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#14: Weathertight Points and Range of Stability

How to check the range of stability with weathertight (TIGHT) points.

GHS gives you the option of modeling downflooding points either as normal openings or weathertight openings. The significance of this distinction is that weathertight points are not usually considered to be downflooding unless they are immersed at the angle of equilibrium.

When evaluating some property of the righting-arm curve, we usually want to look at a range of angles; and this range is often terminated by the angle of downflooding, or the angle of vanishing righting arm, whichever comes first.

If weathertight points are present, they will normally have no effect on this range of angles unless one or more of them is immersed at the equilibrium angle, in which case the range from equilibrium to downflooding is considered to be zero.

However, in some applications, it may be desirable to find the range (from the equilibrium angle to the downflooding angle, or the angle of vanishing righting arm) while treating the weathertight points differently.

There are three cases to consider: 1) Ignore the weathertight points; 2) Take them into consideration only at the equilibrium angle; 3) Take them into consideration at any angle.

Note that this need only comes up when evaluating the range of stability, not when finding area or any other property of the righting-arm curve.

Here is how GHS provides for these three cases (using 20 degrees as an example):

1. Ignore tight points but consider normal downflooding points:
LIMIT ANGLE FROM EQU TO FLD OR RA0 > 20

2. Respect tight points only at equilibrium:
LIMIT ANGLE FROM EQU TO FLD OR RA0 > 20
LIMIT ANGLE FROM EQU TO FLD > 0

3. Respect tight points at any angle:
LIMIT ANGLE FROM EQU TO FLD OR TFLD > 20
LIMIT ANGLE FROM EQU TO RA0 > 20

Note that two LIMIT commands are needed to address case 2. If other limits such as area to downflooding are present, they will respect the weathertight points at equilibrium and so the second LIMIT ANGLE would not really be needed in order to fail the criterion when weathertight points are immersed at equilibrium.

Historical note: Prior to version 10.62 and in versions 10.94 through 10.98, case 1. above was not possible without disabling the tight points.