at 17 kg a hull with out paint it will be under weight. my hulls were 17kg without decks on them, my boat is carrying 1.75 kg of lead.
Have you doubled the deck, or are you trying to go glass either side of the 3mm?
on my boat i had glass on the inside only and split the deck since then i have doubled the deck, as the plans say to and it is very strong now.