**Q:**
What is the difference between damping,
drag, drag caused by current, and drag caused by waves?

REV 7.04

**A:**
First let's us agree that drag and
damping are considered the same. Now, divide damping into
radiation damping and viscous damping. The first of these is
the dissipation of energy though waves being generated by
the vessel moving. Everything else is viscous damping. Now, this
stuff depends on the square of the relative velocity of the
vessel with the relative velocity of the water particles.
So, viscous damping depends on
the vessel motion, the current velocity, and the wave velocity.

**Q:**
Can you put the natural period in a report?

REV 7.04

**A:**
No. This is because there is no natural period.
Here the mass depends on frequency so classical
natural frequecies do not exist. Also there is
the issue of what these would be for a complex
system. Unless the stiffness and mass matrices
are diagonal the natural frequencies are not
really for roll, pitch and heave.

**Q:**
Why does the model of the BALDER have #TUBEs added
for viscous damping when your answer to a FAQ says
to use -CS_CURR 1 1 1?

REV 7.04

**A:**
The #TUBEs are included in the model so that we can show
the efects of using them. See Section XIII of the
the document:
http://bentley.ultramarine.com/hdesk/document/verify.pdf

**Q:**
When I compute pressures on my vessel I find that
the surge radiation damping is zero. Is there something
wrong with my model?

REV 7.02

**A:**
No, you model is ok, it is Strip Theory that is
wrong. Strip Theory is an approximation of the
true solution (3D Diffration) which is valid for
long, slender vessels and part of the appoximation
is neglecting surge radiation damping. If surge is
important, then use 3D diffration.

**Q:**
Can you provide me some write-ups or explanation on the Kato & Tanaka method
(ie quadratic formulation, linearization method etc) for calculating viscous
damping?

REV 7.00

**A:**
The best source for information on the "Tanaka" method is
a paper "Ship Sway, Roll, and Yaw Motions in Oblique Seas"
by Schmidke in
SNAME Transactions Vol 86, 1978.
if I can get it scanned and will send it to you.
As to the
linearization method, MOSES has two of them.
Click here and look in section III.G for the theory and
Here for a more general discussion.

**Q:**
How can I input a 6x6 damping matrix?

REV 7.00

**A:**
In MOSES you model physical things. Thus, there
is no need to input a 6x6 matrix. Consider for
example, a single dashpot. This can be modeled
as either damping added to a GSPR connector or
one term on the diagonal of a #DAMP load attribute.
Now, either of these act at a point, either the
point specified for #DAMP or the ends for the
GSPR. Also this acts in either the element system
or the part system. Now, when the behavior is
converted to the body system, a part of a
6x6 matrix results, but you only need to specify
a convenient subset of the information.

**Q:**
How does MOSES get low frequency damping?

REV 7.00

**A:**
SRESPONSE does not include low frequency damping. The only damping
here is due to viscous effect, so you can claim that this is
conservative. In time domain, there is ample damping from all
the real effects.

**Q:**
How can I add a damping of some percentage of critical?

REV 6.01

**A:**
A problem here is the definition of critical damping.
Critical damping is defined in terms of the natural period, but
there is no "natural period".
See the question above. Thus the first part of the
answer is to ask those asking for 6% Critical to
define critical and then convert this into a real damping
value.

The #DRAG command can then be used to add a diagonal damping matrix. Be careful to add it to the correct point to minimize coupling and you need to change the default to get it to be applied in the frequency domain.

**Q:**
Should I alter the default damping values for really large structures,
such as big TLPs or large floating jettys?

REV 5.10

**A:**
You should turn off the Tanaka damping and use -cs_curr 1 1 1 on
PGEN or &describe piece. This adds some viscous damping to the
problem, which is needed for these types of vessels.

**Q:**
How can I get statistics in the time domain of the force
produced by a **#DRAG** load attribute?

REV 5.06

**A:**
The basic idea behind MOSES is that one models "things" and
these "things" always effect the behavior of the system. Even
Tanaka roll damping effects roll in a time domain simulation.
As a rule, **unless we say it does not work explicitly, it works
all the time**.

Now, obtaining the variation of force with time is accomplished
via post-processing. We do not store the forces on each item as
a function of time since it would be highly inefficient. Instead,
there are two commands,
**ELMFORCE** and **LDGFORCE** which
recompute the forces on beam elements and load groups and make these
available in the Disposition Menu. Here, you can get the statistics.

Also, when I am in doubt as to how something works, I normally build a simple test to see for myself. For example:

**The Data file**

&describe body box pgen plane 0 100 -rect 0 20 50 end &describe load damp *cen 50 0 5 #drag *cen .064*100*10*50 1

**The Data file**

inmodel &instate -loc 0 0 -10 &weight -compute 5 32 29 29 &instate -vel 10 &apply damp 0 &loop i ( 1 2 ) &status loadg_mult tdom -tstep .25 -maxt 10 prcpost traj report loc vel end end &apply damp 100 &endloop &FINIwould have told you quite a bit about how this works in the time domain. In particular, it does two time domains, one with and one without the #DRAG so you can easily see the difference.

**Q:**
How can I account for the effect of bilge keels in MOSES?

REV 5.05

**A:**
Use the
#PLATE command. You will need to create the appropriate
points along the hull for use in this command. There is a sample
dedicated to this topic. Look a the b_keels in the "How to" section
of the samples
Click here.

**Q:**
How can I remove all viscous damping terms in the damping
matrix when computing RAOs?

REV 5.03

**A:**
Do not include any Morrison elements, #PLATE or #AREA commands.
Also, the Tanaka damping must be turned off using the A_TANAKA
BNAME NONE command, and any panel damping needs to be turned
off with the option -cs_cur 0 0 0 .