propeller sizes?

[tata_dado]

Hello, I have Fjord 30 Cabin, with 2 V8 5.7L motors and z- drives. When I drive on 4000 rpm boat speed is about 18 kts. I'm interested in yours expiriences ( if someone here have a simmilar boat) because i think that i have to change propellers in order to achive better speed results. What sizes are you using. Thanks.