Foam density

[redreuben]

Many people looking at smaller yachts <10m and I am thinking multihulls here, are looking for light weight but strong building systems.
In this size range it gets very difficult for foam to compete with gaboon ply and wrc without getting fragile.
It seems all parameters of stiffness and weight can be met except for bashability, the foam hull is extremely tender in compressive strength.
Yet all these comparisons that I have seen use 80k foam.
Has anyone investigated the use of the 100k or 130k foams as a substitute for gaboon ply or wrc whose densities are around 450k ?

 

[pbmaise]

I see no replies, however, I just will give a few cents based upon my experience with foam.

Big crew members are a concern when it comes to foam. The decks in one area of my boat are a sandwich of plywood and foam. Glass and epoxy both sides. The builder made the mistake of trying to span the deck too far with no pillars below. The result was a little flexing.
Over 10 years, heavy foot traffic from heavy crew managed to crush some of this foam and cause slight delamination of the fiberglass above.

I did some core samples and found the builder went with a hatched(correct term?) foam instead of a solid board. The hatched foam is is gapped every 2 inches or so with a gap of about 3/16inch. I gather he was concerned about the curved deck surface.

He really should have gone with solid foam board. The board does bend some, however, would have greatly improved the structural integrity. Those grooves really represent a lot of lost area.

To fix this minor mess I put 3 pillars below. I then ground down the old paint to original fiberglass and added two new layers. This isn't a structurally important area of the boat so I didn't take it all the way down get rid of the foam.

I'm pleased to report the results are as solid as a cement sidewalk. Come on you American's lets bring it on......well maybe not all American's I've seen some really big ones.

Philip

 

[redreuben]

Philip,
You are partly right about the grooves but really it made little difference, the builders biggest mistake was the foam was not thick enough. Second mistake is those grooves should have been full of adhesive. My guess is the foam was not adequately bonded to the ply. The best place for more glass would have been underneath, the topsides you should drill holes and inject some resin perhaps.
Bit off topic but interesting Not sure what the American thing is all about, I'm English/Australian .

 

[groper]

yes mate, most higher load areas such as the bottoms of high speed planing vessels will use higher density foam. Not sure what your question is?

The problem is the high density foams get very expensive, its directly proportional to the density... so expect 160kg/m3 PVC foam to be about double the price of the typical 80kg/m3 for the same size and thickness sheet of foam.

Balsa is the option usually most people go for when high spec cores are required as its cheaper than 80kg/m3 foam but has the toughness of very high density foam up around 200kg/m3 etc... provided its well encapsulated in epoxy and the builder uses due care, the rotting issue isnt a concern... many very nice boats in the water have balsa cored hulls with no troubles.

 

[redreuben]

Groper you may have answered my question-cost.

But what I was getting at is that when replacing thin ply like 6mm with foam sandwich to save weight you can achieve the stiffness with very thin skins leaving a stiff but fragile hull. If you add more glass the weight advantage disappears rapidly. So...why not use a higher density foam rather than more glass (or not quite so much glass) to get the impact strength. I was thinking in purely outcome terms not cost, but I take your point.
As for balsa, not a fan, I would rather strip plank in Paulownia and S-glass it.

 

[groper]

Well yes you are correct.

Alot of the modern multuhulls from grainger, schionning, oram et etc are all done in balsa core with very thin e-glass laminates - typically 600gsm per side. I know of another modern catamaran design, the pacific 40 or solitary island 40, which use 450gsm skins over polycore in most places. They get away with it as balsa, and to a slightly lesser extent polycore, have a much better compressive strength than foam.

The key is to use the higher density cores or higher weight laminates, only where you need the `bashability` and for the rest of it, go with the light weight stuff...

 

[Tungsten]

Good topic,I've been looking for something with more "bashability".

Say you double it so 80 to 160 kg3 so now a 12mm M80 weighs 100g sq/ft so double it to 200g,thats about the same as adding 600g cloth to the 100g piece.

Which gives you more bashability? m80 with 3 layers 600g cloth or m160 with 2 layers.

Another question I have is which type of corecell has more Bashability( I like that word) The A or the M?