Hi everyone,Is it possible to calculate the added masses in the X and Y directions and the added moment of inertia of a given ship design using MAXSURF ??.
Every tiny hint is welcome.
Thank you very much in advance.
In MAXSURF Motions, look at the Global Hydrodynamic Coefficients table in the Results Window (for Strip Theory Analysis)
James
Answer Verified By: Koko
Dear Mr. James Clarkson
Thank you very much for your answer, as always your guidance is very helpful.
Yes you're right, I have performed a strip theory analysis in MAXSURF Motions and the added masses and moment of inertia have been generated in the the Global Hydrodynamic Coefficients table in the Results Window as you have said. But those are not the added masses and added moments of inertia that I am looking for.
I am looking for the added masses and added moment of inertia for the surge, sway, and yaw motions of the ship: m11, m22, and m66 (please refer to the attached image showing the added masses and added inertia moments matrix MA).
8081.tif
Also, I have executed a panel method analysis in the MAXSURF Motions: at this time, added mass 11, added mass 22, and added inertia 66 are among the columns displayed in the Global Hydrodynamic Coefficients table in the Results Window but they are not calculated.
Please can you give more clarifications?
Thank you very much for your continuous help.
Kamal.
Hi Kamal,
Strip theory is a 3 DOF solution, so you will only get results for heave, roll and pitch.
Panel methods are a 6 DOF solution, so you will get all 6 DOF added mass and damping results.
Strip Theory and Panel methods are totally different calculations and as such, they have different results tables.
- Strip Theory added mass / damping are displayed in Global Hydrodynamic Coefficients tab
- Panel Method added mass damping are displayed in Added Mass and Damping tab.
It is probably easier to see the panel methods results in the graph. You can turn on visibility of different data sets in graph using the Graph Option button (Display | Graph Options) or using the icon button.
As with all of MAXSURF, if you double click on the graph area, you will get a pop-up window with the Graph Data. Shift-Ctrl-C copies the data including the headers.
Dear Mr. James Clarkson,
Thank you very much for the answers you gave to my questions.
I am very sorry for the delays in my feedback, I am just busy doing other things in parallel and I don't react to your answers until I apply your guidance in MAXSURF Motion.
I did what you have said and everything is working well but I just have another 3 questions: I need the added masses and inertia of a ship because I am simulating the dynamics of a ship (3DOF, surge, sway, and yaw) while the ship forward speed is equal to 7Kn at calm sea.
So my questions are:
1/- Which spectrum type I have to use to represent calm sea in MAXSURF Motions ?
2/- I have to choose the panel method for the analysis type and as I found , in this method, the forward speed of the ship must be equal to 0 Kn. So my 2nd question is that: If I do the analysis with the panel method and with a speed of 0 Kn, will the calculated added masses and inertia be valid for a ship which is sailing with a 7 Kn forward speed ???.
3/- I did an analysis for my ship with the ITTC (2 Param.) spectrum having a Char. height of 0.001, please see the the attached Graph View of the calculated added mass11 of the ship. My question here is that the added mass 11, as we can see in the Graph, is varying with the encounter frequency, so which value of the added mass I have to use for my simulation ??.
PDF
Thank you very much Mr. Clarkson.
Added mass and inertia are components for calculating the restoring and damping forces on a vessel. However, in calm seas there is nothing to force the vessel from its neutral position, therefore there would be no restoring or damping forces.
In calm seas, it sounds like more of a resistance problem.
What is the big picture? What are you trying to achieve?
Hi Mr. James Clarkson,
Thank you very much for your patience and for your continuous support, I really think that MAXSURF users are very lucky to have a forum like this in which complete novices like me can be guided by experimented naval architects like yourself.
Regarding your questions, let me tell you that I usually perform the simulation of Diesel propulsion of ships in 1 DOF (calculation of the longitudinal velocity of the ship when a given ship is equipped with a given Diesel engine) and I don' take into consideration the sea condition. For the added mass I used to take it as a percentage of the full load displacement of the ship (8% as it can be found in the literature).
This time I have built a model for the simulation of a towing system formed by two ships connected by a towline: this simulation is based on an article in which the authors state the simulation is carried out in still water, nevertheless they include in the data, that they have used, the numerical values of the added masses and added inertia of their ships in 3DOF (surge, sway, and yaw).
To compare the results of the calculations of MAXSURF Motions, I took one quick design of a tug ship available in MAXSURF Modeler sample designs and I modified its length, breadth, depth, displacement mass, and block coefficient to be as close as possible to those of the tug ship used in the article and I have used MAXSURF Motion to calculate the added masses and added moment of inertia in 3DOF of my newly designed tug ship.
The numerical values of the added masses and added moment of inertia given by the authors in the article are:
m11= 33.735 t ; m22=280.342 t ; m66=16163.84 t.m^2
For MAXSURF Motions calculations I tried separately the following spectra: JONSWAP, ITTC, and 1Param, in all of these spectra I have only changed the characteristic height to 0.01 and I got the same results for m11, m22 and m66 at each time. (Please refer to the attached PDF for the graphs of m11, m22, and m66 generated by MAXSURF Motions).
As you can see in the PDF, for each graph the values start from a value close to the one given by the authors but they reach a higher peak before decreasing after that.
So my ultimate goal is to calculate the added masses and added moments of inertia of any ship with any design and any dimensions.
So now that you have the full picture, my question is: For different ships, when I have MAXSURF Motions graphs like the ones shown in the attached PDF, which values should I take for m11, m22, and m66 for my simulations? the starting values, the peak values or the lowest values ??.
I am sorry for this long message but I wanted to be as clear as possible.
Thank you very much Mr. James.