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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,
Thank you marven for your answer! As a matter of fact, my model indeed has an area where there are four faces attached to the same edge, see attached sketch of transverse section. Although I understand why that might be an “incorrect” geometry for DELFTship, shipbuilding-wise this is a perfectly legal configuration. Do you think this could be the cause? Thanks.Attachments:
Thank you Icare and sorry for the late response.
My model is completely enclosed (decked). If the normals point all to the water and I execute the command, the normals will get messed up. I’m not sure whether it has something to do with my model being a stabilized monohull and there are some limitations for that in the software. What puzzles me though, is that when I collapse edges to simplify/edit the model, the flipping of normals is not limited to surrounding surfaces. That wouldn’t be a big deal. Instead great portions of the model get flipped and reversing hundreds of them back again is very time consuming. It’s like if the command would be executed automatically, which in my case is something I don’t want becuase it messes up the normals. I’m just wondering if this is intended behaviour.
About your point: When I insert new faces, I usually can’t always predict in which direction I have to select the corner points in order to get the desired face normal. But that’s no big deal, because I check and correct the new faces right after creating them and they are usually not so many. The sudden change of many already existing and well-oriented faces when collapsing an edge is what worries me.
I made it a habit to check the model by setting the draft in the project settings so high that the whole model gets submerged. By assigning the underwater volume a unique color, I can visually check whether all surface normals are correct, even those above the waterline. This tells me if the calculated displacement volume and other hydrostatic properties are trustworthy. Unfortuntely, not always when a surface has the wrong orientation, the displacement will amount to zero. It can indeed happen that the calculations will just be off slightly, (e.g. being a few cubic meters short) depending on size and location of the ill-oriented face(s). Without means to counter-check the displacement volume, one might be relying on wrong hydrostatic properties without noticing it, due to an only marginal difference. That’s why after I do major edits, I check the model like this just to be sure that all normals are pointing to the water/outwards.
Interesting about the bottle, I forgot about that one. I’m sure that in most cases, DS will orient the faces correctly.
you can send me your model to email@example.com and I can export it to IGES for you. See also this post for notes about IGES export: http://www.delftship.net/DELFTship/index.php/forum/questions/3883-lost-in-transition#5471
OpenGL support depends at first, on whether the hardware you have supports a certain version of OpenGL. Older cards or laptop may have limited functionality. Second, the graphics card/chip manufacturer has to provide a device driver that actually allows your operating system and applications to use that OpenGL functionality. At some point the manufacturers may stop supporting older graphics cards/chips and even the latest available drivers may not support the newest OpenGL functions. This is an app that allows you to check your supported OpenGL version and test it: http://realtech-vr.com/admin/glview
Good luck, Claudio
I’m sorry you haven’t had any luck so far. Have you tried changing the underwater shading color? Does changing the color of layers work? Maybe the Wine implementation of the color dialog has issues.
Have you tried making a simple model file from scratch like a pontoon or opening one of the example files?
For the underwater color problem, you can also check the following:
- Is there a draft specified in Project settings -> Main particulars? If not, enter a meaningful value and see if the color changes.
- Does the vessel have a displacement? If not and you have a draft specified, then check first that the layers making up your underwater hull are checked in the Hydrostatics column of the layers dialog. Check again displacement. If still zero, then your submerged surfaces might have the normals pointing away from the water, i.e. inside. They will be displayed in the color assigned to the layer they are part of, not the one assigned to Shade underwater. Flip them to outside and see whether that changes something. Only surfaces pointing to the water and submerged according to the specified draft will be displayed in the color assigned to Shade underwater.
Exporting IGES Surfaces to Different Layers
In DELFTship, make one layer visible at the time and export it as IGES, one by one. Like this you will have one IGES file for each DS layer and you can import your model layer by layer into Rhino. By exporting your DS layers one by one like this, it will be much easier to handle the surfaces (selecting, joining, grouping etc.) and organize the layers in Rhino. It works very well for me.
Subdivision of IGES Surfaces
One convenience of DELFTship is that you can insert control points only where you need them. If you need to export your model to IGES however, I recommend trying to have as many faces having four edges. This is an IGES limitation I believe. If necessary, you will need to add control points to your model in order to obtain a more regular control net. Although it adds work to your DS model, it will result in larger but also less IGES surfaces, which will simplify working with the model in other programs in subsequent steps.
If faces with five or more edges cannot be avoided but they are planar, an option is to replace the many smaller surfaces the IGES file will have, with one larger planar surface in Rhino.
I hope you may find this helpful.
Hello Emilien, I tried to contact you at firstname.lastname@example.org but it seems that email address is not active anymore. Should you still be looking for a modeller as per your inquiry, feel free to contact me atHidden ContentYou must be logged in to see hidden content.
. I am a professional with both Rhino and DELFTship commercial licenses and might be able to help you.
Best regards, Claudio
As reprensenting a small yard, we design most of our works in Rhino. I seek someone who could help to implement our hull model designed in Rhino into Delftship. IGES model or STL are available. Fees will be.
-Is anyone of you who will be interrested in the job?
-Do you recommand anyone or any compagny where I should oriented myself to?
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