While more apparent in the older boats, this is a problem for all
composite boats.
Fiberglass and similar composites are actually Fiber Reinforced
Plastic FRP. The resins which the fabrics are saturated with are
various types of plastic, with Polyester, Vinylester and Epoxy being
the most common.
These plastics will deform over time, assuming the most natural/
relaxed position. Over the years your upside down boat just gradually
slumped. On boats with reinforcing ribs, the ribs tend to become more
apparent over time due to the same effect.
The suggestions for storing the boat in a sling, putting some weight,
etc are all good. If you put the boat where it will get warm, the
effect should be faster because the plastics do soften in the heat.
120 degrees will tweak almost any hull in a few days (a boat left in
the sun on a 90 degree day will get over 120 degrees). Sand will
absorb and hold the heat well, as does water (water is tricky though,
the boat has to be lined up right for it to work).
You may also just make the thwarts a little wider. Pushing out the
thwarts will splay the boat, lifting the ends, thus giving the boat
some rocker. Making the thwarts wider won't make it illegal for racing
as long as it meets specs (it will probably exceed them being wider
than before).
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