Question:

We want to run stability conditions such that the "weakest axis" can be selected and the condition can be calculated out. Can this be done with the AXIS command?



Answer

The question is, "What is the weakest axis?" The answer is, "It varies".


If you have chosen a certain axis angle, say 30° aft, to be nominally the weakest, then all you have to do is set the axis to that value:

AXIS = 30.0

Or you might pick a representative heel angle and set the axis such that all the inclination at that angle is heel. In a damage case, this might be a few degrees beyond the equilibrium angle. In this case you could set the axis as follows:

HEEL = 0
SOLVE
HEEL = *+3.5
AXIS = *

Another approach would be to run a series of righting arm curves at various axis angles and check the results of each to find the "weakest axis". For example:


MACRO AX
AXIS %1
RA /LIM
/
.AX 10
.AX 20
.AX 30
etc.


Finally, you could find the "weakest axis" at each heel angle. For example,



MACRO RA
AXIS 0
HEEL %1
SOLVE TRIM
AXIS *
RA 0
/
.RA 0
.RA 5
.RA 10
etc.



This would produce a series of 1-line righting-arm tables, each at a different axis angle. The heel angles would be increasing and the trim angles would all be zero. Of course you would have to integrate the area under this "righting- arm curve manually.

Perhaps the middle approach is the most reasonable. You could choose some heel angle, say 20° to be the representative angle for determining the "weakest axis". For example:




AXIS 0
HEEL 20
SOLVE TRIM
AXIS *
HEEL 0
RA /LIM



If you see a righting arm curve where the trim angle exceeds the heel angle in the range of heels which are being used to evaluate stability, you know that it is definitely not being heeled about the weakest axis!

If you get the message, "Need closer angles for accurate Area" then try halving the increment between heel angles. In your case, it stopped just when the angle increment went from 3° to 5°. I think this problem may disappear if you use the weakest axis at perhaps 20° as in the example above. (If you do so, the axis should come out closer to 45°.)

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