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

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.

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.

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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