datatype conversion error.

hi Friends,

i have create one function and its function having parameter number which is in decimal.

but my datatype of this column in table is NVARCHAR.

now m execute this function like..  RETURN FLOAT

select myfunction ('4241338.10',1,60)

formula in function like. '4241338.10'+1/60

but its given error like cant convert Navrchar to number.

if am casting then its given wrong result like .it should be give 4241338.016554but its given me  4.01655492095025.

Please Help.