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Old 30-10-2020, 12:39   #1
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XPS 100lb As Structural Core - Pros Cons

I'm looking to build a light weight 24ft trimaran without throwing money away.
XPS is about 30% the cost of Divinycell. A 4 x 8 x 1/2in thk sheet of 4lb Divinycell h60 is $155 or about $1700 for my main hull. I think it is 120psi compression. The XPS 100psi compression Owens Corning Foamular is only $85 equivalent per sheet (involves hot knifing a 2in thick piece to width). This would save over $1100 on my main hull alone.


So, why don't builders use XPS core, assuming they are planning to build in epoxy anyway?? The mechanical properties seem similar, slightly less compression, but I can compensate for that in thickness.
Pros/Cons of XPS over Divinycell?? obviously cost is a huge pro for XPS!
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Old 31-10-2020, 05:08   #2
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Re: XPS 100lb As Structural Core - Pros Cons

Maybe these threads would interest you?

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https://www.homebuiltairplanes.com/f...ruction.29970/
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Old 31-10-2020, 05:33   #3
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Re: XPS 100lb As Structural Core - Pros Cons

Quote:
Originally Posted by zstine View Post
I'm looking to build a light weight 24ft trimaran without throwing money away.
XPS is about 30% the cost of Divinycell. A 4 x 8 x 1/2in thk sheet of 4lb Divinycell h60 is $155 or about $1700 for my main hull. I think it is 120psi compression. The XPS 100psi compression Owens Corning Foamular is only $85 equivalent per sheet (involves hot knifing a 2in thick piece to width). This would save over $1100 on my main hull alone.


So, why don't builders use XPS core, assuming they are planning to build in epoxy anyway?? The mechanical properties seem similar, slightly less compression, but I can compensate for that in thickness.
Pros/Cons of XPS over Divinycell?? obviously cost is a huge pro for XPS!
The short answer is itís friable.

Under repeated loading and unloading of stress, it breaks down into dust.

Itís not so good for important structural items.

You wonít save a penny using this foam for the hull. In fact, youíll lose a fortune in epoxy, paint, glass and sails and stuff.

The hull is the last place cut corners. Use exterior house paint at the end. Source epoxy carefully. Buy rolls of glass. You can save more with good purchasing than using an inferior core.
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Old 31-10-2020, 05:39   #4
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Re: XPS 100lb As Structural Core - Pros Cons

Second that...... you get what you pay for.
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Old 31-10-2020, 05:50   #5
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Re: XPS 100lb As Structural Core - Pros Cons

Also, keep looking. I’ve seen better prices for divinycell. And ask for quantity discounts. Buy all your materials in one shot.

I’ll share a tidbit of wisdom Chris White shared with me that really made a difference in my boat building life (paraphrased):

“It takes just as much time and money to build a POS as it does to build a good boat. Why not build a good one?” - Chris White
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Old 31-10-2020, 06:25   #6
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Re: XPS 100lb As Structural Core - Pros Cons

People who have tried making surfboards with XPS have experienced more delam issues than with EPS or PU foams. The typical countermeasure is to scarify the surface with a bazillion small holes for a better mechanical bond. A surfboard failing is a lesser order of magnitude than a boat failing. I would seek a more purpose-suited material.
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Old 31-10-2020, 08:50   #7
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Re: XPS 100lb As Structural Core - Pros Cons

Quote:
Originally Posted by Chotu View Post
The short answer is it’s friable.

Under repeated loading and unloading of stress, it breaks down into dust.

It’s not so good for important structural items.

You won’t save a penny using this foam for the hull. In fact, you’ll lose a fortune in epoxy, paint, glass and sails and stuff.

The hull is the last place cut corners. Use exterior house paint at the end. Source epoxy carefully. Buy rolls of glass. You can save more with good purchasing than using an inferior core.

Friable.. gottcha. Thanks! That doesn't show up in technical specs and Divinycell doesn't mention it as an advantageous property in their literature.. at least not that i recall... The 8 year plan is to double this 24ft 'prototype' into a 48ft fast Tri to retire on. What do you think about XPS for cabinets and shelving, nonstructural stuff?

House paint!! I assume you mean in interior spaces only..?
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Old 31-10-2020, 09:28   #8
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Re: XPS 100lb As Structural Core - Pros Cons

Quote:
Originally Posted by zstine View Post
Friable.. gottcha. Thanks! That doesn't show up in technical specs and Divinycell doesn't mention it as an advantageous property in their literature.. at least not that i recall... The 8 year plan is to double this 24ft 'prototype' into a 48ft fast Tri to retire on. What do you think about XPS for cabinets and shelving, nonstructural stuff?

House paint!! I assume you mean in interior spaces only..?
Just talking about saving money. Exterior house paint. It’s INCREDIBLE in how it stands up to the elements. I quickly slapped some on my dagger boards 5 years ago as they were freshly built, just corecell and epoxy and had to be left out in the Florida sun for a year. I planned to sand that junk off and put real paint on. That year turned into 5. They don’t look any different now than they did when I put the “exterior latex” house paint from Home Depot on. I was astounded. I’m just saying save money on cosmetic things, not your core. There is nothing more important than the structural integrity of the boat.

XPS for cabinetry?? Hell yeah! Epoxy is one of the few compatible resins you can use with XPS so you’re good to go. Lucky.

Me? I get an anaphylactic allergic reaction near epoxy after building a 50’ cat from it. Be very careful. It doesn’t smell bad but it’s a harsh chemical. PPE for sure so you don’t end up like me. I have permanent lung damage.

I’m doing some interior cabinetry from XPS myself. BUT I have to use polyester. I’m experimenting with ways to use a tie coat to keep the polyester away from the foam so it doesn’t eat it.

Additionally, for any cabinetry hardware, you’ll need to put in wood blocks or high density corecell or Divinycell in your laminate core to accept the hardware. Don’t forget about that.

PS: polystyrene is friable. Divinycell is not. In case my earlier post wasn’t clear. It’s not a good quality. Basically means the foam will turn to crumbs and dust, especially near the bond line, from repeated flexing like it experiences as the hull of a boat. For something stationary like a cabinet, the foam being friable doesn’t matter since it’s not continuously flexing.
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Old 31-10-2020, 11:49   #9
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Re: XPS 100lb As Structural Core - Pros Cons

[QUOTE=Chotu;3265607]thanks again. good info. I've read through many of your posts regarding your build. sorry that you have become allergic to epoxy, that certainly puts a damper on your project. I appreciate that you have kept an 'out of the box' thinking to keeping your cost down... electric winch alternative to buying a powered Harken/Lewmar off the shelf, buying a used rig, etc... good stuff! And good luck!

This 24ft boat is a prototype that I only need to sail for a few years, but even then, given the info you provided I will avoid XPS in the hull and deck. Hard points for load bearing, noted. I like to use GPO3 (cheap) for backing plates and may use it for 'core' under winches, cleats, etc. The boat is a Tri, but flying a paraglider wing or possibly a hang glider for power. Amas will be quite small as there will be no overturning moment. I intend to get it foiling, especially since the comfort on a boat without the inertia of a standing rig and sails will be like bull riding.

I have a mechanical engineering degree and a basic understanding of aero/hydrodynamics. While I'm no naval Achy, I've sailed for 30 years and have always been interested in boat building, worked at Electric boat for a decade, so I have some knowledge... enough to get me in trouble!
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Old 31-10-2020, 12:01   #10
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Re: XPS 100lb As Structural Core - Pros Cons

Quote:
Originally Posted by Chotu View Post
I’m doing some interior cabinetry from XPS myself. BUT I have to use polyester. I’m experimenting with ways to use a tie coat to keep the polyester away from the foam so it doesn’t eat it.

Duratec has a low VOC polyester that is designed to go over EPS. I used it over EPS with 6oz biaxial cloth and was happy with it... but those were my first couple projects so I don't know if my opinion is all that qualified. The only trouble spots I saw (tore one apart for another reason) is where the resin pooled and exothermed the EPS.

http://duratec1.com/Styrosafe/
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Old 31-10-2020, 12:01   #11
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Re: XPS 100lb As Structural Core - Pros Cons

Quote:
Originally Posted by Chotu View Post
Just talking about saving money. Exterior house paint. Itís INCREDIBLE in how it stands up to the elements. I quickly slapped some on my dagger boards 5 years ago as they were freshly built, just corecell and epoxy and had to be left out in the Florida sun for a year. I planned to sand that junk off and put real paint on. That year turned into 5. They donít look any different now than they did when I put the ďexterior latexĒ house paint from Home Depot on. I was astounded. Iím just saying save money on cosmetic things, not your core. There is nothing more important than the structural integrity of the boat.

XPS for cabinetry?? Hell yeah! Epoxy is one of the few compatible resins you can use with XPS so youíre good to go. Lucky.

Me? I get an anaphylactic allergic reaction near epoxy after building a 50í cat from it. Be very careful. It doesnít smell bad but itís a harsh chemical. PPE for sure so you donít end up like me. I have permanent lung damage.

Iím doing some interior cabinetry from XPS myself. BUT I have to use polyester. Iím experimenting with ways to use a tie coat to keep the polyester away from the foam so it doesnít eat it.

Additionally, for any cabinetry hardware, youíll need to put in wood blocks or high density corecell or Divinycell in your laminate core to accept the hardware. Donít forget about that.

PS: polystyrene is friable. Divinycell is not. In case my earlier post wasnít clear. Itís not a good quality. Basically means the foam will turn to crumbs and dust, especially near the bond line, from repeated flexing like it experiences as the hull of a boat. For something stationary like a cabinet, the foam being friable doesnít matter since itís not continuously flexing.
Chotu, There is a polyester called styrosafe that does not eat xps, I think Duratech makes one too.
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Old 31-10-2020, 12:15   #12
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Re: XPS 100lb As Structural Core - Pros Cons

Quote:
Originally Posted by Chotu View Post
Iím doing some interior cabinetry from XPS myself. BUT I have to use polyester. Iím experimenting with ways to use a tie coat to keep the polyester away from the foam so it doesnít eat it.
This is a back-yard experiment on youtube where a guy paints multiple samples of XPS with cheap Latex paint to see how many coats are required to provide a good barrier to polyester resin. It is far from scientific, but 5 coats of Latex was his finding, i think (i watched many months ago). Seems like a lot of extra work if the above polyesters are compatible and won't set your allergy off.

but here it is...

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Old 31-10-2020, 12:33   #13
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Re: XPS 100lb As Structural Core - Pros Cons

Actually the Styro Safe resin is in fact the product sold by Duratec. No need for home made solutions. Using latex paint may prevent the xps from dissolving but the resin is not going to be bonding to it so it is not actually going to perform as a core.
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Old 31-10-2020, 13:50   #14
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Re: XPS 100lb As Structural Core - Pros Cons

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Chotu, There is a polyester called styrosafe that does not eat xps, I think Duratech makes one too.
WOW!!!!!!

A game changer. Thanks so much for mentioning this. Iím going to get some immediately for use on the cabinetry I wanted to try out of foam.
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Old 31-10-2020, 13:52   #15
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Re: XPS 100lb As Structural Core - Pros Cons

Quote:
Originally Posted by Spot View Post
Duratec has a low VOC polyester that is designed to go over EPS. I used it over EPS with 6oz biaxial cloth and was happy with it... but those were my first couple projects so I don't know if my opinion is all that qualified. The only trouble spots I saw (tore one apart for another reason) is where the resin pooled and exothermed the EPS.

http://duratec1.com/Styrosafe/
Thank you so much!
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