Originally Posted by Wavewacker
U4, your assessment of initial stability is bugging me.
Initial stability is what I want in a power boat
and that comes from a wide flat beam. Sailboats (mono) are designed to heal or roll initially and the flair of the hull provides secondary stability before we get really wet. So, I didn't really follow what you described above. Also, in my mind, if you place weight high on a boat with narrow rounded hull, like a canoe, you're going to get wet. If I'm wrong here, please tell me why I got wet when broadsided by a wake in my canoe (while looking the other way...LOL)
Hi Wave, I just noticed your response.
To make sense of this, you need to keep the issues separate. The problem with using your canoe to understand this phenomenon is that everything is coupled, not separate: you constantly move your high center of gravity to keep the boat balanced. If you don't, you get wet. Canoes, kayaks, and bicycles are tightly coupled, dynamically unstable systems. A yacht is dynamically stable. That is a very big difference.
Lets talk about each issue independently: 1) initial stability, 2) moment of inertia, and 3) center of gravity.
First, initial stability, also known as form stability: stability due to the shape of the hull, turn of the bilge
, beam, water
In protected water, where one only has little waves, then you are right: lots of initial stability is a good thing. Then you and your guests can stroll around and ignore wakes without worrying about wine glasses toppling over.
I was referring to the rather unusual and rare situation of covering serious miles at sea, where the waves are large in comparison to the size of the boat. Its probably far better to just avoid crossing oceans, or at least to ignore the unavoidable discomfort of such crossings. But lets say you really want a boat to be as comfortable as possible (still uncomfortable!) in waves that are much longer -- peak to peak -- than the boat is wide.
A boat with extremely high initial stability will try to stay level with the water -- not with the horizon. Therefore, the boat will rock'n'roll the same as the water under the boat. Bad.
Example: Consider a 36 foot power boat with 15 foot beam is traveling beam to large seas, waves coming from the starboard side. The boat has extremely high initial stability, either very flat bottom or is a catamaran
or (worst of all, as I will explain later) a trimaran
. Consider as a wave peak passes under the boat from abeam: first, the boat will be tilting one way, the angle of heel being the angle of the face of the wave, lets say 20 degree to port. As the peak passes under the boat at 15 knots -- 25 feet per second -- the boat will roll 40 degrees in less than a second! Wine all over the place. Bad.
Second, lets introduce moment of inertia. This is not the center of mass, or the average location of all the masses, but the separation of masses.
One way to reduce the RATE OF motion in waves is to increase the moment of inertia. Think of a spinning top or gyroscope. The spin causes the top to have a very high moment of inertia, and so it tries very hard (actually exerts force) to stay level. It is very hard to make a gyro tilt quickly. Slowly, you can roll it any way you want, but the gyroscopes moment of inertia makes it hard to tilt quickly.
Having masses widely separated, like having a tall mast
and a deep ballasted keel
of a sailboat, provides a high moment of inertia. That is why sailboats have much more gentle motions than powerboats. And that is why adding weight aloft on a powerboat, like adding a tuna tower, reduces the roll rate. Without the keel
(a low center of gravity) you might flip over, but you will flip over nice and gently.
Third, is center of gravity.
You still need positive stability to very large angles of roll (heel): the center of gravity must be low enough that the boat does not tip over. But you need high moment of inertia (weight spread apart) to reduce roll rate.
A boat with low form stability, high moment of inertia, and low center of gravity is what you want to cross oceans.
Of course, the most comfortable way to cross oceans is in the front of a large jet!
Oh: the reason power trimarans are the worst configuration possible at sea is because they combine high initial stability with minimum moment of inertia and a high center of gravity.