**Q:**
Why do I get different Longitudinal Strength
results with MOSES than I get with a different package?

REV 7.06

**A:**
It is probably in the way you defined the weights. Consideer

#WEIGHT *P 1000 -LDIST 0 10this will have a weight with CG located at point *P. Now, for longitudinal strength purposes is weight will be distributed from 0 to 10 in the part system. If point *P has an x location 5 in the part system, then you get a uniform distribution. Otherwise you get a trapezoidal distribution.

**Q:**
Why does KG_ALLOW give the same allowable KG for all
angles?

REV 7.06

**A:**
There are several things that could case this:

- If you do not specify any criteria, then the allowable will be the BM for all cases.
- If you have no wind (or no wind area) then you may get the same results for all angles.
- If you have weights specified then what KG_ALLOW gives is the KG of the additional weight and this can be quite confusing.

**Q:**
We conducted the longitudinal strength assessment of a ship with three
different softwares, and the maximum bending moments, calculated by two are
quite similar. But comparing these maximum bending moments to the ones computed
in the MOSES show large deviations. As you said on your homepage a
MOSES uses a different integration method than that described in the
Principles of Naval Architecture. What kind of integration method is used?

REV 7.06

**A:**
MOSES adds trapezoidal loads on a beam using the exact solution for the bending and
shear in a pin ended beam. In other words, go to the old edition of Roark (fourth edition).
Add the solution of table III.14 and table III.16. This will give you the moment, shear, and
deflection at all points on a simply supported beam with a trapezoidal load applied. Now,
for each load (weights and buoyancies) you simply use this formula to get the total shear,
bending, and deflection.

Notice that since we begin with a pin ended beam we always get zero moment at the ends but the shear will be zero only if the body is in equilibrium under weight and buoyancy.

This algorithm is iron clad; i.e. it is always correct. If you have differences with other programs then the difference is due to the way either the buoyancy or weight is modeled (or of course, the other program could be wrong).

**Q:**
Why does MOSES report different values for GMT and GML
than other programs?

REV 7.03

**A:**
MOSES reports most values in the body system and
most other programs report them in the global
system. GML and GMT can be reported with the
&STATUS
command.

**Q:**
What rule or equation is being used to calculate wind force
when I use the RARM command, or inside the hydrostatics menu
in general?

REV 7.03

**A:**
In general inside the hydrostatics menu
the design wind rules are the same as those
designated by the
&env
command. You can ask
MOSES to report the current designation with
&STATUS ENV
command.

**Q:**
I have a semi-submersible platform with mooring lines and risers. Can I
simulate flooding of one of the columns in static process?

REV 7.02

**A:**
The
STATIC PROCESS
and
HYDROSTATIC
menus are magic in the
sense that they were designed to do very specific traditional things.
Thus they do not have the generality of most of MOSES.
In particular, these menus consider only a single
body and connectors (other than a sling in Static Process)
are ignored.

For your problem you will need to construct your own process:

- Do your flooding,
- Find equilirium, and then issue the command
- &event_store xx

Repeat this until you have stored events for for all situations of interest. You can then use the PRCPOST MENU to report the results.

**Q:**
Why do I get WARNING: Equilibrium Not Found Within Tolerance
when I computer righting arms with RARM?

REV 7.02

**A:**
MOSES rolls about the designated axis and finds
equilibrium at each increment. Here equilibrium
is weight and buoyancy forces. In order to find
equilibrium MOSES is allowed to change heave
and pitch.

The residuals reported along witht he warning message are the residual arms.

This warning occurs when MOSES could not find a z and pitch combination to meet the equilibrium tolerance.

In general if you pick a roll axis of 90 degrees where the roll is done about the vessel y axis then this occurs because you are actually pitching.

**Q:**
If I use the RARM -U_CURRENT command, will MOSES
report the location of the center of pressure?

REV 7.02

**A:**
First, MOSES does not compute the "center of
pressure" as it really does not exist. Instead,
MOSES computes a current force which equilibrates
the wind force and then the net moment of the
wind and the current force will be the heeling
moment.

This is a short description of what the RARM -U_CURRENT does.

MOSES does not report all of this, but you can recapture it if you want.

- First use &INSTATE -COND d, r, p where d, r, and p are the draft, roll, and pitch reported for a given row of output from the RARM command.
Next, use &ENV -wind xx 90 -current 1 90 where xx is the wind you are using with the RARM command.

Next, issue &STATUS FORCE.

Finally issue the two commands &ENV -wind xx 90 -current W/C 90 and &STATUS FORCE. Here W is the beam wind force from the pervious force status and C is the beam current force from the previous status. Now, the wind moment minus for current moment is the healing moment. Feel free to make any interpretation you wish.

**Q:**
Why are both the displacement and waterplane areas from my MOSES
model 2% below the values provided by the designer?

REV 7.01

**A:**
This is normally a result of the surface not being closed.
You wll need to review the panel definition. Whenever you are
having troubles like this you need to issue the command:

&SUMMARY COMPART_SUM PIECES ENDNow, when you look in the output file you will find a "EXTERNAL PIECE SUMMARY" and three of the columns are "Sum Area * Normal". All three of these numbers should be zero (or very near zero) because the surface is supposed to be closed.

**Q:**
Where is the "Center of Rotation" with the
RARM command?

REV 7.00

**A:**
There is not "center of rotation"! The ship is rotated and
equilibrium is found. The vertical position is a result of
the equilibrium. In the results, a draft, roll, and trim
are specified. You can use these with &INSTATE -COND to
set the vessel into position so you can show a picture of it.

**Q:**
Why do I get rubbish for Longitudinal Strength results while
the jacket is tipping during a launch?

REV 6.01

**A:**
Longitudinal Strength is applicable only to a single body. To find what
you watn here,
do not include the jacket model for your check of longitudinal strength
at tip, instead, model your tipping loads at the rocker pins using #weight.
Be sure to give some nominal distance for the load distribution, using -LDIST.
Hopefully, your rocker pins are located forward of the stern, so the rocker
load is applied within the confines of the barge. Then, set up your
equilibrium and ask for bending moments as before. Of course, the better
way to do this is with the structural solver, include barge flexibility, and
request the shear and bending in the barge beam for the tipping load case.

**Q:**
What MOSES commands do I use to obtain tank capacities which shows
ullage and free surface moment data? Also, how do I change the
position of the sounding tube to any position of tank?

REV 6.01

**A:**
Look at the test
TANK_CAP.
The .dat file shows how to define the
sounding tube and the .cif file shows how to get the report
you want.

**Q:**
Is there a way to include the effects of the mooring lines in the
righting arm calculation?

REV 5.10

**A:**
Yes, but since connector forces are not included in any of the hstatic
menu commands you need to manually apply a moment to the body.
Use an #LSET command, and find equilibrium with &equi, not equi_h.
Using &loop command to repeat this for different moment
values you will end up with roll angle versus moment information
that can be converted into roll angle versus righting arm.
Look at the test
C_RARM
for a sample,

**Q:**
Why did MOSES change from using the "LOST BUOYANCY" method when
computing damaged stability to the "ADDED WEIGHT" method?

REV 5.08

**A:**
We changed from "LOST BUOYANCY" because it was not correct!
Yes, the force on the vessel was correct, but if one uses
LOST BUOYANCY, the inertia of the vessel is not correct.
MOSES solves dynamics problems and in many cases the effect
of inertia is more important than displacement.

Now, we would not simply change one inaccuracy for another.
What MOSES does is not the "ADDED WEIGHT" method as you
described it. MOSES simply computes where a flooded compartment
cuts the water plane and "fills" the portion below with water.
An approximate velocity potential is introduced to find an
approximate dynamic pressure and this pressure is integrated
over the wetted surface. Thus, MOSES produces the **exact**
hydrostatic force and an approximate dynamic force. (This is
also the same procedure used for compartments filled with
the **-CORRECT** option.)

When the force computations are viewed as MOSES performs them,
there is no "free surface effect". This is simply an artifact
from viewing the problem as either lost buoyancy or added weight.
**Rest assured, the current MOSES version is correct!**

**Q:**
I have a simple test case where I compare the results of
longitudinal strength to structural results for a box shape.
The structural model is composed of a centerline beam using a
type of WBOX. The results do not compare very nicely,
and the sign convention for shear and moment seem to be
different. Why is this?

REV 5.07

**A:**
First, the sign convention between longitudinal strength and
structural elements are different. This is necessary due to the
programming philosophy for longitudinal strength. The structural
element sign convention can found at this Help Desk, under
MOSES Sign Conventions.
(For details, click here.)

Second, you are comparing an exact solution of hydrostatic bending moments to the results of a finite element structural solution. This is not to say the hydrostatic solution is any better, but simply different. The structural solution has truncation errors that vanish as the model is refined. The structural solution concentrates the hydrostatic pressure at the nodes, since MOSES will analyze any type of structure. When comparing longitudinal strength with structural results, it is not sufficient to match the plane locations with node locations. This certainly must be done, but you also need to minimize the truncation error by increasing the number of nodes. The error goes to zero as the square of the node spacing, but 50 nodes (and therefore planes) will usually provide reasonable results. Finally, you need to remember that load groups are treated differently between the hydrostatic and structural solutions. The use of the -LDIST option on #WEIGHT allows for a uniform distribution of weight over the specified distance, for the hydrostatic solution. The structural solution uses the same weight, but distributed to nodes as specified in the definition of the load group. For testing purposes you may want to use the #ELAT command.

**Q:**
How can I retrieve the weight and buoyancy distribution
along ship length from MOSES?

REV 5.07

**A:**
There is no way in MOSES to summarize the
weight and buoyancy distribution for longitudinal strength calculations.
The formulation in MOSES allows for the shear information to be stored,
but not the individual weight and buoyancy information. This is due to
the type of integration performed to arrive at the shear curve. The
integration technique ensures the shear and moment curve will close
at the ends if the vessel is in equilibrium, and allows for
differences in location where loads are applied. Even if the vessel
is not in equilibrium, the moment curve will close. The traditional
integration method specified in "Principles of Naval Architecture"
does not provide this reliability, but
does allow one to summarize the weight buoyancy distribution. Since we
provide the shear information, which is the interesting part, we chose
what we consider to be the more reliable integration method.

**Q:**
Why does the displacement in the
Curves of Form do not change when I flood a compartment ?

REV 5.06

**A:**
You do not lose displacement because MOSES adds
contents when a compartment is flooded instead of
losing displacement. In the past MOSES did lose
displacement, but this leads to some undesirable
consequences. In particular, the flooding certainly
increases the inertia of the system. Thus, we add
contents.

The "added weight method" has been looked on as not being as good as lost buoyancy, because weight was not correctly added. In particular, we add the correct weight which corresponds to the free surface of the compartment being at the waterline. Thus, for equilibrium, the results are identical, but we also get the increase in inertia.

**Q:**
How can I do
righting arms from negative to positive roll angles?

REV 5.06

**A:**
MOSES performs the righting
arm calculations "about the current state"; i.e. it
rotates the vessel about an axis "adding to the original
state". Thus to do righting arms from say -20 to 20,
you use:

&INSTATE -COND 10 -20 0 RARM 2.5 17Your output will state at the top that the initial roll is -20 and the rolls that are reported should be added to this to get the true angle.

**Q:**
I was looking at the hydrostatic coefficients
generated from MOSES and noticed that the moment
to cause trim by 0.01 degree uses BMl instead of
GML in the computation. This is the same whether
or not I give a value for KG. Does this approximation
continue on when calculating the righting arms and
using the allowable KG macros?

REV 5.06

**A:**
This is conservative and steeped in history. Since we do
not know the value of KG when the CFORM command is
issued, we do not know the GML, therefore BML is the
next best thing. This only occurs in the calculation
of moment to change trim and is not used elsewhere in
the program.

**Q:**
When will EQUI_H and &EQUI give the same result?

REV 5.03

**A:**
When there is no environment and no connectors in the system.