Question:

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.