Some boats are just poor designs and will never handle well. My first boat was an utter pig. You had to hold you breath if you turned to the left. It would just spinout or roll. I just thought that I sucked as a driver (turns out I only kind of suck
) There are lots of good designs out there. I think we all see guys in our clubs that struggle a lot. In most cases I think that the equipment they have has more to do with their lack of success than driving skill. Nobody can make a POS work. It really helps the new guys if we can steer them towards decent equipment rather than having showing up with some freakshow they bought on ebay and try to make it work. Better for them, better for the hobby if they get something that is a proven design.
Lighter boats turn better for the most part. Three bladed props also add corner speed and stability but you give up something on the straightaways. I think you have to have a boat that is straight and doesn't flex to hang all the geometry off of. There is a reason why race car chassis/tubs are so stiff. Hanging a lot of exotic suspension (read sponson/bottom geometry in our case) on something that isn't rigid is just a waste of time and money.
Set up is huge. If you have to set the rudder trim anywhere but zero you are giving up a lot. Better to trim the turn fin or bracket. Clamp a stiff straight edge to the bracket then measure from the centerline of the boat and the straight edge all the way back to the transom. If the measurement gets greater or less shim the turn fin or bracket until it zeros out all the way down and you have a great starting point. .
I find straight versus curved fins to be a matter of driving style. I have seen guys run curved fins with a lot of success but they have to be set exactly. A straight not so much, especially the forward/aft position. If a curved fin is rotated too far back it's adding drag, too far forward and it's a hydrofoil lifting the sponson. I have tried curved fins but I find them to make a boat twitchy and it doesn't suit my driving style. I like smooth and predictable. I set straight fins with about a 5 degree tuck and the bottom of the fin parallel to the bottom and test it. If the right sponson is lifting in the turns I keep adding tuck until it just stops doing that and I'm done.
The turn fin cannot have a rounded edge. I made some like when I started that were rounded and effectively created a wing that caused the boat to pull to the right as I was creating a low pressure area on the right side of the turn fin. Turn fins can be exotic. Making breaks on the side is worthwhile if you are really looking for a small edge. The fin with breaks has less drag as it is running straight because you have less drag area and as the boat turns the aspect ratio opens up and the fin in reality will appear wider to the water for more grip. There is a school of thought in hydrodynamics that making the inside of the surface running in water dished because it creates a bubble that rotates and since air has less density than water there is less drag. The U8 crew actually was doing this some years ago and their boat would actually speed up in the turns. Villwock got nosy and put a straightedge on the inside of the fin and they outlawed it.
All this science doesn't really compensate for a well set up boat, sensible driving and the reliability to finishes heats.
When I coach new guys I try to get them to practice setting an arc through the corner. Set an arc at corner entrance and just let it run through the corners and get the rudder input out of it as soon as you can. A lot of steering inputs in the turns will get even a good handling boat to do something stupid as well as slow you down. Pretty much everybody sees how a boat picks up speed once the rudder gets straight coming off the turn. Having a rudder with any angle other the zero when the boat is going straight is just killing the speed.
The bottom line for me is if the boat won't hold lane 1 wide open in race water I have some work to do.