Slogging
to Windward
by Chuck Merrell
sloggingtowindward@hotmail.com
March 2000
BALLAST: BORING OR BEAUTIFUL?
When I decided to write this column, it didn’t seem like it was going to turn
into a monthly article about boat design. The concept was that I’d pick a subject and
try to inject a bit of humor, controversy and sacred ox goring into the mix-hopefully
strong on entertainment, less so on instruction and information.
I haven’t lost sight of that goal. Those columns will come as time goes on, but
based on what I see in the way of misunderstanding* and misinformation in various boat
interest venues makes it seem like a good thing to take a shot at clearing up a couple of
these muddled areas.
*For example, I recently saw a posting on one of the news groups by some guy whose
email address was near enough: cliffthesquirrel@yahoo.com.
It seems that he wanted to instruct fellow list members that by adding more wood on the
bottom of the boat, the vessel will float higher (because wood floats, ya know!), and make
the boat stiffer because more wood equals more weight. Nice one Cliff, but weight is
weight and a pound of feathers weighs the same as a pound of lead, the only difference
being the volume.
On occasion here in the boat yard, Wombat folk like Cliff show up with their
preconceived and immoveable ideas. However, at the end of the day, with that quality of
thinking, level of understanding and resistance to education, it’s no wonder their
projects wind up looking like the leftovers of Hannibal the Cannibal’s breakfast.
It seems that most folks who are interested in learning and understanding
(rather than making their own answers from the whole cloth of imagination) often get a
little snowed when trying to understand hull stiffness and stability and how the added
influence of ballast plays into the picture. It’s no wonder, because most texts on
the subject start out with such sentences as: The static stability of a vessel is the
moment of the couple formed by the weight and buoyancy.
Wow!
All I wanted to know was: What keeps a sailboat or sailing ship from getting knocked
over in the water when the wind blows from the side and hits the sails, causing it to
heel?
The simple non-technical answer is: It’s the shape and width and weight of the
hull as it’s designed that resists initial heeling. When the boat does heel over (as
the wind blows harder or waves try to capsize the vessel), the resistance of the hull to
the external force gets stronger as the hull tilts to leeward-up to a point.
Ok, is there any way to make the hull even more resistant to being blown over, or
getting turned over by wave action? Also how can a hull be designed to come back from a
knock down or even a roll over?
You can do one of two things, either you can increase the beam of the hull so that it
has a wider base which will make it more difficult to turn over, or if you don’t want
to do that you can lower the center of gravity of the hull, which will make it even more
resistant to heeling or capsizing.
What’s the center of gravity?
The CENTER OF GRAVITY for any three-dimensional
body or system is that point where the body's weight, or mass, may be considered to be
located. The center of gravity of a uniform sphere (baseball), for example, is the center
of the sphere. The center of gravity of an irregular object, however, sometimes even may
be located outside the object itself. When an object is in free motion, the center of
gravity describes a smooth curve around which the rest of the object, or system, may
rotate in a complex manner.
If the irregularly shaped object is a boat hull, the Center of Gravity is
calculated using a mathematical formula that compares the weight of the material making up
the hull above and below the waterline. Usually the CG locates at a point slightly below
the designed waterline in the case of ballasted hulls, and about the same level as the LWL
in high-sided powerboats and un-ballasted hulls.
Fine, so how do I lower the center of gravity and in so doing make the boat more
resistant to heeling forces?
OK. The answer is either to lighten the topsides to reduce weight above the LWL, or add
weight (ballast) inside the boat below the waterline; or both-BUT-past a certain point,
adding more weight does no good and can do a lot of harm. You can only add the amount of
weight/ballast that will lower the boat in the water to the optimum point you designed
into the hull. More ballast than is called for, usually screws up the whole system to a
larger degree but adding weight to the topsides and above is worse. First time builders
and guys who believe that heavier is better overbuild in the name of safety, which tends
to make the boat unsafe. I can tell you stories about overbuilt boats which have resulted
in useless boats and losses in the hundreds of thousands of dollars for the "I think
therefore it is guys".
Keep in mind that the displacement weight of the boat in the water should not total
more than a combination of the weight of the structure plus the weight of the ballast,
plus the weight of the crew and cargo as projected and planned in the design phase. The
drawing below uses TESTBENCH, the hull I drew to illustrate the January Column and shows
that hull with and without ballast. All other weights, structure and load are assumed to
be identical and correct. |
