Keeping myself occupied whilst sitting out the virus problem, I started to go through my filing cabinets. Boy! did I find a lot of stuff that had been forgotten about, rightly so in many cases but a few forgotten "gems" have been found.
One item relevant to the subject of model stability was an OHP (OverHead Projector - from the days before "Powerpoint" and computers) transparency. It had been used many years ago when giving a talk at a club meeting. It brought back memories, including having to lug the monstrous OHP to the meeting room, and I felt it might be a handy addition to this thread.
It concerns the "Righting Lever" often abbreviated to "GZ" and how vessels behave when rolling over. Using a modified diagram from the beginning of this thread, it shows the two forces acting on a heeled hull. These being the Weight force, acting vertically downwards from the Center of Gravity and the Buoyancy force acting vertically upwards from the Center of the Immersed Hull Volume.
When the hull is at rest, these two forces would be in line and balance each other out. When heeled to one side their Lines of Action move apart but are still parallel. Provided the Center of Buoyancy moves further to the heeled side of the hull than the Center of Gravity does, then a "Couple" (the term for a pair of parallel forces that do not act along the same line) is created that will try to counter the heeling action. In this case the the perpendicular separation of the two lines of action is called the "Righting Lever" (GZ) and can be used to describe how powerfully the forces act to return the hull to the upright position.
As you might expect, as the angle of heel starts to increase the value of GZ also increases. But, the second diagram shows that there is a limit to how far you can go. When the listing angle exceed a certain value then the Buoyancy force can start to move inwards and the value of GZ decreases. Heeling further makes things worse until the two lines of action swap sides and the model capsizes. This point being termed, quite appropriately, the "Vanishing Angle".
What does this mean for us modelers? Not sure about you but I like to have no doubts about my models stability so my standard test is to push the completed model down on one side until the edge of the hull is at the waters surface. If, when released from this position, the model springs smartly back upright and returns, maybe after a few oscillation, to the original state, I'm happy. Should the model ever be caught out by rough conditions, then even the sight of it rolling with water at the edge of the deck isn't a worry as it should still be in its "Range of Stability".
To be honest, most of my models have such low Centers of Gravity (build light but strong especially any superstructure who's function is usually only to look pretty and keep the water out and put dense ballast as low as possible inside the hull) that they can roll on their "beam ends" (90 degrees to upright) and still know which way is up!
Glynn Guest
P.S. Left the OHP transparency in it's original had drawn form to remind me of the life before computers and drawing programs!
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Topic: Stability - how to avoid rolling upside down (Read 12393 times)