maxsurf problem:density affect the bend moment

[ageny]

in the hydromax, i changed the density of the fluid in the cargo hold(i define it as a tank), then after the longitudinal analysis, i found the bend moment was also changed!

 

[Andrew Mason]

If you keep the volume of fluid in the tank constant and change the density of the fluid, the weight of fluid in the tank will increase or decrease. Hence the bending moment and shear forces will change.

 

[ageny]

thank u for reply.but i have keep the mass in the tank. for example, when i use the fluid of density 3 t/m3, the quantity of the tank is 46%, then i change the density to 1.5t/m3, at the same time i change the quantity to 92%.but the bend moment is also changed

 

[Andrew Mason]

If you can email the design file (.msd) and the Hydromax file (.hmd) to support@formsys.com I will have a look at the problem for you.

 

[hello]

maybe the LGC have been changed

 

[Navaldesign]

Changes in Bending moment

Unless the tank or hold is of a vertical extruded shape, which is highly impropable in a real ship, changing the level of the fluid in the tank, even keeping the total weight unchanged, usually means that you change both the vertical and longitudinal position of CG. Consequentely there is a change in the bending moment. I suggest that whithin Hydromax you check the LCG in the two different conditions. I believe you will see there is a difference.

 

[Andrew Mason]

I tried this with a Hydromax design, added a box shaped tank and ran the longitudinal strength with the tank full with density of 0.5, and half full with a density of 1.0. Results for weight, bending moment and shear force were all within 0.1% of one another - the difference can be attributed to the difference in VCG affecting the LCG with trim as well as the fact that the fluid in the half full tank undergoes a longitudinal shift with trim.

As I said before, if you think you have a problem email me a model, but so far I have not been able to reproduce any problem based on what you have described.

 

[Navaldesign]

Hi Andrew,

the box shaped tank is exactly what i defined as a particular case of tank because the LCG will not change by modifying the level of the liquid. The same goes for any tank that has two symmetry planes, one longitudinal and one transversal, because these two symmetry properties determine the non variation of LGG along with the VCG. Try instead forming f.e. a tank in the bow, where the trapezium shape of the hul will affect the LCG. Rising the level of the liquid yet keeping the total weight unchanged, not only results
in rising the VCG but also results in moving the LCG forwards. So bending moment changes.

 

[Andrew Mason]

NavalDesign,

Sorry, I was addressing the original question, not your reply.

I agree that the trapezoidal tank will have a clear change in LCB as the VCB rises, but as I said in my previous post, if the vessel has any trim there is also a subtle shift in LCB when you raise the VCG in a completely parallel box shaped tank. There is also a longitudinal shift in the fluid in a half full tank with trim that does not occur in a full tank with a fluid that is half as dense.

The combination of these two factors can result in small differences to the LCB and therefore bending moment, however this difference should be extremely small in most cases.