You do know how to open a door for a serious debate, don't you?
There's more than "Bigger is better" when it comes to what engine and going fast. There's also a lot of things to look at when dealing with what kind of hull to get.
Let's start with the hull question, what kind of hull do I buy/build? This is answered by looking at what do you want to do with the boat. You need to look at the answers to the following questions:
- Do you want to be able to turn both ways or just run laps in one direction?
- Are you looking at getting a very large boat or just something that can handle a gas engine?
- Is the Umatilla River, where you're looking at running, fairly smooth or is it rough and shallow? I'm thinking Coe Street Park as one of the few places that would be possible to run due to the width of the river at that point
The reason these questions are important is that they will determine what style boat is better for you:
- If you don't mind running laps in one direction, a hydroplane would be fine, as long as the water is fairly smooth. If you want to be able to turn both left and right, go with a cat.
- You need to remember, speed and size means a wider turning radius. You said you want at least a 48" boat so, that said, you can plan on a controlled turn of at least, roughly speaking 10ft
- I'm assuming the Umatilla is the river you were referring to. That said, a hydroplane doesn't like more than an inch or two of chop as a general rule. If the river is normally rougher than that, look at getting a cat
As far as engines, there are Bonzi, Hanson and several others. While they are all created more or less equal, it really depends on how much of a modded engine you want. One thing to remember is that, generally speaking, the more modding, the more it's going to cost and the more finicky it's going to be