Benn is correct, get professional assistance. A good place to start might be:
SNAME European Regional Office
c/o Elkco Marine Group
61, Poseidonos Avenue
Palaio Faliro, 17561
GREECE
Telephone + 30 210 4528200
Fax +30 210 4526260
Email: sname@elkco.gr
Key Contact: Petros Lalangas
SERO Director
Email: plala@otenet.gr
SNAME has a very comprehensive library of technical documents which address the issues you are dealing with. The library also includes several good studies on proplusion efficiency and propeller selection for "trawler style" yachts in the same size range.
There is very good information available. Though it might cost $20 or $30, it is certainly worth it.