Maxprop wrote:
No indication of CoG movement on the graphs I have, but I'd assume that it
would have to change, unless one puts all the additional weight in the
bilge.


Well, it makes the calculations simpler if you assume the Center of
Gravity doesn't change, even though that would be almost impossible in
real life.



.... Bottom line: don't overload a 39'
Beneteau. And I'd suspect the same admonition might apply to similar
boats, such as Jeanneaus, DuFours, Catalinas, Hunters, etc.= the plastic
fantastics.



Probably true of any boat that is shaped like a normal sailboat. The only
way to be able to keep stability constant is to load all the weight
increasingly lower down so as to maintain the relationship between the
centers of bouyancy & gravity. Actually, as the beam/disp ratio drops, the
lever arm between CG and CB needs to increase... very unlikely!


Not sure I comprehend that concept, but I'll assume you know what you're
talking about here.


Not sure if I can explain, but here's a try:
When you look at stability, two things help. Weight down low and beam.
As the boat heels from 0 to 90 degrees, beam loses it's effectiveness &
weight down low gains. The lever arm between the center of bouyancy &
center of gravity is determined by these, so if you increase weight you
increase the leverage of the boat's beam but sink the hull deeper in the
water which decreases the lateral shift in bouyancy. Unless that weight
is down real low, you're reducing both modes of static stability.

Another way to look at it is by the Capsize Screening Ratio, a number
wich tells how likely the boat is to roll upside down and stay that
way... heavy + narrow = good... but if you're rightside up and want to
stay that way, the same relationship holds true which makes heavy +
narrow = not so good!




... What was interesting is that there seemed to be an over-center
point, beyond which the stability of the boat fell off precipitously. It
was not a smooth curve, rather it peaked, then plunged.

Maybe like a sine wave, with the peak for the point at which the gunwhale
immerses?


Sounds a bit extreme, or tongue-in-cheek.


Not at all. Consider a hull shaped like a cylinder, with a weight at the
bottom. This will have zero stability due to beam; called initial
stability or form stability. It will have very very little righting
moment at low angles of heel, then as it approaches 45 the righting
moment starts increasing steeply, then nears max somewhat short of 90
and increases slowly to the max at 90... a sine wave. A diagram could
explain this much much better.

Most monohulls have elements of a sine curve in the stability.



Ever hear of the Plimsoll marks? For commercial ships, these figures are
published and are supposed to be kept by the captain. For naval vessels,
they keep a whole book of figures on stability.


No, never heard of them, but it might be a good idea for recreational boats
to keep such figures handy as well. Then again, perhaps it's difficult to
exceed those critical figures in terms of load.


Depends on what you're carrying. In the old days, sailors used to love
to carry lumber because it's bouyant and when stacked up & strapped down
on deck, the boat was very safe (although a PITA to work around the deck
load).

About Plimsoll marks-

http://amchouston.home.att.net/plimsoll.htm

The story I heard was that the basic idea behind Plimsoll marks came
from a lowly insurance clerk who tabulated ship losses & their loading,
but could not get anybody to pay attention... ship captains being rather
set in their ways...



Aw, heck, I want it all, dammit. g

If you figure out how, let me know.


Still working on it. No solution in sight.


I don't want it all. I just want a time travel machine.

Fresh Breezes- Doug King