Took the refurbished MiniCups out on Lake Hall near Tallahassee today.
Good wind although a little gusty. I was in the older green boat
(Tadpole) that was less strong and I could feel the cockpit sides flex
compared to the strengthened orange boat (Satsuma). The rudder box was
bending under stress in spite of my doubling the wood thickness and
fibreglassing parts of it. Suddenly, CRAAAAACK... and the mast bent
waaaay over. Obviously, the mast step had given way. I was able to
sail on the other tack back to the landing.
My son sailed the other boat in increasing winds and it was waaaaay
cool. We sailed together and it was like a rocket with a huge wake
boiling behind, both of us leaning way out to balance her, waves
sluicing over the bow and even water coming over the side into the
cockpit at times. A really wild ride.
Got home and did sabre saw surgery on the injured boat. Found that rot
had nothing to do with it. The problem was the bathroom underlayment I
had built her from had simply split along a weak line. I'll have to
repair her by putting a piece of marine ply over that side of the mast
box and epoxy it in place followed by glass. Unfortunately, I expect
the other boat to eventually suffer the same fate giving me less
confidence in the boat till then.

Decided to oipen up the other side of the bow to inspect the mast
step/dagger board box. I am glad I did because I found the Gorilla
Glue I used when I began these boats has not held where it is immersed
in water. Somehow water is entering through the dagger board box and
has weakened the glue joining the box to the bottom. I cleaned out as
much of the old glue as I can and I plan to inject very thin epoxy into
the crack and then use a fillet of epoxy over the joint followed by
glass over that.
When I put the access ports on the bows of boat #2 I found no evidence
of water entering and I think this is because I started to use epoxy
much more extensively when I assembled it although it also has some
gorilla glue.
As far as bathroom underlayment goes, I would never use it again for
anything except firewood. After building the MiniCups, I built a
nesting dinghy from 1/4" marine ply and it has had a lot of use and it
shows no signs of material failure like the MiniCups.