Exposing the myth Seacast is lighter than marine plywood

[Capt Terry]

There isn’t a lot of CSC interest in pouring transoms. But, I wanted to advise and see if anyone could refute any of this. After a recent Seacast pour of my transom, I examined the following statement from the Seacast website
“Seacast is .03 pounds lighter than a marine plywood transom, which will not alter the balance of your boat.”
(.03 pounds mean nothing without noting the volume being weighed. A more accurate statement would be the density of Seacast is about 0.03 lbs/cu.in. (confirmed by weights by volumes quoted on the internet and weighing a Seacast spacer).

I estimated/calculated the volume of the wood in my ’76 Seafari at about 3100 cu.in. I achieved nearly the same volume estimating I had poured 13.5 gallons to fill the transom.

Per a West Systems chart a ¾” x 2’ x 4’ epoxy coated panel weighs 17 lbs, therefore the wood density would be 0.02 lbs/cu.in. or 35 lbs/cu.ft. Another source shows 38 lbs/cu.ft.

The ¾” x 2’ x 4’ panel in Seacast would weigh 26 lbs and the density would be 0.03 lbs/cu.in. or 51 lbs/cu.ft. BTW the density of Arjay is slightly higher at 55 lbs/cu.ft.

CONCLUSION: For my 3100 cu.in. volume I replaced 62 - 68 lbs of wood with 93 lbs of Seacast.