With the growing popularity of small boats made out of rigid foam insulation I thought I’d see what information I could find about it. Foam used in houses has to meet standards so I looked up the standards to see if they contained anything useful. There is one standard for the United States and another for Canada where I live so it’s a bit confusing although both seem to use the same testing procedure. Both measure the same qualities and they are min or max, products may exceed the standard. The “compression resistance” tells how hard the foam is, the “flexural strength” how bendable, and the “density” how heavy. I’d use them to rank foams for comparison. There are three kinds of foam. The familiar bead board is EPS for expanded foam; the closed cell is XPS for extruded foam; and the not so familiar ISO for polyISOcyanurate which is usually sprayed or faced with foil The XPS is probably the best for making boats covered with fabric although I used ISO and removed one face. The EPS is being used for heavier boats covered with fiberglass. The US standard for EPS and XPS is called ASTM C578. In Canada it’s CAN/UCL S701.
The following table shows the US standard and where one manufacturer’s products fit in it. Type IV has been used for foam boats featured in Duckworks articles.
Here is the relevant Canadian standard, showing how the products can be compared. Since this table was printed a denser Type 4 foam has been added but I was not able to find a table which included all four types.
The Canadian and American standards are not standards for load-bearing structural material so the measurements are not useful in that sense. I think for boatbuilding we need to know how much it weights and how much it bends so I went to three home improvement stores to weigh and bend foam. The foam I measured was made in Canada to the Canadian standard but can be cross referenced to the American standard using the tables above. To measure the bend I pushed on the short edge until it stopped and measured the change in length, in effect measuring the length of the chord of the arc it made. I went by feel so it was at best an estimate. Ideally I would buy a bunch of foam and put weights on them and measure how far they deflected before they broke but for that I’d need a research grant. My method was free. I calculated the height of the arc by substituting the curved arc for the straight hypotenuse in the Pythagorean formula so the height and the corresponding radius of curvature in the table are slightly less than the actual numbers. Consider it a small margin of safety. To measure weight I used a kitchen food scale which goes up to 10 pounds. Note that 1/5” lauan and okoume plywood weigh 1 lb per sq ft for comparison. The numbers went into a computer spreadsheet. I would use them to compare and to rank type and thickness of foam. The “radius” column is the radius of curvature of the arc in the foam. It could be compared to the radius of curvature of plywood when thinking of substituting foam for plywood in a small boat design as I did for small foam canoe I made. The thicker the foam board the stronger it is but less bendable to the shape of a boat, just like plywood. Note on cost: the Canadian dollar was worth seventy-five cents American when the cost was calculated.
I hope the numbers will help fellow experimental boatbuilders choose the best foam for small, lightweight, durable craft.
Bill in his foam boat