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Modelhovercraft - n/a
25-Dec-07, 03:18 AM
What do you think is best used on a hovercraft for buoyancy?

This expanding foam stuff that comes in an aerosol can?



http://images.truckinweb.com/tech/0511tr_view_24_z.jpg



or the simple sheet polystyrene?



http://myweb.tiscali.co.uk/ironmammoth/tmmp/images/18%20x%20 (http://myweb.tiscali.co.uk/ironmammoth/tmmp/images/18%20x%2036%20sheet%20side.jpg) 36%20sheet%20side.jpg

tonybroad - n/a
25-Dec-07, 08:41 AM
polystyrene is a simple and inexpensive way of creating buoyancy, just make sure it is fixed in well, beware polyester resin melts it !



you can also get Polyurethane sheet which you can fibreglass directly onto with polyester resin which melts polystyrene



there is also polyethelyne sheet which is a plasticised foam, doesn't absorb water and can be glassed onto - bit pricy though



most aerosol foam is rubbish as it dries in air so the middle stays sticky



there is also two-pack liquid Polyurethane expanding foam which sets chemically rather than in air and gives a fairly consistent rate of expansion, one other advantage is it sticks really well



poured into the bottom of the plenum it rises evenly and sticks top and bottom hulls together giving good rigidity



depending on temperature i've found it expands 20-25 times it's original volume so i work on 5 litres of the stuff supporting 100kg buoyancy - it's not cheap at 35-40 for 5 litres but i love the stuff



i use a combination of polyurethane sheet between the floor and bottom hull with expanding foam around the bottom of the plenum



hope that helps



Tony

GaryH - n/a
25-Dec-07, 11:30 AM
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depending on temperature i've found it expands 20-25 times it's original volume
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if you use 2 part keep the tubs indoors until moments before you mix it. if foams much better. i have been told (but hav'nt tried it) to warm it in an oven as you will get more expansion per gram of product. but beware the warmer it is the faster it expands. so get organised and have EVERYTHING you need to hand before you mix it

kach22i - n/a
28-Dec-07, 02:40 PM
APD, both examples of foam you posted absorb water and should not be used below the water line on any watercraft.



Extruded (NOT expanded) polystyrene sheets are good for exposed to water areas found on watercraft and below grade applications on buildings.



Example link/info:

MOISTURE RESISTANCE AND MECHANICAL PROPERTIES

http://www.diversifoam.com/xeps.htm (www.diversifoam.com/xeps.htm)