Metacenter

[Roger Long]

My one and only being on the other side of our great (but troubled) nation, I've been spending this Saturday night exercising my liver and writing another installment of my stability piece.

You can now read about the mythical and mysterious metacenter here:

http://www.rogerlongboats.com/Stability.htm

We're now poised to get to the good stuff.

 

[Ross]

It is not a wonder that I had trouble getting my head around that concept.

 

[Roger Long]

Well, do you understand it now?

I just added a section on rolling period to the end of the metacenter page so you might want to look at it again.

 

[Ross]

That helps. I'll ruminate for a while and see how I come out. Will you get into the damping effect of the keel?

 

[Ross]

A pendulum or a metronome changes its period as you move a mass relative to the pivot or center of rotation. A floating round, straight log has its center of mass at its center of rotation and has no righting moment. When a man straddles the log his body mass changes the center of mass of the man/log system. Man learned this early and created outriggers and secondary floating logs. This made for form stability. Man discovered that chopping a hollow in the log made a place to carry things and made the log more stable by changing the center of mass.

How am I doing so far?

 

[Roger Long]

Will you get into the damping effect of the keel?

That gets into vessel motion which is a different topic. Perhaps I'll get that far. In the meantime, you can read this from the University National Oceanographicl Laboratory System site. Some of it is applicable to sailboats.

http://www.unols.org/publications/manuals/SBCompendium/motion.pdf

 

[Ross]

That one is gonna take several more readings. Thanks.

 

[All U Get]

Hi Roger,
I've been putting off thinking about this so as to concentrate on celestial nav, but why isn't the metacenter (MC) also the center of gravity (CG)? I can grasp the center of boyancy (CB) acting as a lever to the CG to keep the boat upright but what causes the MC to be above the CG?
All U Get

I have been still thinking of this and after four days I have another question. Is the metacenter the point of center for the shape. I seem to remember some excitement at the Portsmouth Naval Shipyard during a refit when the weight inside a sub was moved and the result was a list to one side. The next time I take a tour I'll have several question for the young seamen.
All U Get