Balsa VS foam core

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dennisail
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Balsa VS foam core

Post by dennisail » Sat Feb 23, 2013 2:19 pm

What re the pros and cons of these and could foam be substituted for balsa core without redesign? Is the foam used in boat building water proof or does it soak water like balsa?

rexd666
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Re: Balsa VS foam core

Post by rexd666 » Sat Feb 23, 2013 10:34 pm

The different cores have different properties, which means different design. I believe the most significant is shear strength as this is the main force exerted on the core. Foam will tend to be thicker than the balsa for same usage, but lighter and costs more.

Foam is closed cell, so won't soak up water. Stories about balsa rotting, foam turning to pulp etc are all out there. Both are valid choices.

groper
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Re: Balsa VS foam core

Post by groper » Sun Feb 24, 2013 9:57 pm

The main difference in design terms, is the compressive strength of foam being rather poor compared to balsa. The shear strength is generally adequate for foam sandwich cored panels to be used in structural marine applications. The shear strength of typical 80kg/m3 PVC or SAN foam is not as high as balsa, but this is of no consequence to us because the thin laminates used in typical cruising yachts do not demand excessive shear properties. That is, in average yachts, the panels strength is generally limited by the laminate, not the core.

However this is not the case when its comes to compressive strength. Foam is more prone to impact damage as it cannot resist point loads (such as someone jumping off a wharf onto your deck) so its easier to ding etc. In a structural application, It cannot resist the wave slamming loads like balsa can. So in applications such as high speed planing powerboats, 80kg/m3 foam is not sufficient for the hull bottom panels. Instead, you typically need to go upto minimum 130kg/m3 density foam to get enough compressive strength for this application. Of course you could alternatively goto balsa to get the compressive strength needed and save money, as the high density foams get very expensive.

Hope that helps...

44c
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Re: Balsa VS foam core

Post by 44c » Tue Feb 26, 2013 6:53 pm

The only real "con" of balsa is that it absolutely must be kept dry. Structurally it's an excellent material, with some supeior properties to foam.

You do have to use correct practises to ensure water doesn't get in though.

puremajek
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Re: Balsa VS foam core

Post by puremajek » Wed Feb 27, 2013 10:03 am

In my humble opinion, foam would not be the preference for the uninitiated unless you are very confident with the sealing issue raised by 44C. This though comes at cost.

(I left the word 'not' out, which may explain the next post)
Last edited by puremajek on Wed Feb 27, 2013 1:42 pm, edited 1 time in total.
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sea_bee
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Re: Balsa VS foam core

Post by sea_bee » Wed Feb 27, 2013 12:32 pm

puremajek wrote:In my humble opinion, foam would be the preference for the uninitiated unless you are very confident with the sealing issue raised by 44C. This though comes at cost.
This sounds like a polite way of saying "Your gonna make a boat. It's gonna get wet"

puremajek
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Re: Balsa VS foam core

Post by puremajek » Wed Feb 27, 2013 1:43 pm

See Bee

Thank you, amazing what one word does.
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Corley
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Re: Balsa VS foam core

Post by Corley » Wed Feb 27, 2013 2:54 pm

Balsa is generally a superior core material in it's properties but must be kept dry with modern epoxies and following the manufacturers instructions as to core preperation and tie coats it should not be an issue. It would not be wise to make a blanket assumption that foam core will be adequate as a one to one exchange for balsa in all locations. There are different densities of foam core and their usage is dependent on location for example in areas exposed to slamming you would use a higher density foam. Good idea to contact the designer of the boat to check or if your designing the boat yourself run some numbers to compare the different core properties.

A compromise composite material could be considered too like Durakore which consists of a balsa core sandwiched between plywood veneers. This assists with keeping water out of the core material and provides an alternative to constuction in standard strip planking timbers like Western Red Cedar. I've heard this is how the Geforce catamarans are constructed at Noosa Marine, strip plank durakore.

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sea_bee
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Re: Balsa VS foam core

Post by sea_bee » Wed Feb 27, 2013 5:02 pm

I would assume that any 10 year old cored hull weighs more than it did when it was launched.

Back in the 1970's, ferro-cement was the wonder material for boat building. I can see the day coming when cored hulls (of any kind) will have a reputation similar to that of ferro-cement today.

groper
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Re: Balsa VS foam core

Post by groper » Thu Feb 28, 2013 6:49 pm

I made a PVC foam cored kiteboard many years ago... i put a hole in it about the size of a 10cent peice from hitting some rocks one day... it went right through the laminate and into the core about half way through so you you could see exposed foam etc... i never bothered to fill the hole with resin as i wanted to see how long it would last... it gets used frequaently and hole is always full of water when in use... the damage has never spread or delaminated around the original damaged area. The foam does not absorb water, the data sheet says < 1%. I have absolute faith in foam cored composites and their longevity, just doint expect them to withstand impacts very well.

Foam is lighter than balsa, except where the high density stuff is required. It also absorbs less resin in the surface of the material for laminate adhesion. You also cannot infuse composites using balsa, unless your prepared to pay an even greater weight penalty with even more resin absorbtion as the vacuum sucks it in deeper - despite the coatings they say mitigates this...

Food for thought...

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