Propeller pitch for 1984 23 Open Fish?

[Fr. Frank]

...the Mirage Plus stinks on my 1984 23 cc. It drives the boat into the water, not matter how I tilt the engine.

Normally, that's caused by stern-lift and thrust-angle. If you pick up the stern, the bow rides lower. It makes for a softer ride. However, if can also be cause by a lack of hull rocker, or by hook in the hull.

The Mirage is fine in flat water (although it still rides low, on all 3 chines).

That is, in fact, to a certain extent, how the hull is designed to ride. At higher speeds, instead of the hull rising onto a pad, you get less wetted surface by the increased deflection of the water off each chine, giving you a larger air pocket outboard of each hard-chine. That is also why the hull does NOT have a flat pad anywhere on the hull. But if the hull will not rise onto the aft-most portions of all 3 chines, I'd be willing to bet you've got more than one issue, including a lack of rocker, or effective hook in your hull.

On the plus side it gives me the same top end speed of 40 mph. However, in heavy seas it simply plows into the waves, with no lift, poor stability, and soaks everyone, constantly.

I've never heard anyone wax eloquently about the dry ride of a SeaCraft, only a soft ride that doesn't beat you up. I agree that the 20 ad 23 hulls can be damp in a chop. But lack of stability is a problem that can be dangerous, and needs to be addressed.
What exactly does this lack of stability feel/look like? Usually, lack of stability is a problem that is masked or revealed by different propellers, but never caused by them. So even if you don't feel that lack with the old prop, it may still be there waiting to bite you at a time when you need stability badly.

I am going back to the 15-20 year old SS 17 pitch prop. That old prop gives great lift, and eats up heavy seas, riding high. Frankly, I don't know what I was thinking going to the Mirage. I ride on all 3 chines, or at least 2, at all times with the Mirage. Perhaps this has to do with the fact that the engine is not on a bracket, and is mounted direct, but no matter. The old prop rules, and the Mirage Plus is a no-go. I pop up on 1 chine in less than 5-7 seconds with the old prop.

It sounds like you equate "lift" solely with bow lift. For 80-90% of most boaters, that's what they want most of the time. in mild to moderate chop, a SeaCraft will generally ride through that stuff effortlessly, even with the bow trimmed up higher.

However, in dangerously big or really choppy seas, too much bow lift causes the hull to smack through waves on the bottom instead of cutting through them with the sharper bow. That's dangerous, even if it is drier. In moderate to heavy chop, or in seas greater than 2'-3', it is much safer to trim the bow down and cut through the waves, so that the motor is pushing the hull straight forward instead of up and forward.

After having said all of that, the old SS Yamaha SWS propellers are still one of the best designs out there.