Just my .02 again. Seacast markets the heck out of their stuff so people naturally see it more and think its the standard. The boat manufacturers use the Arjay and Nicacore pourables. I doubt any boat manufacturer uses Seacast. DRJ, if you have the small transom (like the one in tsubaki's post), I guarantee you'll use much less than I did. Just guessing, I'd say you'd use around 10 gallons. Do what I did, measure the transom and lay it out on grid paper and use 1.75 inches as your width. The smaller transom means you have the luxery of probably reaching the limits of the entire inside of the transom with an 18" chainsaw. I had to resort to the metal pry bar method which was labor intensive, and would be moreso if your wood wasn't completely rotten. I was lucky, my wood was shot. Finally, if you can't get the middle piece of fiberglass out, leave it in there. You want all the wood out. It will make it difficult to shopvac the pieces out but pour around it. Once you coat it with resin, the pourable compound should adhere to it fine. Call around - you may be able to pick up the Nidacore or Arjay locally and save shipping.
Maury