I used to have a 4.3 mtr glass boat, and generally found it OK up to 15 knots. Even 20 knots was OK in the afternoon, provided that the wind was going in the same direction I was on the return trip home. Example was from Manly to Mud Island or Harries Artificial reef where typically the wind would swing N/NE in the afternoon.
A very respected service centre told me however that my outboard showed evidence of too much salt spray - fair amount of surface rust as a result. Once we had checked that nothing was causing excessive spray behind the boat, we concluded that I was pushing my boat in ordinary conditions, slowing down to a snail pace which causes a large rooster tail just behind the motor. Too much ingestion of salt spray is obviously detrimental to the health of your motor. That is another reason why it is important to learn how to plan your trips.
Scalem