4 Blade Propeller Problem ?

Our gliding club operates a Rallye tug fitted with a Hoffman four blade
propeller. On repeated occasions it has had to be remove for rectification
to serious cracking of the fibre sheathing in the hub area. This can be
after as little as 150 hours flying over a 6 month period. Also, it is not
unique having occurred with a second bought principally to avoid grounding
the tug. This is in direct contrast to the four blade Hoffman unit fitted
to our Pawnee which has given several years of trouble free service.

Is anyone aware of operators having similar problems with the fibre covering
on Hoffman four blade propellers and, if so, has a solution been found?