And now for a little alternate universe theory:
It is pretty standard to go to a smaller impeller cut with a relatively light 18'.
A heavy boat needs the launch of a larger impeller and takes a hit in out-of-the-hole performance when a smaller cut is installed. However, a smaller impeller will allow the engine RPM higher (assuming it makes power there).
A (relatively) lighter boat like your's won't take such an out-of-the-hole hit in performance like the heavy boat will. So, if the impeller cut and engine power curve complement each other, the boat will ultimately be faster. There will be a loss of out-of-the-hole and for a hot rod (non-race) kind of boat, you will really notice it when pulling skiers.
I wouldn't say you you got burned, I just don't think they looked at the big picture.
Looking at your impeller, I have a couple thoughts. A stock impeller is a pretty crude casting. It is terribly prone to cavitation. The vanes are really thick and don't cut through the water very well. There is a huge knob on the hub, just between the shaft and the root of the vanes which disrupts flow. The radius between the vanes and the hub/shroud cavitate terribly. If you paint the impeller black, you will quickly see the cavitation burn in these areas. This cavitation acts like slipping the clutch while you are driving your car uphill at full-throttle. The RPM increased, but thrust was lost.
When you installed the loader, the water pressure at the inlet eye of the impeller increased, cavitation was reduced, and the RPM was pulled down. This is good, because the impeller is now working harder.
However, the concern is that you didn't pick up speed. With the installation of the loader, drag was increased. There is a balancing act between drag and speed. Too much drag and the boat is slow. Not loading the pump hard enough and the boat is slow.
So, where do you go from here? You will need a 4' straight edge to see how far the loader ramps hang below the keel. Place the straight edge on the fiberglass hull, to project the keel line into the pump opening. Do not try to rotate the straight edge to touch the back of the intake. All you care about is the keel line. Now you can take a measurement to the loader ramps. Just eyeballing it is deceptive.
Because you don't have a shoe, the back of the intake is not doing a whole lot to pick up water. Being a little more aggressive with the depth of the loader ramps can be beneficial. Too aggressive and the loader will shadow the back of the intake all while increasing drag. You can trim the loader ramps to fine tune it to your application. Be sure to resharpen the leading edges.
If you are open to spending more, detailing the front of your impeller will make it bite harder, equalavent to a full impeller cut size simply by reducing cavitation. An inducer (pre-impeller) will also increase the out-of-the-hole performance in much the same way, by increasing the water pressure at the impeller inlet eye, therefore reducing cavitation. The downside of installing an inducer in your boat will be the removal of the pump shaft to machine it for the inducer. That means everything has to come apart.
Hopefully this gives you enough to stew over for a minute. Post up if you have more questions.
Cheers,
Joe
