The standard formula to use is Square root of (Weight in tons x HP ) x Constant = Speed in Knots. The constant for a planning hull is probably between 2.5 to 3
So if you know the all up weight (include absolutely everything ) of you rib and given that you will know your existing top speed and hp you can determine the constant for your boat. So then you change the hp to give the speed you want. Easy with a spreadsheet. All a bit theoretical I know but will give you a decent result to start from and you can play with the HP to get any ( within reason) speed you want.