.


Gross Tonnage - is the internal volume in cubic feet of the vessel
minus certain spaces above the main or tonnage deck like stacks and
ventilators which are called "exemptions" .


Net Registered Tonnage - is obtained by deducting from the gross tonnage
the volume of space that can't be used for paying cargo or passengers,
that is to say the space occupied by the engines, the crew's quarter,
the stores, etc.


Displacement Tonnage - is the actual weight of the water "displaced" by
the ship and is usually quoted in long tons of 2240 lbs.


Light Displacement Tonnage - is the weight with nothing in it.


Loaded Displacement Tonnage - is the fully loaded weight to the maximum
and is on her Summer draft in Salt Water..


Deadweight Tonnage - is the difference between Light and Loaded
Displacement Tonnage....the Actual carrying capacity of the vessel.


Panama & Suez Canal Tonnages - these are different to the international
ones. There used to be a lot of variations between countries and they
thought they were being conned, so they came up with their own for
everyone.


Simplified Measurement System - The USCG decided that all this was to
much for the bureaucrats to deal with for yachts so they came up with
this formula.
(Now pay attention Capt. Schlemiel).


Take the horizontal distance between the outboard ends of the boat not
including rudders and bow sprits.
Multiply that by the maximum beam outside to outside.
Multiply that by the distance from the sheer line not including bulwarks
or cap rails to the outside bottom of the hull not including the keel.
Add the volume of the deck house/cabin top.
Multiply by .5 for sailboats and .67 for power boats.
Divide by 100.


This will give you the "Gross Tonnage". Net tonnage is 90% of gross for
sailboats and 80% for power boats.


It should be obvious to those of you that have gotten this far that
"tonnage" has nothing to do with anything except in the mind of some
government bureaucrat.


Enough to make you want a drink ain't it?


--
Glenn Ashmore

I'm building a 45' cutter in strip/composite. Watch my progress (or lack
there of) at: http://www.rutuonline.com
Shameless Commercial Division: http://www.spade-anchor-us.com

rhys wrote:
....

Buddy of mine in a Wallstrom-designed ketch checked with Wallstrom
himself who said it should displace 24,000 lbs. About 30% of that is
in the cutaway forefoot full keel.

Well, in the slings it weighs 36,000 lbs., which should create a new
category of "liveaboard displacement".

Still sails properly and is only a couple of inches south of its
waterline, however.


Its easy to do a little "napkin math" to figure the immersion factor.
Assume a 40 foot LWL, a 12 foot Beam at the WL, multiply and take 70%
(the pointy ends factor) gives about 350 square feet. Divide by 12 and
its 30 cubic feet of water displaced per inch of immersion. At 64
pounds per cubic foot, and we're at a little under 2000 pounds per inch.

Many 48 footers are at this figure, a Westsail 42 is about 1500 pounds
per inch. My lightweight catamaran has the volume of these boats, but
the immersion number is 700 pounds per inch, and three inches down would
seriously affect performance. No wonder I favor lightweight ground
tackle, etc.

On Tue, 15 Feb 2005 11:45:49 -0400, Terry Spragg
wrote:

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
....
Terry K

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.

Brian Whatcott Altus OK

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

This is because weather is usually worse in winter therefore higher
freeboard required especially winter North Atlantic
wrote in message
oups.com...
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

That could be a factor but I wonder . . .

It happens that temperature has a greater effect on water
density than salinity.

Dominic wrote:
This is because weather is usually worse in winter therefore higher
freeboard required especially winter North Atlantic