I cannot find the source of the required specific weight of the hull material in the AC45 or AC50, so I might be wrong about that.I did find in the rules of the AC72 catamarans: 0.9kg/m2 skin density is required, and the fibers should be carbon.In the new AC75 monohulls, the "hull surface" needs to have at least 2kg/m2. Not sure if this means weight of skins or the whole sandwich structure in the hull walls.So that was for cutting edge lightweight designs.And then I found this for the Volvo Ocean race boats:
"The hull weight varies from 7..4kg/m2 to 11kg/m2 and the deck is 5.7kg/m2"The boats (monohulls) are built in carbon.So that is a huge difference. But the first two figures are minimum according to the rules, and the second figures are the actual hull material weight used in practice. Maybe the difference in practice are not that large. Because the hull, deck and rig (not including keel) will weigh approximately 6800kg of the Volvo 70. And the weight of the AC72 catamaran is 5900kg (no keel needed).And I guess these weights scale linearly with size. So a 35 foot boat built at strong as the Volvo 70 should have 3.7kg/m2 to 5.5kg/m2 hull weight, built from carbon. Pretty far from the Elementarry 35 which I think had 450g/m2 glass each side of foam, so around 2kg/m2 composite weight, with very little carbon in the hulls.On Mon, Oct 22, 2018 at 5:25 PM Björn <bjornmail@gmail.com> wrote:| Mr. Trinardo did it vs H100, right? Can you specify what "low density PVC" you used?
He compared to Divinycell H60.I compared to Divinycell MX 7-7. (It is the only foam core sold by my local supplier.)I found a datasheet: (German language...)
| How does a material with lower compressive strength crush a material with higher compressive strength? Is the paper lying? Do we literally know nothing.
Not true. H60 is specified 0.9MPa (0.7 MPa) compressive strength. (The parentheses contains the minimum value.)Finnfoam F700 is specified >= 0..7MPa. So they have the same minimum spec, and in this testcase the XPS-samples where stronger than the PVC foam-samples.My PVC foam, Divinycell MX 7-7, is specified 0.8MPa (0.65 MPa) compressive strength. So it is actually expected to be weaker than the Finnfoam most of the time.One thing I've noticed is that the XPS seems to rebound better from deformation. The Divinycell seems to get permanently deformed from the same deformation.The shear strength is only 0.3MPa in the specs of F700. And it is 0.6MPa in the specs of MX 7-7.. So on paper, looks like a huge difference. I'll try to setup a test for shear strength.| Yep. While I don't think anyone here is a composite engineer, I think having discussions of many possible foam substitutions might make sense for the budget builder. Although, perhaps contra indicated in a discussion on building 10-30m+ boats, which seems a rich mans hobby.I would like to make a 10m boat though.. Or maybe even larger. It's so cheap to buy a used Cat or Tri up to 10m here (a used 10m Cat starts at 5000 USD, my 5.5m cat cost 1000USD) so its not worth the effort to build anything smaller imo (even though it would be lighter). But from 10m and up it would make sense. Because then the used boats are expensive. And we are also getting to the size where the Proa hulls are large enough to fit people, which makes the boat so much more usable, since it's possible to go into a hull sleep in it and so on. Smaller, and it's just possible to reach down with an arm, and the hulls are useless for everything else than flotation. Trimarans can have accommodation from about 6m hulls.| It is kinda funny how there does not seem, to me, to be expected levels of certain types of strength for boat designsI think it's because of the amount of parameters involved, and the level of certainty needed to be able to publish recommendation.I did some some simple calculations on flat sandwich panels based on that paper which is available (which you sent me to), and used by the guys on Boatdesign. What I found was that for a panel under load, with a certain tensile thickness/strength of skins, and certain thickness/shear strength of the core, the core will fail first if the panel is small, and the skins will fail first if the panel is large. Pretty interesting, and still kind of counter intuitive, even though I have done the math and am sure it's correct.If a panel is not flat, but with a radius, the properties changes. My catamaran has thin-walled hulls without core. The cross section under the waterline is semicircular, and above the waterline the areas are flatter. It's easy to push a dent on the flatter areas with a finger (but it springs back). The shape of the hull seems to make it strong enough for the loads in heavy waves even with this old thin-walled glass+polyester design.In the regulations for the last Americas cup, it was stated that the panels for the hulls (45 feet) needed to weigh at least 3.2kg/m2 if I remember right. And that was with carbon fibre skins, honeycomb core. I think the Harryproa hulls are built lighter, even though the panels are glass+foam.On Mon, Oct 22, 2018 at 2:13 PM '.' eruttan@yahoo.com [harryproa] <harryproa@yahoogroups.com.au> wrote:
| 96 kg/m3 - Nice. Twice the density of XPS gives it better strength of course. But only half the compressive strength of similar density PVC foam (on paper).
Yep. While I don't think anyone here is a composite engineer, I think having discussions of many possible foam substitutions might make sense for the budget builder. Although, perhaps contra indicated in a discussion on building 10-30m+ boats, which seems a rich mans hobby.
| And the shear strength is only 0.55MPA, which is 1/3 of H100. So on paper it doesn't look ideal. But maybe good enough.
It is kinda funny how there does not seem, to me, to be expected levels of certain types of strength for boat designs, or perhaps my search skills are fail.
Like, it seems, I really don't know how we know how strong a boat needs to be, nor how to stress it, or load it, to PROVE its as strong as it needs to be. Except to go sail it till it breaks and make it stronger. Which suggests individual, case by case experience based knowledge. Which seems so last century.
And so, we do not know if a chosen core is way stronger than needed or too weak. We can only eventually know, perhaps, if it is too weak by its failure, as that fail may have failed in every case because the storms do not come in measured sizes.
It also seems that, given a reasonable array of potential cores, and the relative costs, selection is based on relative criteria of PERCEIVED value, not the objective requirements. Because there is no objective requirements data.
For example Rob has often mentioned H100, because it is about the same price or cheaper for him in AU. Other than cost, and the perceived value of a better material, and reasonable disregard of the small weight gain, H100 is not NEEDED, in that its performance level is not required.
| Would be interesting to compare XPS, PIR, PVC foam in compression, tension and shear. Maybe the cheaper products will not be as inferior as it looks on paper.
We don't even know what the paper is telling us. As pointed out in the boat design threads, perhaps only compressive strength matters in a foam core. And even if we had enough good data from enough of the various cores and layups to get a reasonable statistical expectation of typical or minimum performance, of a home builder, we don't know how to translate that into building a boat.
And IF we had all that, 'a boat' is not a HP, which is an entirely different boat design, mostly, than your typical multihull, or mono, if the words 'typical' can be applied to boats in this case.
Its a rather complicated domain.
| I compared my Finnfoam samples to low density PVC foam in a vice, just like the Trinardo guy, with the same result. But I'm a bit too busy at work, so I haven't played that much with it yet.
Mr. Trinardo did it vs H100, right? Can you specify what "low density PVC" you used?
How does a material with lower compressive strength crush a material with higher compressive strength? Is the paper lying? Do we literally know nothing.
InsaneLaughter.mp3
At this point I need to build a boat and go sailing to get away from the insanity of thinking about how to build a boat.
Posted by: Rob Denney <harryproa@gmail.com>
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