Piet,
The bigger the boat the less likely it will get finished... 30 foot is
a big boat and a major project in both time and money... Talk to folks
who have built a boat this large before starting... Go into it with
your eyes wide open... The plywood and the fiberglass is only a small
part of the total cost...
I just finished two 8 foot dingies in stitch and glue, and while it
took only a few months of my spare time (all my spare time and then
some) it was a major job...
I am right on the verge of starting on my retirement boat, a power
cat... It is planned to take one year, 4 to 6 full time hired workers,
and a quarter million USD to complete...

denny