John on here developed an app to help with this, not sure if it was ever released but I have the beta version on my phone as I tested it.
As others have said above your boat creates a hump of water in front of you. However the speed you can travel at before you start to climb the hump relates to you boat length. So assuming you have about 4.85m of hull in the water you can only travel at 5.3 knots before you start to fight against the hump.
Once over it though you're away
So if you're in a 6 knot limit there's no point in doing 6 knots as you're wasting fuel and creating loads of wash
Here's a pic of the app assuming that you have 4.85m of hull in the water...