GHS FAQ

Question QA120 :
I am having some trouble with the GROUND command. I know the l,t,v of the
grounding location (ice contact for an ice breaker), but don't know the
reaction force. I get an error "Depth must be defined" when I include the
command in my run file. What is wrong?

Answer :

The thing to remember about a ground point is that it establishes a distance
to the ground at that point relative to present waterplane. Therefore the
waterplane must be defined when you give the GROUND command. This was the
cause of the error message.

There are various ways to define the waterplane (heel, trim and depth) and the
one to use depends on what you know. Basically, you set the parameters whose
values you know, and you solve for the others.

In the icebreaker case perhaps you want to assume that the surface of the ice
is initially a certain distance above the point where it contacts the stem.
Let's call this distance "d". Then, after establishing your free-floating
condition, you would give commands such as,

GROUND "Ice reaction" *, l, t, v /pen:d
SOLVE
STATUS

This solves for equilibrium, generating a ground force sufficient to lift the
bow and eliminate most of the "penetration" distance d. If you want to see
what the penetration distance is you can do a

GROUND REPORT

This also works nicely in Load Editor. Press Ctr-Tab once or twice to get to
the Ground Point screen. Then Press Insert to add a new ground point. Type
its name and location. Then put in your distance "d" for the penetration and
press Ctr-Q to solve. You will see the penetration reduced to a much smaller
number and the resulting ground reaction showing. Also notice the new trim
and GM. If you want to go back to free-floating, make the penetration a
negative number large enough to let the bow down to its free-floating position
without coming into contact with the ground again. Remember that the
penetration distance is the vertical (waterplane-normal) distance that the
ground point is driven into the ground. If negative, it is the gap (distance
normal to the waterplane) between the ground point and the ground.