DEAD THREAD, THE END: [bolger] Re: Physics (and Water Ballast)
I couldn't resist to say something here. While I do agree that there
are some reduntant postings here, they're no problem for me. The boss
though made the right thing in this very case ("physics") because
there are people who know the physics here and people who don't, and
this is not the place to organize physics lectures. It'd be like
putting up an astronomer vs. astrologist thread on the web: everybody
would have something to say in a field that doesn't have any room for
"opinions".
I do pay for Internet, but the cost is one local call. What I did when
I registered to this list was to select the "read on the web" option,
so I don't download everything. I read the titles and then only the
messages that I deem interesting. It takes 10 minutes in the evening.
Best, Pippo
are some reduntant postings here, they're no problem for me. The boss
though made the right thing in this very case ("physics") because
there are people who know the physics here and people who don't, and
this is not the place to organize physics lectures. It'd be like
putting up an astronomer vs. astrologist thread on the web: everybody
would have something to say in a field that doesn't have any room for
"opinions".
I do pay for Internet, but the cost is one local call. What I did when
I registered to this list was to select the "read on the web" option,
so I don't download everything. I read the titles and then only the
messages that I deem interesting. It takes 10 minutes in the evening.
Best, Pippo
--- Inbolger@egroups.com, freedem@e... wrote:
> Ive enjoyed learning and reasoning out these problems It
would
> be best but time consuming to do exparaments and answer every
question
> for ourselves. It has been said that a few weeks of
exparamentation
> can avoid half a day spent in a libary on research ;-)
On Wed, 8 Nov 2000, Samson family wrote:
responding too. Many people read in Digest, and the reposting of entire
messages is a nightmare that makes participation prohibitive.
Edit wildly! Snip! Snip!
Thanks.
> I'll reiterate that in some parts of the world (the UK in particular)A good strategy for this is to snip all non-essential material that one is
> email is NOT free - and we have to pay for local phone calls in order
> to download our email. A significant amount of time, and therefore
> money, is spent downloading stuff that is of little interest to
> ANYONE, apart perhaps from the person who submitted it.
responding too. Many people read in Digest, and the reposting of entire
messages is a nightmare that makes participation prohibitive.
Edit wildly! Snip! Snip!
Thanks.
At the risk of repeating what several members have said before
(evidenced by the number who have voted with their feet/unsubscribe
button)this list has a had a pretty low signal to noise ratio in
recent times.
I'll reiterate that in some parts of the world (the UK in particular)
email is NOT free - and we have to pay for local phone calls in order
to download our email. A significant amount of time, and therefore
money, is spent downloading stuff that is of little interest to
ANYONE, apart perhaps from the person who submitted it.
I'll support the moderator in his pleas that we keep on-topic (i.e.
Phil Bolger's boats) and keep our thoughts to ourselves unless we
genuinely believe that they are of interest in terms of the list's
topic.
Ivanhoe Flacktower has obviously written in anger, but we ought to
reflect on what sparked his ire in the first place.
Bill Samson
"Blessed is he who, having nothing to say, remains silent."
(evidenced by the number who have voted with their feet/unsubscribe
button)this list has a had a pretty low signal to noise ratio in
recent times.
I'll reiterate that in some parts of the world (the UK in particular)
email is NOT free - and we have to pay for local phone calls in order
to download our email. A significant amount of time, and therefore
money, is spent downloading stuff that is of little interest to
ANYONE, apart perhaps from the person who submitted it.
I'll support the moderator in his pleas that we keep on-topic (i.e.
Phil Bolger's boats) and keep our thoughts to ourselves unless we
genuinely believe that they are of interest in terms of the list's
topic.
Ivanhoe Flacktower has obviously written in anger, but we ought to
reflect on what sparked his ire in the first place.
Bill Samson
"Blessed is he who, having nothing to say, remains silent."
--- Inbolger@egroups.com, flacktower@f... wrote:
> Nice to see that our earnest list boss has has put the squash on
this
> thread. For once, I agree with him doing that. There are just too
> many people here on the RYAN/VANDERWAART list who go on and on
about
> subjects they don't know doggie doo about. It certainly is
arrogant
> of them to think the rest of us are interested in their half-baked
> theories, silly comments and erroneous information. I wonder if
> these foot shooters have a life away from answering mailing list
> postings? Bloody doubtful if they'll ever get around to building
> anything, let alone a boat.
>
> Ivanhoe Flacktower
>
> WEST HAM UNITED FC RULES!!
>http://www.westhamunited.co.uk/
> GO HAMMERS! GO IAN!!
--- Inbolger@egroups.com, flacktower@f... wrote:
while in my case you are right so far this year that I have not built
a boat the great pleasure recived and enjoyed with the mental games
brought up by such discussions as "Physics" is second only to the
pleasure recived due to finding a wife and our two dogs, for this list
member. Ive enjoyed learning and reasoning out these problems It would
be best but time consuming to do exparaments and answer every question
for ourselves. It has been said that a few weeks of exparamentation
can avoid half a day spent in a libary on research ;-)
When I first joined this list it was likely the biggest in
membership and most active in the egroups on boating. Boat design
still has fewer people but has become rather more active. except this
month the extended dicussion ment there were more than twice as many
post here as there.
IMHO I would like to thank those that post and comment and wish you
every happyness especally warm indian summer days to get another board
attached to the boat abuilding or another afternoons excursion.
Mr Flacktower may I ask you to write about your boats and on the water
adventures, because here in northern utah it gettin right down chilly
and thats just outside let alone on the water
thankyou
jeffery
ps I do still expect to get a boat or two don
> Nice to see that our earnest list boss has has put the squash onthis
> thread. For once, I agree with him doing that. There are just tooMr Flacktower
> many people here on the RYAN/VANDERWAART list who go on and on about
> subjects they don't know doggie doo about. It certainly is arrogant
> of them to think the rest of us are interested in their half-baked
> theories, silly comments and erroneous information. I wonder if
> these foot shooters have a life away from answering mailing list
> postings? Bloody doubtful if they'll ever get around to building
> anything, let alone a boat.
>
> Ivanhoe Flacktower
>
while in my case you are right so far this year that I have not built
a boat the great pleasure recived and enjoyed with the mental games
brought up by such discussions as "Physics" is second only to the
pleasure recived due to finding a wife and our two dogs, for this list
member. Ive enjoyed learning and reasoning out these problems It would
be best but time consuming to do exparaments and answer every question
for ourselves. It has been said that a few weeks of exparamentation
can avoid half a day spent in a libary on research ;-)
When I first joined this list it was likely the biggest in
membership and most active in the egroups on boating. Boat design
still has fewer people but has become rather more active. except this
month the extended dicussion ment there were more than twice as many
post here as there.
IMHO I would like to thank those that post and comment and wish you
every happyness especally warm indian summer days to get another board
attached to the boat abuilding or another afternoons excursion.
Mr Flacktower may I ask you to write about your boats and on the water
adventures, because here in northern utah it gettin right down chilly
and thats just outside let alone on the water
thankyou
jeffery
ps I do still expect to get a boat or two don
Nice to see that our earnest list boss has has put the squash on this
thread. For once, I agree with him doing that. There are just too
many people here on the RYAN/VANDERWAART list who go on and on about
subjects they don't know doggie doo about. It certainly is arrogant
of them to think the rest of us are interested in their half-baked
theories, silly comments and erroneous information. I wonder if
these foot shooters have a life away from answering mailing list
postings? Bloody doubtful if they'll ever get around to building
anything, let alone a boat.
Ivanhoe Flacktower
WEST HAM UNITED FC RULES!!
http://www.westhamunited.co.uk/
GO HAMMERS! GO IAN!!
thread. For once, I agree with him doing that. There are just too
many people here on the RYAN/VANDERWAART list who go on and on about
subjects they don't know doggie doo about. It certainly is arrogant
of them to think the rest of us are interested in their half-baked
theories, silly comments and erroneous information. I wonder if
these foot shooters have a life away from answering mailing list
postings? Bloody doubtful if they'll ever get around to building
anything, let alone a boat.
Ivanhoe Flacktower
WEST HAM UNITED FC RULES!!
http://www.westhamunited.co.uk/
GO HAMMERS! GO IAN!!
--- Inbolger@egroups.com, GHC <ghartc@p...> wrote:
> Ok, let's call this a dead thread.
>
> Gregg Carlson
> moderator
>
> At 11:10 PM 11/6/2000 +0200, you wrote:
> >On Mon, 6 Nov 2000, Brand, Christopher wrote:
> >
> >> Example of water ballast "working" below the surface of the
water: Take a
> >> hollow lead sphere that is just more than neutrally bouyant. It
floats
> just
> >> awash. This lead sphere has a watertight bulkhead that cuts it
in half and
> >> one hemisphere has a valve that can allow water to enter. If
that
> >> hemisphere is allowed to flood with water, the sphere will sink
and it will
> >> orient itself with the flooded half at the bottom (toward the
center of the
> >> earth). If the sphere is turned so that the flooded half is at
the top, it
> >> will right itself powerfully.
> >>
> >> This behavior is in spite of the fact that all the water ballast
is below
> >> the "waterline". The reason is the separation of the center of
gravity and
> >> the center of bouyancy. CG is located in the lower hemisphere,
the CB is
> >> located in the center of the dividing bulkhead disk. These must
(by vitrue
> >> of the forces acting upon them) vertically align themselves.
> >
> >First, the empty half's mass is less than the flooded half's.
Second,
> >its buoyancy is positive, whereas that of the flooded half is zero.
> >No wonder the empty half comes up on top; it has both gravity and
> >buoyancy on its side.
> >
> >But I don't think I have said that a boat ballasted with water will
> >float bottom-up...:-)
> >
> >
> >Sakari Aaltonen
> >
> >
> >
> >
> >Bolger rules!!!
> >- no cursing
> >- stay on topic
> >- use punctuation
> >- add your comments at the TOP and SIGN your posts
> >- add some content: send "thanks!" and "ditto!" posts off-list.
> >
> >
In extreme weather in our little trawler we used to sling weights over on a
22ft line so they hung from the tips of the trawl rig, each about 8 feet out
the side.
So the weight was about 15 feet out from the centerline, about the same
below sealevel.
You may have seen trawlers with tyres hanging out by the rig. Thse were part
of the line to prevent jerking the rig.
Weights were about 150kg on a 16 ton 38ft boat and effective esp in
damping rolling.
We would have used more weight but had to manhandle them
from stowage. used 2 small 4 cyl engine blocks.
We winched them back.
Jeff
She was called the "Income", although I wanted to
rename her
"Syd Fishous".
Especially as she never lived up to the old name. Perhaps " Income?"
At least she always came home.
22ft line so they hung from the tips of the trawl rig, each about 8 feet out
the side.
So the weight was about 15 feet out from the centerline, about the same
below sealevel.
You may have seen trawlers with tyres hanging out by the rig. Thse were part
of the line to prevent jerking the rig.
Weights were about 150kg on a 16 ton 38ft boat and effective esp in
damping rolling.
We would have used more weight but had to manhandle them
from stowage. used 2 small 4 cyl engine blocks.
We winched them back.
Jeff
She was called the "Income", although I wanted to
rename her
"Syd Fishous".
Especially as she never lived up to the old name. Perhaps " Income?"
At least she always came home.
----- Original Message -----
From: <KF4call@...>
To: <bolger@egroups.com>
Sent: Monday, November 06, 2000 7:57 AM
Subject: Re: [bolger] Re: Physics (and Water Ballast)
Well here is an idea "borrowed" from Tristan Jones:
It is quite usual to reef by lowering the sail area exposed to the
wind...Jones suggests another form of "reefing", by increasing the ballast
(rather than decreasing sail area) when more stability is needed. What he
suggests is a large diameter PVC tube with caps on both ends. In windy
conditions it could be filled with seawater, capped and suspended just below
the actual waterline parallel to the windward side.
As the boat heels, the downward force of the water, as it is moved
above
the waterline, will contribute weight (or perhaps mass?) to help right the
boat by opposing the heeling movement. He SAID it would work, but then he
was also talking about a catamaran. Sort of like pumping water to the
windward ballast tank. Any takers?
Best regards, Warren
Bolger rules!!!
- no cursing
- stay on topic
- use punctuation
- add your comments at the TOP and SIGN your posts
- add some content: send "thanks!" and "ditto!" posts off-list.
Christopher:
You have absolutely nailed this subject. Your last two paragraphs were
particularly appropriate. I have included them here for those who did not
make it that far in your original post:
Chuck
You have absolutely nailed this subject. Your last two paragraphs were
particularly appropriate. I have included them here for those who did not
make it that far in your original post:
Chuck
> I wish Mr. Bolger would stop saying that water ballast isn't effectiveuntil
> it is raised above the waterline by heeling because it just isn't true.The
> idea of subtracting the space with the waterballast is intuitivelyand
> appealing, but fallacious. There is a powerfull bouyant force pressing
> upward on those parts of the boat, some portion of that force the equal
> opposite force to the weight of the water ballast falling toward thecenter
> of the earth.
>
> Stop worrying about anything but the RELATIVE POSITIONS of the CENTER OF
> GRAVITY and the CENTER OF BOUYANCY. THESE ARE THE ONLY TWO THINGS THAT
> AFFECT STATIC STABILITY.
>
> Apologies for my venting/shouting. This has been an interesting thread.
>
> Christopher Brand
Sakari:
With all due respect, you don't want your ballast under water: you want it
in the boat. If it is in the boat, it is not under water.
Chuck
With all due respect, you don't want your ballast under water: you want it
in the boat. If it is in the boat, it is not under water.
Chuck
>
> An item's mass does not depend on whether it is under water or above,
> but its weight does. (Otherwise, life vests, say, would be useless.)
>
> So, a water ballast under water has mass, but weighs nothing, that is,
> the force is zero. Only when it, or part of it, rises above water is
> there a force.
>
>
> Sakari Aaltonen
>
Ok, let's call this a dead thread.
Gregg Carlson
moderator
At 11:10 PM 11/6/2000 +0200, you wrote:
Gregg Carlson
moderator
At 11:10 PM 11/6/2000 +0200, you wrote:
>On Mon, 6 Nov 2000, Brand, Christopher wrote:just
>
>> Example of water ballast "working" below the surface of the water: Take a
>> hollow lead sphere that is just more than neutrally bouyant. It floats
>> awash. This lead sphere has a watertight bulkhead that cuts it in half and
>> one hemisphere has a valve that can allow water to enter. If that
>> hemisphere is allowed to flood with water, the sphere will sink and it will
>> orient itself with the flooded half at the bottom (toward the center of the
>> earth). If the sphere is turned so that the flooded half is at the top, it
>> will right itself powerfully.
>>
>> This behavior is in spite of the fact that all the water ballast is below
>> the "waterline". The reason is the separation of the center of gravity and
>> the center of bouyancy. CG is located in the lower hemisphere, the CB is
>> located in the center of the dividing bulkhead disk. These must (by vitrue
>> of the forces acting upon them) vertically align themselves.
>
>First, the empty half's mass is less than the flooded half's. Second,
>its buoyancy is positive, whereas that of the flooded half is zero.
>No wonder the empty half comes up on top; it has both gravity and
>buoyancy on its side.
>
>But I don't think I have said that a boat ballasted with water will
>float bottom-up...:-)
>
>
>Sakari Aaltonen
>
>
>
>
>Bolger rules!!!
>- no cursing
>- stay on topic
>- use punctuation
>- add your comments at the TOP and SIGN your posts
>- add some content: send "thanks!" and "ditto!" posts off-list.
>
>
On Mon, 6 Nov 2000, Brand, Christopher wrote:
its buoyancy is positive, whereas that of the flooded half is zero.
No wonder the empty half comes up on top; it has both gravity and
buoyancy on its side.
But I don't think I have said that a boat ballasted with water will
float bottom-up...:-)
Sakari Aaltonen
> Example of water ballast "working" below the surface of the water: Take aFirst, the empty half's mass is less than the flooded half's. Second,
> hollow lead sphere that is just more than neutrally bouyant. It floats just
> awash. This lead sphere has a watertight bulkhead that cuts it in half and
> one hemisphere has a valve that can allow water to enter. If that
> hemisphere is allowed to flood with water, the sphere will sink and it will
> orient itself with the flooded half at the bottom (toward the center of the
> earth). If the sphere is turned so that the flooded half is at the top, it
> will right itself powerfully.
>
> This behavior is in spite of the fact that all the water ballast is below
> the "waterline". The reason is the separation of the center of gravity and
> the center of bouyancy. CG is located in the lower hemisphere, the CB is
> located in the center of the dividing bulkhead disk. These must (by vitrue
> of the forces acting upon them) vertically align themselves.
its buoyancy is positive, whereas that of the flooded half is zero.
No wonder the empty half comes up on top; it has both gravity and
buoyancy on its side.
But I don't think I have said that a boat ballasted with water will
float bottom-up...:-)
Sakari Aaltonen
Congratulations Chris!
In the wee hours of this morning I gave up my attempt to compose a
similar reply to the myths about water ballast having no weight when
positioned below the waterline and to acknowledge Chris Krumm's
excellent synopsis. You have done both in fine fashion. No need for
apologies, this thread was begging for a proper knot, and it took two
Chrises to tie it!
Using actual hull examples might help put this whole business in
perspective. I strongly reccomend Mike O'Brien's article "Shoal-Draft
Sharpies, Simple, self-righting, and roomy" in "WoodenBoat" No. 114,
September/October 1993, pages 64 to 75. Mike uses examples from
Bolger, Chapelle, Beebe, Herreshoff and Kirby. Of particular interest
is the part "Surprising Stability Curves" on pages 72 for Kirby's 31'
Norwalk Islands Sharpie with graphs and diagrams to show the
interaction of vertical center of gravity (VCG) with center of
buoyancy (CB) as the righting arm changes with heel angle.
Note that the VCG is fixed with respect to the hull configuration and
is the center of rotation while the CB moves as the underwater hull
geometry changes with heel angle. I believe this concept has been
overlooked in most of the previous discussions.
One more example regarding water versus lead or some other heavier
material ballast. As Chris Krumm pointed out, higher density material
"takes less volume than an equal mass of water; hence, it can
typically be stuffed lower in the hull and help lower the boat's
center of gravity." Martha Jane (isn't that who started much of this
thread?) has water ballast tanks roughly six inches deep, so the VCG
of its 500 pounds is about three inches above the bottom of the tanks.
Since lead is more than eleven times as heavy as an equal volume of
water 500 pounds would be just over one-half inch spread out over the
tank bottoms, and its VCG would be about one-quarter inch above the
tank bottom, a difference of 2 3/4 in. With total boat weight
(displacement) of 2500 lb, replacing 500 lb of water with 500 lb of
lead would not change the total weight but would lower 20% of the
total weight 2 3/4 inch thus lowering the total VCG a bit over 1/2
in. Not a lot, but it would give another five plus inches of depth to
stuff more whatever in the tanks. Anyone want to try?
John Gerty - MJ "Zephyr"
In the wee hours of this morning I gave up my attempt to compose a
similar reply to the myths about water ballast having no weight when
positioned below the waterline and to acknowledge Chris Krumm's
excellent synopsis. You have done both in fine fashion. No need for
apologies, this thread was begging for a proper knot, and it took two
Chrises to tie it!
Using actual hull examples might help put this whole business in
perspective. I strongly reccomend Mike O'Brien's article "Shoal-Draft
Sharpies, Simple, self-righting, and roomy" in "WoodenBoat" No. 114,
September/October 1993, pages 64 to 75. Mike uses examples from
Bolger, Chapelle, Beebe, Herreshoff and Kirby. Of particular interest
is the part "Surprising Stability Curves" on pages 72 for Kirby's 31'
Norwalk Islands Sharpie with graphs and diagrams to show the
interaction of vertical center of gravity (VCG) with center of
buoyancy (CB) as the righting arm changes with heel angle.
Note that the VCG is fixed with respect to the hull configuration and
is the center of rotation while the CB moves as the underwater hull
geometry changes with heel angle. I believe this concept has been
overlooked in most of the previous discussions.
One more example regarding water versus lead or some other heavier
material ballast. As Chris Krumm pointed out, higher density material
"takes less volume than an equal mass of water; hence, it can
typically be stuffed lower in the hull and help lower the boat's
center of gravity." Martha Jane (isn't that who started much of this
thread?) has water ballast tanks roughly six inches deep, so the VCG
of its 500 pounds is about three inches above the bottom of the tanks.
Since lead is more than eleven times as heavy as an equal volume of
water 500 pounds would be just over one-half inch spread out over the
tank bottoms, and its VCG would be about one-quarter inch above the
tank bottom, a difference of 2 3/4 in. With total boat weight
(displacement) of 2500 lb, replacing 500 lb of water with 500 lb of
lead would not change the total weight but would lower 20% of the
total weight 2 3/4 inch thus lowering the total VCG a bit over 1/2
in. Not a lot, but it would give another five plus inches of depth to
stuff more whatever in the tanks. Anyone want to try?
John Gerty - MJ "Zephyr"
--- Inbolger@egroups.com, "Brand, Christopher" <cbrand@a...> wrote:
> "So, a water ballast under water has mass, but weighs nothing, that
is,
> the force is zero. Only when it, or part of it, rises above water is
> there a force."
>
> This is a myth, and unfortunately Mr. Bolger is guilty of
perpetuating it.
> Chris Krumm's excellent synopsis covers the issue accurately.
>
> Once the water ballast does not weigh (note the intentional use of
weight -
> force of gravity upon an object) less when it is positioned below
the
> waterline than when it is above the waterline. The force of
gravity is the
> same regardless.
>
> "(Otherwise, life vests, say, would be useless.)"
>
> This, and one other example will serve to prove this point. The
life vest
> lifts a person to the surface, not because gravity acts differently
upon it
> when it is below water than above, but because the water it
displaces weighs
> MORE (and is fluid and can therefore flow around and under the life
vest).
>
> Example of water ballast "working" below the surface of the water:
Take a
> hollow lead sphere that is just more than neutrally bouyant. It
floats just
> awash. This lead sphere has a watertight bulkhead that cuts it in
half and
> one hemisphere has a valve that can allow water to enter. If that
> hemisphere is allowed to flood with water, the sphere will sink and
it will
> orient itself with the flooded half at the bottom (toward the
center of the
> earth). If the sphere is turned so that the flooded half is at the
top, it
> will right itself powerfully.
>
> This behavior is in spite of the fact that all the water ballast is
below
> the "waterline". The reason is the separation of the center of
gravity and
> the center of bouyancy. CG is located in the lower hemisphere, the
CB is
> located in the center of the dividing bulkhead disk. These must
(by vitrue
> of the forces acting upon them) vertically align themselves.
>
> You could ballast such a sphere effectively with life vests,
ping-pong
> balls, or packing peanuts instead of water. As long as they have
more mass
> than the air in the other hemisphere. Radical as it may sound,
items used
> for floatation can act as ballast if they have mass (as Chris Krumm
> indirectly pointed out). It is only a queston of degrees of
effectiveness
> (the water would shift the center of gravity much farther than
packing
> peanuts).
>
> I wish Mr. Bolger would stop saying that water ballast isn't
effective until
> it is raised above the waterline by heeling because it just isn't
true. The
> idea of subtracting the space with the waterballast is intuitively
> appealing, but fallacious. There is a powerfull bouyant force
pressing
> upward on those parts of the boat, some portion of that force the
equal and
> opposite force to the weight of the water ballast falling toward
the center
> of the earth.
>
> Stop worrying about anything but the RELATIVE POSITIONS of the
CENTER OF
> GRAVITY and the CENTER OF BOUYANCY. THESE ARE THE ONLY TWO THINGS
THAT
> AFFECT STATIC STABILITY.
>
> Apologies for my venting/shouting. This has been an interesting
thread.
>
> -----Original Message-----
> From: Sakari Aaltonen [mailto:sakaria@c...]
> Sent: Sunday, November 05, 2000 10:55 PM
> To:bolger@egroups.com
> Subject: Re: [bolger] Re: Physics (and Water Ballast)
>
>
> I think the distinction is important. What a boat does is not
directly
> determined by the masses of all the items, but rather by their
weights.
> That's because, as you say, masses are not forces; weights are.
>
> An item's mass does not depend on whether it is under water or
above,
> but its weight does. (Otherwise, life vests, say, would be
useless.)
>
> So, a water ballast under water has mass, but weighs nothing, that
is,
> the force is zero. Only when it, or part of it, rises above water is
> there a force.
>
>
> Sakari Aaltonen
>
>
>
>
> Bolger rules!!!
> - no cursing
> - stay on topic
> - use punctuation
> - add your comments at the TOP and SIGN your posts
> - add some content: send "thanks!" and "ditto!" posts off-list.
"So, a water ballast under water has mass, but weighs nothing, that is,
the force is zero. Only when it, or part of it, rises above water is
there a force."
This is a myth, and unfortunately Mr. Bolger is guilty of perpetuating it.
Chris Krumm's excellent synopsis covers the issue accurately.
Once the water ballast does not weigh (note the intentional use of weight -
force of gravity upon an object) less when it is positioned below the
waterline than when it is above the waterline. The force of gravity is the
same regardless.
"(Otherwise, life vests, say, would be useless.)"
This, and one other example will serve to prove this point. The life vest
lifts a person to the surface, not because gravity acts differently upon it
when it is below water than above, but because the water it displaces weighs
MORE (and is fluid and can therefore flow around and under the life vest).
Example of water ballast "working" below the surface of the water: Take a
hollow lead sphere that is just more than neutrally bouyant. It floats just
awash. This lead sphere has a watertight bulkhead that cuts it in half and
one hemisphere has a valve that can allow water to enter. If that
hemisphere is allowed to flood with water, the sphere will sink and it will
orient itself with the flooded half at the bottom (toward the center of the
earth). If the sphere is turned so that the flooded half is at the top, it
will right itself powerfully.
This behavior is in spite of the fact that all the water ballast is below
the "waterline". The reason is the separation of the center of gravity and
the center of bouyancy. CG is located in the lower hemisphere, the CB is
located in the center of the dividing bulkhead disk. These must (by vitrue
of the forces acting upon them) vertically align themselves.
You could ballast such a sphere effectively with life vests, ping-pong
balls, or packing peanuts instead of water. As long as they have more mass
than the air in the other hemisphere. Radical as it may sound, items used
for floatation can act as ballast if they have mass (as Chris Krumm
indirectly pointed out). It is only a queston of degrees of effectiveness
(the water would shift the center of gravity much farther than packing
peanuts).
I wish Mr. Bolger would stop saying that water ballast isn't effective until
it is raised above the waterline by heeling because it just isn't true. The
idea of subtracting the space with the waterballast is intuitively
appealing, but fallacious. There is a powerfull bouyant force pressing
upward on those parts of the boat, some portion of that force the equal and
opposite force to the weight of the water ballast falling toward the center
of the earth.
Stop worrying about anything but the RELATIVE POSITIONS of the CENTER OF
GRAVITY and the CENTER OF BOUYANCY. THESE ARE THE ONLY TWO THINGS THAT
AFFECT STATIC STABILITY.
Apologies for my venting/shouting. This has been an interesting thread.
-----Original Message-----
From: Sakari Aaltonen [mailto:sakaria@...]
Sent: Sunday, November 05, 2000 10:55 PM
To:bolger@egroups.com
Subject: Re: [bolger] Re: Physics (and Water Ballast)
I think the distinction is important. What a boat does is not directly
determined by the masses of all the items, but rather by their weights.
That's because, as you say, masses are not forces; weights are.
An item's mass does not depend on whether it is under water or above,
but its weight does. (Otherwise, life vests, say, would be useless.)
So, a water ballast under water has mass, but weighs nothing, that is,
the force is zero. Only when it, or part of it, rises above water is
there a force.
Sakari Aaltonen
Bolger rules!!!
- no cursing
- stay on topic
- use punctuation
- add your comments at the TOP and SIGN your posts
- add some content: send "thanks!" and "ditto!" posts off-list.
the force is zero. Only when it, or part of it, rises above water is
there a force."
This is a myth, and unfortunately Mr. Bolger is guilty of perpetuating it.
Chris Krumm's excellent synopsis covers the issue accurately.
Once the water ballast does not weigh (note the intentional use of weight -
force of gravity upon an object) less when it is positioned below the
waterline than when it is above the waterline. The force of gravity is the
same regardless.
"(Otherwise, life vests, say, would be useless.)"
This, and one other example will serve to prove this point. The life vest
lifts a person to the surface, not because gravity acts differently upon it
when it is below water than above, but because the water it displaces weighs
MORE (and is fluid and can therefore flow around and under the life vest).
Example of water ballast "working" below the surface of the water: Take a
hollow lead sphere that is just more than neutrally bouyant. It floats just
awash. This lead sphere has a watertight bulkhead that cuts it in half and
one hemisphere has a valve that can allow water to enter. If that
hemisphere is allowed to flood with water, the sphere will sink and it will
orient itself with the flooded half at the bottom (toward the center of the
earth). If the sphere is turned so that the flooded half is at the top, it
will right itself powerfully.
This behavior is in spite of the fact that all the water ballast is below
the "waterline". The reason is the separation of the center of gravity and
the center of bouyancy. CG is located in the lower hemisphere, the CB is
located in the center of the dividing bulkhead disk. These must (by vitrue
of the forces acting upon them) vertically align themselves.
You could ballast such a sphere effectively with life vests, ping-pong
balls, or packing peanuts instead of water. As long as they have more mass
than the air in the other hemisphere. Radical as it may sound, items used
for floatation can act as ballast if they have mass (as Chris Krumm
indirectly pointed out). It is only a queston of degrees of effectiveness
(the water would shift the center of gravity much farther than packing
peanuts).
I wish Mr. Bolger would stop saying that water ballast isn't effective until
it is raised above the waterline by heeling because it just isn't true. The
idea of subtracting the space with the waterballast is intuitively
appealing, but fallacious. There is a powerfull bouyant force pressing
upward on those parts of the boat, some portion of that force the equal and
opposite force to the weight of the water ballast falling toward the center
of the earth.
Stop worrying about anything but the RELATIVE POSITIONS of the CENTER OF
GRAVITY and the CENTER OF BOUYANCY. THESE ARE THE ONLY TWO THINGS THAT
AFFECT STATIC STABILITY.
Apologies for my venting/shouting. This has been an interesting thread.
-----Original Message-----
From: Sakari Aaltonen [mailto:sakaria@...]
Sent: Sunday, November 05, 2000 10:55 PM
To:bolger@egroups.com
Subject: Re: [bolger] Re: Physics (and Water Ballast)
I think the distinction is important. What a boat does is not directly
determined by the masses of all the items, but rather by their weights.
That's because, as you say, masses are not forces; weights are.
An item's mass does not depend on whether it is under water or above,
but its weight does. (Otherwise, life vests, say, would be useless.)
So, a water ballast under water has mass, but weighs nothing, that is,
the force is zero. Only when it, or part of it, rises above water is
there a force.
Sakari Aaltonen
Bolger rules!!!
- no cursing
- stay on topic
- use punctuation
- add your comments at the TOP and SIGN your posts
- add some content: send "thanks!" and "ditto!" posts off-list.
Chris:
Since you agree with ME, I heartily endorse your synopsis. (grin)
Chuck
Since you agree with ME, I heartily endorse your synopsis. (grin)
Chuck
> Hi all -....snip...
> I've read through most of the "Physics" thread, and am just going to
> put my bit in. acouple days ago Chuck pointed out the discrepancy in
> Ernie's logic regarding adding balloons of water outside the hull vs.
> lead sinkers.
>
>
> Chris Krumm
>
>
> It's pretty simple. Mass is mass - whether a kilogram of H2O or ofI think the distinction is important. What a boat does is not directly
> lead (or feathers). Float a hull at a specified waterline. The mass
> of the displaced water (the immersed volume) will equal the mass of
> the boat. Doesn't matter whether the total mass of the boat is made
> up of wood, lead and passengers; or fiberglass, water jugs and
> passengers. If the hulls are of identical size and form, and weigh
> the same, they'll float at the same static waterline. For salt water,
> take the immersed volume in cubic feet and multiply by 64#/cubic foot
> to get the weight of the boat. For fresh water, multiply immersed
> volume by 62.4#/cubic foot to get weight of boat.
>
> The combined masses of all the items comprising the boat act at a
> single point, the center of gravity (CG). 50# of ballast at each side
> of the hull has the same effect regarding static stability and
> righting moment as 100# of ballast at the centerline, provided the
> ballast is the same distance from the bow and above/below the
> waterline. The center of gravity of either combination will be the
> same.
>........................
> Note I cross over between mass and weight pretty indicriminately -
> mass is more typically associated with SI units (metric). Weight is
> more commonly referred to in British units. Weight is a force - mass
> (in slugs) x acceleration (due to gravity). Long winded, but I hope
> complete...
>
> Chris Krumm
determined by the masses of all the items, but rather by their weights.
That's because, as you say, masses are not forces; weights are.
An item's mass does not depend on whether it is under water or above,
but its weight does. (Otherwise, life vests, say, would be useless.)
So, a water ballast under water has mass, but weighs nothing, that is,
the force is zero. Only when it, or part of it, rises above water is
there a force.
Sakari Aaltonen
Hi Chris,
As long as the center of gravity is on the "good" side of the center of
buoyancy (i.e. adding to the force tending to right the vessel) all is well.
When it is not, the vessel capsizes.
With a circular midships section the center of buoyancy (cb) won't move much
as she heels, only the center of gravity(cg). With a Bolger Box, the cb
moves towards the deck as she heels thus amplifying (perhaps) the effect of
ballast. Water ballast when submerged only tends to damp rapid movement by
adding mass. If her heeling over lifts the tank of water ballast out of the
water, and if there is a horizontal distance between the center of gravity
of the tank and the cb, the water ballast will exert a righting force on the
vessel.
Water ballast won't make her stiffer until she is heeled way over but it may
prevent capsize. In a box shaped hull, her shape provides the initial
stiffness. In a fully developed (molded) hull with slack bilges only a
massive lead mine on the bottom will make her stiff. But the high sailing
rig required to push all that through the water tends to fight the
stiffness. The design of yachts, alas, requires compromise.
Jim
krumly@...wrote:
As long as the center of gravity is on the "good" side of the center of
buoyancy (i.e. adding to the force tending to right the vessel) all is well.
When it is not, the vessel capsizes.
With a circular midships section the center of buoyancy (cb) won't move much
as she heels, only the center of gravity(cg). With a Bolger Box, the cb
moves towards the deck as she heels thus amplifying (perhaps) the effect of
ballast. Water ballast when submerged only tends to damp rapid movement by
adding mass. If her heeling over lifts the tank of water ballast out of the
water, and if there is a horizontal distance between the center of gravity
of the tank and the cb, the water ballast will exert a righting force on the
vessel.
Water ballast won't make her stiffer until she is heeled way over but it may
prevent capsize. In a box shaped hull, her shape provides the initial
stiffness. In a fully developed (molded) hull with slack bilges only a
massive lead mine on the bottom will make her stiff. But the high sailing
rig required to push all that through the water tends to fight the
stiffness. The design of yachts, alas, requires compromise.
Jim
krumly@...wrote:
> Hi all -
> I've read through most of the "Physics" thread, and am just going to
> put my bit in. acouple days ago Chuck pointed out the discrepancy in
> Ernie's logic regarding adding balloons of water outside the hull vs.
> lead sinkers.
>
> It's pretty simple. Mass is mass - whether a kilogram of H2O or of
> lead (or feathers). Float a hull at a specified waterline. The mass
> of the displaced water (the immersed volume) will equal the mass of
> the boat. Doesn't matter whether the total mass of the boat is made
> up of wood, lead and passengers; or fiberglass, water jugs and
> passengers. If the hulls are of identical size and form, and weigh
> the same, they'll float at the same static waterline. For salt water,
> take the immersed volume in cubic feet and multiply by 64#/cubic foot
> to get the weight of the boat. For fresh water, multiply immersed
> volume by 62.4#/cubic foot to get weight of boat.
>
> The combined masses of all the items comprising the boat act at a
> single point, the center of gravity (CG). 50# of ballast at each side
> of the hull has the same effect regarding static stability and
> righting moment as 100# of ballast at the centerline, provided the
> ballast is the same distance from the bow and above/below the
> waterline. The center of gravity of either combination will be the
> same. Again, it doesn't matter whether this ballast is a tank of
> water, a pig of lead, or a couple of passengers. As pointed out in an
> earlier post (by Gregg Carlson, I believe), the boat with the 50#
> chunks of ballast at each chine will have a higher polar moment of
> inertia and will both take longer to both get rolling and damp that
> rolling.
>
> Pro's of high density ballast such as lead are that it takes less
> volume than an equal mass of water; hence, it can typically be stuffed
> lower in the hull and help lower the boats center of gravity. Cons of
> lead are that you have to carry that mass at all times - on the
> trailer, on downwind sail legs, et al.
>
> Pro's of water ballast are that you can adjust the volume (mass)
> required, and pump it to different tanks in the boat (to windward when
> sailing upwind, to leeward in light winds if a small amount of heel
> improves your hull's performance, or aft when surfing down wind in
> order to keep the bow up). Con's are all the plumbing and vulnerable
> stuff needed to move this water around and in/out of the boat.
>
> Note I cross over between mass and weight pretty indicriminately -
> mass is more typically associated with SI units (metric). Weight is
> more commonly referred to in British units. Weight is a force - mass
> (in slugs) x acceleration (due to gravity). Long winded, but I hope
> complete...
>
> Chris Krumm
>
>
> Bolger rules!!!
> - no cursing
> - stay on topic
> - use punctuation
> - add your comments at the TOP and SIGN your posts
> - add some content: send "thanks!" and "ditto!" posts off-list.
Well here is an idea "borrowed" from Tristan Jones:
It is quite usual to reef by lowering the sail area exposed to the
wind...Jones suggests another form of "reefing", by increasing the ballast
(rather than decreasing sail area) when more stability is needed. What he
suggests is a large diameter PVC tube with caps on both ends. In windy
conditions it could be filled with seawater, capped and suspended just below
the actual waterline parallel to the windward side.
As the boat heels, the downward force of the water, as it is moved above
the waterline, will contribute weight (or perhaps mass?) to help right the
boat by opposing the heeling movement. He SAID it would work, but then he
was also talking about a catamaran. Sort of like pumping water to the
windward ballast tank. Any takers?
Best regards, Warren
It is quite usual to reef by lowering the sail area exposed to the
wind...Jones suggests another form of "reefing", by increasing the ballast
(rather than decreasing sail area) when more stability is needed. What he
suggests is a large diameter PVC tube with caps on both ends. In windy
conditions it could be filled with seawater, capped and suspended just below
the actual waterline parallel to the windward side.
As the boat heels, the downward force of the water, as it is moved above
the waterline, will contribute weight (or perhaps mass?) to help right the
boat by opposing the heeling movement. He SAID it would work, but then he
was also talking about a catamaran. Sort of like pumping water to the
windward ballast tank. Any takers?
Best regards, Warren
Hi all -
I've read through most of the "Physics" thread, and am just going to
put my bit in. acouple days ago Chuck pointed out the discrepancy in
Ernie's logic regarding adding balloons of water outside the hull vs.
lead sinkers.
It's pretty simple. Mass is mass - whether a kilogram of H2O or of
lead (or feathers). Float a hull at a specified waterline. The mass
of the displaced water (the immersed volume) will equal the mass of
the boat. Doesn't matter whether the total mass of the boat is made
up of wood, lead and passengers; or fiberglass, water jugs and
passengers. If the hulls are of identical size and form, and weigh
the same, they'll float at the same static waterline. For salt water,
take the immersed volume in cubic feet and multiply by 64#/cubic foot
to get the weight of the boat. For fresh water, multiply immersed
volume by 62.4#/cubic foot to get weight of boat.
The combined masses of all the items comprising the boat act at a
single point, the center of gravity (CG). 50# of ballast at each side
of the hull has the same effect regarding static stability and
righting moment as 100# of ballast at the centerline, provided the
ballast is the same distance from the bow and above/below the
waterline. The center of gravity of either combination will be the
same. Again, it doesn't matter whether this ballast is a tank of
water, a pig of lead, or a couple of passengers. As pointed out in an
earlier post (by Gregg Carlson, I believe), the boat with the 50#
chunks of ballast at each chine will have a higher polar moment of
inertia and will both take longer to both get rolling and damp that
rolling.
Pro's of high density ballast such as lead are that it takes less
volume than an equal mass of water; hence, it can typically be stuffed
lower in the hull and help lower the boats center of gravity. Cons of
lead are that you have to carry that mass at all times - on the
trailer, on downwind sail legs, et al.
Pro's of water ballast are that you can adjust the volume (mass)
required, and pump it to different tanks in the boat (to windward when
sailing upwind, to leeward in light winds if a small amount of heel
improves your hull's performance, or aft when surfing down wind in
order to keep the bow up). Con's are all the plumbing and vulnerable
stuff needed to move this water around and in/out of the boat.
Note I cross over between mass and weight pretty indicriminately -
mass is more typically associated with SI units (metric). Weight is
more commonly referred to in British units. Weight is a force - mass
(in slugs) x acceleration (due to gravity). Long winded, but I hope
complete...
Chris Krumm
I've read through most of the "Physics" thread, and am just going to
put my bit in. acouple days ago Chuck pointed out the discrepancy in
Ernie's logic regarding adding balloons of water outside the hull vs.
lead sinkers.
It's pretty simple. Mass is mass - whether a kilogram of H2O or of
lead (or feathers). Float a hull at a specified waterline. The mass
of the displaced water (the immersed volume) will equal the mass of
the boat. Doesn't matter whether the total mass of the boat is made
up of wood, lead and passengers; or fiberglass, water jugs and
passengers. If the hulls are of identical size and form, and weigh
the same, they'll float at the same static waterline. For salt water,
take the immersed volume in cubic feet and multiply by 64#/cubic foot
to get the weight of the boat. For fresh water, multiply immersed
volume by 62.4#/cubic foot to get weight of boat.
The combined masses of all the items comprising the boat act at a
single point, the center of gravity (CG). 50# of ballast at each side
of the hull has the same effect regarding static stability and
righting moment as 100# of ballast at the centerline, provided the
ballast is the same distance from the bow and above/below the
waterline. The center of gravity of either combination will be the
same. Again, it doesn't matter whether this ballast is a tank of
water, a pig of lead, or a couple of passengers. As pointed out in an
earlier post (by Gregg Carlson, I believe), the boat with the 50#
chunks of ballast at each chine will have a higher polar moment of
inertia and will both take longer to both get rolling and damp that
rolling.
Pro's of high density ballast such as lead are that it takes less
volume than an equal mass of water; hence, it can typically be stuffed
lower in the hull and help lower the boats center of gravity. Cons of
lead are that you have to carry that mass at all times - on the
trailer, on downwind sail legs, et al.
Pro's of water ballast are that you can adjust the volume (mass)
required, and pump it to different tanks in the boat (to windward when
sailing upwind, to leeward in light winds if a small amount of heel
improves your hull's performance, or aft when surfing down wind in
order to keep the bow up). Con's are all the plumbing and vulnerable
stuff needed to move this water around and in/out of the boat.
Note I cross over between mass and weight pretty indicriminately -
mass is more typically associated with SI units (metric). Weight is
more commonly referred to in British units. Weight is a force - mass
(in slugs) x acceleration (due to gravity). Long winded, but I hope
complete...
Chris Krumm
Chris.
Ill keep your excellent summary.
You mention the mass/weight confusion.
Its also a fact that predicting behaviour of a
boat under say water ballasting involves inertial as well as gravitational
effects.
Throw wave action into the mix, and wild variation in surface drag as the
boat leaps in and out of the air/water interface.
In the end you have to go out there and test.
Jeff Gilbert
Original Message -----
From: <krumly@...>
To: <bolger@egroups.com>
Sent: Monday, November 06, 2000 4:30 AM
Subject: [bolger] Re: Physics (and Water Ballast)
Hi all -
I've read through most of the "Physics" thread, and am just going to
put my bit in. acouple days ago Chuck pointed out the discrepancy in
Ernie's logic regarding adding balloons of water outside the hull vs.
lead sinkers.
It's pretty simple. Mass is mass - whether a kilogram of H2O or of
lead (or feathers). Float a hull at a specified waterline. The mass
of the displaced water (the immersed volume) will equal the mass of
the boat. Doesn't matter whether the total mass of the boat is made
up of wood, lead and passengers; or fiberglass, water jugs and
passengers. If the hulls are of identical size and form, and weigh
the same, they'll float at the same static waterline. For salt water,
take the immersed volume in cubic feet and multiply by 64#/cubic foot
to get the weight of the boat. For fresh water, multiply immersed
volume by 62.4#/cubic foot to get weight of boat.
The combined masses of all the items comprising the boat act at a
single point, the center of gravity (CG). 50# of ballast at each side
of the hull has the same effect regarding static stability and
righting moment as 100# of ballast at the centerline, provided the
ballast is the same distance from the bow and above/below the
waterline. The center of gravity of either combination will be the
same. Again, it doesn't matter whether this ballast is a tank of
water, a pig of lead, or a couple of passengers. As pointed out in an
earlier post (by Gregg Carlson, I believe), the boat with the 50#
chunks of ballast at each chine will have a higher polar moment of
inertia and will both take longer to both get rolling and damp that
rolling.
Pro's of high density ballast such as lead are that it takes less
volume than an equal mass of water; hence, it can typically be stuffed
lower in the hull and help lower the boats center of gravity. Cons of
lead are that you have to carry that mass at all times - on the
trailer, on downwind sail legs, et al.
Pro's of water ballast are that you can adjust the volume (mass)
required, and pump it to different tanks in the boat (to windward when
sailing upwind, to leeward in light winds if a small amount of heel
improves your hull's performance, or aft when surfing down wind in
order to keep the bow up). Con's are all the plumbing and vulnerable
stuff needed to move this water around and in/out of the boat.
Note I cross over between mass and weight pretty indicriminately -
mass is more typically associated with SI units (metric). Weight is
more commonly referred to in British units. Weight is a force - mass
(in slugs) x acceleration (due to gravity). Long winded, but I hope
complete...
Chris Krumm
Bolger rules!!!
- no cursing
- stay on topic
- use punctuation
- add your comments at the TOP and SIGN your posts
- add some content: send "thanks!" and "ditto!" posts off-list.
Ill keep your excellent summary.
You mention the mass/weight confusion.
Its also a fact that predicting behaviour of a
boat under say water ballasting involves inertial as well as gravitational
effects.
Throw wave action into the mix, and wild variation in surface drag as the
boat leaps in and out of the air/water interface.
In the end you have to go out there and test.
Jeff Gilbert
Original Message -----
From: <krumly@...>
To: <bolger@egroups.com>
Sent: Monday, November 06, 2000 4:30 AM
Subject: [bolger] Re: Physics (and Water Ballast)
Hi all -
I've read through most of the "Physics" thread, and am just going to
put my bit in. acouple days ago Chuck pointed out the discrepancy in
Ernie's logic regarding adding balloons of water outside the hull vs.
lead sinkers.
It's pretty simple. Mass is mass - whether a kilogram of H2O or of
lead (or feathers). Float a hull at a specified waterline. The mass
of the displaced water (the immersed volume) will equal the mass of
the boat. Doesn't matter whether the total mass of the boat is made
up of wood, lead and passengers; or fiberglass, water jugs and
passengers. If the hulls are of identical size and form, and weigh
the same, they'll float at the same static waterline. For salt water,
take the immersed volume in cubic feet and multiply by 64#/cubic foot
to get the weight of the boat. For fresh water, multiply immersed
volume by 62.4#/cubic foot to get weight of boat.
The combined masses of all the items comprising the boat act at a
single point, the center of gravity (CG). 50# of ballast at each side
of the hull has the same effect regarding static stability and
righting moment as 100# of ballast at the centerline, provided the
ballast is the same distance from the bow and above/below the
waterline. The center of gravity of either combination will be the
same. Again, it doesn't matter whether this ballast is a tank of
water, a pig of lead, or a couple of passengers. As pointed out in an
earlier post (by Gregg Carlson, I believe), the boat with the 50#
chunks of ballast at each chine will have a higher polar moment of
inertia and will both take longer to both get rolling and damp that
rolling.
Pro's of high density ballast such as lead are that it takes less
volume than an equal mass of water; hence, it can typically be stuffed
lower in the hull and help lower the boats center of gravity. Cons of
lead are that you have to carry that mass at all times - on the
trailer, on downwind sail legs, et al.
Pro's of water ballast are that you can adjust the volume (mass)
required, and pump it to different tanks in the boat (to windward when
sailing upwind, to leeward in light winds if a small amount of heel
improves your hull's performance, or aft when surfing down wind in
order to keep the bow up). Con's are all the plumbing and vulnerable
stuff needed to move this water around and in/out of the boat.
Note I cross over between mass and weight pretty indicriminately -
mass is more typically associated with SI units (metric). Weight is
more commonly referred to in British units. Weight is a force - mass
(in slugs) x acceleration (due to gravity). Long winded, but I hope
complete...
Chris Krumm
Bolger rules!!!
- no cursing
- stay on topic
- use punctuation
- add your comments at the TOP and SIGN your posts
- add some content: send "thanks!" and "ditto!" posts off-list.