I think you should build from XPS, and I say that without any intention of being snide.
You have faith in XPS, you feel the cost savings is worthwhile, and you want to give it a go. And really, how else would we learn about how it performs on a larger multihull over time? The only way we gain real-world experience is with real-world experience.
Go for it!
---
If you wanted to convince /me/ to consider it, though, you'd have to address five items you've not yet really responded to. We could argue for weeks about the other points you bring up, but I don't see the utility in that.
Maybe one of us is right, maybe both, maybe neither, but most of the points do not deal with the main criteria I'd use for choosing a core material. Finnfoam may have loads of experience with thermal cycling, but if XPS doesn't have the elongation and impact resistance of linear PVC or SAN, then Finnfoam's experience, while valid, doesn't really apply to my choice.
THE STANDARD
For me, CoreCell is the gold standard for now. That might change when it's time to build and I do more research, but for now, it's my favorite because:
- Good fatigue resistance
- The strength, weight, and temperature resistance of cross-liked PVC
- The elongation and impact resistance of linear PVC
I'd also consider a linear PVC because the boat would rarely, if ever, require the heat resistance that might be important in the tropics.
Here's the big point, for me, about linear PVC or SAN like CoreCell from the previously-mentioned CompositesWorld article: "Linear or ductile PVC foams, made with a different polymer formulation, are more elastic than crosslinked varieties and are widely used in marine applications, where they offer high deflection before failure and excellent impact resistance."
THE COMPARISON
So, given that, how does XPS compare to linear PVC or SAN?
1. Percentage savings. How much money will XPS save in terms of total boat cost?
2. Weight savings. How much less would the boat weigh with XPS?
3. Compressive deflection before failure. How does XPS compare to linear PVC and SAN specifically in terms of deflection before failure?
This will happen when the sandwich is impacted or subjected to enough bending moment to move the whole sandwich. That's the difference between recovering from being overstressed (assuming the skins don't fail), and not recovering. Assuming the same comrpessive strength, and a force that moves the sandwich past its design goal in deflection without damaging the skins, one panel will remain useful, the other not.
4. Strength in tension. This is not a primary design consideration because most of the time the core is under compression or shear, but it can be put under tension depending upon how the greater structure is stressed. Such as a box beam.
5. Tensile deflection before failure. Where does XPS fail, compared to the others, when put under tension in a foam-core composite?
THE WORK (or.. the cop-out?)
I don't have the equipment and time to do dozens of trials for each test, each one involving the exact same forces, and then analyzing the results statistically. I'm happy to use the information already out there, so in that sense I'm being lazy.
If the industry has done enough testing and witnessed enough failures to say that linear PVC and SAN are better at dealing with impact, and the materials' mechanical properties support the assertion, that's good enough for me.
On the other hand, you might care enough about XPS to go through the work of testing. Or perhaps be curious enough.
THE POINT
In any case, the question isn't about aircraft, Finnfoam, or other things.
It's specifically about how much money and/or weight will XPS save, relative to linear PVC and SAN, and how does it compare in: tension, tensile deflection before failure, and compressive deflection before failure.
If XPS is superior in these areas, or it saves 40% of the boat cost, wow -- either outcome would be impressive.
If XPS is not superior in these areas, or the savings is 5% or 10%, then it's a tougher sell.
- Mike
On May 4, 2018 6:48:00 PM UTC, "mcrawf@nuomo.com [harryproa]" <harryproa@yahoogroups.com.au> wrote:
><<Did you actually read my post.?>>
>
> Yes. Fully.
>
> I think differences come down to two sets of assumptions.
>
>I'm assuming that the hull will indeed be overstressed while it's in the water, perhaps in ways that we aren't aware, so I'd want the most resilient core material available.
Then why not use a steel core?
One has to compromise. You want a material that meets the guestimated needs.
My point here is there has not been an assertion of an engineering reason for choosing such an expensive core over a less expensive option. Rob has admitted there is a localized economic reason for his choice if H80. Not an engineering one.
And, your statement "most resilient core material available" is factually untrue, with respect, H80 is simply one grade available. They make higher grade material that is provably more resilient.
How is H80 better than H60 or H200? Why choose H80?
I mean all the above with diplomacy. If that does not come through, that is my failing in phrasing. But the points stand.
>There are significant differences in delamination, particularly due to impact, between different core materials. The stress is a given, the amount of damage from that stress is the variable.
I disagree. Only in the case of an overstress should one expect delamination. It is obvious that the carbon fiber and fiberglass anecdotes delaminated due to repeated over stress of the core, who's properties were not known. To suggest XPS tends to delaminate based on these anecdotes is silly.
>I'm also assuming that water will find its way into the core. XPS may indeed be waterproof (I use it in my dock float, for example), but that doesn't mean it will make for a good core material when water gets in between the sandwich skins, particularly if submerged for long periods of time and then subjected to thermal cycling. Rick's experience being a good example.
Again I disagree. Are you saying all the boats made of XPS are known to delaminate due to thermal cycling? Is Finnfoam aware of that? Because i think they would laugh at you. You know, it is known to thermal cycle in their homeland.
>But I'm a conservative guy. Many others have tested these materials for compression, tension, shear, delamination under stress, delamination under impact, and the effects of water intrusion. Including their manufacturers. There's a lot of data out there, much of it that I couldn't reproduce through my own destructive testing.
That seems strange. These are ASTM standards, right? If you did not get the same results you probably should contact them to see how you got your test wrong, right? Unless you are suggesting there third party testing was lies. Are you suggesting that?
What grade of XPS did you destructively test?
Posted by: Arto Hakkarainen <ahakkara@yahoo.com>
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