Re: how to clarify ballast!
Jim,
Your statement that "Each water tank is fully supported by the weight
of the water it displaces," and "{as the boat heels} one water tank
simply sinks deeper still supported by it surrounding water," is
misleading. You could pump out the water and substitute an equal
weight in lead, and as long as the weights were placed in the same
location the boat's stability would not be significantly altered.
In a given hull, ballast placed inside the hull does not change the
shape of the hull, so the added weight alone is not displacing it's
own volume with an equal amount of water; the volume of the entire
hull is displacng the added weight.
Only if water ballast were added outside the hull would the water
ballast tank be "fully supported by the weight of the water it
displaces."
The trouble with comparing "inside" v "outside" ballast is that there
is no such thing as 'all else being equal' in the comparision. All
else is not equal, and can't be equal, because in tacking on outside
ballast the shape of the hull is changed. Tacking on one square foot
of water ballast outside a hull adds one square foot of underwater
volume to that hull. Since the medium is water, the volume of the
water ballast equals the volume of the water it has to displace, so
there is no change in the vessel's waterline. Unbolt that tank and
put it inside and the vessel will now displace 64 more pounds of
seawater - it floats deeper in the water. Now its just a matter of
determining where to put that weight.
Put it centerline and low in the hull and it will have maximum
effect. Split it in two and place it outboard to either side and it's
like putting two equal weights on either side, but below the axle, of
a wheel. A small rotation in the wheel is easy to make because the
force of the weight going down tends to counteract the force of the
weight going up. But the more the wheel is turned (the more the boat
heels) the less the effect.
John O'Neill
Your statement that "Each water tank is fully supported by the weight
of the water it displaces," and "{as the boat heels} one water tank
simply sinks deeper still supported by it surrounding water," is
misleading. You could pump out the water and substitute an equal
weight in lead, and as long as the weights were placed in the same
location the boat's stability would not be significantly altered.
In a given hull, ballast placed inside the hull does not change the
shape of the hull, so the added weight alone is not displacing it's
own volume with an equal amount of water; the volume of the entire
hull is displacng the added weight.
Only if water ballast were added outside the hull would the water
ballast tank be "fully supported by the weight of the water it
displaces."
The trouble with comparing "inside" v "outside" ballast is that there
is no such thing as 'all else being equal' in the comparision. All
else is not equal, and can't be equal, because in tacking on outside
ballast the shape of the hull is changed. Tacking on one square foot
of water ballast outside a hull adds one square foot of underwater
volume to that hull. Since the medium is water, the volume of the
water ballast equals the volume of the water it has to displace, so
there is no change in the vessel's waterline. Unbolt that tank and
put it inside and the vessel will now displace 64 more pounds of
seawater - it floats deeper in the water. Now its just a matter of
determining where to put that weight.
Put it centerline and low in the hull and it will have maximum
effect. Split it in two and place it outboard to either side and it's
like putting two equal weights on either side, but below the axle, of
a wheel. A small rotation in the wheel is easy to make because the
force of the weight going down tends to counteract the force of the
weight going up. But the more the wheel is turned (the more the boat
heels) the less the effect.
John O'Neill
--- Inbolger@yahoogroups.com, Jim Pope <jpope@a...> wrote:
> If the water ballast is carried in two tanks arranged to be close
to the
> waterline when filled but located at the hull's extreme beam, her
> initial stability is form stability only. And the form inducing
that
> stability is the shape of the hull without the water ballast tanks.
> Now heel the hull around its center of rotation.
>
> One water tank simply sinks deeper still supported by it
surrounding
> water, but the other tank is being lifted out of the water and can
exert
> its full weight as a righting force. Going back to the beginning
then,
> we can look at the two filled tanks with the hull on an even keel
as
> weight sinking her lower in the water, big ship style, but when she
> heels then the upper tank becomes righting force ballast, trying to
> crank her back upright, yacht style.
>
> Both approaches to ballast work well. In troubled time, the lead
mine
> will sink you, however, and at the haul out ramp the water tanks
can be
> emptied helping you a whole lot. But the plumbing is one more thing
to
> be maintained and, like everything else, it too can fail at
> inappropriate times. (on a boat very little fails at the right time)
>
> I hope that didn't simply muddy the water further,
> Jim
stormpetrel2002 wrote:
ship employs ballast it is usually to improve the way that she would
handle if she were otherwise left empty. That is to say, to bury the
propeller and rudder and to put enough of the ship's underbody under the
water so that she can be steered even though she might be steaming
across the wind. Nowadays that ballast is almost always water and the
tanks into which it is put are usually filled right up so that it
doesn't slosh around. The point of that ballasting, however, is to make
the ship heavier so that it will handle better.
In our yachts the purpose is not to make the boat heavier. We ballast
our boats in order to make them more stable. To that end, a deep keel
with a small amount of dense metal on its bottom end, rigidly attached
to the boat's bottom and forced to rotate with the hull as the boat
heels would probably be the most effective. For lots of reasons, we
don't want to do exactly that. So plate the boat's bottom with a layer
of dense metal. That works too, but to get the same righting force that
close to the boats fore and aft center of rotation your metal plate will
will have to actually be heavier than the dense metal bulb on the deep keel.
Now lets take another look at the idea of water ballast for stability.
If the water ballast is carried in two tanks arranged to be close to the
waterline when filled but located at the hull's extreme beam, her
initial stability is form stability only. And the form inducing that
stability is the shape of the hull without the water ballast tanks.
Each water tank is fully supported by the weight of the water it
displaces. Now heel the hull around its center of rotation.
One water tank simply sinks deeper still supported by it surrounding
water, but the other tank is being lifted out of the water and can exert
its full weight as a righting force. Going back to the beginning then,
we can look at the two filled tanks with the hull on an even keel as
weight sinking her lower in the water, big ship style, but when she
heels then the upper tank becomes righting force ballast, trying to
crank her back upright, yacht style.
Both approaches to ballast work well. In troubled time, the lead mine
will sink you, however, and at the haul out ramp the water tanks can be
emptied helping you a whole lot. But the plumbing is one more thing to
be maintained and, like everything else, it too can fail at
inappropriate times. (on a boat very little fails at the right time)
I hope that didn't simply muddy the water further,
Jim
> --- Inbolger@yahoogroups.com, Lincoln Ross <lincolnr@r...> wrote:Good points all, but maybe another thought might be useful. When a big
> > The water outside the boat really can't tell what's inside the boat!
> All
> > that matters is the hull shape and the center of mass of the hull. Just
> > figure that out and forget whether the ballast is feathers or uranium
> > until you're trying to see how low you can get it in the boat. While
> > correct, that bit about water ballast acting like it isn't there is a
> > red herring.
ship employs ballast it is usually to improve the way that she would
handle if she were otherwise left empty. That is to say, to bury the
propeller and rudder and to put enough of the ship's underbody under the
water so that she can be steered even though she might be steaming
across the wind. Nowadays that ballast is almost always water and the
tanks into which it is put are usually filled right up so that it
doesn't slosh around. The point of that ballasting, however, is to make
the ship heavier so that it will handle better.
In our yachts the purpose is not to make the boat heavier. We ballast
our boats in order to make them more stable. To that end, a deep keel
with a small amount of dense metal on its bottom end, rigidly attached
to the boat's bottom and forced to rotate with the hull as the boat
heels would probably be the most effective. For lots of reasons, we
don't want to do exactly that. So plate the boat's bottom with a layer
of dense metal. That works too, but to get the same righting force that
close to the boats fore and aft center of rotation your metal plate will
will have to actually be heavier than the dense metal bulb on the deep keel.
Now lets take another look at the idea of water ballast for stability.
If the water ballast is carried in two tanks arranged to be close to the
waterline when filled but located at the hull's extreme beam, her
initial stability is form stability only. And the form inducing that
stability is the shape of the hull without the water ballast tanks.
Each water tank is fully supported by the weight of the water it
displaces. Now heel the hull around its center of rotation.
One water tank simply sinks deeper still supported by it surrounding
water, but the other tank is being lifted out of the water and can exert
its full weight as a righting force. Going back to the beginning then,
we can look at the two filled tanks with the hull on an even keel as
weight sinking her lower in the water, big ship style, but when she
heels then the upper tank becomes righting force ballast, trying to
crank her back upright, yacht style.
Both approaches to ballast work well. In troubled time, the lead mine
will sink you, however, and at the haul out ramp the water tanks can be
emptied helping you a whole lot. But the plumbing is one more thing to
be maintained and, like everything else, it too can fail at
inappropriate times. (on a boat very little fails at the right time)
I hope that didn't simply muddy the water further,
Jim
--- Inbolger@yahoogroups.com, Lincoln Ross <lincolnr@r...> wrote:
from the tendency to view water in the context of the medium the boat
is floating in.
A better way would be to look at an otherwise empty boat, say an
unballasted dinghy . The dinghy is filled with air. As you add water,
as anyone who has dumped a dinghy can attest, the boat will sit lower
in the water. Add enough and eventually the seats will be awash.
This happens because initially the air filled dinghy is displacing only
it's weight in the water.
If you were to dump an equal VOLUME of lead, steel or bricks into the
bottom of three identical dinghies they would also sit lower than when
empty. Yet the amount of water displaced will vary in each case because
the specific gravity of each material varies. Draft will be greater
with a cubic foot of lead than with a cubic foot of brick. (or a cubic
foot of water)
Note that I haven't said an equal weight of these materials. When
building a boat, you will have a given space to devote to ballast.
Space is volume not weight.
Water acts in the same fashion, but as it's specific gravity is lower
than lead or steel it will not displace as much water outside the boat
for the same volume of space allotted to ballast inside the boat. But
it will still increase displacement, or rather reduce the preexisting
reserve buoyancy the empty boat hull started with.
This is true of anything you put into a boat. Extra timbers, a deck the
boat wasn't designed for, several heavy friends, all are additional
ballast subtracting from the hull's original reserve buoyancy. And if
you think about it, the heavy friends are roughly the same specific
gravity as water, the timbers and new deck probably less (would float
by themselves) and yet they affect the amount of water the boat draws.
Hope this isn't too confusing.
Peter
> The water outside the boat really can't tell what's inside the boat! AllI think the difficulty people have understanding water as ballast comes
> that matters is the hull shape and the center of mass of the hull. Just
> figure that out and forget whether the ballast is feathers or uranium
> until you're trying to see how low you can get it in the boat. While
> correct, that bit about water ballast acting like it isn't there is a
> red herring.
from the tendency to view water in the context of the medium the boat
is floating in.
A better way would be to look at an otherwise empty boat, say an
unballasted dinghy . The dinghy is filled with air. As you add water,
as anyone who has dumped a dinghy can attest, the boat will sit lower
in the water. Add enough and eventually the seats will be awash.
This happens because initially the air filled dinghy is displacing only
it's weight in the water.
If you were to dump an equal VOLUME of lead, steel or bricks into the
bottom of three identical dinghies they would also sit lower than when
empty. Yet the amount of water displaced will vary in each case because
the specific gravity of each material varies. Draft will be greater
with a cubic foot of lead than with a cubic foot of brick. (or a cubic
foot of water)
Note that I haven't said an equal weight of these materials. When
building a boat, you will have a given space to devote to ballast.
Space is volume not weight.
Water acts in the same fashion, but as it's specific gravity is lower
than lead or steel it will not displace as much water outside the boat
for the same volume of space allotted to ballast inside the boat. But
it will still increase displacement, or rather reduce the preexisting
reserve buoyancy the empty boat hull started with.
This is true of anything you put into a boat. Extra timbers, a deck the
boat wasn't designed for, several heavy friends, all are additional
ballast subtracting from the hull's original reserve buoyancy. And if
you think about it, the heavy friends are roughly the same specific
gravity as water, the timbers and new deck probably less (would float
by themselves) and yet they affect the amount of water the boat draws.
Hope this isn't too confusing.
Peter
The water outside the boat really can't tell what's inside the boat! All
that matters is the hull shape and the center of mass of the hull. Just
figure that out and forget whether the ballast is feathers or uranium
until you're trying to see how low you can get it in the boat. While
correct, that bit about water ballast acting like it isn't there is a
red herring.
that matters is the hull shape and the center of mass of the hull. Just
figure that out and forget whether the ballast is feathers or uranium
until you're trying to see how low you can get it in the boat. While
correct, that bit about water ballast acting like it isn't there is a
red herring.