Dave Doe wrote:
Nope, never did. I'm saying it's stability is indeed improved (more
mass to move)

That would be an issue of inertia... dynamic stability, if
you like the term.


Dave Doe wrote:
ie stability.


There is a difference between static stability ei righting
moment, and dynamic stability or roll resistance. The first
is relatively simple, the latter vastly complex and
influenced by underwater foils, distribution of mass,
distribution of hull volume & reserve bouyancy, etc etc.

Correct me if I'm wrong, Dave: you started out by saying
that water ballast cannot generate righting moment.



... but question its righting ability vs lead keel - and
posed a question about that.


Well, I don't think that anybody has claimed that water
ballast provides equal righting moment to external lead
ballast. But it can provide significant righting moment,
especially if the boat is designed from the start to utilize
water ballast effectively.


Provide some evidence.


That lots & lots of water ballast boats are out there
sailing? That I've personally sailed about a dozen boats
with water ballast, and found them to have no significant
difference in the way they sail compared to lead ballasted
boats?

For example, we owned & sailed a 19' water ballasted sloop
for eleven years. Many times at club get-togethers, people
would say "Is a water ballast boat stable enough"? I would
say, "Step on the gun'l and see." People would always
conclude, after this very real test, that our boat was just
as stable as 21 foot & 22 foot boats with lead ballasted
swing keels & keel/centerboards. It's a question of how the
boat is designed, not the material used for ballast.


The problem is that water in the ballast has the same density of the
water it is in

Well there you go again. The water ballast is not "in water"
it is in the boat.


Where is the boat?


Floating.

AGAIN-
If the water ballast did not "weigh anything" then the boat
would not get lower in the water when the ballast tank is
filled.


Let me propose this example- a cooler full of ice & beer is
lighter than water (hence lighter than water ballast) yet
can be pretty heavy when you're carrying it down the dock.
If you put it in the lowest possible location in the boat,
right down against the hull, and tie it in securely (to the
handles, so you can still open the lid of course), will this
improve the boat's stability?


What is your point?

That water ballast works just fine.


... I think you need to consider the
basics when considering a sailing vessel;

We have done exactly that, Dave.

.... water on or about the
centerline is a waste of potential that is even more easily achieved
otherwise.

Really? Like what?

.... It is the same density of the stuff you're moving through -
it's a very major factor, as you're floating on it.

Actually, it's not a factor at all. Water is heavy. Put it
down low in the boat, and it functions as ballast.


.... Consider the
obvious. When the boat is on an angle the only force the water can
exert is on the air below it and not the water (it exerts no effective
force on the water if you can get your head around that).

Wrong, wrong, and wrong.

Does the force of gravity have some sort of sixth sense that
the water ballast is "exerting force on the air below it"?

No, gravity pulls downward equally on the water ballast at
all times, just as it does on the hull, the crew, the beer,
and for that matter, the water that the boat is floating in.



Consider the absurd, a boat of no mass other than its water ballast. It
will sit in the water, level with the ballast waterline. If you are to
heel it - well you work it out. The maths is easy, consider the water
ballast as a "solid" (as it cannot move).

The ballast lowers the center of gravity of the boat. As the
boat heels, the center of bouyancy shifts to the low side.
The center of bouyancy pushes up, the center of gravity
pushes down, when the boat is level they are in alignment.
When the boat is heeled, there is a lever arm between the
forces which is the force we call "righting moment."

A given displacement & a given lever arm will give the same
righting moment, whether the ballast is lead or feathers. It
is true that lead can be placed lower in the boat, but that
doesn't change the basic physics of stability.





Short of moving magically moving the water ballast from one side to the
other - same as the tack - water ballast is a crock.


Wrong. Just look at the plain facts.

It's a lot heavier than air, but look at "what you're doing" and the
alternatives.

Come on - do my maths (example question posed already, no answers yet)


I have given you many sensible answers, and tried to explain
the physics in easy terms. I thought you were getting the
point, but no you reply that 'it's a crock.' So good bye, Dave.

Fresh Breezes- Doug King