I think and have experience myself that it depends on the boat. My first boat was a bfg 4.75 sea nyph and she could handle the waters well. The sides of the boat were above the knees and never had trouble going to the peak or the traps off Wollongong, we are talking between 9-12 kilometres from shore in 15-20 knot winds, no higher or further!!!! the boat also had a 70 hp on the back and it had the power to bring me home in a hurry. As the others said experience and common sense plays a big factor I have been out with mates and when I use to see conditions change above the 15-20 knot I always brought the anchor up and headed home but as always, some idiot tries to talk you out of it cause we're catching fish. These are the people that you don't go out in their boat cause they take stupid chances for a lousy fish!! There were many times when we made plans to go out to the traps but decided not to venture out especially when you have a wind coming from the north, a swell coming from the south and a current thats doing it's on little thing and the sea is like a washing machine. It is always good to go out with someone who has small boat experience and check out your boat in the water and be able to show you your boat limitations expecially on a day with 15-20 knot winds. I can assure you, you won't have to go out too far offshore too find out if she is good or not!!
Cheers