Building a New Ruuder
I appreciate the feedback. I hope more people ask questions. This is probably far more detail than you need but, hey its fun and details are the key.
"...about 4 times its liquid volume" is vague. But its the only description from the supplier, US Composites. Turns out that the expanded volume depends on the ambient temperature as well as very much on the temperature of the two parts just before mixing. As a result they have to do the "...about..." thing since its different for everybody.
I initially calculated the volume I needed to fill the fiberglass shell by averaging the width, length, and height. I basically divided the shell into 1" wide rectangles and then averaged the depth and length every inch along its length. This gave me about 20 rectangles with their respective average depths and lengths. So, 1" times average depth times average length on each of the 20 gave me a very accurate volume. This is a very easy way to measure volumes of abstract shapes. Its actually the foundation of using calculus. The smaller you make your rectangles the more acurate your measure.
The volume was just under a cubic foot, good enough to place an order for the product. When I finished the shell I was going to fill it with water and measure its volume more accurately. But, as I was blissfully filling I realized that the shell was expanding out from the weight of the water. I would not get an accurate measure and I could damage the shell. So, how to measure an accurate volume? My 3 and 4 year old nephews are playing in the garage with me in a big box full of packing "peanut styrofoam". Well thank you nephews, I pour the packing peanuts into the shell, pour them into a box and measure the volume they fill in the box. The volume came to 1,662 cubic inches give or take, (.962 cubic feet).
It just so happens that the 16lb two part foam is sold in 1 ft^3 amounts. It basically comes in two 1 gallon containers. So, I was lucky I guess that the volume I needed happend to be about what was sold. (Although I think the very helpful and friendly people at US Composites might help out with different quanities than what''s listed, since its obvious from the product packaging that it came from bulk containers.)
Anyway, I was concerned that I might be right on the edge of "...enough". Furthermore, the two containers from US Composites were not of equal volume. "Part A" had about 1 pint less volume than "Part B". So I called them back to find out the deal.
They said that the mix should be by weight NOT volume, and that the weights were equal as I saw them (Part A is heavier). They also gave me some pointers to work with: to get the maximum expansion, warm the two parts in a water bath to a temperature in the mid 90s F. Mix small batches by weight so that the final volume of each batch is about 1.5 quarts. So after finishing the the entire process I''ve mixed 5-6 batches of the foam and allowed each pour to expand, harden, and cool. Remember that this foam, like epoxy, is exothermic, it gives off mucho heat and can damage epoxy laminated fiberglass. (i.e. if you mixed all of the two parts you''d have a huge expansion and this could get very hot, somewhere in the high 190''s.)
I was very satisfied with the outcome. The foam did exactly what it was suppossed to do, expanded into all the nooks and cranny''s and stuck like epoxy to everything inside the shell. After my 4th pour I was pretty good at estimating the last pour. My last pour expanded out of the top of the shell about 1" and was easy to shave off with a rasp before it hardened completely (about 5 minutes after pouring). I still have a couple of cups of the 2 parts left.
Hope this helps.