This tool is being offered for download at the KonTiki website for use by entrants to KonTiki, but hsould be useful to any Scout raft designers. Note that KonTiki is held on sheltered, still, inland waters, so the buoyancy is calculated with that in mind.
The tool is metric, but for rough estimation purposes for Imperial units, 1kg = approximately 2 pounds, and 100mm = almost exactly 4 inches.
Release notes from the website:
Alan Ford has arranged this tool as a safety measure to ensure that the raft being built will have sufficient buoyancy provided for by barrels, before being launched. This will aid you and also ensure that the raft is sufficiently manoeuvrable should you need to be towed by a rescue vessel.
If you have any questions or would like Alan to review it, please e-mail it to firstname.lastname@example.org when you have populated the final version or bring it to Kontiki on a memory stick, Alan will go through it at the event.