Re: prop decision
Well, with a damaged prop it's hard to get a baseline. I'm assuming you have an aluminum prop. You could use a hammer and block of wood to backup the blades and reshape the areas that are "bent". Then use a file to take any burrs off the edges. If you have areas on the leading edge that are missing chunks of material you can have the prop welded and reconditioned as long as it's not too far gone. They can even re-pitch the prop down or up from where it's at now with limitations. With the performance you're reporting, it seems as if the prop is repairable. If you feel any vibration you need to get it fixed or replaced....which is why you are asking these questions I presume.
What you are describing with regard to riding nose high at less than 20 mph is just indicative of an I/O. I'm afraid it's just the nature of the beast. You have all of the weight of the engine and outdrive in the rear, so when you take off or slow down the nose rares up until you are on or off plane. Trimming all the way down is the best you can do in this case. When the outdrive is down/in, the propeller is pushing downward past horizontal which tends to raise the stern. When you are up on plane trimming up a bit raises the nose up so that less of the hull is in contact with the water and that allows the rpm to come up and hence speed goes up correspondingly. Unfortunately there is no way to change the speed at which you come off plane or get on plane with a prop, it is purely a function of the hull design and weight distribution of the boat. The rpm at which you get on plane or come off plane is affected by the prop, but not the speed...it's basically a constant for your setup. Getting as much weight forward as possible is your only option.
Think of a prop as a gear. A boat is like a car, but with only one gear. That gear has to perform through the entire rpm range of the engine. If the gear is too big (ie: pitch too large) the motor will struggle to get the boat out of the hole, but will have more top end speed. If the gear is too small (ie: pitch too small), the boat will snap out of the hole but have less top end and you run the risk of over revving the motor. Diameter affects the performance in a similar fashion, because as the diameter is reduced or increased, the surface area in contact with the water decreases and increases.
A 4 blade prop will have more surface area in contact with the water, and hence more push to get you up on plane...hole shot. However, you still have to size it such that you don't over-rev at WOT under a "normal" load. I like to size mine such that with a light load (just me in the boat with a normal load of fuel), I'm close to or right at the maximum WOT recommended for the motor. This way I know that it is never possible to over-rev the engine, and I have basically maximized the overall performance.
One of the characteristics of a 4-blade will be that you can hold a set speed easier when towing a skier/boarder. With an I/O it's hard to hold that 12-15 mph speed because the boat is in that transition between plane and off plane, so you are constantly playing with the throttle to stay at a constant speed for a skier/boarder. A 4-blade may help in that situation.
I have a 454 big block inboard and just changed from a 14 x14 3-blade to a 13 x 16 4-blade. I did not notice any drastic difference in hole shot, it's probably a little quicker out of the hole. I like the fact that I can put 8-9 adults in it and still get up and go. I could probably get more speed out of it with a 14 x 16 but with that prop it would take a while to get up and go with 8-9 people. The I/O and Inboard are an apples oranges comparison, but the principles are the same.
You'll just have to try a couple different props to see what works with how you want to use the boat, keeping in mind that you want the WOT rpm to be in upper end of the 4400-4800 range with a light load.
