View Full Version : foam sandwich

tonybroad - n/a
18-Feb-06, 07:33 PM
I'm sandwiching some 1" sheet foam between the floor and the bottom hull, mainly to stiffen the floor up but it will also contribute to buoyancy so i don't have to stick as much in the plenum

i can't decide what is best to bond it with - sikaflex will only stick where i put it and i'd like full coverage over the whole surface of the foam - i could use resin but i'm really looking for something that foams very slightly to fill any irregularities - weight is also an issue and is putting me off using resin

has anyone used the liquid two pack rapid expanding stuff as a bonding agent, can it be spread thinly ?? i'm concerned about not having control over the expansion and getting the two halves together quickly enough

suggestions please


Keith Oakley - n/a
18-Feb-06, 08:37 PM
Prob with home brew 2 pot is if you get mix wrong its either too brittle or soft and water absorbant. I also doubt you get it to spread 1 inch thick without serious voids. Better to use ready made PU sheets (from Glasplies?) and bond wet resin onto it. However with the usual brown foam the cells next to the skin break up after a while with vibration.

Best foam I ever used was Unitex Marines 'Unifoam Blueboard' (6.5lb/cu ft density) sold at the time for structural boat building. My 1976 4 seater had an 8 * 4ft totally flat floor of 1 inch board of this stuff with 1.5 oz csm (polyester resin) either side - applied wet. Its still intact. 1997 complete F25 built of 10mm of same foam with 1 layer 150g cloth (epoxy resin) is still solid despite folk walking all over it and it being lighter than standard kip grp F25.

It was expensive but not sure if its still available. Looking at co website http://www.trelleborg.com/appliedtechnology/ tonight they seemed to have been taken over and making different stuff. Might be worth a call.

tonybroad - n/a
18-Feb-06, 09:07 PM
Thanks Keith - i already have some PU foam sheet from glasplies - i just wondered how much of it would actually stick if i used resin -

i suppose i could put the foam on the last wet glass layer when i lay up the hull and introduce an extra wet layer on the floor just prior to assembly, i could always throw a couple of dustbins worth of warm water in the top hull to compress it all together

Nice to know resin has worked for you - i'll just have to suffer the extra weight of glassing it in and shed a few pounds myself to compensate http://hovercraft.org.uk/images/icons/smiley_icons/icon_lol.gif http://hovercraft.org.uk/images/icons/smiley_icons/icon_lol.gif

Keith Oakley - n/a
18-Feb-06, 09:20 PM
You might want to try putting microballoons as a filler in the resin thats in contact with the foam sheet. The surface on those cut foam sheets is very rough and it takes a lot of resin to fill in all those cavities = more weight.

Mart366 - n/a
18-Feb-06, 10:59 PM
Hi Tony,

you could use 2 pack foam, like anything tough it has pro,s an cons,


you can mix as little or as much as you like, being a liquid, it will to some degree find its own level, and until the reaction point is reached, flow uniformly, once the expansion starts, it then continues until its prevented, either by time(cures) or by filling the area required, or by a lid/ cap over the filled area.

if you get any high or low spots, this can be filled by adding more liquid foam or cutting off the excess.


As others have said, its not watertight. the method we used, (lloyds approved) was to use a layer or more of grp over the top of the foam. we wet the foam thouroughly and then layed 1 300 or 600 csm mat over the top. this would make the foam watertight.


nickyd - n/a
19-Feb-06, 02:43 PM

You could use the PU foam and lay up directly on it then make yourself a homebuilt autoclave with a wooden box lined with foil and a heater. Then bag the lay up and attach a suction pump to it. This will then suck out all the unecesary resin leaving you with a much lighter and stronger finished product.

I've seen this done with 6mm foam and carbon fibre and it worked really well.


jar2 - n/a
19-Feb-06, 03:49 PM
I might as well stick in my 2p's worth here Tony!

I've build two PU foam hulls in the last year and I've found that if you use the correct grade PU foam sheet (at least 4lbs/ft2 or 65 Kg/m3) the cell size is so small that it doesn't absorb much resin at all (finished weight should be around 4.5Kg/m2 with a couple of layers of 6oz glass both sides). Before glassing this foam it is very important to remove all of the foam particles from the open cells on the surface (a vacuum cleaner is good at this if you can get it out of the house!). If you don't prep the surface then the resin doesn't 'lock' into the surface foam cells properly - resulting in a weak bond.

I have also used 60kg/m3 PVC foam and found it to be much stronger than PU of a similar grade. However, the downside is that the cell size is much larger (more resin needed) and it's quite difficult to sand or form.