Since your running an off brand prop it's really hard to make comparisons. All props are different in how they perform. So a three blade this won't be anything like a three blade that. Most with Bajas run Merc props of the Mirage or Bravo designations. So comparing your prop to one of them will be a crap shoot. Find someone that'll let you borrow a 23 pitch Mirage for an hour. Then you'll have a solid base line for us to go on. It might even fit your desires. Once you've ran that Mirage, or even a Bravo, it's easy to say what will put you where you want to be from experience of known brand props.

Engine specs for operating rpm range are there to ensure the engine is rum in the peak power bandwidth, and not have the engine over-revving into valve float etc.

Unless you plan to run a few diff props depending on load etc, propping to the higher rpm is far better, so when you are loaded up, you aren't lugging the engine hard. Especially if you get a 4 blade that bites hard. Lugging will create really high cylinder pressures, with more time per power stroke (at lower rpm).

I prefer to run a bit higher, with a smaller pitch, but still within the engine safety margin for the components. Easier on the drive, easier on the 3000-3200 rpm cruise. (More intake vacuum...engine is not working as hard)
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Hope you get it dialed in.

This is honestly probably great advice. I don’t expect to be changing props, at least not every day. Maybe for a weekend here or there. But now that you say it, I suppose it’d be more critical to have all of the throttle available when the boats loaded, and just don’t go full throttle when she’s light, than the other way around. Thanks for chiming in!

Alright, I had some issues that kept me off the water for a bit but I'm back! I did get to try that 4 blade prop, and it was indeed a Rev 4 21P. Funny though, I really don't like it. Boat gets on plane in 6-7 seconds, which is comparable to my 3 blade, I don't have any more low speed control which I thought the 4 blade would do way better at, and it shaved a bit off the top end. It's basically been running pretty darn close to MPH = RPM; getting about 44 mph at about 4400 RPM. Which is still ludicrous prop slip, if we want to talk about that... And this Rev 4 prop is in perfect condition, no nicks, and I put a brand new hub in it. But I digress... The motor can still over-rev, even with 5 people on the boat (I think the most I'd ever realistically have is 6 or 7), it could turn 5k+ rpm (should be WOT 4200-4600). I have a local shop that has a prop "demo" program to let you try before you buy, and they offer the Bravo 4 blade 22P and a Mirage 3 blade 23P that I might want to try, to limit more rpm. But I'm afraid of losing TOO much holeshot...

I know they can be frowned upon but I do also own a SE Sport 400 hydrofoil that I'm not opposed to trying, to help with holeshot. Frankly, I kind of want the thing anyways because the boat porpoises really bad at trim, I don't think I'm getting to trim up at speed as much as the motor really wants.

I'm certainly not an expert...porpoising is from bow lift that cannot sustain and stalls out, in a constant cycle.

21P @4400 with 44mph you're still pushing 25% slip.

You now have more data with the 2nd prop and a 4 blade at that giving you slip. Maybe you have 1.65:1 Bravo 1 gears?
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I agree, and I have no quantifiable way to justify this, but I *feel* like my range of trim is incredibly small before I hit porpoising territory. I’m aware different props handle this different with varying degrees of bow/stern lift, so perhaps it’s all part of the same question.

I am pretty darn certain on the 1.5:1 bravo gears. Unless there’s a difference in the lower, as it’s replacement SEI lower, but to my knowledge all bravo one lowers are the same? As for the upper, the reason I haven’t been on the water in weeks is because my whole clutch mechanism failed and I couldn’t shift out of forward gear. I just rebuilt the whole upper myself and am 100% sure it’s the 27/32 gear set for 1.5:1 bravo one.

So, I've got a dumb follow-up question on prop slip numbers here... I'm going to confirm the tachometer with a timing light / digital tach, and I've already double-verified GPS speed that is giving me the wonky 25-28% prop slip numbers, that we all agree is odd. So long as that's accurate, the only things remaining (as far as I can tell) that could influence prop slip are things I can't change; hull design / condition, depth the sterndrive is mounted at, etc.

So here's the dumb question. Can I just accept that I'm getting 25% prop slip and prop it accordingly? For example, the prop selector on Mercury's website, calculated from my current performance numbers, recommends a Mirage Plus 25P. 25P sounds crazy to me, but perhaps its not - if I assume it's slipping substantially? I know I have to go up in pitch, as I reach my WOT rating at 3/4 throttle, if not less. My WOT should be 4600 and I can easily blow past 5K. According to google, 1" of pitch changes roughly 150-200 rpm; If I assume my actual current WOT is around 5200, I need to drop 600 rpm to get to my rated 4600 rpm; that'd be 3" or 4" of pitch increase from my current 21P prop - putting me dead on the 25P that Mercury's selector picked. Logically, seems to check out... Thoughts?

How you go about determining where to trim the drive while you are doing your testing?

Here is a page I wrote years ago explaining the process.

https://www.go-fast.com/Knowledge-ba...your-prop-slip

Yeah, that's effectively what I'm doing. In near perfectly calm water, getting up to my WOT rpm (which, again, I'm achieving at ~3/4 throttle). Then trimming it out. At full down position it feels a little rough, almost like a cavitation. Just a bump up from full down rectifies that. Trimming up from there, I can basically get 2-3mph more than where I started, before I start porpoising. I seem to have a pretty limited trim range between full down and porpoising.

Just because the throttle lever is at 3/4 does not mean that the carb is not at WOT. Most carbs and throttle bodies will be at WOT when the throttle lever is at 75-90% of the throw.

The stock prop that would have come on your boat OEM was most likely a 23" Mirage

BTW, Baja's will porpoise easily with 3 blade props and minimal trim. I had a 1993 24 Outlaw. 23 Mirage Plus was fastest prop, but a 24 Bravo 1 was better for every day cruising.