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April 26, 2013 at 00:03 #36672
I am trying to estimate the weight of my hypothetical foam core hull covered with two layers of epoxy fiberglass fabric from outside and inside the hull. I am getting some sort of inconsistent results. Please help me with figuring out what is wrong with my calculations and Delftship Free numbers.
In order to do this I am working with a 1′ x 1′ x 2″ thick polystyrene foam which is readily available. After calculating all weights I am trying to come up with foam/fiberglass sandwich average density to input in the layer density in the Delftship Free.
Here are the calculations for one 1′ x 1′ x 2″ foam patch with two surfaces skinned with epoxy fiberglass:
foam (60 psi) …. 2.2 lb/ft³
glass cloth ……… 9.0 oz/yd² (two layers used for each skin, multiplier 4x is used)
epoxy ……………… 4.0 oz/yd² (multiplier 2x is used for total weight)
After converting glass fiber and epoxy weights to lb/ft² and applying it to 1′ x 1′ x 2″ foam covered with two skins:
foam ………………. 0.3667 lb (1’x1’x2″ @ 2.2 lb/ft³)
glass cloth …….. 4×0.0625 lb (1’x1′ patch)
epoxy …………….. 2×0.0278 lb (1’x1′ wet surface)
total 0.672 lb per 1’x1’x2″ panel or 0.672×6 = 4.03 lb/ft³
This is what I calculated for a hypothetical 2″ thick fiberglass foam board covered with two layers with 9 oz/yd² glass fabric and epoxy on each side.
Using this density of 4.03 lb/ft³ my question is what is the number I need to input in the corresponding field of the layers dialog? I remember reading somewhere a while ago that the density must be a *ratio* of hull material density to the density of the liquid (63.99 lb/ft³ seawater and 62.43 lb/ft³ for fresh water). So if this is correct the number I need to input for sea water is:
4.03/63.99=0.063 (dimensionless) — is this correct?
Presuming it is correct I plug these numbers in the test model which is simple 8’x2’x2″ board. I attached this file your review (the latest 32-bit version). The resulting weight is shown as 0.011 T which is 24.64 lb. After I divide this total weight by the board volume I get: 24.64/(8x2x0.1667)=9.24 lb/ft³. This is twice the original density.
I wonder where I made an error and if someone would help me to figure this problem out.
I would appreciate very much if someone would correct me if I am wrong.
May 2, 2013 at 20:48 #36678
Well, it has been a week or so, but I got exactly zero replies. I guess not so many people have time to go methodically through the above calculations. In this case my next iteration would be to claim that layer density calculations are not correct in Delftship. Or, I am doing something fundamentally wrong, which not that unusual… Would any one help me in sorting all this out? Thank you.
May 3, 2013 at 22:16 #36679Jesus CruzParticipant
The data that you must enter in the dialog box is the weigth of the layer per cubic feet and the thikness of the layer in feet, that way the system will calculate the weigth and center of gravity of the layer…
June 10, 2013 at 23:57 #36741
I tried to use values as suggested, I used 4.03 lb/ft³ in layers property and the answer – weight of the panel – was 0.3T It is definitely not correct. Then I used relative density of the sandwich component 4.03/63.99=0.063 AND reduced thickness of the panel to 1″ instead of 2″ Miraculously the weight of the panel now is 4.48 lb (0.002T) I consider this to be close to the original 4.03 lb/ft³ value because of whatever rounding errors. I still have not received a convincing answer what values should I use for layer density. I attached 2’x4’x2″ foam sandwich model – really just one flat surface with 4 corners with 2″ draft.
Can anyone help, please?Attachments:
June 11, 2013 at 01:01 #36742giorgio zuppinParticipant
Hello! Maybe I can help, but take with prejudice -i.e. “cum grano salis” as romans said.
I’v taken your sketch, converted in metrics (because I’m lazy), most important: have reduced your box in a plain sheet.
Layers are… well, layers: containing objects defined by surfaces, the thickness of those surf. (as assigned on layer prop. box) combined with the extent in square meters gives a cubic figure. The weight now comes out assigning relative density.
Ok. sorry… to make it easy; assuming a mean density of composite fiberglass of 1.529 gr/cub.cm, a value for relative density must lay from 0.8 to 0.4 or less. Obviously it depends by the fibers – glass to carbon, etc. – And by the sandwich material – balsa, foam, honeycomb, etc. – I have assigned for our ex. a value of 0.504 and 25mm. ( 1″).
The result seems plausible: at least is the method I currently use and well, it works.
June 13, 2013 at 17:24 #36743
Thank you all who read and helped me in figuring out this puzzle (at least for me).
The numbers were correct all along, here are the conclusion:
1. Layer density must be relative to the water density, either fresh or salt.
2. Model should not be symmetrically replicated, meaning only half of the model should be present on the screen
3. With small relative values there is an error in rounding off, at least with displayed precision numbers.
Here is the backup calculations:
Foam density 4.03 lb/ft³
Salt water density 63.99 lb/ft³
Relative density 4.03/63.99=0.06298
After calculations in Delftship:
Calculated panel weight 0.005T = 0.005 * 2240 = 11.2 lb
Theoretical panel weight @4.03 lb/ft³ = 2′ * 8′ * 0.1667′ * 4.03 lb/ft³= 10.74 lb
Difference 0.45 lb (4%)
Conclusion: In order to have correct layer weight calculations, the density must be entered in relative to the density of fresh/salt water. For example, if the foam sandwich density is 4.03 lb/ft³ and the salt water density is 63.99 lb/ft³ then the density number in the layer field should be 4.03/63.99=0.06298
Thank you all for the help, hopefully I am correct.
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