Foam vs Tree

Matt · 05-11-2007, 03:48 PM

A lot of people talk about foam being a wonder product in boatbuilding.

My hydrolift has a lot of foam in it - pretty much the entire boat is foam cored, as are the large stringers, bulkheads etc etc all foam. And to be fair, they've all held together so far.

So I decided to do some research about foam vx tree

Tree is a unidirectional composite which sounds cool, plus that baker bloke swears by it, and he built a few boats.

I noticed it's possible to deform a foam cored panel on one side only - a load on one side doesn not necessarily induce a bend in the other side. This implied to me that the foam was being compressed - not sure I like the sound of that.

So anyway, a bit of googling later on a few materials sites and I found someone has done the hard work for me. They sum it up pretty well:
...foams are fundamentally weaker than balsa core.....

Oh dear.

End grain balsa is typically 3!!!! times stronger under compressive loads.
Twice as strong under tensile loads.
and 3 times as strong under shear
OK. So are they lighter (ie, can I get more strength for the same weight?)
Er, no. Foam often requires MORE adhesive than the balse core, so even though the core materials weigh similar, the finished panel weighs more. This means for 2 panels of the same weight, balsa wins both ways!

OK, are the foam cores more resistant to water ingress? We know balsa doesn't like water.

Ask anyone who's ever owned a dell quay dory about waterlogged foam.
Or hydraulic failures where a water logged foam core hydraulics itself into goo. Nice.

Looks like it's 1 nil to tree then.

05-11-2007, 04:07 PM

Quote:

Originally Posted by Matt

A lot of people talk about foam being a wonder product in boatbuilding.

My hydrolift has a lot of foam in it - pretty much the entire boat is foam cored, as are the large stringers, bulkheads etc etc all foam. And to be fair, they've all held together so far.

So I decided to do some research about foam vx tree

Tree is a unidirectional composite which sounds cool, plus that baker bloke swears by it, and he built a few boats.

I noticed it's possible to deform a foam cored panel on one side only - a load on one side doesn not necessarily induce a bend in the other side. This implied to me that the foam was being compressed - not sure I like the sound of that.

So anyway, a bit of googling later on a few materials sites and I found someone has done the hard work for me. They sum it up pretty well:
...foams are fundamentally weaker than balsa core.....

Oh dear.

End grain balsa is typically 3!!!! times stronger under compressive loads.
Twice as strong under tensile loads.
and 3 times as strong under shear
OK. So are they lighter (ie, can I get more strength for the same weight?)
Er, no. Foam often requires MORE adhesive than the balse core, so even though the core materials weigh similar, the finished panel weighs more. This means for 2 panels of the same weight, balsa wins both ways!

OK, are the foam cores more resistant to water ingress? We know balsa doesn't like water.

Ask anyone who's ever owned a dell quay dory about waterlogged foam.
Or hydraulic failures where a water logged foam core hydraulics itself into goo. Nice.

Looks like it's 1 nil to tree then.

what type of foam are you talking about pu, hd, builders, a&b?

Matt · 05-11-2007, 04:18 PM

The 1st article I was reading doesnt state which foam - although does sight all it's mechanical properties. That seems to be a bit of an omission.

jw. · 05-11-2007, 04:57 PM

Quote:

Originally Posted by Matt

End grain balsa is typically 3!!!! times stronger under compressive loads.

I'd put money on it being much stronger than that. I bet if you took a piece of balsa 50mm cube and stamped on it you wouldn't break it. Foam would be powder in no time. Having said that, balsa is very variable in density so you'd need to state exactly what density you are referring to as the comparison.

I've always felt foam has such little strength that its use is little more than a former to mould over.

Jon Fuller · 05-11-2007, 05:08 PM

There are certainly plenty of different density foams available for use as composite structure core.
Divinycell make some highly expensive options

http://www.diabgroup.com/americas/u_...u_prods_2.html

However, I have a gut feeling about foam, which is possibly founded at least slightly on seeing so many waterlogged foam filled cavities on boats over the years. This is supposedly impossible now, with all these fancy closed cell foams.

The real decider though, is that the likes of Peter Hledin (Skater boats USA) and me oul mate Steve Baker still consider good old balsa to be superior. Those two opinions alone are enough for me.

Matt · 05-11-2007, 05:33 PM

OK, so to compare like for like as much as possible, lets use 80kg/m3 density divinycell H vs 88kg/m3 balsa (all I can find right now for balsa).

Tensile Strength Foam : 363psi
Compressive Strength Foam : 203 psi

Tensile Strength Balsa : 990 psi
Compressive Strength Balsa :831 psi

Or, let's look at a spruce stringer (units are in Mpa this time)

Tensile Strength : 90Mpa
Compressive Strength : 16-55Mpa
Density about 500kg/m3

Compare to the most dense divinycell H foam (250kg/m3)
Compressive Strength : 6.2Mpa
Tensile Strength : 9.2 Mpa

In a stringer, the length would be constant, so doubling the area of the foam stringer to make the weight like for like, the spruce stringer is much stronger.

Or to put it another way, the 250kg/m3 foam stringer would need to be about 10 times the cross section to have the same tensile strength, or 3+ times to have the same compressive strength.

I think it's as jw says - foam is really just a former for the shape.

RossM · 05-11-2007, 09:59 PM

Interesting read!
more info....

Jon Fuller · 06-11-2007, 09:32 AM

we were looking at the balsa spec the other day, and the density options were 110, or 150 kg/m3

We will be using Baltek 'contourkore' balsa core. They do a huge range of densities, sizes and extras such as fillet strip

http://files.alcancomposites.com/dow...data_sheet.pdf

Cookee · 08-11-2007, 08:50 AM

We use all three - wood, balsa and foam and use the materials in the appropriate places, for instance the deck on our race boat is all foam, but the hull has a lot of balsa in it where the compression requires it.

Damon2100S · 08-11-2007, 11:10 AM

http://nida-core.com/english/fusion.htm

Ultimate strength, if you have the application to use it, I have used this in infused floor panels 2x3m unsupported. Absolutely fantastic combination of foam resin and glass.

Nidacore is fantastic but pricey.

Jon Fuller · 08-11-2007, 11:19 AM

Quote:

Originally Posted by Cookee

We use all three - wood, balsa and foam and use the materials in the appropriate places, for instance the deck on our race boat is all foam, but the hull has a lot of balsa in it where the compression requires it.

What did kitten use where it sits on the trailer.

Damon2100S · 08-11-2007, 11:21 AM

Or if you want to prevent water penetration you can use this

http://nida-core.com/english/nidaprod_honeycombs.htm

Once infused each cell becomes independant and fluid can only penetrate into punctured cells. If you had a major side impact casing surface fracture the long term damage would be minimal compared to HDF of end grain balsacore (which we all know looooves moisture!)

The world of composites is growing at an alarming rates, google nanocomposites.