The notion that the CG must be below the CB is completely correct for a
submarine, because a submarine has no waterplane, and so the CB does not
shift to the low side when it rolls or heels. In that case, the center
of gravity is like a pendulum under the CB and that, and that alone,
gives a submerged body stability, or more correctly it will roll to the
angle such that the CG is under the CB.
The difference for a floating boat is that it has a waterplane with
vertical or even outward sloping sides; this means that when it heels,
the low side picks up more buoyancy while the high side loses some,
making the CB shift sideways to the low side. This gives it the
righting moment. The more it heels, the farther the CB shifts to the
side, up to the point where the deck edge immerses.
Actually, now that I think about it, the best example may be a floating
cylinder: its waterplane does /not/ change when it heels or rolls, so
it will /not/ have a shift of CB, and just like a submarine, it will
roll until the CG is under the CB. However, any conventionally shaped
hull can, and most do, have the CG well above the CG, including yachts.
So it all comes back to the shape of the hull, thus the "form" stability
name.
Chuck, no offense, but it's physically impossible for CB to be a foot
above the waterline, while displacement always equals buoyancy, as per
Achimedes. For a racing yacht with a 50% ballast ratio the CG of the
keel alone may be 4' below the WL (my 35-1 is 3.5' below WL, per keel
drawing), but the CG of the other half of the weight, including rig and
sails, is well above the WL, which typically puts the total CG just
about at the WL, thus above the CB. There are some yachts that have CG
below CB, but it's not a requirement for positive stability.
Neil Gallagher
Weatherly, 35-1
Glen Cove, NY
On 12/5/2013 8:39 PM, Chuck S wrote:
Dwight,
Read Neil's email below?
Chuck
Resolute
1990 C&C 34R
Atlantic City, NJ
------------------------------------------------------------------------
*From: *"dwight" <dwight...@gmail.com>
*To: *cnc-list@cnc-list.com
*Sent: *Thursday, December 5, 2013 8:27:05 PM
*Subject: *Re: Stus-List Now Stability - was List halyards again( 10
aloft =1on the rail )
I know it just seemed so obvious: was there someone who thought
differently?
------------------------------------------------------------------------
*From:*CnC-List [mailto:cnc-list-boun...@cnc-list.com] *On Behalf Of
*Steve Thomas
*Sent:* December 5, 2013 8:51 PM
*To:* cnc-list@cnc-list.com
*Subject:* Re: Stus-List Now Stability - was List halyards again( 10
aloft =1on the rail )
That is exactly what he was saying as I read it.
-----Original Message-----
*From:* CnC-List [mailto:cnc-list-boun...@cnc-list.com]*On Behalf Of
*dwight
*Sent:* Thursday, December 05, 2013 7:44 PM
*To:* cnc-list@cnc-list.com
*Subject:* Re: Stus-List Now Stability - was List halyards again( 10
aloft = 1on the rail )
Forgive me for this Chuck, but some things seem obvious to
me...however, sometimes I get confused...but if buoyancy is ever less
than displacement would that not be a submarine or sunken vessel and
if COB is below COG would she not be inclined to topple over???
------------------------------------------------------------------------
*From:*CnC-List [mailto:cnc-list-boun...@cnc-list.com] *On Behalf Of
*Chuck S
*Sent:* December 5, 2013 8:38 PM
*To:* cnc-list@cnc-list.com
*Subject:* Re: Stus-List Now Stability - was List halyards again( 10
aloft = 1 on the rail )
Neil,
Sorry, I'm heavily biased by life experience, logic, hundreds of books
on boats. Read Skene's Elements of Sailboat Design.
Center of buoyancy on a keelboat is always above center of gravity.
On my boat the CB is a foot above the WL while the CG is 4 feet
below. The Drawings prove it. Buoyancy must exceed displacement
also. Sailboats face different requirements than cruise ships so
should never be compared.
Another source:
http://www.engineeringtoolbox.com/centre-gravity-buoyancy-d_1286.html
Chuck
*/Resolute/*
1990 C&C 34R
Atlantic City, NJ
------------------------------------------------------------------------
*From: *"Neil Gallagher" <njgallag...@optonline.net>
*To: *cnc-list@cnc-list.com
*Sent: *Thursday, December 5, 2013 4:49:53 PM
*Subject: *Re: Stus-List Now Stability - was List halyards again( 10
aloft = 1 on the rail )
Marek:
Picture a large cruise ship: its draft might be 10 m, so its vertical
center of buoyancy is a little more than 1/2 way from the keel to the
waterline, say 6m above the keel. The center of gravity is not only
above the center of buoyancy, it's well above the waterline, sometimes
20 m above the keel, yet unless it happens to buzz too close to an
Italian island, it will stay upright. Or picture a small sailing
dinghy without anyone on it, the CB again is below the waterline,
while the CG is well above, but they don't capsize until the sheets
are pulled in.
A yacht's form stability is the same as a ship's; for sure, adding a
keel to a yacht improves the stability by lowering CG, but it does not
necessarily put the CG below the CB, in fact it usually doesn't.
(There is a point called the metacenter, which is an imaginary point
on centerline through which the buoyancy force always acts, that is
the point which must stay above the CG for positive stability...but
now we're getting deep into naval architecture.)
Check out the illustration below:
http://www.troldand.dk/en/?The_Yacht:Stability#.UqDzkvIo5Ik
Neil Gallagher
Weatherly, 35-1
Glen Cove, NY
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