## Re: water ballast revisited

Expand Messages
• John: This is an excellent visualization! Everybody: Read this! I don t think that you can reduce the water ballast question to math anyway. You have
Message 1 of 9 , Feb 29, 2000
John:

This is an excellent visualization! Everybody: Read this! I don't think
that you can reduce the water ballast question to math anyway. You have
succeeded in putting the whole thing in a nutshell. Thanks.

Chuck Leinweber
Duckworks Magazine
http://www.duckworksmagazine.com

----- Original Message -----
From: John Paul Bullen <drjpbullen@...>
To: <boatdesign@onelist.com>
Sent: Wednesday, March 01, 2000 8:20 AM
Subject: [boatdesign] water ballast revisited

> From: John Paul Bullen <drjpbullen@...>
>
> Hi all,
> Sorry to revisit this subject while everyone has moved to a higher plane,
but
> I am a "bear of little brain" and it took a while to think it through.I
> provide this for the benefit of people like myself who get cold sweats and
an
> attack of the vapours when they see a mathematical formula.
> Unfortunately, without maths the talk is longwinded, sorry.
>
> A couple of weeks ago someone wrote that water ballast only works when
above
> the water level. I suggest the following "thought experiment" to
demonstrate
> that this need not always be the case.
>
> Imagine a length of pipe with the ends blanked off to represent the hull.
> Chuck it in the water and it floats. Adjust the density of the wall of the
> pipe so that it floats mostly submerged. (It has no top or bottom side in
this
> condition as the centre of gravity and the centre of bouyancy are along
the
> centre of the long axis of the pipe) Now glue a small sachet of water
inside
> the pipe. It will now float mostly submerged, but with the sachet of water
at
> the bottom, and below water level. Interesting!!
>
> Now, to complete the mind game, remove the sachet of water, and this time
> glue it onto the outside of the pipe. Intuitively, in this condition the
water
> will only exert a righting effect when it is lifted above water level.
Even
> more interesting!
>
> How does one explain this, and what does it mean for boat design?
> I'm no expert on any of this stuff, but two things that need to be
remembered
> are:
> 1)that water ballast can be used to move (lower) the centre of gravity of
a
> hull
> 2)that the relationship of the C of G compared to the C of B is what
provides
> the righting effect of the ballast
> 3)most hulls take a shape more like a somewhat squashed pipe, and when
this
> flattened shape starts to heal the centre of bouyancy shifts to the
leeward
> side increasing the righting effect. The extreme example of this is the
> catamaran when it lifts a hull the centre of bouyancy shifts to somewhere
in
> the hull that remains in the water.
>
> hope this helps, JohnPaul
>
>
>
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>
• ... Yes. Yes, yes, yes, yes, yes. Yes.
Message 2 of 9 , Feb 29, 2000
Chuck Leinweber wrote:
>
> From: "Chuck Leinweber" <chuck@...>
>
> John:
>
> This is an excellent visualization! Everybody: Read this! I don't think
> that you can reduce the water ballast question to math anyway. You have
> succeeded in putting the whole thing in a nutshell. Thanks.
>

Yes.

Yes, yes, yes, yes, yes.

Yes.

> Chuck Leinweber
> Duckworks Magazine
> http://www.duckworksmagazine.com
>
> ----- Original Message -----
> From: John Paul Bullen <drjpbullen@...>
> To: <boatdesign@onelist.com>
> Sent: Wednesday, March 01, 2000 8:20 AM
> Subject: [boatdesign] water ballast revisited
>
> > From: John Paul Bullen <drjpbullen@...>
> >
> > Hi all,
> > Sorry to revisit this subject while everyone has moved to a higher plane,
> but
> > I am a "bear of little brain" and it took a while to think it through.I
> > provide this for the benefit of people like myself who get cold sweats and
> an
> > attack of the vapours when they see a mathematical formula.
> > Unfortunately, without maths the talk is longwinded, sorry.
> >
> > A couple of weeks ago someone wrote that water ballast only works when
> above
> > the water level. I suggest the following "thought experiment" to
> demonstrate
> > that this need not always be the case.
> >
> > Imagine a length of pipe with the ends blanked off to represent the hull.
> > Chuck it in the water and it floats. Adjust the density of the wall of the
> > pipe so that it floats mostly submerged. (It has no top or bottom side in
> this
> > condition as the centre of gravity and the centre of bouyancy are along
> the
> > centre of the long axis of the pipe) Now glue a small sachet of water
> inside
> > the pipe. It will now float mostly submerged, but with the sachet of water
> at
> > the bottom, and below water level. Interesting!!
> >
> > Now, to complete the mind game, remove the sachet of water, and this time
> > glue it onto the outside of the pipe. Intuitively, in this condition the
> water
> > will only exert a righting effect when it is lifted above water level.
> Even
> > more interesting!
> >
> > How does one explain this, and what does it mean for boat design?
> > I'm no expert on any of this stuff, but two things that need to be
> remembered
> > are:
> > 1)that water ballast can be used to move (lower) the centre of gravity of
> a
> > hull
> > 2)that the relationship of the C of G compared to the C of B is what
> provides
> > the righting effect of the ballast
> > 3)most hulls take a shape more like a somewhat squashed pipe, and when
> this
> > flattened shape starts to heal the centre of bouyancy shifts to the
> leeward
> > side increasing the righting effect. The extreme example of this is the
> > catamaran when it lifts a hull the centre of bouyancy shifts to somewhere
> in
> > the hull that remains in the water.
> >
> > hope this helps, JohnPaul
> >
> >
> >
> > ____________________________________________________________________
> > Get your own FREE, personal Netscape WebMail account today at
> http://webmail.netscape.com
> >
> > ------------------------------------------------------------------------
> > Get a FREE \$15 BN.com gift certificate when you join carclub.com.
> > As a member, you can save up to \$550 a year if you own a car, and
> > thousands more when you're buying one.
> > http://click.egroups.com/1/1945/1/_/446985/_/951861037/
> > ------------------------------------------------------------------------
> >
> >
>
> ------------------------------------------------------------------------
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• Hi all, Sorry to revisit this subject while everyone has moved to a higher plane, but I am a bear of little brain and it took a while to think it through.I
Message 3 of 9 , Mar 1, 2000
Hi all,
Sorry to revisit this subject while everyone has moved to a higher plane, but
I am a "bear of little brain" and it took a while to think it through.I
provide this for the benefit of people like myself who get cold sweats and an
attack of the vapours when they see a mathematical formula.
Unfortunately, without maths the talk is longwinded, sorry.

A couple of weeks ago someone wrote that water ballast only works when above
the water level. I suggest the following "thought experiment" to demonstrate
that this need not always be the case.

Imagine a length of pipe with the ends blanked off to represent the hull.
Chuck it in the water and it floats. Adjust the density of the wall of the
pipe so that it floats mostly submerged. (It has no top or bottom side in this
condition as the centre of gravity and the centre of bouyancy are along the
centre of the long axis of the pipe) Now glue a small sachet of water inside
the pipe. It will now float mostly submerged, but with the sachet of water at
the bottom, and below water level. Interesting!!

Now, to complete the mind game, remove the sachet of water, and this time
glue it onto the outside of the pipe. Intuitively, in this condition the water
will only exert a righting effect when it is lifted above water level. Even
more interesting!

How does one explain this, and what does it mean for boat design?
I'm no expert on any of this stuff, but two things that need to be remembered
are:
1)that water ballast can be used to move (lower) the centre of gravity of a
hull
2)that the relationship of the C of G compared to the C of B is what provides
the righting effect of the ballast
3)most hulls take a shape more like a somewhat squashed pipe, and when this
flattened shape starts to heal the centre of bouyancy shifts to the leeward
side increasing the righting effect. The extreme example of this is the
catamaran when it lifts a hull the centre of bouyancy shifts to somewhere in
the hull that remains in the water.

hope this helps, JohnPaul

____________________________________________________________________
Get your own FREE, personal Netscape WebMail account today at http://webmail.netscape.com
• Good example. Now the quiz: suppose that your thick-walled tube has a gouge on the outside - just a depression, not a hole. No water sachet. Which way will the
Message 4 of 9 , Mar 2, 2000
Good example.

Now the quiz: suppose that your thick-walled tube has a gouge on the
outside - just a depression, not a hole. No water sachet. Which way
will the tube float?

Peter
• Good example. Now the quiz: suppose that your thick-walled tube has one side shaved flat on the outside. No water sachet. Which way will the tube float? Peter
Message 5 of 9 , Mar 2, 2000
Good example.

Now the quiz: suppose that your thick-walled tube has one side shaved
flat on the outside. No water sachet. Which way will the tube float?

Peter
• Shaved side up, it s lighter. I saved a glass har from the recycling last night, maybe I ll get my kids together and do a science fair project this weekend.
Message 6 of 9 , Mar 2, 2000
Shaved side up, it's lighter.

I saved a glass har from the recycling last night, maybe I'll get my kids
together and do a science fair project this weekend. Report to follow.
However I do not plan to shave the glass.

--- Peter & Polly Vanderwaart <pvanderw@...> wrote:
> From: "Peter & Polly Vanderwaart" <pvanderw@...>
>
> Good example.
>
> Now the quiz: suppose that your thick-walled tube has one side shaved
> flat on the outside. No water sachet. Which way will the tube float?
>
> Peter
>
>
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• Wouldn t it float flat side up? The C of G being moved towards the round side? However the gouged one stumps me. Seems the surface area of the tube would
Message 7 of 9 , Mar 2, 2000
Wouldn't it float flat side up? The C of G being moved towards the round
side?
However the gouged one stumps me. Seems the surface area of the tube would
increase so the tupe material would weigh more, however it would now
displace less water? Hmm... float lower in the water with the gouge down?
That's my guess.
djb

Peter & Polly Vanderwaart wrote:

> From: "Peter & Polly Vanderwaart" <pvanderw@...>
>
> Good example.
>
> Now the quiz: suppose that your thick-walled tube has one side shaved
> flat on the outside. No water sachet. Which way will the tube float?
>
> Peter
>
> ------------------------------------------------------------------------
> GET A NEXTCARD VISA, in 30 seconds! Get rates
> as low as 0.0% Intro APR and no hidden fees.
> Apply NOW!
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• Wouldn t it float flat side up? The C of G being moved towards the round side? However the gouged one stumps me a bit. Seems the surface area of the tube would
Message 8 of 9 , Mar 2, 2000
Wouldn't it float flat side up? The C of G being moved towards the round
side?
However the gouged one stumps me a bit. Seems the surface area of the tube
would increase so the tube material would weigh more, however it would now
displace less water? Hmm... float lower in the water with the gouge down?
That's my guess.
djb

Peter & Polly Vanderwaart wrote:

> From: "Peter & Polly Vanderwaart" <pvanderw@...>
>
> Good example.
>
> Now the quiz: suppose that your thick-walled tube has one side shaved
> flat on the outside. No water sachet. Which way will the tube float?
>
> Peter
>
> ------------------------------------------------------------------------
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SOME CLICKS THAT COUNT!!
Feed someone.
http://www.thehungersite.com/
Save a little rain forest.
http://rainforest.care2.com/front.html/player12296

Simplicity Boats (& mirror sites)
http://members.tripod.com/simplicityboats/index.html
http://members.xoom.com/simpleboats/index.html
Here's my latest boat:
http://members.tripod.com/simplicityboats/featherwind.html
http://members.xoom.com/simpleboats/featherwind.html
Quasi esoteric musical instruments
http://unicornstrings.com
• Peter, I sent the thought experiment to Jim Michalak for his take on it. Here tis. ... I agree with the comments. It took me a while to figure out because I
Message 9 of 9 , Mar 2, 2000
Peter,
I sent the thought experiment to Jim Michalak for his take on it. Here
'tis.

> Jim,
> This is from a discussion on the boat design list. I found your
> article and graphs on ballast interesting, and I'd love any comments a
> real engineer might add to this water ballast thought experiment.
> Thanks,
> David

I agree with the comments. It took me a while to figure out because I
had also seen so many times that only the water ballast above the
waterline counts. That is true only if a tank is added to the outside of
the normal hull. In that case the tank has neutral weight because the
added ballast is exactly balanced by the buoyancy water that tank
displaces, until the tank is lifted out of the water, of course.

If the tank is internal to the normal hull then that is not usually
true. The ballast is just weight and could be any material with the same
weight and cg location, the boat has no way of knowing if the tank has
water or sand or concrete or lead inside. The added weight is still
balanced by an equal amount of displaced water but now the diplacement
is spread out over the normal hull.

JIm

Peter & Polly Vanderwaart wrote:

> From: "Peter & Polly Vanderwaart" <pvanderw@...>
>
> Good example.
>
> Now the quiz: suppose that your thick-walled tube has one side shaved
> flat on the outside. No water sachet. Which way will the tube float?
>
> Peter
>
> ------------------------------------------------------------------------
> GET A NEXTCARD VISA, in 30 seconds! Get rates
> as low as 0.0% Intro APR and no hidden fees.
> Apply NOW!
> http://click.egroups.com/1/975/1/_/446985/_/952009009/
> ------------------------------------------------------------------------

--

SOME CLICKS THAT COUNT!!
Feed someone.
http://www.thehungersite.com/
Save a little rain forest.
http://rainforest.care2.com/front.html/player12296

Simplicity Boats (& mirror sites)
http://members.tripod.com/simplicityboats/index.html
http://members.xoom.com/simpleboats/index.html
Here's my latest boat:
http://members.tripod.com/simplicityboats/featherwind.html
http://members.xoom.com/simpleboats/featherwind.html
Quasi esoteric musical instruments
http://unicornstrings.com
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