Mass Moment of Inertia

[Marcus]

Hi Everybody,

I have got a simple question. Is it possible to calculate the mass moment of inertia of a ship in DelftShip? As far as I know I can set a LoadingCondition to get my centre of Gravity for x, y and z as well as my GM and the bending moment. Is there a way to get more Information for the mass inertia of my ship? If not is there a recommended procedure for an export to another programme?

Thank you very much.

Kind Regards Marcus

[Maarten]

Hi Marcus,

DELFTship does not do dynamic calculations- so while you can get displacement, CoG etc, inertia is not included.

I can not recommend another package, as this is not something we nor,ally deal with.

[Claudio Boezio]

To calculate the total mass moment of inertia, the mass moments of inertia of all masses constituting the ship need to be known. This includes also cargo, tank fillings etc. The calculation for that can be easily added to a weight calculation spreadsheet. You’ll need to extend it so that for each mass item it includes its mass moments of inertia around its centre of gravity and the formulae to calculate the mass moments of inertia around the ship’s centre of gravity or other defined point, e.g. ship’s origin. The latter is similar to the Steiner component when calculating area moment of inertia, A · d^2. For mass moment I think it is m · d^2, but please look it up yourself. Be careful to calculate the distances to the relative axis correctly (simple trigonometry). The book “Principles of Yacht Design” has some info about this in Chapter 5, “Added Resistance in Waves”.

If you don’t know the mass moment of a component, you can approximate it by assuming it to be box shaped. Moments of single components around their own axes are usually small. Rhinoceros can calculate the area/mass (don’t rmemember) moments of surfaces of any shape. So if you are capable of scripting, you can design the hull structure with Rhino and write yourself a script that exports all the inertia information to a *.csv file, that you then can import into your weight calculation spreadsheet.

It sounds complicated but trust me, once it’s set up, you just enter the weights and cg mass moments and voilà, the spreadsheet calculates the ship’s cg and its mass moment of inertia or radii of gyration.

Hope this helps and fair winds,

Claudio

[Author]

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