Stanley,
Yes it’s all plywood. What looks like varnish actually is 3 layers of clear epoxy coating on the inside and outside. This makes it both watertight and water proof.
It has 4 casters to enable manoeuvring it around, and a ball valve tap to drain the water. A lid keeps the water in at high speeds, but isn’t necessary for engines this size. The sloped front enables lifting the prop out of the water. The largest engine I’ve run in it is my 1960 fastwin 18.
