I completed my hatches for Picara. Made a light wooden frame with
one inch ribs spaced about every 6 inches. Cut regular 1 inch bead
board foam about 1/8 inch over sized and glued it inside the ribs
with epoxy as a force fit, glassed both sides to complete a rain
tight hatch. Seems plenty strong for it's purpose. Will also
insulate from the hot sun as needed. About 7 pounds for the 32 X 38
inch aft hatch.
Thinking of making a take down Dink in three parts with the same
construction ideas, might be able to stow it aft of the footwell on
my Picara "Mattie Cooper" -- would be more canoe size, 30 inches X
11 feet than full size Dink. Might use 2 inch foam and ribs with
enough plywood on top of the ribs to support 400 pounds or so. Will
be light weight. Most of the cost would be in epoxy and cloth,
guessing at about 2 gallon epoxy.
The glassing on both sides of the foam seem to add a lot of
strength, just used 4 oz cloth for the hatches -- 6 oz might be
better for a small boat. The thicker the foam core the stronger the
lay up is and the more rigid the foam keeps the ribs.
The foam sanded well with a rotary sanded, but extra care must be
used, because it will eat the foam very quickly. Also uneven spots
and the edge glueing was prepared with a thin epoxy putty to fill
gaps and also to seal the surface before glass was added. The glass
glued to the ribs every 6 inches or so seems to be important to get
a strong structure.
The foam cut the best with a metal cutting blade in a jig saw.
I am sure better foams might be used, but after the foam is sealed
in the glass it seems to fill the bill for hatches at $ 6 per sheet.